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Valenčič T, Ansdell P, Brownstein CG, Spillane PM, Holobar A, Škarabot J. Motor unit discharge rate modulation during isometric contractions to failure is intensity- and modality-dependent. J Physiol 2024; 602:2287-2314. [PMID: 38619366 DOI: 10.1113/jp286143] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2023] [Accepted: 03/25/2024] [Indexed: 04/16/2024] Open
Abstract
The physiological mechanisms determining the progressive decline in the maximal muscle torque production capacity during isometric contractions to task failure are known to depend on task demands. Task-specificity of the associated adjustments in motor unit discharge rate (MUDR), however, remains unclear. This study examined MUDR adjustments during different submaximal isometric knee extension tasks to failure. Participants performed a sustained and an intermittent task at 20% and 50% of maximal voluntary torque (MVT), respectively (Experiment 1). High-density surface EMG signals were recorded from vastus lateralis (VL) and medialis (VM) and decomposed into individual MU discharge timings, with the identified MUs tracked from recruitment to task failure. MUDR was quantified and normalised to intervals of 10% of contraction time (CT). MUDR of both muscles exhibited distinct modulation patterns in each task. During the 20% MVT sustained task, MUDR decreased until ∼50% CT, after which it gradually returned to baseline. Conversely, during the 50% MVT intermittent task, MUDR remained stable until ∼40-50% CT, after which it started to continually increase until task failure. To explore the effect of contraction intensity on the observed patterns, VL and VM MUDR was quantified during sustained contractions at 30% and 50% MVT (Experiment 2). During the 30% MVT sustained task, MUDR remained stable until ∼80-90% CT in both muscles, after which it continually increased until task failure. During the 50% MVT sustained task the increase in MUDR occurred earlier, after ∼70-80% CT. Our results suggest that adjustments in MUDR during submaximal isometric contractions to failure are contraction modality- and intensity-dependent. KEY POINTS: During prolonged muscle contractions a constant motor output can be maintained by recruitment of additional motor units and adjustments in their discharge rate. Whilst contraction-induced decrements in neuromuscular function are known to depend on task demands, task-specificity of motor unit discharge behaviour adjustments is still unclear. In this study, we tracked and compared discharge activity of several concurrently active motor units in the vastii muscles during different submaximal isometric knee extension tasks to failure, including intermittent vs. sustained contraction modalities performed in the same intensity domain (Experiment 1), and two sustained contractions performed at different intensities (Experiment 2). During each task, motor units modulated their discharge rate in a distinct, biphasic manner, with the modulation pattern depending on contraction intensity and modality. These results provide insight into motoneuronal adjustments during contraction tasks posing different demands on the neuromuscular system.
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Affiliation(s)
- Tamara Valenčič
- School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, UK
| | - Paul Ansdell
- Department of Sport, Exercise and Rehabilitation, Northumbria University, Newcastle upon Tyne, UK
| | - Callum G Brownstein
- School of Biomedical, Nutritional and Sport Sciences, Newcastle University, Newcastle upon Tyne, UK
| | - Padraig M Spillane
- Department of Sport, Exercise and Rehabilitation, Northumbria University, Newcastle upon Tyne, UK
| | - Aleš Holobar
- Faculty of Electrical Engineering and Computer Science, University of Maribor, Maribor, Slovenia
| | - Jakob Škarabot
- School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, UK
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2
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Magnuson JR, Dalton BH, McNeil CJ. The orderly recruitment of motor units may be modified when a muscle is acting as an antagonist. J Appl Physiol (1985) 2023; 135:519-526. [PMID: 37439237 DOI: 10.1152/japplphysiol.00203.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2023] [Revised: 06/20/2023] [Accepted: 07/07/2023] [Indexed: 07/14/2023] Open
Abstract
Despite the perceived importance of antagonist muscle activity, it is unknown if motor unit (MU) behavior at recruitment differs when a muscle acts as an antagonist versus agonist. Fourteen healthy participants performed ramped, isometric elbow flexor or extensor contractions to 50% or 100% maximal voluntary contraction (MVC) torque. Surface and fine-wire intramuscular electromyographic (EMG) recordings were sampled from biceps and triceps brachii. During agonist contractions, low-threshold MUs (recruited at <10% MVC torque) were sampled in all participants, with a total of 107 and 90 for biceps and triceps brachii, respectively. For ramped MVCs, antagonist surface EMG coactivation (% amplitude during agonist MVC) was 8.3 ± 6.6% for biceps and 15.2 ± 7.3% for triceps brachii. However, antagonist single MU activity was recorded from only four participants, with only one of these individuals having antagonist MUs recorded from both muscles. All antagonist MUs were successfully detected during agonist contractions, but many (∼40%) had a recruitment threshold >10% MVC torque. For MUs recorded during both agonist and antagonist contractions, discharge rate at recruitment was seemingly lower for antagonist than agonist contractions. Coexistence of typical levels of surface EMG-derived coactivation with scant antagonist MU recordings suggests that coactivation in these muscles is primarily the result of cross talk. Based on the limited antagonist MU data detected, MUs recruited early during an agonist contraction are not necessarily among those first recruited during an antagonist contraction. These findings highlight the possibility of a modification of orderly recruitment when a motoneuron pool is acting as an antagonist.NEW & NOTEWORTHY Modest levels of coactivation are widely considered essential for appropriate motor control; however, minimal attention has been given to recruitment patterns of motor units (MUs) from antagonist muscles. Despite the successful recording of many low-threshold MUs during agonist contractions, we recorded no antagonist MUs in most participants. Of the units recorded, only ∼60% matched those recruited at <10% of maximal torque when the muscle acted as an agonist, which suggests a modified recruitment order for antagonist MUs.
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Affiliation(s)
- Justine R Magnuson
- School of Health and Exercise Sciences and Centre for Heart, Lung and Vascular Health, The University of British Columbia, Kelowna, British Columbia, Canada
| | - Brian H Dalton
- School of Health and Exercise Sciences and Centre for Heart, Lung and Vascular Health, The University of British Columbia, Kelowna, British Columbia, Canada
| | - Chris J McNeil
- School of Health and Exercise Sciences and Centre for Heart, Lung and Vascular Health, The University of British Columbia, Kelowna, British Columbia, Canada
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Rodriguez-Falces J, Place N. Sarcolemmal Excitability, M-Wave Changes, and Conduction Velocity During a Sustained Low-Force Contraction. Front Physiol 2021; 12:732624. [PMID: 34721063 PMCID: PMC8554155 DOI: 10.3389/fphys.2021.732624] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2021] [Accepted: 09/14/2021] [Indexed: 11/20/2022] Open
Abstract
This study was undertaken to investigate whether sarcolemmal excitability is impaired during a sustained low-force contraction [10% maximal voluntary contraction (MVC)] by assessing muscle conduction velocity and also by analyzing separately the first and second phases of the muscle compound action potential (M wave). Twenty-one participants sustained an isometric knee extension of 10% MVC for 3min. M waves were evoked by supramaximal single shocks to the femoral nerve given at 10-s intervals. The amplitude, duration, and area of the first and second M-wave phases were computed. Muscle fiber conduction velocity, voluntary surface electromyographic (EMG), perceived effort, MVC force, peak twitch force, and temperature were also recorded. The main findings were: (1) During the sustained contraction, conduction velocity remained unchanged. (2) The amplitude of the M-wave first phase decreased for the first ~30s (−7%, p<0.05) and stabilized thereafter, whereas the second phase amplitude increased for the initial ~30s (+7%, p<0.05), before stabilizing. (3) Both duration and area decreased steeply during the first ~30s, and then more gradually for the rest of the contraction. (4) During the sustained contraction, perceived effort increased fivefold, whereas knee extension EMG increased by ~10%. (5) Maximal voluntary force and peak twitch force decreased (respectively, −9% and −10%, p<0.05) after the low-force contraction. Collectively, the present results indicate that sarcolemmal excitability is well preserved during a sustained 10% MVC task. A depression of the M-wave first phase during a low-force contraction can occur even in the absence of changes in membrane excitability. The development of fatigue during a low-force contraction can occur without alteration of membrane excitability.
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Affiliation(s)
- Javier Rodriguez-Falces
- Department of Electrical and Electronical Engineering, Public University of Navarre, Pamplona, Spain
| | - Nicolas Place
- Institute of Sport Sciences, University of Lausanne, Lausanne, Switzerland
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Delgadillo JD, Sundberg CW, Kwon M, Hunter SK. Fatigability of the knee extensor muscles during high-load fast and low-load slow resistance exercise in young and older adults. Exp Gerontol 2021; 154:111546. [PMID: 34492255 DOI: 10.1016/j.exger.2021.111546] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2021] [Revised: 08/08/2021] [Accepted: 08/26/2021] [Indexed: 10/20/2022]
Abstract
Resistance exercise training is a cornerstone in preventing age-related declines in muscle mass and strength, and fatigability of limb muscle is important to this adaptive response. It is unknown, however, whether fatigability and the underlying mechanisms differ between different resistance exercise protocols in young and older adults. The purpose of this study was to quantify the fatigability of the knee extensors and identify the mechanisms in 20 young (22.2 ± 1.3 yr, 10 women) and 20 older adults (73.8 ± 5.4 yr, 10 women) elicited by a single session of high- and low-load resistance exercise. One leg completed a high-load protocol with contractions performed as fast as possible (HL-fast, ~80% 1 Repetition Max, 1RM), and the contralateral leg a low-load protocol performed with slow contractions (LL-slow, ~30% 1RM, 6 s concentric, 6 s eccentric). Each exercise involved four sets of eight repetitions. Before and immediately following each set, maximal voluntary isometric contractions (MVC) were performed, and voluntary activation and contractile properties quantified using electrical stimulation. The reduction in MVC was greater following the LL-slow (20%) than the HL-fast (12%, P = 0.004), with no age or sex differences. Similarly, the reduction in the amplitude of the involuntary electrically-evoked twitch was greater in the LL-slow (14%) than the HL-fast (7%, P = 0.014) and correlated with the reduction in MVC (r = 0.546, P < 0.001), whereas voluntary activation decreased only for the LL-slow protocol (5%, P < 0.001). Thus, low-load resistance exercise with slow contractions induced greater fatigability within the muscle than a more traditional high-load resistance protocol for both young and older men and women.
