1
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Zero AM, Paris MT, Rice CL. Differential effects of stimulation frequency on isometric and concentric isotonic contractile function in human quadriceps. J Appl Physiol (1985) 2024; 137:111-124. [PMID: 38841755 DOI: 10.1152/japplphysiol.00075.2024] [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: 01/24/2024] [Revised: 04/26/2024] [Accepted: 05/28/2024] [Indexed: 06/07/2024] Open
Abstract
Electrically evoked contractions are used to assess the relationship between frequency input and contractile output to characterize inherent muscle function, and these have been done mostly with isometric contractions (i.e., no joint rotation). The purpose was to compare the electrically stimulated frequency and contractile function relationship during isometric (i.e., torque) with isotonic (i.e., concentric torque, angular velocity, and mechanical power) contractions. The knee extensors of 16 (5 female) young recreationally active participants were stimulated (∼1-2.5 s) at 14 frequencies from 1 to 100 Hz. This was done during four conditions, which were isometric and isotonic at loads of 0 (unloaded), 7.5%, and 15% isometric maximal voluntary contraction (MVC), and repeated on separate days. Comparisons across contractile parameters were made as a % of 100 Hz. Independent of the load, the mechanical power-frequency relationship was rightward shifted compared with isometric torque-frequency, concentric torque-frequency, and velocity-frequency relationships (all P ≤ 0.04). With increasing load (0%-15% MVC), the isotonic concentric torque-frequency relationship was shifted leftward systematically from 15 to 30 Hz (all P ≤ 0.04). Conversely, the same changes in load caused a rightward shift in the velocity-frequency relationship from 1 to 40 Hz (all P ≤ 0.03). Velocity was leftward shifted of concentric torque in the unloaded isotonic condition from 10 to 25 Hz (all P ≤ 0.03), but concentric torque was leftward shifted of velocity at 15% MVC isotonic condition from 10 to 50 Hz (all P ≤ 0.03). Therefore, isometric torque is not a surrogate to evaluate dynamic contractile function. Interpretations of evoked contractile function differ depending on contraction type, load, and frequency, which should be considered relative to the specific task.NEW & NOTEWORTHY In whole human muscle, we showed that the electrically stimulated power-frequency relationship was rightward shifted of the stimulated isometric torque-frequency relationship independent of isotonic load, indicating that higher stimulation frequencies are needed to achieve tetanus. Therefore, interpretations of evoked contractile function differ depending on contraction type (isometric vs. dynamic), load, and frequency. And thus, isometric measures may not be appropriate as a surrogate assessment when evaluating dynamic isotonic contractile function.
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Affiliation(s)
- Alexander M Zero
- Faculty of Health Sciences, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
| | - Michael T Paris
- Faculty of Health Sciences, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
- Faculty of Health, School of Kinesiology & Health Science, York University, Toronto, Ontario, Canada
| | - Charles L Rice
- Faculty of Health Sciences, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
- Department of Anatomy and Cell Biology, Schulich School of Medicine & Dentistry, The University of Western Ontario, London, Ontario, Canada
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2
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Zong Y, Lu Z, Chen M, Xie Q, Zhou P. MScanFit motor unit number estimation of human anconeus muscle. Muscle Nerve 2022; 65:460-463. [PMID: 34993997 DOI: 10.1002/mus.27487] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2021] [Revised: 12/23/2021] [Accepted: 12/31/2021] [Indexed: 11/11/2022]
Abstract
INTRODUCTION/AIMS Motor unit number estimation (MUNE) studies of the anconeus muscle are very limited, although the information they provide is useful for neurophysiological investigations. The objective of this study was to estimate the number of motor units in the anconeus muscle. METHODS Compound muscle action potential scans of the anconeus muscle were recorded from 11 healthy participants, all of whom were tested on two occasions. MUNE was determined from the MScanFit program. RESULTS The average MUNE of the anconeus muscle was 55.09 ± 3.27 (mean ± standard error of the mean) for the first test and 54.64 ± 3.70 for the retest, demonstrating excellent measurement reliability, with an intraclass correlation coefficient of 0.90. DISCUSSION A relatively low motor unit number is found in the anconeus, a muscle not comprehensively studied in literature.
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Affiliation(s)
- Ya Zong
- Department of Rehabilitation Medicine, Ruijin Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China
| | - Zhiyuan Lu
- Faculty of Rehabilitation Engineering, University of Health and Rehabilitation Sciences, Qingdao, China
| | - Maoqi Chen
- Faculty of Rehabilitation Engineering, University of Health and Rehabilitation Sciences, Qingdao, China
| | - Qin Xie
- Department of Rehabilitation Medicine, Ruijin Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China
| | - Ping Zhou
- Faculty of Rehabilitation Engineering, University of Health and Rehabilitation Sciences, Qingdao, China
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3
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Kirk EA, Gilmore KJ, Rice CL. Anconeus motor unit firing rates during isometric and muscle-shortening contractions comparing young and very old adults. J Neurophysiol 2021; 126:1122-1136. [PMID: 34495770 DOI: 10.1152/jn.00219.2021] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
With effects of aging, voluntary neural drive to the muscle, measured as motor unit (MU) firing rate, is lower in older adults during sustained isometric contractions compared with young adults, but differences remain unknown during limb movements. Therefore, our purpose was to compare MU firing rates during both isometric and shortening contractions between two adult age groups. We analyzed intramuscular electromyography of single-MU recordings in the anconeus muscle of young (n = 8, 19-33 yr) and very old (n = 13, 78-93 yr) male adults during maximal voluntary contractions (MVCs). In sustained isometric and muscle-shortening contractions during limb movement, MU trains were linked with elbow joint kinematic parameters throughout the contraction time course. The older group was 33% weaker and 10% slower during movements than the young group (P < 0.01). In isometric contractions, median firing rates were 42% lower (P < 0.01) in the older group (18 Hz) compared with the young group (31 Hz), but during shortening contractions firing rates were higher for both age groups and not statistically different between groups. As a function of contraction time, firing rates at MU recruitment threshold were 39% lower in the older group, but the firing rate decrease was attenuated threefold throughout shortening contraction compared with the young group. At the single-MU level, age-related differences during isometric contractions (i.e., pre-movement initiation) do not remain constant throughout movement that comprises greater effects of muscle shortening. Results indicate that neural drive is task dependent and during movement in older adults it is decreased minimally.NEW & NOTEWORTHY Changes of neural drive to the muscle with adult aging, measured as motor unit firing rates during limb movements, are unknown. Throughout maximal voluntary efforts we found that, in comparison with young adults, firing rates were lower during isometric contraction in older adults but not different during elbow extension movements. Despite the older group being ∼33% weaker across contractions, their muscles can receive neural drive during movements that are similar to that of younger adults.
