Reliability of Corticospinal and Motoneuronal Excitability Evaluation During Unfatiguing and Fatiguing Cycling Exercise

INTRODUCTION

Central nervous system excitability depends on the task performed, muscle group solicited, and contraction type. However, little is known on corticospinal and motoneuronal excitability measured during locomotor exercise. This study aimed at determining the reliability of motor-evoked potentials (MEP) and thoracic motor-evoked potentials (TMEP) in dynamic mode during unfatiguing and fatiguing cycling exercise.

METHODS

Twenty-two participants completed four visits. Visit 1 comprised familiarization and a maximal incremental test to determine maximal power output (Wmax). The remaining visits encompassed unfatiguing evaluations, which included 8 brief bouts of moderate (50% Wmax) and high intensity cycling (80% Wmax). In each bout, a set of two TMEPs, five MEPs and one M-max were obtained. Subsequently, a fatiguing exercise to exhaustion at 80% Wmax was performed, with four sets of measurements 3 min through the exercise and 4 additional sets at exhaustion, both measured at 50% Wmax.

RESULTS

Intraclass correlation coefficients (ICCs) for 5, 10, 15 and 20 MEP·Mmax-1 revealed excellent reliability at both intensities and during fatiguing exercise (ICC ≥ 0.92). TMEP·Mmax-1 showed ICCs ≥0.82 for moderate and high intensity, and it was not affected by fatigability. Overall standard error of measurement was 0.090 [0.083, 0.097] for MEP·Mmax-1 and 0.114 [0.105, 0.125] for TMEP·Mmax-1. A systematic bias associated to the number of stimulations, especially at high intensity, suggested the evaluation itself may be influenced by fatigability. A mean reduction of 8% was detected in TMEP·Mmax-1 at exhaustion.

CONCLUSIONS

Motoneuronal and corticospinal excitability measured in dynamic mode presented good to excellent reliability in unfatiguing and fatiguing exercise. Further studies inducing greater fatigability must be conducted to assess the sensitivity of central nervous system excitability during cycling.

Effects of prolonged vibration to the flexor carpi radialis muscle on intracortical excitability

Prolonged local vibration (LV) can induce neurophysiological adaptations thought to be related to long-term potentiation or depression. Yet, how changes in intracortical excitability may be involved remains to be further investigated as previous studies reported equivocal results. We therefore investigated the effects of 30 min of LV applied to the right flexor carpi radialis muscle (FCR) on both short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). SICI and ICF were measured through transcranial magnetic stimulation before and immediately after 30 min of FCR LV (vibration condition) or 30 min of rest (control condition). Measurements were performed during a low-intensity contraction (n = 17) or at rest (n = 7). No significant SICI nor ICF modulations were observed, whether measured during isometric contractions or at rest (p = 0.2). Yet, we observed an increase in inter-individual variability for post measurements after LV. In conclusion, while intracortical excitability was not significantly modulated after LV, increased inter-variability observed after LV may suggest the possibility of divergent responses to prolonged LV exposure.

Do soleus responses to transcutaneous spinal cord stimulation show similar changes to H-reflex in response to Achilles tendon vibration?

INTRODUCTION/PURPOSE

Recently, the use of transcutaneous spinal cord stimulation (TSCS) has been proposed as a viable alternative to the H-reflex. The aim of the current study was to investigate to what extent the two modes of spinal cord excitability investigation would be similarly sensitive to the well-known vibration-induced depression.

METHODS

Fourteen healthy participants (8 men and 6 women; age: 26.7 ± 4.8 years) were engaged in the study. The right soleus H-reflex and TSCS responses were recorded at baseline (PRE), during right Achilles tendon vibration (VIB) and following 20 min of vibration exposure (POST-VIB). Care was taken to match H-reflex and TSCS responses amplitude at PRE and to maintain effective stimulus intensities constant throughout time points.

RESULTS

The statistical analysis showed a significant effect of time for the H-reflex, with VIB (13 ± 5% of maximal M-wave (Mmax) and POST-VIB (36 ± 4% of Mmax) values being lower than PRE-values (48 ± 6% of Mmax). Similarly, TSCS responses changed over time, VIB (9 ± 5% of Mmax) and POST-VIB (27 ± 5% of Mmax) values being lower than PRE-values (46 ± 6% of Mmax). Pearson correlation analyses revealed positive correlation between H-reflex and TSCS responses PRE-to-VIB changes, but not for PRE- to POST-VIB changes.

CONCLUSION

While the sensitivity of TSCS seems to be similar to the gold standard H-reflex to highlight the vibratory paradox, both responses showed different sensitivity to the effects of prolonged vibration, suggesting slightly different pathways may actually contribute to evoked responses of both stimulation modalities.