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Affiliation(s)
- Jose D Delgadillo
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, USA
| | - Christopher W Sundberg
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, USA; Athletic and Human Performance Research Center, Marquette University, Milwaukee, WI, USA
| | - Minhyuk Kwon
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, USA; Department of Kinesiology & Health Promotion, California State Polytechnic University, Pomona, CA, USA
| | - Sandra K Hunter
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, USA; Athletic and Human Performance Research Center, Marquette University, Milwaukee, WI, USA.
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Enoka RM, Farina D. Force Steadiness: From Motor Units to Voluntary Actions. Physiology (Bethesda) 2021; 36:114-130. [DOI: 10.1152/physiol.00027.2020] [Citation(s) in RCA: 25] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Voluntary actions are controlled by the synaptic inputs that are shared by pools of spinal motor neurons. The slow common oscillations in the discharge times of motor units due to these synaptic inputs are strongly correlated with the fluctuations in force during submaximal isometric contractions (force steadiness) and moderately associated with performance scores on some tests of motor function. However, there are key gaps in knowledge that limit the interpretation of differences in force steadiness.
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Affiliation(s)
- Roger M. Enoka
- Department of Integrative Physiology, University of Colorado Boulder, Colorado
| | - Dario Farina
- Department of Bioengineering, Faculty of Engineering, Imperial College London, London, United Kingdom
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Do skeletal muscle motor units and microvascular units align to help match blood flow to metabolic demand? Eur J Appl Physiol 2021; 121:1241-1254. [PMID: 33538852 DOI: 10.1007/s00421-021-04598-4] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2020] [Accepted: 01/10/2021] [Indexed: 10/22/2022]
Abstract
PURPOSE We explore the motor unit recruitment and control of perfusion of microvascular units in skeletal muscle to determine whether they coordinate to match blood flow to metabolic demand. METHODS The PubMed database was searched for historical, current and relevant literature. RESULTS A microvascular, or capillary unit consists of 2-20 individual capillaries. Individual capillaries within a capillary unit cannot increase perfusion independently of other capillaries within the unit. Capillary units perfuse a short segment of approx. 12 muscle fibres located beside each other. Motor units consist of muscle fibres that can be dispersed widely within the muscle volume. During a contraction, where not all motor units are recruited, muscle fibre contraction will result in increased perfusion of associated capillaries as well as all capillaries within that capillary unit. Perfusion of the entire capillary unit will result in an increased blood flow delivery to muscle fibres associated with active motor unit plus approximately 11 other inactive muscle fibres within the same region. This will result in an overperfusion of the muscle resulting in blood flow in excess of the muscle fibre needs. CONCLUSIONS Given the architecture of the capillary units and the dispersed nature of muscle fibres within a motor unit, during submaximal contractions, where not all motor units are recruited, there will be a greater perfusion to the muscle than that predicted by the number of active muscle fibres. Such overperfusion brings into question if blood flow and metabolic demand are as tightly matched as previously assumed.
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Otieno LA, Semmler JG, Sidhu SK. Single joint fatiguing exercise decreases long but not short-interval intracortical inhibition in older adults. Exp Brain Res 2020; 239:47-58. [PMID: 33098654 DOI: 10.1007/s00221-020-05958-w] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2020] [Accepted: 10/13/2020] [Indexed: 12/18/2022]
Abstract
Ageing is accompanied by neuromuscular changes which may alter fatigue in older adults. These changes may include changes in corticospinal excitatory and inhibitory processes. Previous research has suggested that single joint fatiguing exercise decreases short-(SICI) and long-(LICI) interval intracortical inhibition in young adults. However, this is yet to be established in older adults. In 19 young (23 ± 4 years) and 18 older (69 ± 5 years) adults, SICI (2 ms interstimulus interval; ISI) and LICI (100 ms ISI) were measured in a resting first dorsal interosseous (FDI) muscle using transcranial magnetic stimulation (TMS) before and after a 15 min sustained submaximal contraction at 25% of their maximum EMG. Subsequent ten 2-min contractions held at 25% EMG were also performed to sustain fatigue for a total of 30 min, while SICI and LICI were taken immediately after each contraction. There was no change in SICI post-fatiguing exercise compared to baseline in both young and older adults (P = 0.4). Although there was no change in LICI post-fatiguing exercise in younger adults (P = 1.0), LICI was attenuated in older adults immediately post-fatiguing exercise and remained attenuated post-fatigue (PF)1 and PF2 (P < 0.05). Contrary to previous studies, the lack of change in SICI and LICI in young adults following a sustained submaximal EMG contraction suggests that GABA modulation may be dependent on the type of fatiguing task performed. The reduction in LICI in older adults post-fatiguing exercise suggests an age-related decrease in GABAB-mediated activity with sustained submaximal fatiguing exercise.
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Affiliation(s)
- Lavender A Otieno
- Discipline of Physiology, Adelaide Medical School, The University of Adelaide, S433, Helen Mayo South, Frome Rd, Adelaide, South Australia, 5005, Australia
| | - John G Semmler
- Discipline of Physiology, Adelaide Medical School, The University of Adelaide, S433, Helen Mayo South, Frome Rd, Adelaide, South Australia, 5005, Australia
| | - Simranjit K Sidhu
- Discipline of Physiology, Adelaide Medical School, The University of Adelaide, S433, Helen Mayo South, Frome Rd, Adelaide, South Australia, 5005, Australia.
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8
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Martinez-Valdes E, Negro F, Falla D, Dideriksen JL, Heckman CJ, Farina D. Inability to increase the neural drive to muscle is associated with task failure during submaximal contractions. J Neurophysiol 2020; 124:1110-1121. [DOI: 10.1152/jn.00447.2020] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Motor unit firing and contractile properties during a submaximal contraction until failure were assessed with a new tracking technique. Two distinct phases in firing behavior were observed, which compensated for changes in twitch area and predicted time to failure. However, the late increase in firing rate was below the rates attained in the absence of fatigue, which points to an inability of the central nervous system to sufficiently increase the neural drive to muscle with fatigue.
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Affiliation(s)
- Eduardo Martinez-Valdes
- Centre of Precision Rehabilitation for Spinal Pain (CPR Spine), School of Sport, Exercise and Rehabilitation Sciences, College of Life and Environmental Sciences, University of Birmingham, Birmingham, United Kingdom
| | - Francesco Negro
- Department of Clinical and Experimental Sciences, Research Centre for Neuromuscular Function and Adapted Physical Activity “Teresa Camplani,” Università degli Studi di Brescia, Brescia, Italy
| | - Deborah Falla
- Centre of Precision Rehabilitation for Spinal Pain (CPR Spine), School of Sport, Exercise and Rehabilitation Sciences, College of Life and Environmental Sciences, University of Birmingham, Birmingham, United Kingdom
| | - Jakob Lund Dideriksen
- Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Aalborg, Denmark
| | - C. J. Heckman
- Department of Physiology, Northwestern University, Chicago, Illinois
| | - Dario Farina
- Department of Bioengineering, Imperial College London, Royal School of Mines, London, United Kingdom
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9
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Brownstein CG, Espeit L, Royer N, Lapole T, Millet GY. Fatigue-induced changes in short-interval intracortical inhibition and the silent period with stimulus intensities evoking maximal versus submaximal responses. J Appl Physiol (1985) 2020; 129:205-217. [PMID: 32584668 DOI: 10.1152/japplphysiol.00282.2020] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
Abstract
During fatiguing exercise, previous studies have employed transcranial magnetic stimulation (TMS) paradigms eliciting either maximal or submaximal short-interval intracortical inhibition (SICI) and silent period (SP) durations. However, the effect of using either approach on the change in these variables with fatigue is unknown. This study examined the effects of using conditioning stimulus (CS, experiment A) and single-pulse TMS intensities (experiment B) that elicit maximal and submaximal SICI and SP duration (MaxSICI vs. SubmaxSICI in experiment A, MaxSP vs. SubmaxSP in experiment B) on the change in these measures with fatigue. In both experiments, participants performed a 10-min sustained isometric knee-extension contraction at a constant level of EMG, with measurements taken with maximal and submaximal intensities at baseline and every 2.5 min throughout the task. Immediately after the 10-min contraction (i.e., without recovery), responses were also measured at the same absolute force level as at baseline. In experiment A, no change in SICI was observed with either CS intensity throughout the EMG task (P > 0.05). However, an 18% decrease in SICI (i.e., less inhibition) was observed at the same absolute force only with the MaxSICI CS intensity (P < 0.01), with no change in SubmaxSICI (P = 0.72). In experiment B, the magnitude of increase in SP with fatigue was similar for both stimulus intensities (stimulus × time interaction: P = 0.44). These results suggest that CS intensities eliciting maximum SICI are more sensitive in detecting fatigue-induced reductions in SICI, whereas increases in SP are detectable with TMS intensities evoking maximal or submaximal SPs.NEW & NOTEWORTHY This study compared the change in silent period (SP) and short-interval intracortical inhibition (SICI) with conditioning stimulus and single-pulse transcranial magnetic stimulation (TMS) intensities (for SICI and SP, respectively) eliciting maximal and submaximal SICI and SP during fatiguing exercise. The results showed that changes in SICI were only detectable with intensities evoking maximal responses, with no difference between intensities for SP. These findings highlight the importance of maximizing SICI with appropriate intensities before measuring SICI during fatiguing exercise.