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Affiliation(s)
- Eric A Kirk
- School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, Ontario, Canada
| | - Kevin J Gilmore
- Department of Surgery, Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada
| | - Charles L Rice
- School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, Ontario, Canada.,Department of Anatomy and Cell Biology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, Canada
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4
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Zero AM, Rice CL. Post-activation potentiation induced by concentric contractions at three speeds in humans. Exp Physiol 2021; 106:2489-2501. [PMID: 34569107 DOI: 10.1113/ep089613] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2021] [Accepted: 09/23/2021] [Indexed: 11/08/2022]
Abstract
NEW FINDINGS What is the central question of this study? Is the degree of in human muscle affected by different shortening velocities, or contraction type? What are the main findings and their importance? The PAP response following maximal concentric contractions was independent of velocity. Slow and moderate velocity maximal contractions produced PAP responses like those from maximal isometric contractions when matched for contraction duration. Despite contraction type differences in cross-bridge and Ca2+ kinetics, maximal contractions, regardless of contraction modality, likely generate sufficient Ca2+ to induce maximal PAP. ABSTRACT Post-activation potentiation (PAP) is the acute enhancement of contractile properties following a brief (<10 s) high-intensity contraction. Compared with isometric contractions, little is known about the PAP response induced by concentric conditioning contractions (CCs) and the effect of velocity. In the dorsiflexors of 11 participants, twitch responses were measured following 5 s of maximal effort concentric CCs at each of 10, 20 and 50°/s. Concentric PAP responses were compared to a maximal isometric voluntary contraction (MVC) matched for contraction time. Additionally, concentric CCs were compared to isometric CCs matched for mean torque, contraction area and time. The PAP response following maximal concentric CCs was independent of velocity and there was no difference in the PAP response between concentric CCs and an isometric MVC. During maximal contractions, regardless of contraction modality, there is likely sufficient Ca2+ to induce a similar full PAP response, and thus there was no difference between speeds or contraction type. Following concentric CCs there was a significantly larger peak twitch torque than following their isometric torque matches (49-58%), and faster maximal rates of torque development at the three speeds (62-77%). However, these responses are likely related to greater EMG in concentric contractions, 125-129% of isometric maximum compared to 38-54%, and not to contraction modality per se. Thus, PAP responses following maximal concentric CCs are not affected by velocity and responses are not different from an isometric MVC. This indicates maximal CCs of 5 s produce a maximal PAP response independent of contraction type (isometric vs. concentric) or shortening velocity.
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Affiliation(s)
- Alexander M Zero
- School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, ON, Canada
| | - Charles L Rice
- School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, ON, Canada.,Department of Anatomy and Cell Biology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, ON, Canada
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5
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The relationship of agonist muscle single motor unit firing rates and elbow extension limb movement kinematics. Exp Brain Res 2021; 239:2755-2766. [PMID: 34240233 DOI: 10.1007/s00221-021-06168-8] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2021] [Accepted: 06/26/2021] [Indexed: 10/20/2022]
Abstract
This study explored the relationship between single motor unit (MU) firing rates (FRs) and limb movement velocity during voluntary shortening contractions when accounting for the effects of time course variability between different kinematic comparisons. Single MU trains recorded by intramuscular electromyography in agonist muscles of the anconeus (n = 15 participants) and lateral head of the triceps brachii (n = 6) were measured during each voluntary shortening contraction. Elbow extension movements consisted of a targeted velocity occurring along the sagittal plane at 25, 50, 75 and 100% of maximum velocity. To account for the effect of differences in contraction time course between parameters, each MU potential was time locked throughout the shortening muscle contraction and linked with separated kinematic parameters of the elbow joint. Across targeted movement velocities, instantaneous FRs were significantly correlated with elbow extension rate of torque development (r = 0.45) and torque (r = 0.40), but FRs were not correlated with velocity (r = 0.03, p = n.s.). Instead, FRs had a weak indirect relationship with limb movement velocity and position assessed through multiple correlation of the stepwise kinematic progression. Results show that voluntary descending synaptic inputs correspond to a more direct relationship between agonist muscle FRs and torque during shortening contractions, but not velocity. Instead, FRs were indirectly correlated to preparing the magnitude of imminent movement velocity of the lagging limb through torque.
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6
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Oliveira AS, Negro F. Neural control of matched motor units during muscle shortening and lengthening at increasing velocities. J Appl Physiol (1985) 2021; 130:1798-1813. [PMID: 33955258 DOI: 10.1152/japplphysiol.00043.2021] [Citation(s) in RCA: 20] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Modulation of movement velocity is necessary during daily life tasks, work, and sports activities. However, assessing motor unit behavior during muscle shortening and lengthening at different velocities is challenging. High-density surface electromyography (HD-sEMG) is an established method to identify and track motor unit behavior in isometric contractions. Therefore, we used this methodology to unravel the behavior of the same motor units in dynamic contractions at low contraction velocities. Velocity-related changes in tibialis anterior motor unit behavior during concentric and eccentric contractions at 10% and 25% maximum voluntary isometric contraction were assessed by decomposing HD-sEMG signals recorded from the tibialis anterior muscle of eleven healthy participants at 5°/s, 10°/s, and 20°/s. Motor units extracted from the dynamic contractions were tracked across different velocities at the same load levels. On average, 14 motor units/participant were matched across different velocities, showing specific changes in discharge rate modulation. Specifically, increased velocity led to an increased rate of change in discharge rate (e.g., discharge rate slope, P = 0.025), recruitment and derecruitment discharge rates (P = 0.003 and P = 0.001), and decreased recruitment angles (P = 0.0001). Surprisingly, the application of the motor unit extraction filters calculated from 20°/s onto the recordings at 5°/s and 10°/s revealed that >92% of motor units recruited at the highest velocity were active on both lower velocities, indicating no additional recruitment of motor units. Our results suggest that motor unit rate coding rather than recruitment is responsible for controlling muscle shortening and lengthening contractions at increasing velocities against a constant load.NEW & NOTEWORTHY The control of movement velocity is accomplished by the modulation of the neural drive to muscle and its variation over time. In this study, we tracked motor units decomposed from HD-sEMG across shortening and lengthening contractions at increasing velocities in two submaximal load levels. We demonstrate that concentric and eccentric contractions of the tibialis anterior muscle at slow velocities are achieved by specific motor unit rate coding strategies rather than distinct recruitment schemes.
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Affiliation(s)
| | - Francesco Negro
- Department of Clinical and Experimental Sciences, Università degli Studi di Brescia, Brescia, Italy
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7
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Spencer A, Davis K, Jacobs C, Johnson D, Ireland ML, Noehren B. Decreased quadriceps force steadiness following anterior cruciate ligament reconstruction is associated with altered running kinematics. Clin Biomech (Bristol, Avon) 2020; 72:58-62. [PMID: 31835112 DOI: 10.1016/j.clinbiomech.2019.11.021] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/28/2019] [Revised: 09/16/2019] [Accepted: 11/26/2019] [Indexed: 02/07/2023]
Abstract
BACKGROUND Anterior cruciate ligament reconstruction is associated with quadriceps dysfunction and altered knee mechanics, but the relationship between these outcomes is not clear. Inclusion of metrics such as the stability of torque output could provide additional insights into the relationship between quadriceps dysfunction and knee mechanics. The purposes of this study were to: (1) measure the difference in quadriceps force steadiness between anterior cruciate ligament reconstructed and contralateral limbs; and (2) assess the relationship of quadriceps force steadiness and peak torque with knee flexion excursion during running. METHODS Twenty-eight participants (14 female, age 20 (5) years) underwent quadriceps strength testing and gait analysis. Force steadiness was measured with the standard deviation and coefficient of variation of the participants' detrended torque. Knee flexion excursion was calculated during the stance phase of running trials. Differences between limbs for force steadiness and peak torque were assessed with paired sample t-tests, and a Pearson's product-moment correlation coefficient determined the relationship between pairs of variables. FINDINGS Anterior cruciate ligament reconstructed limbs presented with a significant deficit in relative force steadiness compared to the contralateral limb (4.03 (1.04) % and 3.58 (1.41) % (P < .05), respectively). In addition, the relationship of quadriceps strength and force steadiness with knee flexion excursion was altered in anterior cruciate ligament reconstructed limbs due to the inability of the quadriceps to sustain a controlled force output. INTERPRETATION These results suggest that considering both the magnitude and quality of force production can provide important insight into comprehensive quadriceps function.
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Affiliation(s)
- Alex Spencer
- College of Health Sciences, Department of Rehabilitation Sciences, University of Kentucky, Lexington, KY 40508, USA.