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Affiliation(s)
- Callum G Brownstein
- Inter-University Laboratory of Human Movement Science, Université Claude Bernard Lyon 1 and Université Jean Monnet-Saint-Etienne, EA 7424, Saint-Etienne, France
| | - Loïc Espeit
- Inter-University Laboratory of Human Movement Science, Université Claude Bernard Lyon 1 and Université Jean Monnet-Saint-Etienne, EA 7424, Saint-Etienne, France
| | - Nicolas Royer
- Inter-University Laboratory of Human Movement Science, Université Claude Bernard Lyon 1 and Université Jean Monnet-Saint-Etienne, EA 7424, Saint-Etienne, France
| | - Thomas Lapole
- Inter-University Laboratory of Human Movement Science, Université Claude Bernard Lyon 1 and Université Jean Monnet-Saint-Etienne, EA 7424, Saint-Etienne, France
| | - Guillaume Y Millet
- Inter-University Laboratory of Human Movement Science, Université Claude Bernard Lyon 1 and Université Jean Monnet-Saint-Etienne, EA 7424, Saint-Etienne, France
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10
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Deering RE, Cruz M, Senefeld JW, Pashibin T, Eickmeyer S, Hunter SK. Impaired Trunk Flexor Strength, Fatigability, and Steadiness in Postpartum Women. Med Sci Sports Exerc 2018; 50:1558-1569. [PMID: 29554014 PMCID: PMC6045430 DOI: 10.1249/mss.0000000000001609] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
PURPOSE To determine whether postpartum women (vaginal and cesarean delivery) have deficits in trunk flexor strength, fatigability and steadiness, compared with nulligravid women, up to 26 wk postpartum. We hypothesized that postpartum women would be weaker, more fatigable, and have greater torque fluctuations than controls, with cesarean delivery showing greater deficits than vaginal delivery. METHODS Twenty-two control women (nulligravid) and 29 postpartum women (20-40 yr, 19 who delivered via vaginal birth, 13 via Caesarian section) participated. Postpartum women were tested 8 to 10 wk and 24 to 26 wk postpartum. Control women were tested 16 to 18 wk apart. Maximal voluntary isometric contractions (MVC) were performed at multiple trunk positions with the trunk flexor muscles. To determine trunk flexor fatigability, subjects performed intermittent isometric contractions at 50% MVC (6-s contraction, 4-s rest) in upright sitting until task failure. An MVC was performed during the fatiguing task (one per minute) and at 10 and 20 min of recovery. RESULTS At 8 and 26 wk, postpartum women (groups pooled) were weaker at all trunk angles (38% and 44% respectively, P < 0.05) than controls despite no differences in handgrip strength. Postpartum women were more fatigable (71% and 52% respectively) and had greater torque fluctuations than controls (P < 0.05). At 8 wk postpartum, women who had a cesarean delivery, were 59% more fatigable (P = 0.004) than the vaginal delivery group, with no difference between delivery types at 26 wk postpartum. CONCLUSIONS Musculoskeletal recovery, including trunk flexor muscle strength and fatigability, is incomplete at 26 wk postpartum. These findings provide a rationale for future studies to address outcomes of rehabilitation programs specifically targeted at improving strength and fatigability of the trunk flexor muscles after pregnancy and childbirth.
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Affiliation(s)
- Rita E. Deering
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI
| | - Meredith Cruz
- Department of Obstetrics & Gynecology, Medical College of Wisconsin, Milwaukee, WI
| | - Jonathon W. Senefeld
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI
| | - Tatyana Pashibin
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI
| | - Sarah Eickmeyer
- Department of Rehabilitation Medicine, University of Kansas, Kansas City, KS
| | - Sandra K. Hunter
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI
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Abstract
Performance fatigability is characterized as an acute decline in motor performance caused by an exercise-induced reduction in force or power of the involved muscles. Multiple mechanisms contribute to performance fatigability and originate from neural and muscular processes, with the task demands dictating the mechanisms. This review highlights that (1) inadequate activation of the motoneuron pool can contribute to performance fatigability, and (2) the demands of the task and the physiological characteristics of the population assessed, dictate fatigability and the involved mechanisms. Examples of task and population differences in fatigability highlighted in this review include contraction intensity and velocity, stability and support provided to the fatiguing limb, sex differences, and aging. A future challenge is to define specific mechanisms of fatigability and to translate these findings to real-world performance and exercise training in healthy and clinical populations across the life span.
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Affiliation(s)
- Sandra K Hunter
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin 53201
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12
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Potvin JR, Fuglevand AJ. A motor unit-based model of muscle fatigue. PLoS Comput Biol 2017; 13:e1005581. [PMID: 28574981 PMCID: PMC5473583 DOI: 10.1371/journal.pcbi.1005581] [Citation(s) in RCA: 71] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2017] [Revised: 06/16/2017] [Accepted: 05/15/2017] [Indexed: 11/18/2022] Open
Abstract
Muscle fatigue is a temporary decline in the force and power capacity of skeletal muscle resulting from muscle activity. Because control of muscle is realized at the level of the motor unit (MU), it seems important to consider the physiological properties of motor units when attempting to understand and predict muscle fatigue. Therefore, we developed a phenomenological model of motor unit fatigue as a tractable means to predict muscle fatigue for a variety of tasks and to illustrate the individual contractile responses of MUs whose collective action determines the trajectory of changes in muscle force capacity during prolonged activity. An existing MU population model was used to simulate MU firing rates and isometric muscle forces and, to that model, we added fatigue-related changes in MU force, contraction time, and firing rate associated with sustained voluntary contractions. The model accurately estimated endurance times for sustained isometric contractions across a wide range of target levels. In addition, simulations were run for situations that have little experimental precedent to demonstrate the potential utility of the model to predict motor unit fatigue for more complicated, real-world applications. Moreover, the model provided insight into the complex orchestration of MU force contributions during fatigue, that would be unattainable with current experimental approaches.
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Affiliation(s)
- Jim R. Potvin
- Department of Kinesiology, McMaster University, Hamilton, Ontario, Canada
- * E-mail:
| | - Andrew J. Fuglevand
- Department of Physiology, College of Medicine, University of Arizona, Tucson, Arizona, United States of America
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13
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Chen YC, Lin YT, Chang GC, Hwang IS. Paradigm Shifts in Voluntary Force Control and Motor Unit Behaviors with the Manipulated Size of Visual Error Perception. Front Physiol 2017; 8:140. [PMID: 28348530 PMCID: PMC5346555 DOI: 10.3389/fphys.2017.00140] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2016] [Accepted: 02/23/2017] [Indexed: 12/13/2022] Open
Abstract
The detection of error information is an essential prerequisite of a feedback-based movement. This study investigated the differential behavior and neurophysiological mechanisms of a cyclic force-tracking task using error-reducing and error-enhancing feedback. The discharge patterns of a relatively large number of motor units (MUs) were assessed with custom-designed multi-channel surface electromyography following mathematical decomposition of the experimentally-measured signals. Force characteristics, force-discharge relation, and phase-locking cortical activities in the contralateral motor cortex to individual MUs were contrasted among the low (LSF), normal (NSF), and high scaling factor (HSF) conditions, in which the sizes of online execution errors were displayed with various amplification ratios. Along with a spectral shift of the force output toward a lower band, force output with a more phase-lead became less irregular, and tracking accuracy was worse in the LSF condition than in the HSF condition. The coherent discharge of high phasic (HP) MUs with the target signal was greater, and inter-spike intervals were larger, in the LSF condition than in the HSF condition. Force-tracking in the LSF condition manifested with stronger phase-locked EEG activity in the contralateral motor cortex to discharge of the (HP) MUs (LSF > NSF, HSF). The coherent discharge of the (HP) MUs during the cyclic force-tracking predominated the force-discharge relation, which increased inversely to the error scaling factor. In conclusion, the size of visualized error gates motor unit discharge, force-discharge relation, and the relative influences of the feedback and feedforward processes on force control. A smaller visualized error size favors voluntary force control using a feedforward process, in relation to a selective central modulation that enhance the coherent discharge of (HP) MUs.
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Affiliation(s)
- Yi-Ching Chen
- School of Physical Therapy, Chung Shan Medical UniversityTaichung, Taiwan; Physical Therapy Room, Chung Shan Medical University HospitalTaichung, Taiwan
| | - Yen-Ting Lin
- Physical Education Room, Asian University Taichung, Taiwan
| | - Gwo-Ching Chang
- Department of Information Engineering, I-Shou University Kaohsiung, Taiwan
| | - Ing-Shiou Hwang
- Institute of Allied Health Sciences, College of Medicine, National Cheng Kung UniversityTainan, Taiwan; Department of Physical Therapy, College of Medicine, National Cheng Kung UniversityTainan, Taiwan
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14
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Sex Differences in Neuromuscular Fatigability of the Knee Extensors Post-Stroke. Brain Sci 2017; 7:brainsci7010008. [PMID: 28085089 PMCID: PMC5297297 DOI: 10.3390/brainsci7010008] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2016] [Revised: 12/19/2016] [Accepted: 01/06/2017] [Indexed: 11/17/2022] Open
Abstract
BACKGROUND AND PURPOSE Despite the implications of optimizing strength training post-stroke, little is known about the differences in fatigability between men and women with chronic stroke. The purpose of this study was to determine the sex differences in knee extensor muscle fatigability and potential mechanisms in individuals with stroke. METHODS Eighteen participants (10 men, eight women) with chronic stroke (≥6 months) and 23 (12 men, 11 women) nonstroke controls participated in the study. Participants performed an intermittent isometric contraction task (6 s contraction, 3 s rest) at 30% of maximal voluntary contraction (MVC) torque until failure to maintain the target torque. Electromyography was used to determine muscle activation and contractile properties were assessed with electrical stimulation of the quadriceps muscles. RESULTS Individuals with stroke had a briefer task duration (greater fatigability) than nonstroke individuals (24.1 ± 17 min vs. 34.9 ± 16 min). Men were more fatigable than women for both nonstroke controls and individuals with stroke (17.9 ± 9 min vs. 41.6 ± 15 min). Individuals with stroke had less fatigue-related changes in muscle contractile properties and women with stroke differed in their muscle activation strategy during the fatiguing contractions. CONCLUSIONS Men and women fatigue differently post-stroke and this may be due to the way they neurally activate muscle groups.