| | - Kylie Davis
- College of Health Sciences, Department of Rehabilitation Sciences, University of Kentucky, Lexington, KY 40508, USA
| | - Cale Jacobs
- College of Medicine, Department of Orthopedic Surgery and Sports Medicine, University of Kentucky, Lexington, KY 40508, USA
| | - Darren Johnson
- College of Medicine, Department of Orthopedic Surgery and Sports Medicine, University of Kentucky, Lexington, KY 40508, USA
| | - Mary Lloyd Ireland
- College of Medicine, Department of Orthopedic Surgery and Sports Medicine, University of Kentucky, Lexington, KY 40508, USA
| | - Brian Noehren
- College of Health Sciences, Department of Rehabilitation Sciences, University of Kentucky, Lexington, KY 40508, USA; College of Medicine, Department of Orthopedic Surgery and Sports Medicine, University of Kentucky, Lexington, KY 40508, USA
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8
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Kumar RI, Mallette MM, Cheung SS, Stashuk DW, Gabriel DA. A method for editing motor unit potential trains obtained by decomposition of surface electromyographic signals. J Electromyogr Kinesiol 2020; 50:102383. [DOI: 10.1016/j.jelekin.2019.102383] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2019] [Revised: 12/04/2019] [Accepted: 12/23/2019] [Indexed: 12/13/2022] Open
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9
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Wernbom M, Aagaard P. Muscle fibre activation and fatigue with low-load blood flow restricted resistance exercise-An integrative physiology review. Acta Physiol (Oxf) 2020; 228:e13302. [PMID: 31108025 DOI: 10.1111/apha.13302] [Citation(s) in RCA: 45] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2019] [Revised: 05/12/2019] [Accepted: 05/17/2019] [Indexed: 12/13/2022]
Abstract
Blood flow-restricted resistance exercise (BFRRE) has been shown to induce increases in muscle size and strength, and continues to generate interest from both clinical and basic research points of view. The low loads employed, typically 20%-50% of the one repetition maximum, make BFRRE an attractive training modality for individuals who may not tolerate high musculoskeletal forces (eg, selected clinical patient groups such as frail old adults and patients recovering from sports injury) and/or for highly trained athletes who have reached a plateau in muscle mass and strength. It has been proposed that achieving a high degree of muscle fibre recruitment is important for inducing muscle hypertrophy with BFRRE, and the available evidence suggest that fatiguing low-load exercise during ischemic conditions can recruit both slow (type I) and fast (type II) muscle fibres. Nevertheless, closer scrutiny reveals that type II fibre activation in BFRRE has to date largely been inferred using indirect methods such as electromyography and magnetic resonance spectroscopy, while only rarely addressed using more direct methods such as measurements of glycogen stores and phosphocreatine levels in muscle fibres. Hence, considerable uncertainity exists about the specific pattern of muscle fibre activation during BFRRE. Therefore, the purpose of this narrative review was (1) to summarize the evidence on muscle fibre recruitment during BFRRE as revealed by various methods employed for determining muscle fibre usage during exercise, and (2) to discuss reported findings in light of the specific advantages and limitations associated with these methods.
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Affiliation(s)
- Mathias Wernbom
- Center for Health and Performance, Department of Food and Nutrition and Sport Science University of Gothenburg Gothenburg Sweden
- Department of Health and Rehabilitation, Institute of Neuroscience and Physiology, Sahlgrenska Academy University of Gothenburg Gothenburg Sweden
| | - Per Aagaard
- Department of Sports Sciences and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC) University of Southern Denmark Odense M Denmark
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10
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Miyamoto T, Kizuka T, Ono S. The Influence of Contraction Types on the Relationship Between the Intended Force and the Actual Force. J Mot Behav 2019; 52:687-693. [PMID: 31665979 DOI: 10.1080/00222895.2019.1680947] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
It has been demonstrated that the intended force (subjective estimation of force) does not always match to actual force without external feedback. The purpose of this study was to compare the influence of ballistic and tonic contractions on the relationship between the intended and actual force. Subjects produced isometric force at requested percentages of their MVC (20, 40, 60 and 80%) based on subjective estimation of force under two conditions (tonic and ballistic conditions). The tonic condition was to maintain force production, whereas the ballistic condition was to produce force as fast as possible. As a result, the actual force amplitude, the coefficient of variance and EMG amplitude were larger under the ballistic contraction compared with the tonic condition, even the same intended force levels. These results suggest that different motor unit activity and control systems in the ballistic and tonic contractions could alter the relationship between the intended force and the actual force.
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Affiliation(s)
- Takeshi Miyamoto
- Graduate School of Comprehensive Human Science, University of Tsukuba, Tsukuba, Ibaraki, Japan
| | - Tomohiro Kizuka
- Faculty of Health and Sport Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan
| | - Seiji Ono
- Faculty of Health and Sport Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan
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11
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Josephson MD, Rose WC, Knight CA. Evidence of bilinearity in the relationship between rate of neuromuscular excitation and rate of force development. J Electromyogr Kinesiol 2019; 49:102355. [PMID: 31525632 DOI: 10.1016/j.jelekin.2019.102355] [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: 07/02/2019] [Revised: 08/31/2019] [Accepted: 09/07/2019] [Indexed: 11/16/2022] Open
Abstract
The purpose was to examine the relationship between the rate of neural excitation (rate of rise in the electromyogram, EMG) and the rate of isometric force development (RFD) to determine whether surface EMG measures can detect nonlinearity that is expected due to underlying motor unit discharge behavior and the summation of progressively larger motor unit potentials throughout recruitment. Due to interest in obtaining a change point, a bilinear model was hypothesized to provide the best fit of the EMG-RFD relationship compared to a linear model, exponential model and log-transformed data. 21 young adult participants performed isometric dorsiflexion contractions to 40% of their maximal voluntary contraction (MVC) force. Contractions were performed in RFD conditions ranging from slow (20 %MVC/s) to fast (peak volitional rate). The Akaike Information Criterion supported nonlinear best fit models in 16 of the 21 participants with the greatest overall support for the bilinear model (n = 13). The bilinear models indicated a mean change point at 204%MVC/s. The present data do not identify the specific motor unit control mechanisms at play and the influence of amplitude cancellation on the electromyogram must be carefully considered.
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Affiliation(s)
- Micah D Josephson
- Doctor of Physical Therapy Program, Alvernia University, Reading, PA, USA.
| | - William C Rose
- Department of Kinesiology and Applied Physiology, University of Delaware, Newark, DE, USA
| | - Christopher A Knight
- Department of Kinesiology and Applied Physiology, University of Delaware, Newark, DE, USA
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12
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Penasso H, Thaller S. Model-based analysis of fatigued human knee extensors : Effects of isometrically induced fatigue on Hill-type model parameters and ballistic contractions. Eur J Appl Physiol 2018; 118:1447-1461. [PMID: 29730804 PMCID: PMC6028922 DOI: 10.1007/s00421-018-3875-2] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2017] [Accepted: 04/23/2018] [Indexed: 02/05/2023]
Abstract
This study investigated the effect of isometrically induced fatigue on Hill-type muscle model parameters and related task-dependent effects. Parameter identification methods were used to extract fatigue-related parameter trends from isometric and ballistic dynamic maximum voluntary knee extensions. Nine subjects, who completed ten fatiguing sets, each consisting of nine 3 s isometric maximum voluntary contractions with 3 s rest plus two ballistic contractions with different loads, were analyzed. Only at the isometric task, the identified optimized model parameter values of muscle activation rate and maximum force generating capacity of the contractile element decreased from [Formula: see text] to [Formula: see text] Hz and from [Formula: see text] to [Formula: see text] N, respectively. For all tasks, the maximum efficiency of the contractile element, mathematically related to the curvature of the force-velocity relation, increased from [Formula: see text] to [Formula: see text]. The model parameter maximum contraction velocity decreased from [Formula: see text] to [Formula: see text] m/s and the stiffness of the serial elastic element from [Formula: see text] to [Formula: see text] N/mm. Thus, models of fatigue should consider fatigue dependencies in active as well as in passive elements, and muscle activation dynamics should account for the task dependency of fatigue.
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Affiliation(s)
- Harald Penasso
- Institute of Sport Science, University of Graz, Mozartgasse 14, 8010, Graz, Austria.
| | - Sigrid Thaller
- Institute of Sport Science, University of Graz, Mozartgasse 14, 8010, Graz, Austria
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13
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Gilmore KJ, Kirk EA, Doherty TJ, Rice CL. Effect of very old age on anconeus motor unit loss and compensatory remodelling. Muscle Nerve 2017; 57:659-663. [DOI: 10.1002/mus.25982] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2017] [Revised: 09/18/2017] [Accepted: 10/03/2017] [Indexed: 12/13/2022]
Affiliation(s)
- Kevin J. Gilmore
- School of Kinesiology, Canadian Centre for Activity and Aging, Faculty of Health SciencesThe University of Western Ontario, 1490 Richmond Street London Ontario CanadaN6G 2M3
| | - Eric A. Kirk
- School of Kinesiology, Canadian Centre for Activity and Aging, Faculty of Health SciencesThe University of Western Ontario, 1490 Richmond Street London Ontario CanadaN6G 2M3
| | - Timothy J. Doherty
- Department of Clinical Neurological SciencesThe University of Western OntarioLondon Ontario Canada
- Department of Physical Medicine and RehabilitationThe University of Western OntarioLondon Ontario Canada
| | - Charles L. Rice
- School of Kinesiology, Canadian Centre for Activity and Aging, Faculty of Health SciencesThe University of Western Ontario, 1490 Richmond Street London Ontario CanadaN6G 2M3
- Department of Anatomy and Cell Biology, Schulich School of Medicine and DentistryThe University of Western OntarioLondon Ontario Canada
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14
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Celli A, Bonucci P. The anconeus-triceps lateral flap approach for total elbow arthroplasty in rheumatoid arthritis. Musculoskelet Surg 2016; 100:73-83. [PMID: 27900699 DOI: 10.1007/s12306-016-0417-0] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2016] [Accepted: 06/29/2016] [Indexed: 01/25/2023]
Abstract
BACKGROUND The posterior approaches to the elbow are considered as to allow an excellent joint exposure for total elbow arthroplasty. One complication that is well recognized is the insufficiency of the extensor mechanism in particular with the patients with poor tendon quality as in the rheumatoid diseases. The purpose of this paper is to present a new triceps-splitting exposure for total elbow arthroplasty used in rheumatoid patients with their preliminary results. METHODS Fifteen consecutive patients with rheumatoid disease at grades III to V of the Larsen grading scale underwent total elbow replacement using the new triceps-splitting exposure called anconeus-triceps lateral flap and it preserves the integrity of the medial proper triceps tendon. The assessments were performed with a minimum follow-up of 2 years. RESULTS The mean Mayo Elbow Performance Score increased from 24 points to 95 points at a mean follow-up of 38 months. The pain was calculated using the visual analogue score, it had a mean pre-operative value of 8.9 points and it became 0.5 points. Although elbow motion in flexion-extension and pronosupination was allowed from the day after surgery, we did not observe any insufficiency or secondary detachments of the triceps tendon reporting grade 4 to 5 according to the Medical Research Council scale. CONCLUSIONS These preliminary outcomes suggest that the decision to preserve the medial proper triceps tendon insertion allows to start an earlier active unrestricted rehabilitation programme. This new triceps management improves the surgical exposure of the olecranon surface. LEVEL OF EVIDENCE Level IV.