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15
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Taylor JL, Amann M, Duchateau J, Meeusen R, Rice CL. Neural Contributions to Muscle Fatigue: From the Brain to the Muscle and Back Again. Med Sci Sports Exerc 2016; 48:2294-2306. [PMID: 27003703 PMCID: PMC5033663 DOI: 10.1249/mss.0000000000000923] [Citation(s) in RCA: 312] [Impact Index Per Article: 39.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
: During exercise, there is a progressive reduction in the ability to produce muscle force. Processes within the nervous system as well as within the muscles contribute to this fatigue. In addition to impaired function of the motor system, sensations associated with fatigue and impairment of homeostasis can contribute to the impairment of performance during exercise. This review discusses some of the neural changes that accompany exercise and the development of fatigue. The role of brain monoaminergic neurotransmitter systems in whole-body endurance performance is discussed, particularly with regard to exercise in hot environments. Next, fatigue-related alterations in the neuromuscular pathway are discussed in terms of changes in motor unit firing, motoneuron excitability, and motor cortical excitability. These changes have mostly been investigated during single-limb isometric contractions. Finally, the small-diameter muscle afferents that increase firing with exercise and fatigue are discussed. These afferents have roles in cardiovascular and respiratory responses to exercise, and in the impairment of exercise performance through interaction with the motor pathway, as well as in providing sensations of muscle discomfort. Thus, changes at all levels of the nervous system, including the brain, spinal cord, motor output, sensory input, and autonomic function, occur during exercise and fatigue. The mix of influences and the importance of their contribution vary with the type of exercise being performed.
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Affiliation(s)
- Janet L Taylor
- 1Neuroscience Research Australia, Sydney, AUSTRALIA; 2School of Medical Sciences, the University of New South Wales, Sydney, AUSTRALIA; 3Department of Medicine, University of Utah, Salt Lake City, UT; 4Laboratory of Applied Biology and Neurophysiology, ULB Neuroscience Institute, Université Libre de Bruxelles, Brussels, BELGIUM; 5Human Physiology Research Group Vrije Universiteit Brussel, Brussels, BELGIUM; 6School of Public Health, Tropical Medicine and Rehabilitation Sciences, James Cook University, Queensland, AUSTRALIA; and 7School of Kinesiology, and Department of Anatomy and Cell Biology, The University of Western Ontario, London, CANADA
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16
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Gould JR, Cleland BT, Mani D, Amiridis IG, Enoka RM. Motor unit activity in biceps brachii of left-handed humans during sustained contractions with two load types. J Neurophysiol 2016; 116:1358-65. [PMID: 27334949 DOI: 10.1152/jn.00147.2016] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2016] [Accepted: 06/18/2016] [Indexed: 11/22/2022] Open
Abstract
The purpose of the study was to compare the discharge characteristics of single motor units during sustained isometric contractions that required either force or position control in left-handed individuals. The target force for the two sustained contractions (24.9 ± 10.5% maximal force) was identical for each biceps brachii motor unit (n = 32) and set at 4.7 ± 2.0% of maximal voluntary contraction (MVC) force above its recruitment threshold (range: 0.5-41.2% MVC force). The contractions were not sustained to task failure, but the duration (range: 60-330 s) was identical for each motor unit and the decline in MVC force immediately after the sustained contractions was similar for the two tasks (force: 11.1% ± 13.7%; position: 11.6% ± 9.9%). Despite a greater increase in the rating of perceived exertion during the position task (task × time interaction, P < 0.006), the amplitude of the surface-recorded electromyogram for the agonist and antagonist muscles increased similarly during the two tasks. Nonetheless, mean discharge rate of the biceps brachii motor units declined more during the position task (task × time interaction, P < 0.01) and the variability in discharge times (coefficient of variation for interspike interval) increased only during the position task (task × time interaction, P < 0.008). When combined with the results of an identical study on right-handers (Mottram CJ, Jakobi JM, Semmler JG, Enoka RM. J Neurophysiol 93: 1381-1392, 2005), the findings indicate that handedness does not influence the adjustments in biceps brachii motor unit activity during sustained submaximal contractions requiring either force or position control.
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Affiliation(s)
- Jeffrey R Gould
- Department of Integrative Physiology, University of Colorado, Boulder, Colorado; and
| | - Brice T Cleland
- Department of Integrative Physiology, University of Colorado, Boulder, Colorado; and
| | - Diba Mani
- Department of Integrative Physiology, University of Colorado, Boulder, Colorado; and
| | - Ioannis G Amiridis
- Department of Physical Education and Sport Sciences, Aristotle University of Thessaloniki, Serres, Greece
| | - Roger M Enoka
- Department of Integrative Physiology, University of Colorado, Boulder, Colorado; and
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17
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Short-interval cortical inhibition and intracortical facilitation during submaximal voluntary contractions changes with fatigue. Exp Brain Res 2016; 234:2541-51. [PMID: 27165508 DOI: 10.1007/s00221-016-4658-9] [Citation(s) in RCA: 49] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2015] [Accepted: 04/20/2016] [Indexed: 10/21/2022]
Abstract
This study determined whether short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) change during a sustained submaximal isometric contraction. On 2 days, 12 participants (6 men, 6 women) performed brief (7-s) elbow flexor contractions before and after a 10-min fatiguing contraction; all contractions were performed at the level of integrated electromyographic activity (EMG) which produced 25 % maximal unfatigued torque. During the brief 7-s and 10-min submaximal contractions, single (test) and paired (conditioning-test) transcranial magnetic stimuli were applied over the motor cortex (5 s apart) to elicit motor-evoked potentials (MEPs) in biceps brachii. SICI and ICF were elicited on separate days, with a conditioning-test interstimulus interval of 2.5 and 15 ms, respectively. On both days, integrated EMG remained constant while torque fell during the sustained contraction by ~51.5 % from control contractions, perceived effort increased threefold, and MVC declined by 21-22 %. For SICI, the conditioned MEP during control contractions (74.1 ± 2.5 % of unconditioned MEP) increased (less inhibition) during the sustained contraction (last 2.5 min: 86.0 ± 5.1 %; P < 0.05). It remained elevated in recovery contractions at 2 min (82.0 ± 3.8 %; P < 0.05) and returned toward control at 7-min recovery (76.3 ± 3.2 %). ICF during control contractions (conditioned MEP 129.7 ± 4.8 % of unconditioned MEP) decreased (less facilitation) during the sustained contraction (last 2.5 min: 107.6 ± 6.8 %; P < 0.05) and recovered to 122.8 ± 4.3 % during contractions after 2 min of recovery. Both intracortical inhibitory and facilitatory circuits become less excitable with fatigue when assessed during voluntary activity, but their different time courses of recovery suggest different mechanisms for the fatigue-related changes of SICI and ICF.
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18
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Hasson CJ, Gelina O, Woo G. Neural Control Adaptation to Motor Noise Manipulation. Front Hum Neurosci 2016; 10:59. [PMID: 26973487 PMCID: PMC4771770 DOI: 10.3389/fnhum.2016.00059] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2015] [Accepted: 02/08/2016] [Indexed: 12/02/2022] Open
Abstract
Antagonistic muscular co-activation can compensate for movement variability induced by motor noise at the expense of increased energetic costs. Greater antagonistic co-activation is commonly observed in older adults, which could be an adaptation to increased motor noise. The present study tested this hypothesis by manipulating motor noise in 12 young subjects while they practiced a goal-directed task using a myoelectric virtual arm, which was controlled by their biceps and triceps muscle activity. Motor noise was increased by increasing the coefficient of variation (CV) of the myoelectric signals. As hypothesized, subjects adapted by increasing antagonistic co-activation, and this was associated with reduced noise-induced performance decrements. A second hypothesis was that a virtual decrease in motor noise, achieved by smoothing the myoelectric signals, would have the opposite effect: co-activation would decrease and motor performance would improve. However, the results showed that a decrease in noise made performance worse instead of better, with no change in co-activation. Overall, these findings suggest that the nervous system adapts to virtual increases in motor noise by increasing antagonistic co-activation, and this preserves motor performance. Reducing noise may have failed to benefit performance due to characteristics of the filtering process itself, e.g., delays are introduced and muscle activity bursts are attenuated. The observed adaptations to increased noise may explain in part why older adults and many patient populations have greater antagonistic co-activation, which could represent an adaptation to increased motor noise, along with a desire for increased joint stability.
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Affiliation(s)
- Christopher J. Hasson
- Neuromotor Systems Laboratory, Department of Physical Therapy, Movement and Rehabilitation Sciences, Northeastern UniversityBoston, MA, USA
| | - Olga Gelina
- Neuromotor Systems Laboratory, Department of Physical Therapy, Movement and Rehabilitation Sciences, Northeastern UniversityBoston, MA, USA
| | - Garrett Woo
- Neuromotor Systems Laboratory, Department of Physical Therapy, Movement and Rehabilitation Sciences, Northeastern UniversityBoston, MA, USA
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19
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Héroux ME, Butler AA, Gandevia SC, Taylor JL, Butler JE. Time course of human motoneuron recovery after sustained low-level voluntary activity. J Neurophysiol 2016; 115:803-12. [DOI: 10.1152/jn.00950.2015] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2015] [Accepted: 11/25/2015] [Indexed: 11/22/2022] Open
Abstract
Motoneurons often fire repetitively and for long periods. In sustained voluntary contractions the excitability of motoneurons declines. We provide the first detailed description of the time course of human motoneuron recovery after sustained activity at a constant discharge rate. We recorded the discharge of single motor units (MUs, n = 30) with intramuscular wire electrodes inserted in triceps brachii during weak isometric contractions. Subjects ( n = 15) discharged single MUs at a constant frequency (∼10 Hz) with visual feedback for prolonged durations (3–7 min) until rectified surface electromyogram (sEMG) of triceps brachii increased by ∼100%. After a rest of 1–2, 15, 30, 60, 120, or 240 s, subjects briefly resumed the contraction with the target MU at the same discharge rate. Each MU was tested with three to four rest periods. The magnitude of sEMG was increased when contractions were resumed, and the target motoneuron discharged at the test frequency following rest intervals of 2–60 s ( P = 0.001–0.038). The increased sEMG indicates that greater excitatory drive was needed to discharge the motoneuron at the test rate. The increase in EMG recovered exponentially with a time constant of 28 s but did not return to baseline even after a rest period of ∼240 s. Thus the decline in motoneuron excitability from a weak contraction takes several minutes to recover fully.
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Affiliation(s)
- Martin E. Héroux
- Neuroscience Research Australia and University of New South Wales, Sydney, New South Wales, Australia
| | - Annie A. Butler
- Neuroscience Research Australia and University of New South Wales, Sydney, New South Wales, Australia
| | - Simon C. Gandevia
- Neuroscience Research Australia and University of New South Wales, Sydney, New South Wales, Australia
| | - Janet L. Taylor
- Neuroscience Research Australia and University of New South Wales, Sydney, New South Wales, Australia
| | - Jane E. Butler
- Neuroscience Research Australia and University of New South Wales, Sydney, New South Wales, Australia
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20
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Yoon T, Doyel R, Widule C, Hunter SK. Sex differences with aging in the fatigability of dynamic contractions. Exp Gerontol 2015; 70:1-10. [PMID: 26159162 DOI: 10.1016/j.exger.2015.07.001] [Citation(s) in RCA: 45] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2015] [Revised: 07/01/2015] [Accepted: 07/02/2015] [Indexed: 01/18/2023]
Abstract
This study determined the sex difference with aging in fatigability of the elbow flexor muscles during a dynamic fatiguing task, and explored the associated mechanisms. We compared fatigability of the elbow flexor muscles in 18 young (20.2 ± 1 years: 9 men) and 36 old adults (73.5 ± 1 years: 16 men) during and in recovery from repeated dynamic contractions (~60°/s) with a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque until failure. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (an increase in the superimposed twitch, SIT) and the peak rate of muscle relaxation. Time to failure was briefer for the men than the women (6.1 ± 2.1 vs. 9.7 ± 5.5 min, respectively; P=0.02) with no difference between young and old adults (7.2 ± 2.9 vs. 8.4 ± 5.2 min, respectively, P=0.45) and no interaction (P>0.05). The relative decline in peak relaxation rate with fatigability was similar for young and old adults (P=0.11), but greater for men than women (P=0.046). Supraspinal fatigue increased for all groups and was associated with the time to failure (P<0.05). Regression analysis however, indicated that the time to failure was best predicted by the peak relaxation rate (baseline values and slowing with fatigability) (r(2)=0.55). Rate-limiting contractile mechanisms (e.g. excitation-contraction coupling) were responsible for the increased fatigability of the elbow flexors of men compared with women for a dynamic fatiguing task of slow angular velocity, and this sex difference was maintained with aging. The age difference in fatigability for the dynamic task was diminished for both sexes relative to what is typically observed with isometric fatiguing contractions.
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Affiliation(s)
- Tejin Yoon
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, United States; Department of Kinesiology and Integrative Physiology, Michigan Technological University, Houghton, MI, United States
| | - Ryan Doyel
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, United States; Creighton University School of Medicine, Omaha, NE, United States
| | - Claire Widule
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, United States; Uniformed Services University of the Health Sciences, F. Edward Hebert School of Medicine, Bethesda, MD, United States
| | - Sandra K Hunter
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI, United States.
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21
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Pascoe MA, Holmes MR, Stuart DG, Enoka RM. Discharge characteristics of motor units during long-duration contractions. Exp Physiol 2014; 99:1387-98. [PMID: 25016025 DOI: 10.1113/expphysiol.2014.078584] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The purpose of the study was to determine how long humans could sustain the discharge of single motor units during a voluntary contraction. The discharge of motor units in first dorsal interosseus of subjects (27.8 ± 8.1 years old) was recorded for as long as possible. The task was terminated when the isolated motor unit stopped discharging action potentials, despite the ability of the individual to sustain the abduction force. Twenty-three single motor units were recorded. Task duration was 21.4 ± 17.8 min. When analysed across discharge duration, mean discharge rate (10.6 ± 1.8 pulses s(-1)) and mean abduction force (5.5 ± 2.8% maximum) did not change significantly (discharge rate, P = 0.119; and abduction force, P = 0.235). In contrast, the coefficient of variation for interspike interval during the initial 30 s of the task was 22.2 ± 6.0% and increased to 31.9 ± 7.0% during the final 30 s (P < 0.001). All motor units were recruited again after 60 s of rest. Although subjects were able to sustain a relatively constant discharge rate, the cessation of discharge was preceded by a gradual increase in discharge variability. The findings also showed that the maximal duration of human motor unit discharge exceeds that previously reported for the discharge elicited in motor neurons by intracellular current injection in vitro.
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Affiliation(s)
- Michael A Pascoe
- Department of Integrative Physiology, University of Colorado, Boulder, CO, USA
| | - Matthew R Holmes
- Department of Integrative Physiology, University of Colorado, Boulder, CO, USA
| | - Douglas G Stuart
- Department of Physiology, University of Arizona, Tucson, AZ, USA
| | - Roger M Enoka
- Department of Integrative Physiology, University of Colorado, Boulder, CO, USA
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22
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Vanden Noven ML, Pereira HM, Yoon T, Stevens AA, Nielson KA, Hunter SK. Motor Variability during Sustained Contractions Increases with Cognitive Demand in Older Adults. Front Aging Neurosci 2014; 6:97. [PMID: 24904410 PMCID: PMC4033244 DOI: 10.3389/fnagi.2014.00097] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2014] [Accepted: 05/05/2014] [Indexed: 01/26/2023] Open
Abstract
To expose cortical involvement in age-related changes in motor performance, we compared steadiness (force fluctuations) and fatigability of submaximal isometric contractions with the ankle dorsiflexor muscles in older and young adults and with varying levels of cognitive demand imposed. Sixteen young (20.4 ± 2.1 year: 8 men, 9 women) and 17 older adults (68.8 ± 4.4 years: 9 men, 8 women) attended three sessions and performed a 40 s isometric contraction at 5% maximal voluntary contraction (MVC) force followed by an isometric contraction at 30% MVC until task failure. The cognitive demand required during the submaximal contractions in each session differed as follows: (1) high-cognitive demand session where difficult mental math was imposed (counting backward by 13 from a 4-digit number); (2) low-cognitive demand session which involved simple mental math (counting backward by 1); and (3) control session with no mental math. Anxiety was elevated during the high-cognitive demand session compared with other sessions for both age groups but more so for the older adults than young adults (p < 0.05). Older adults had larger force fluctuations than young adults during: (1) the 5% MVC task as cognitive demand increased (p = 0.007), and (2) the fatiguing contraction for all sessions (p = 0.002). Time to task failure did not differ between sessions or age groups (p > 0.05), but the variability between sessions (standard deviation of three sessions) was greater for older adults than young (2.02 ± 1.05 vs. 1.25 ± 0.51 min, p < 0.05). Thus, variability in lower limb motor performance for low- and moderate-force isometric tasks increased with age and was exacerbated when cognitive demand was imposed, and may be related to modulation of synergist and antagonist muscles and an altered neural strategy with age originating from central sources. These data have significant implications for cognitively demanding low-force motor tasks that are relevant to functional and ergonomic in an aging workforce.
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Affiliation(s)
- Marnie L Vanden Noven
- Exercise Science Program, Department of Physical Therapy, Marquette University , Milwaukee, WI , USA
| | - Hugo M Pereira
- Exercise Science Program, Department of Physical Therapy, Marquette University , Milwaukee, WI , USA
| | - Tejin Yoon
- Exercise Science Program, Department of Physical Therapy, Marquette University , Milwaukee, WI , USA
| | - Alyssa A Stevens
- Exercise Science Program, Department of Physical Therapy, Marquette University , Milwaukee, WI , USA
| | - Kristy A Nielson
- Department of Psychology, Marquette University , Milwaukee, WI , USA
| | - Sandra K Hunter
- Exercise Science Program, Department of Physical Therapy, Marquette University , Milwaukee, WI , USA
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23
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Rybar MM, Walker ER, Kuhnen HR, Ouellette DR, Hunter SK, Hyngstrom AS, Hyngstrom AS. The stroke-related effects of hip flexion fatigue on over ground walking. Gait Posture 2014; 39:1103-8. [PMID: 24602975 PMCID: PMC4007512 DOI: 10.1016/j.gaitpost.2014.01.012] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/23/2013] [Revised: 12/19/2013] [Accepted: 01/22/2014] [Indexed: 02/02/2023]
Abstract
Individuals post stroke often rely more on hip flexors for limb advancement during walking due to distal weakness but the effects of muscle fatigue in this group is not known. The purpose of this study was to quantify how stroke affects the influence of hip flexor fatigue on over ground walking kinematics and performance and muscle activation. Ten individuals with chronic stroke and 10 without stroke (controls) participated in the study. Maximal walking speed, walking distance, muscle electromyograms (EMG), and lower extremity joint kinematics were compared before and after dynamic, submaximal fatiguing contractions of the hip flexors (30% maximal load) performed until failure of the task. Task duration and decline in hip flexion maximal voluntary contraction (MVC) and power were used to assess fatigue. The stroke and control groups had similar task durations and percent reductions in MVC force following fatiguing contractions. Compared with controls, individuals with stroke had larger percent reductions in maximal walking speed, greater decrements in hip range of motion and peak velocity during swing, greater decrements in ankle velocity and lack of modulation of hip flexor EMG following fatiguing dynamic hip flexion contractions. For a given level of fatigue, the impact on walking function was more profound in individuals with stroke than neurologically intact individuals, and a decreased ability to up regulate hip flexor muscle activity may contribute. These data highlight the importance of monitoring the effect of hip flexor muscle activity during exercise or performance of activities of daily living on walking function post stroke.