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Affiliation(s)
- A Celli
- Department of Orthopaedic Surgery, Hesperia Hospital, Via Emilia Est 380\1, 41124, Modena, Italy.
| | - P Bonucci
- Department of Orthopaedic Surgery, Hesperia Hospital, Via Emilia Est 380\1, 41124, Modena, Italy
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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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Celli A. A new posterior triceps approach for total elbow arthroplasty in patients with osteoarthritis secondary to fracture: preliminary clinical experience. J Shoulder Elbow Surg 2016; 25:e223-31. [PMID: 27422461 DOI: 10.1016/j.jse.2016.04.003] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/24/2015] [Revised: 03/23/2016] [Accepted: 04/05/2016] [Indexed: 02/01/2023]
Abstract
BACKGROUND During the past decade, total elbow arthroplasty (TEA) procedures have increased because of an increase in the number of trauma patients. Even though most current posterior approaches to the elbow provide excellent joint exposure, they involve the risk of extensor mechanism injury and of eventual insufficiency, particularly in patients with osteoarthritis (OA) secondary to fracture. I describe a new triceps exposure approach for TEA, the anconeus-triceps lateral flap, which has proved valuable in patients with distal humeral and olecranon fracture malunion, and its preliminary results at a minimum follow-up of 24 months. METHODS Twenty consecutive patients with OA due to distal humeral and olecranon fracture malunion underwent TEA by the anconeus-triceps lateral flap approach, which preserves the olecranon insertion of the medial portion of the triceps proper tendon. RESULTS At a mean follow-up of 33 months, the mean Mayo Elbow Performance Score rose from 41.3 to 94.3. The mean pain score on the visual analog scale fell from 7.1 to 1.1. There were no patients with insufficiency, secondary detachment of the triceps tendon, or grade 4 to 5 of the Medical Research Council scale. DISCUSSION These preliminary data suggest that preservation of the insertion of the medial portion of the triceps proper tendon enables earlier active rehabilitation. Moreover, the new approach provides optimum exposure of the olecranon also in patients with OA secondary to intra-articular fracture of the distal humerus and olecranon, where scarring and bone deformity usually hamper joint exposure.
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Affiliation(s)
- Andrea Celli
- Shoulder and Elbow Unit, Department of Orthopaedic Surgery, Hesperia Hospital, Modena, Italy.
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Cowling BL, Harwood B, Copithorne DB, Rice CL. Rate modulation of human anconeus motor units during high-intensity dynamic elbow extensions. J Appl Physiol (1985) 2016; 121:475-82. [PMID: 27283910 DOI: 10.1152/japplphysiol.00131.2016] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2016] [Accepted: 06/06/2016] [Indexed: 11/22/2022] Open
Abstract
Investigations of high-intensity isometric fatiguing protocols report decreases in motor unit firing rates (MUFRs), but little is known regarding changes in MUFRs following fatigue induced by high-intensity dynamic contractions. Our purpose was to evaluate MUFRs of the anconeus (an accessory elbow extensor) and elbow extension power production as a function of time to task failure (TTF) during high-velocity fatiguing concentric contractions against a moderately heavy resistance. Fine-wire intramuscular electrode pairs were inserted into the anconeus to record MUs in 12 male participants (25 ± 3 yr), over repeated sessions on separate days. MUs were tracked throughout a three-stage, varying load dynamic elbow extension protocol designed to extend the task duration for >1 min thereby inducing substantial fatigue. Mean MUFRs and peak power were calculated for three relative time ranges: 0-15% TTF (beginning), 45-60% TTF (middle) and 85-100% TTF (end). Mean duration of the overall fatigue protocol was ∼80 s. Following the protocol, isometric maximum voluntary contraction (MVC), highest velocity at 35% MVC load, and peak power decreased 37, 60, and 64% compared with baseline, respectively. Data from 20 anconeus MUs tracked successfully throughout the protocol indicated a reduction in MUFRs in relation to power loss from 36 Hz/160 W (0-15% TTF) to 28 Hz/97 W (45-60% TTF) to 23 Hz/43 W (85-100% TTF). During these high-intensity maximal effort concentric contractions, anconeus MUFRs decreased substantially (>35%). Although the absolute MUFRs were higher in the present study than those reported previously for other muscles during sustained high-intensity isometric tasks, the relative decrease in MUFRs was similar between the two tasks.
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Affiliation(s)
- Brianna L Cowling
- School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and
| | - Brad Harwood
- School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and
| | - David B Copithorne
- School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and
| | - Charles L Rice
- School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and Department of Anatomy and Cell Biology, The University of Western Ontario, London, Ontario, Canada
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Stevens DE, Smith CB, Harwood B, Rice CL. In vivo measurement of fascicle length and pennation of the human anconeus muscle at several elbow joint angles. J Anat 2014; 225:502-9. [PMID: 25223934 DOI: 10.1111/joa.12233] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/11/2014] [Indexed: 12/31/2022] Open
Abstract
Ultrasound imaging has facilitated the reliable measure of the architectural variables fascicle length (LF ) and pennation angle (PA), at rest and during static and dynamic contractions in many human skeletal muscles in vivo. Despite its small size and very modest contribution to elbow extension torque, the anconeus muscle has proven a useful model for the study of neuromuscular function in health and disease. Recent single motor unit (MU) studies in the anconeus have reported discrete and identifiable individual trains of MU potentials from intramuscular electromyography (EMG) recordings during dynamic elbow extensions. It is unknown whether the anconeus has unique architectural features related to alterations in LF and PA throughout the elbow joint range of motion that may help explain these high-quality recordings. Previous anatomical studies have investigated this muscle in cadavers and at mainly one elbow joint angle. The purpose of this study was to measure in vivo PA and LF of the anconeus muscle in a relaxed state at different degrees of elbow flexion using ultrasonography. Ultrasound images were collected from 10 healthy males (25 ± 3 years) at 135°, 120°, 90°, 45°, and 0° of elbow flexion. Average values of LF decreased by 6 mm (10%), 6 mm (12%), and 4 mm (9%) from 135-120°, 120-90°, and 90-45° of elbow flexion, respectively, whereas average PA values increased by 1° (9%), 1° (8%), and 2° (14%) from 135-120°, 120-90°, and 45-0°, respectively. The results indicate that anconeus muscle architecture is dynamic, undergoing moderate changes with elbow joint excursion that are similar to other limb muscles reported elsewhere. The data obtained here are more comprehensive and representative of architectural changes at various elbow joint positions than those data reported in cadaveric studies. Furthermore, the results of this study indicate that despite experiencing similar relative changes in muscle architecture to other skeletal muscles about the elbow joint, the minimal absolute changes in LF of the anconeus likely contribute to the clarity of intramuscular EMG previously reported in this muscle.