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Affiliation(s)
- Megan M. Rybar
- Department of Physical Therapy, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA
| | - Eric R. Walker
- Department of Biomedical Engineering, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA
| | - Henry R. Kuhnen
- Department of Biomedical Engineering, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA
| | - Daniel R. Ouellette
- Department of Physical Therapy, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA
| | - Sandra K. Hunter
- Department of Physical Therapy, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA
| | - Allison S. Hyngstrom
- Department of Physical Therapy, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA
| | - Allison S Hyngstrom
- Department of Physical Therapy, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA.
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24
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Kisiel-Sajewicz K, Siemionow V, Seyidova-Khoshknabi D, Davis MP, Wyant A, Ranganathan VK, Walsh D, Yan JH, Hou J, Yue GH. Myoelectrical manifestation of fatigue less prominent in patients with cancer related fatigue. PLoS One 2013; 8:e83636. [PMID: 24391800 PMCID: PMC3877402 DOI: 10.1371/journal.pone.0083636] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2013] [Accepted: 11/05/2013] [Indexed: 11/18/2022] Open
Abstract
PURPOSE A lack of fatigue-related muscle contractile property changes at time of perceived physical exhaustion and greater central than peripheral fatigue detected by twitch interpolation technique have recently been reported in cancer survivors with fatigue symptoms. Based on these observations, it was hypothesized that compared to healthy people, myoelectrical manifestation of fatigue in the performing muscles would be less significant in these individuals while sustaining a prolonged motor task to self-perceived exhaustion (SPE) since their central fatigue was more prominent. The purpose of this study was to test this hypothesis by examining electromyographic (EMG) signal changes during fatiguing muscle performance. METHODS Twelve individuals who had advanced solid cancer and cancer-related fatigue (CRF), and 12 age- and gender-matched healthy controls performed a sustained elbow flexion at 30% maximal voluntary contraction till SPE. Amplitude and mean power frequency (MPF) of EMG signals of the biceps brachii, brachioradialis, and triceps brachii muscles were evaluated when the individuals experienced minimal, moderate, and severe fatigue. RESULTS CRF patients perceived physical "exhaustion" significantly sooner than the controls. The myoelectrical manifestation of muscular fatigue assessed by EMG amplitude and MPF was less significant in CRF than controls. The lower MPF even at minimal fatigue stage in CRF may indicate pathophysiologic condition of the muscle. CONCLUSIONS CRF patients experience less myoelectrical manifestation of muscle fatigue than healthy individuals near the time of SPE. The data suggest that central nervous system fatigue plays a more important role in limiting endurance-type of motor performance in patients with CRF.
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Affiliation(s)
- Katarzyna Kisiel-Sajewicz
- Department of Biomedical Engineering, the Lerner Research Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
- Department of Kinesiology, Faculty of Physiotherapy, University School of Physical Education in Wroclaw, Wroclaw, Poland
| | - Vlodek Siemionow
- Department of Biomedical Engineering, the Lerner Research Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
- Department of Physical Medicine and Rehabilitation, the Neurological Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
| | - Dilara Seyidova-Khoshknabi
- The Harry R. Horvitz Center for Palliative Medicine, the Taussig Cancer Center, the Cleveland Clinic, Cleveland, Ohio, United States of America
| | - Mellar P. Davis
- The Harry R. Horvitz Center for Palliative Medicine, the Taussig Cancer Center, the Cleveland Clinic, Cleveland, Ohio, United States of America
| | - Alexandria Wyant
- Department of Biomedical Engineering, the Lerner Research Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
| | - Vinoth K. Ranganathan
- Department of Biomedical Engineering, the Lerner Research Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
- Department of Physical Medicine and Rehabilitation, the Neurological Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
| | - Declan Walsh
- The Harry R. Horvitz Center for Palliative Medicine, the Taussig Cancer Center, the Cleveland Clinic, Cleveland, Ohio, United States of America
| | - Jin H. Yan
- Institute of Affective and Social Neuroscience, Shenzhen University, Shenzhen; Department of Psychology, Tsinghuan University, Beijing, China
- * E-mail: (GHY); (JHY)
| | - Juliet Hou
- Department of Physical Medicine and Rehabilitation, the Neurological Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
| | - Guang H. Yue
- Department of Biomedical Engineering, the Lerner Research Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
- Department of Physical Medicine and Rehabilitation, the Neurological Institute, the Cleveland Clinic, Cleveland, Ohio, United States of America
- Kessler Foundation Research Center, West Orange, New Jersey, United States of America
- * E-mail: (GHY); (JHY)
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Abstract
Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.
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Affiliation(s)
- C J Heckman
- Northwestern University, Evanston, Illinois, USA.
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Kelly LA, Racinais S, Cresswell AG. Discharge properties of abductor hallucis before, during, and after an isometric fatigue task. J Neurophysiol 2013; 110:891-8. [PMID: 23678020 DOI: 10.1152/jn.00944.2012] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Abductor hallucis is the largest muscle in the arch of the human foot and comprises few motor units relative to its physiological cross-sectional area. It has been described as a postural muscle, aiding in the stabilization of the longitudinal arch during stance and gait. The purpose of this study was to describe the discharge properties of abductor hallucis motor units during ramp and hold isometric contractions, as well as its discharge characteristics during fatigue. Intramuscular electromyographic recordings from abductor hallucis were made in 5 subjects; from those recordings, 42 single motor units were decomposed. Data were recorded during isometric ramp contractions at 60% maximum voluntary contraction (MVC), performed before and after a submaximal isometric contraction to failure (mean force 41.3 ± 15.3% MVC, mean duration 233 ± 116 s). Motor unit recruitment thresholds ranged from 10.3 to 54.2% MVC. No significant difference was observed between recruitment and derecruitment thresholds or their respective discharge rates for both the initial and postfatigue ramp contractions (all P > 0.25). Recruitment threshold was positively correlated with recruitment discharge rate (r = 0.47, P < 0.03). All motor units attained similar peak discharge rates (14.0 ± 0.25 pulses/s) and were not correlated with recruitment threshold. Thirteen motor units could be followed during the isometric fatigue task, with a decline in discharge rate and increase in discharge rate variability occurring in the final 25% of the task (both P < 0.05). We have shown that abductor hallucis motor units discharge relatively slowly and are considerably resistant to fatigue. These characteristics may be effective for generating and sustaining the substantial level of force that is required to stabilize the longitudinal arch during weight bearing.
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Affiliation(s)
- Luke A Kelly
- Department of Exercise and Sport Science, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, Doha, Qatar
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Pascoe MA, Gould JR, Enoka RM. Motor unit activity when young and old adults perform steady contractions while supporting an inertial load. J Neurophysiol 2012; 109:1055-64. [PMID: 23221403 DOI: 10.1152/jn.00437.2012] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The purpose of the study was to compare the discharge characteristics of biceps brachii motor units of young and old adults when they performed steady, submaximal contractions while the arm supported different inertial loads. Young (28 ± 4 yr; n = 16) and old (75 ± 4 yr; n = 14) adults performed steady contractions with the elbow flexors at target forces set at either small (11.7 ± 4.4% maximum) or large (17.8 ± 6.5% maximum) differences below the recruitment threshold force of the motor unit (n = 40). The task was to maintain an elbow angle at 1.57 rad until the motor unit was recruited and discharged action potentials for ∼120 s. Time to recruitment was longer for the larger target force difference (187 ± 227 s vs. 23 ± 46 s, P < 0.001). Once recruited, motor units discharged action potentials either repetitively or intermittently, with a greater proportion of motor units exhibiting the repetitive pattern for old adults. Discharge rate at recruitment and during the steady contraction was similar for the two target force differences for old adults but was greater for the small target force difference for young adults. Discharge variability was similar at recruitment for the two age groups but less for the old adults during the steady contraction. The greatest difference between the present results and those reported previously when the arm pulled against a rigid restraint was that old adults modulated discharge rate less than young adults across the two contraction intensities for both load types.
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Affiliation(s)
- Michael A Pascoe
- Department of Integrative Physiology, University of Colorado, Boulder, Colorado 80045, USA.
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Yoon T, Schlinder-Delap B, Hunter SK. Fatigability and recovery of arm muscles with advanced age for dynamic and isometric contractions. Exp Gerontol 2012; 48:259-68. [PMID: 23103238 DOI: 10.1016/j.exger.2012.10.006] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2012] [Revised: 09/21/2012] [Accepted: 10/19/2012] [Indexed: 01/22/2023]
Abstract
This study determined whether age-related mechanisms can increase fatigue of arm muscles during maximal velocity dynamic contractions, as it occurs in the lower limb. We compared elbow flexor fatigue of young (n=10, 20.8±2.7 years) and old men (n=16, 73.8±6.1 years) during and in recovery from a dynamic and an isometric postural fatiguing task. Each task was maintained until failure while supporting a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (superimposed twitch, SIT) and muscle relaxation. Time to failure was longer for the old men than for the young men for the isometric task (9.5±3.1 vs. 17.2±7.0 min, P=0.01) but similar for the dynamic task (6.3±2.4 min vs. 6.0±2.0 min, P=0.73). Initial peak rate of relaxation was slower for the old men than for the young men, and was associated with a longer time to failure for both tasks (P<0.05). Low initial power during elbow flexion was associated with the greatest difference (reduction) in time to failure between the isometric task and the dynamic task (r=-0.54, P=0.015). SIT declined after both fatigue tasks similarly with age, although the recovery of SIT was associated with MVIC recovery for the old (both sessions) but not for the young men. Biceps brachii and brachioradialis EMG activity (% MVIC) of the old men were greater than that of the young men during the dynamic fatiguing task (P<0.05), but were similar during the isometric task. Muscular mechanisms and greater relative muscle activity (EMG activity) explain the greater fatigue during the dynamic task for the old men compared with the young men in the elbow flexor muscles. Recovery of MVC torque however relies more on the recovery of supraspinal fatigue among the old men than among the young men.
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Affiliation(s)
- Tejin Yoon
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI 53201, United States
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Neurophysiological characterization of the New Anatomy and motor control that results from neurological injury or disease. Clin Neurol Neurosurg 2012; 114:447-54. [DOI: 10.1016/j.clineuro.2012.01.013] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2011] [Revised: 01/10/2012] [Accepted: 01/11/2012] [Indexed: 12/14/2022]
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Ritzmann R, Gollhofer A, Kramer A. The influence of vibration type, frequency, body position and additional load on the neuromuscular activity during whole body vibration. Eur J Appl Physiol 2012; 113:1-11. [PMID: 22538279 DOI: 10.1007/s00421-012-2402-0] [Citation(s) in RCA: 118] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2012] [Accepted: 04/04/2012] [Indexed: 11/26/2022]
Abstract
This study aimed to assess the influence of different whole body vibration (WBV) determinants on the electromyographic (EMG) activity during WBV in order to identify those training conditions that cause highest neuromuscular responses and therefore provide optimal training conditions. In a randomized cross-over study, the EMG activity of six leg muscles was analyzed in 18 subjects with respect to the following determinants: (1) vibration type (side-alternating vibration (SV) vs. synchronous vibration (SyV), (2) frequency (5-10-15-20-25-30 Hz), (3) knee flexion angle (10°-30°-60°), (4) stance condition (forefoot vs. normal stance) and (5) load variation (no extra load vs. additional load equal to one-third of the body weight). The results are: (1) neuromuscular activity during SV was enhanced compared to SyV (P < 0.05); (2) a progressive increase in frequency caused a progressive increase in EMG activity (P < 0.05); (3) the EMG activity was highest for the knee extensors when the knee joint was 60° flexed (P < 0.05); (4) for the plantar flexors in the forefoot stance condition (P < 0.05); and (5) additional load caused an increase in neuromuscular activation (P < 0.05). In conclusion, large variations of the EMG activation could be observed across conditions. However, with an appropriate adjustment of specific WBV determinants, high EMG activations and therefore high activation intensities could be achieved in the selected muscles. The combination of high vibration frequencies with additional load on an SV platform led to highest EMG activities. Regarding the body position, a knee flexion of 60° and forefoot stance appear to be beneficial for the knee extensors and the plantar flexors, respectively.
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Affiliation(s)
- Ramona Ritzmann
- Institute of Sport and Sport Science, University of Freiburg, Schwarzwaldstraße 175, 79117 Freiburg, Germany.
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31
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Stroke-related changes in neuromuscular fatigue of the hip flexors and functional implications. Am J Phys Med Rehabil 2012; 91:33-42. [PMID: 22157434 DOI: 10.1097/phm.0b013e31823caac0] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
OBJECTIVE The aim of this study was to compare stroke-related changes in hip flexor neuromuscular fatigue of the paretic leg during a sustained isometric submaximal contraction with those of the nonparetic leg and controls and to correlate fatigue with clinical measures of function. DESIGN Hip torques were measured during a fatiguing hip flexion contraction at 20% of the hip flexion maximal voluntary contraction in the paretic and nonparetic legs of 13 people with chronic stroke and 10 age-matched controls. In addition, the participants with stroke performed a fatiguing contraction of the paretic leg at the absolute torque equivalent to 20% maximal voluntary contraction of the nonparetic leg and were tested for self-selected walking speed (10-m Walk Test) and balance (Berg). RESULTS When matching the nonparetic target torque, the paretic hip flexors had a shorter time to task failure compared with the nonparetic leg and controls (P < 0.05). The time to failure of the paretic leg was inversely correlated with the reduction of hip flexion maximal voluntary contraction torque. Self-selected walking speed was correlated with declines in torque and steadiness. Berg-Balance scores were inversely correlated with the force fluctuation amplitude. CONCLUSIONS Fatigue and precision of contraction are correlated with walking function and balance after stroke.
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Keller ML, Pruse J, Yoon T, Schlinder-Delap B, Harkins A, Hunter SK. Supraspinal fatigue is similar in men and women for a low-force fatiguing contraction. Med Sci Sports Exerc 2012; 43:1873-83. [PMID: 21364478 DOI: 10.1249/mss.0b013e318216ebd4] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
PURPOSE This study determined the contribution of supraspinal fatigue to the sex difference in neuromuscular fatigue for a low-intensity fatiguing contraction. Because women have greater motor responses to arousal than men, we also examined whether cortical and motor nerve stimulation, techniques used to quantify central fatigue, would alter the sex difference in muscle fatigue. METHODS In study 1, cortical stimulation was elicited during maximal voluntary contractions (MVC) before and after a submaximal isometric contraction at 20% MVC with the elbow flexor muscles in 29 young adults (20 ± 2.6 yr, 14 men). In study 2, 10 men and 10 women (19.1 ± 2.9 yr) performed a fatiguing contraction in the presence and absence of cortical and motor nerve stimulation. RESULTS Study 1: Men had a briefer time to task failure than women (P = 0.009). Voluntary activation was reduced after the fatiguing contraction (P < 0.001) similarly for men and women. Motor-evoked potential area and the EMG silent period increased similarly with fatigue for both sexes. Peak relaxation rates, however, were greater for men than women and were associated with time to task failure (P < 0.05). Force fluctuations, RPE, HR, and mean arterial pressure increased at a greater rate for men than for women during the fatiguing contraction (P < 0.05). Study 2: Time to task failure, force fluctuations, and all other physiological variables assessed were similar for the control session and stimulation session (P > 0.05) for both men and women. CONCLUSIONS Supraspinal fatigue was similar for men and women after the low-force fatiguing contraction, and the sex difference in muscle fatigue was associated with peripheral mechanisms. Furthermore, supraspinal fatigue can be quantified in both men and women without influencing motor performance.
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Affiliation(s)
- Manda L Keller
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI 53201, USA
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Yoon T, Schlinder-Delap B, Keller ML, Hunter SK. Supraspinal fatigue impedes recovery from a low-intensity sustained contraction in old adults. J Appl Physiol (1985) 2011; 112:849-58. [PMID: 22174405 DOI: 10.1152/japplphysiol.00799.2011] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
This study determined the contribution of supraspinal fatigue and contractile properties to the age difference in neuromuscular fatigue during and recovery from a low-intensity sustained contraction. Cortical stimulation was used to evoke measures of voluntary activation and muscle relaxation during and after a contraction sustained at 20% of maximal voluntary contraction (MVC) until task failure with elbow flexor muscles in 14 young adults (20.9 ± 3.6 yr, 7 men) and 14 old adults (71.6 ± 5.4 yr, 7 men). Old adults exhibited a longer time to task failure than the young adults (23.8 ± 9.0 vs. 11.5 ± 3.9 min, respectively, P < 0.001). The time to failure was associated with initial peak rates of relaxation of muscle fibers and pressor response (P < 0.05). Increments in torque (superimposed twitch; SIT) generated by transcranial magnetic stimulation (TMS) during brief MVCs, increased during the fatiguing contraction (P < 0.001) and then decreased during recovery (P = 0.02). The increase in the SIT was greater for the old adults than the young adults during the fatiguing contraction and recovery (P < 0.05). Recovery of MVC torque was less for old than young adults at 10 min post-fatiguing contraction (75.1 ± 8.7 vs. 83.6 ± 7.8% of control MVC, respectively, P = 0.01) and was associated with the recovery of the SIT (r = -0.59, r(2) = 0.35, P < 0.001). Motor evoked potential (MEP) amplitude and the silent period elicited during the fatiguing contraction increased less for old adults than young adults (P < 0.05). The greater fatigue resistance with age during a low-intensity sustained contraction was attributable to mechanisms located within the muscle. Recovery of maximal strength after the low-intensity fatiguing contraction however, was impeded more for old adults than young because of greater supraspinal fatigue. Recovery of strength could be an important variable to consider in exercise prescription of old populations.