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Affiliation(s)
- Daniel E Stevens
- School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, ON, Canada
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Stevens DES, Harwood B, Power GA, Doherty TJ, Rice CL. Anconeus motor unit number estimates using decomposition-based quantitative electromyography. Muscle Nerve 2014; 50:52-9. [PMID: 24123180 DOI: 10.1002/mus.24092] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2013] [Revised: 10/02/2013] [Accepted: 10/04/2013] [Indexed: 11/06/2022]
Abstract
INTRODUCTION Motor unit number estimates (MUNEs) provide important information in health, aging, and disease, and can be determined using decomposition-enhanced spike-triggered averaging (DE-STA). Discrimination of surface-detected motor unit potentials (S-MUPs) has been limited to contractile forces of ∽30% maximum voluntary contraction (MVC), which is insufficient to recruit a representative sample of the entire MU pool in most muscles. Unique features of the anconeus may permit MUNEs at high muscle activation levels. METHODS In 10 men (25 ± 3 years), anconeus MUNEs were performed using DE-STA at 10%, 30%, and 50% root-mean-square of MVC (RMS(MVC)). RESULTS The mean compound muscle action potential of the anconeus was ∽6 mV, and average S-MUP amplitudes were ∽100 μV, 145 μV, and 235 μV at 10%, 30%, and 50% RMS(MVC), resulting in low average MUNEs of 58, 38, and 25, respectively. CONCLUSIONS Elbow extensor force-EMG relationships suggest full recruitment of the anconeus MU pool at 50% RMS(MVC), thus providing a representative sample for MUNE.
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Affiliation(s)
- Daniel E S Stevens
- Canadian Centre for Activity and Aging, School of Kinesiology, Faculty of Health Sciences, University of Western Ontario, London, Ontario, Canada
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Harwood B, Rice CL. Short interspike intervals and double discharges of anconeus motor unit action potentials for the production of dynamic elbow extensions. J Neurophysiol 2014; 111:2039-46. [PMID: 24554783 DOI: 10.1152/jn.00412.2013] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Incidence of double discharges (DDs; >100 Hz) and short interspike intervals (ISIs; >50 to <100 Hz) is reported to vary widely among different muscles and tasks, with a higher incidence in motor unit (MU) trains of fast muscles and for the production of fast contractions in humans. However, it is unclear whether human muscles with a large composition of slower motor units exhibit DDs or short ISIs when activated with maximal synaptic drive, such as those required for maximal velocity dynamic contractions. Thus the purpose of this study was to determine the effect of increasing peak contraction velocity on the incidence of DDs and short ISIs in the anconeus muscle. Seventeen anconeus MUs in 10 young males were recorded across dynamic elbow extensions ranging from low submaximal velocities (16% of maximal velocity) up to maximal velocities. A low incidence of DDs (4%) and short ISIs (29%) was observed among the 583 MU trains recorded. Despite the low incidence in individual MU trains, a majority (71% and 94%, respectively) of MUs exhibited at least one DD or short ISI. The number of short ISIs shared no variance with MU recruitment threshold (R(2) = 0.02), but their distribution was skewed toward higher peak velocities (G = -1.26) and a main effect of peak elbow extension velocity was observed (P < 0.05). Although a greater number of short ISIs was observed with increasing velocity, the low incidence of DDs and short ISIs in the anconeus muscle is likely related to the function of the anconeus as a stabilizer rather than voluntary elbow extensor torque and velocity production.
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Affiliation(s)
- B Harwood
- Canadian Centre for Activity and Aging, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and
| | - C L Rice
- Canadian Centre for Activity and Aging, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; and Department of Anatomy and Cell Biology, The University of Western Ontario, London, Ontario, Canada
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Fukutani A, Miyamoto N, Kanehisa H, Yanai T, Kawakami Y. Potentiation of isokinetic torque is velocity-dependent following an isometric conditioning contraction. SPRINGERPLUS 2013; 2:554. [PMID: 24255848 PMCID: PMC3825088 DOI: 10.1186/2193-1801-2-554] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/05/2013] [Accepted: 10/17/2013] [Indexed: 11/16/2022]
Abstract
Not only twitch torque but also the maximal voluntary concentric torque increases after a high-intensity contraction (conditioning contraction). The purpose of this study was to test the hypothesis that the increase in the maximal voluntary concentric torque induced by a conditioning contraction is prominent when tested at fast angular velocities conditions. Twelve healthy male participants performed the maximal voluntary isometric plantar flexion for six seconds as a conditioning contraction. Before and after the conditioning contraction, peak torques during the maximal voluntary concentric plantar flexions were measured at 30°/s (slow) and 180°/s (fast), each of which was carried out in a separate condition. Isometric twitch torque was also recorded before and after the conditioning contraction in each of the two velocity conditions to confirm the extent of the positive effect of the conditioning contraction. The extent of increase in isometric twitch torque was similar between the two velocity conditions, whereas the maximal voluntary concentric torque increased significantly only in the fast velocity condition (p = 0.003). These results support the hypothesis and indicate that the maximal voluntary concentric torque can be potentiated by the conditioning contraction if the joint angular velocity during the maximal voluntary concentric contraction is sufficiently high.