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Affiliation(s)
- Tejin Yoon
- Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin 53201, USA
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Vieira TMM, Loram ID, Muceli S, Merletti R, Farina D. Recruitment of motor units in the medial gastrocnemius muscle during human quiet standing: is recruitment intermittent? What triggers recruitment? J Neurophysiol 2011; 107:666-76. [PMID: 21994258 DOI: 10.1152/jn.00659.2011] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The recruitment and the rate of discharge of motor units are determinants of muscle force. Within a motoneuron pool, recruitment and rate coding of individual motor units might be controlled independently, depending on the circumstances. In this study, we tested whether, during human quiet standing, the force of the medial gastrocnemius (MG) muscle is predominantly controlled by recruitment or rate coding. If MG control during standing was mainly due to recruitment, then we further asked what the trigger mechanism is. Is it determined internally, or is it related to body kinematics? While seven healthy subjects stood quietly, intramuscular electromyograms were recorded from the MG muscle with three pairs of wire electrodes. The number of active motor units and their mean discharge rate were compared for different sway velocities and positions. Motor unit discharges occurred more frequently when the body swayed faster and forward (Pearson R = 0.63; P < 0.0001). This higher likelihood of observing motor unit potentials was explained chiefly by the recruitment of additional units. During forward body shifts, the median number of units detected increased from 3 to 11 (P < 0.0001), whereas the discharge rate changed from 8 ± 1.1 (mean ± SD) to 10 ± 0.9 pulses/s (P = 0.001). Strikingly, motor units did not discharge continuously throughout standing. They were recruited within individual, forward sways and intermittently, with a modal rate of two recruitments per second. This modal rate is consistent with previous circumstantial evidence relating the control of standing to an intrinsic, higher level planning process.
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Affiliation(s)
- Taian M M Vieira
- School of Physical Education and Sports, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil.
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35
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Pascoe MA, Holmes MR, Enoka RM. Discharge characteristics of biceps brachii motor units at recruitment when older adults sustained an isometric contraction. J Neurophysiol 2011; 105:571-81. [PMID: 21160000 PMCID: PMC3059164 DOI: 10.1152/jn.00841.2010] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2010] [Accepted: 12/10/2010] [Indexed: 11/22/2022] Open
Abstract
The purpose of this study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit. The discharge times of 27 single motor units were recorded from the biceps brachii in 11 old adults (78.8 ± 5.9 yr). The target force was set at either a relatively small (6.6 ± 3.7% maximum) or large (11.4 ± 4.5% maximum) difference below the recruitment threshold force and the contraction was sustained until the motor unit was recruited and discharged action potentials for about 60 s. The time to recruitment was longer for the large target-force difference (P = 0.001). At recruitment, the motor units discharged repetitively for both target-force differences, which contrasts with data from young adults when motor units discharged intermittently at recruitment for the large difference between recruitment threshold force and target force. The coefficient of variation (CV) for the first five interspike intervals (ISIs) increased from the small (18.7 ± 7.9) to large difference (35.0 ± 10.2%, P = 0.008) for the young adults, but did not differ for the two target force differences for the old adults (26.3 ± 14.7 to 24.0 ± 13.1%, P = 0.610). When analyzed across the discharge duration, the average CV for the ISI decreased similarly for the two target-force differences (P = 0.618) in old adults. These findings contrast with those of young adults and indicate that the integration of synaptic input during sustained contractions differs between young and old adults.
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Affiliation(s)
- Michael A Pascoe
- Neurophysiology of Movement Laboratory, Carlson 202G 354 UCB, Department of Integrative Physiology, University of Colorado, Boulder, CO 80309-0354, USA.
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Farina D, Holobar A, Merletti R, Enoka RM. Decoding the neural drive to muscles from the surface electromyogram. Clin Neurophysiol 2010; 121:1616-23. [PMID: 20444646 DOI: 10.1016/j.clinph.2009.10.040] [Citation(s) in RCA: 219] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2009] [Revised: 10/02/2009] [Accepted: 10/22/2009] [Indexed: 10/19/2022]
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Dideriksen JL, Farina D, Baekgaard M, Enoka RM. An integrative model of motor unit activity during sustained submaximal contractions. J Appl Physiol (1985) 2010; 108:1550-62. [PMID: 20360437 DOI: 10.1152/japplphysiol.01017.2009] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The purpose of the study was to expand a model of motor unit recruitment and rate coding (30) to simulate the adjustments that occur during a fatiguing contraction. The major new components of the model were the introduction of time-varying parameters for motor unit twitch force, recruitment, discharge rate, and discharge variability, and a control algorithm that estimates the net excitation needed by the motoneuron pool to maintain a prescribed target force. The fatigue-induced changes in motor unit activity in the expanded model are a function of changes in the metabolite concentrations that were computed with a compartment model of the intra- and extracellular spaces. The model was validated by comparing the simulation results with data available from the literature and experimentally recorded in the present study during isometric contractions of the first dorsal interosseus muscle. The output of the model was able to replicate a number of experimental findings, including the time to task failure for a range of target forces, the changes in motor unit discharge rates, the skewness and kurtosis of the interspike interval distributions, discharge variability, and the discharge characteristics of newly recruited motor units. The model output provides an integrative perspective of the adjustments during fatiguing contractions that are difficult to measure experimentally.
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Affiliation(s)
- Jakob L Dideriksen
- Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7 D-3, DK-9220 Aalborg, Denmark
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Jesunathadas M, Marmon AR, Gibb JM, Enoka RM. Recruitment and derecruitment characteristics of motor units in a hand muscle of young and old adults. J Appl Physiol (1985) 2010; 108:1659-67. [PMID: 20339011 DOI: 10.1152/japplphysiol.00807.2009] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The significant decline in motor neuron number after approximately 60 yr of age is accompanied by a remodeling of the neuromuscular system so that average motor unit force increases and the ability of old adults to produce an intended force declines. One possible explanation for the loss of movement precision is that the remodeling increases the difference in recruitment forces between successively recruited motor units in old adults and this augments force variability at motor unit recruitment. The purpose of the study was to compare the forces and discharge characteristics of motor units in a hand muscle of young and old adults at motor unit recruitment and derecruitment. The difference in recruitment force between pairs of motor units did not differ between young (n=54) and old adults (n=56; P=0.702). However, old adults had a greater proportion of contractions in which motor units discharged action potentials transiently before discharging continuously during the ramp increase in force (young: 0.32; old: 0.41; P=0.045). Force variability at motor unit recruitment was greater for old adults compared with young adults (P<or=0.010), but discharge rate and discharge variability did not differ between age groups (P>or=0.729). These results suggest that the difference in force between the recruitment of successive motor units does not differ between age groups, but that motor unit recruitment may be more transient and could contribute to the greater variability in force observed in old adults during graded ramp contractions.
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Affiliation(s)
- Mark Jesunathadas
- Department of Integrative Physiology, University of Colorado, Boulder, CO, USA.
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Yoon T, Keller ML, De-Lap BS, Harkins A, Lepers R, Hunter SK. Sex differences in response to cognitive stress during a fatiguing contraction. J Appl Physiol (1985) 2009; 107:1486-96. [PMID: 19729594 DOI: 10.1152/japplphysiol.00238.2009] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
This study compared the time to task failure for a submaximal fatiguing contraction in the presence and absence of a cognitive stressor in men and women. In study 1, 10 men and 10 women (22 +/- 3 yr of age) performed an isometric fatiguing contraction at 20% maximal voluntary contraction force until task failure with the elbow flexor muscles during two separate sessions. Subjects performed a mental-math task during one of the fatiguing contractions that aimed to increase anxiety and stress (stressor session). Salivary cortisol and reported levels of arousal (visual analog scale for anxiety, and State-Trait Anxiety Inventory scores) were elevated during the stressor session compared with a control session for both sexes (P < 0.05). Time to task failure, however, was briefer during the stressor session compared with control (P = 0.005) but more so for the women (27.3 +/- 20.1%) than the men (8.6 +/- 23.1%) (P = 0.03). The briefer time to task failure was associated with target force (r(2) = 0.21) and accompanied by a higher mean arterial pressure, heart rate, and rate-pressure product during the fatiguing contraction in the stressor session compared with control in women. In study 2 (11 men and 8 women, 20 +/- 3 yr of age), time to task failure was similar for a fatiguing contraction with simple mental-math that did not increase stress (mental-attentiveness session) and control for both men and women. The greater change in fatigability of women than men with performance of a cognitive stressor involved initial strength and increases in indexes of sympathetic neural activity and cardiac work compared with control conditions.
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Affiliation(s)
- Tejin Yoon
- Exercise Science Program, Dept. of Physical Therapy, Marquette Univ., P.O. Box 1881, Milwaukee, WI 53201, USA
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40
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Riley ZA, Baudry S, Enoka RM. Reflex inhibition in human biceps brachii decreases with practice of a fatiguing contraction. J Neurophysiol 2008; 100:2843-51. [PMID: 18667549 DOI: 10.1152/jn.90244.2008] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The purpose of the study was to examine the influence of practice on time to failure of a submaximal contraction with the elbow flexor muscles and on reflex inhibition from brachioradialis afferents onto biceps brachii motor neurons. Fifteen subjects practiced sustaining an isometric contraction (20% of maximum) with the elbow flexors until failure. Spike-triggered stimulation was used to assess the influence of radial nerve stimulation on the discharge of single motor units in biceps brachii before and after three practice sessions. Time to failure increased from 760 +/- 333 s in session 1 to 1,103 +/- 415 s in session 3 (P < 0.03) and was accompanied by a slower rate of increase in electromyographic (EMG) activity of the short head of biceps brachii (P < 0.05). Stimulation of the radial nerve prolonged the interspike interval before practice (n = 56; 7.2 +/- 6.8 ms; P < 0.001), and this effect was reduced after practice (n = 62; 2.3 +/- 3.6 ms; P < 0.01). The reduction was greater for motor units in the short head of biceps brachii than for those in the long head (P < 0.05) and was associated with a slower rate of increase in EMG (r = 0.57, P = 0.03). The decrease in reflex inhibition was the main predictor of the increase in time to failure (r(2) = 0.60, P = 0.001). These results demonstrate that practice reduced an antagonistic inhibition and improved the ability of the muscles to perform a synergistic action of elbow flexion.
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Affiliation(s)
- Zachary A Riley
- Department of Integrative Physiology, 354 UCB, Boulder, CO 80309-0354, USA.
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