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Affiliation(s)
- Atsuki Fukutani
- Faculty of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama, 359-1192 Japan ; Japan Society for the Promotion of Science, 5-3-1 kojimachi, Chiyoda-ku, Tokyo, 102-8472 Japan
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Harwood B, Dalton BH, Power GA, Rice CL. Motor unit properties from three synergistic muscles during ramp isometric elbow extensions. Exp Brain Res 2013; 231:501-10. [PMID: 24081681 DOI: 10.1007/s00221-013-3714-y] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2013] [Accepted: 09/16/2013] [Indexed: 11/24/2022]
Abstract
Many tasks require synergistic activation of muscles that possess different architectural, mechanical, and neural control properties. However, investigations of the motor unit (MU) mechanisms which modulate force are mostly restricted to individual muscles and low forces. To explore the pattern of MU recruitment and discharge behavior among three elbow extensors (lateral and long heads of the triceps brachii, and anconeus) during ramp isometric contractions, recruitment thresholds of 77 MUs in five young men were determined and corresponding MU discharge rates were tracked in 1-s epochs over forces ranging from 0 to 75 % of maximal voluntary isometric force (MVC). Across all forces, MUs in the lateral head discharged at higher rates than those in the anconeus (p < 0.001, Δ = 0.23). When all MUs were considered, recruitment thresholds in the long head of the triceps brachii were higher than the lateral head (p < 0.05, Δ = 0.70) with a trend (p = 0.08, Δ = 0.48) for higher recruitment thresholds in the long head compared with the anconeus. Together, these data indicate a potential mechanical disadvantage of the long head of the triceps brachii at 0° shoulder flexion. However, among low-threshold MUs (<10 % MVC), recruitment thresholds were lower in the anconeus than in both heads of the triceps brachii consistent with the expected twitch contractile and fiber type differences among these muscles. These findings illustrate the importance of considering synergistic relations among muscles used for a coordinated task, and the sensitivity of synergies to muscle architectural, mechanical, and possibly specific synaptic input factors.
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Affiliation(s)
- B Harwood
- Department of Physiology, University of Arizona, Tucson, AZ, 85721, USA,
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Surface electromyography for assessing triceps brachii muscle activities: A literature review. Biocybern Biomed Eng 2013. [DOI: 10.1016/j.bbe.2013.09.001] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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Harwood B, Choi I, Rice CL. Reduced motor unit discharge rates of maximal velocity dynamic contractions in response to a submaximal dynamic fatigue protocol. J Appl Physiol (1985) 2012; 113:1821-30. [PMID: 23085960 DOI: 10.1152/japplphysiol.00879.2012] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Fatigability is highly task dependent wherein motor unit (MU) discharge rates and recruitment thresholds are affected differently depending on whether contractions are performed at maximal or submaximal intensities. Although much is described for isometric tasks, the behavior of MU properties during the production of maximal velocity dynamic contractions following submaximal fatiguing contractions is unknown. In seven young men, we evaluated changes in MU recruitment thresholds and MU discharge rates of the anconeus muscle during both submaximal and maximal dynamic elbow extensions following a submaximal dynamic fatiguing protocol of moderate intensity to velocity task failure. Velocity and power of the maximal dynamic contractions declined ∼45 and ∼55%, respectively, but these variables were unchanged for the submaximal target velocity contractions. Discharge rates of the 12 MUs at task failure were unchanged for submaximal dynamic contractions, but were decreased ∼20% for maximal dynamic and ballistic isometric contractions at task failure. MU recruitment thresholds of submaximal dynamic contractions decreased 52% at task failure, but were similar throughout the fatiguing protocol for maximal contractions. These findings support the concept of a common neural mechanism responsible for the relative declines in MU discharge rate associated with submaximal fatigability in both isometric and dynamic contractions.
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Affiliation(s)
- B Harwood
- School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
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Harwood B, Rice CL. Changes in motor unit recruitment thresholds of the human anconeus muscle during torque development preceding shortening elbow extensions. J Neurophysiol 2012; 107:2876-84. [PMID: 22378176 DOI: 10.1152/jn.00902.2011] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Rate of torque development and the subsequent movement velocity are modulated by motor unit (MU) properties, primarily MU discharge rate and MU recruitment threshold (MURT). In isometric conditions, MURTs have been shown to decrease with increased rates of torque development. It is unclear whether this relationship is similar in the production of dynamic shortening contractions. Using fast joint velocities to drive the system, we aimed to determine how anconeus MURTs recorded during the torque production phase preceding movement were affected in relation to the resultant peak elbow extension velocity. Recruitment thresholds of 17 MUs from 9 young men were tracked throughout non-isokinetic dynamic elbow extensions with velocities ranging from 64°/s to 500°/s at a constant resistance of 25% of maximal voluntary isometric contraction and during isometric elbow extensions (0°/s). Relative MURTs decreased ∼50% from the slowest (<25% of maximal velocity) to the fastest (>75% of maximal velocity) resultant velocity ranges (P < 0.05). Although a significant (P < 0.001) but weak (r = -0.27, R(2) = 0.08) relationship was observed between MURT and resultant peak elbow extension velocity for the group, only 7 of the 17 MUs displayed significant moderate (r = -0.40, R(2) = 0.17) to strong (r = -0.85, R(2) = 0.73) negative MURT-velocity relationships. These data indicate variable responses of MURTs with increasing resultant peak velocity, which may be related to the intrinsic properties of individual MUs.
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Affiliation(s)
- B Harwood
- Canadian Centre for Activity and Aging, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada
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