Age-Related Differences between Old and Very Old Men in Performance and Fatigability Are Evident after Cycling but Not Isometric or Concentric Single-Limb Tasks

PURPOSE

This study aimed to compare performance and fatigability between young (n = 13; 18-30 yr), old (n = 13; 60-80 yr), and very old (n = 12; >80 yr) men during a single-joint isometric (ISO) and concentric (CON) task performed on an isokinetic dynamometer and a cycling (BIKE) task.

METHODS

Participants randomly performed incremental tasks consisting of stages of 75 contractions (i.e., 120 s, 0.8 s on/0.8 s off) for ISO and CON and 120 s at 37.5 rpm (similar duty cycle) for BIKE. Increments were set as a percentage of body weight. Knee extensor maximal force, voluntary activation, and twitch amplitude were measured at baseline, after each stage, and at task failure (five out of eight contractions below the target force or 6 s in a row at a cadence <37.5 rpm).

RESULTS

Compared with young men, performance (number of stages) was 24% and 40% lower in old and very old men in ISO, 54% and 59% lower in CON, and 36% and 60% lower in BIKE (all P < 0.05). Performance of old and very old differed only in BIKE (P < 0.01). For the last common stages performed, compared with young, force loss was greater for very old men in ISO and for old and very old men in BIKE (all P < 0.05). Overall, for the last common stage performed and task failure, old and very old men presented similar force loss, alterations in voluntary activation, and twitch amplitude.

CONCLUSIONS

Our findings reveal that, with workloads relative to body weight, differences in performance between old and very old men could only be observed during BIKE (i.e., the more ecologically valid task). Results from isometric or concentric conditions might not be transferable to dynamic exercise with large muscle masses.

How about running on Mars? Influence of sensorimotor coherence on running and spatial perception in simulated reduced gravity

Motor control, including locomotion, strongly depends on the gravitational field. Recent developments such as lower-body positive pressure treadmills (LBPPT) have enabled studies on Earth about the effects of reduced body weight (BW) on walking and running, up to 60% BW. The present experiment was set up to further investigate adaptations to a more naturalistic simulated hypogravity, mimicking a Martian environment with additional visual information during running sessions on LBPPT. Twenty-nine participants performed three sessions of four successive five-min runs at preferred speed, alternating Earth- or simulated Mars-like gravity (100% vs. 38% BW). They were displayed visual scenes using a virtual reality headset to assess the effects of coherent visual flow while running. Running performance was characterized by normal ground reaction force and pelvic accelerations. The perceived upright and vection (visually-induced self-motion sensation)in dynamic visual environments were also investigated at the end of the different sessions. We found that BW reduction induced biomechanical adaptations independently of the visual context. Active peak force and stance time decreased, while flight time increased. Strong inter-individual differences in braking and push-off times appeared at 38% BW, which were not systematically observed in our previous studies at 80% and 60% BW. Additionally, the importance given to dynamic visual cues in the perceived upright diminished at 38% BW, suggesting an increased reliance on the egocentric body axis as a reference for verticality when the visual context is fully coherent with the previous locomotor activity. Also, while vection was found to decrease in case of a coherent visuomotor coupling at 100% BW (i.e., post-exposure influence), it remained unaffected by the visual context at 38% BW. Overall, our findings suggested that locomotor and perceptual adaptations were not similarly impacted, depending on the -simulated- gravity condition and visual context.

Neuromuscular adjustments to unweighted running: the increase in hamstring activity is sensitive to trait anxiety

Introduction: Originally developed for astronauts, lower body positive pressure treadmills (LBPPTs) are increasingly being used in sports and clinical settings because they allow for unweighted running. However, the neuromuscular adjustments to unweighted running remain understudied. They would be limited for certain lower limb muscles and interindividually variable. This study investigated whether this might be related to familiarization and/or trait anxiety. Methods: Forty healthy male runners were divided into two equal groups with contrasting levels of trait anxiety (high, ANX⁺, n = 20 vs. low, ANX^-, n = 20). They completed two 9-min runs on a LBPPT. Each included three consecutive 3-min conditions performed at 100%, 60% (unweighted running), and 100% body weight. Normal ground reaction force and electromyographic activity of 11 ipsilateral lower limb muscles were analyzed for the last 30 s of each condition in both runs. Results: Unweighted running showed muscle- and stretch-shortening cycle phase-dependent neuromuscular adjustments that were repeatable across both runs. Importantly, hamstring (BF, biceps femoris; STSM, semitendinosus/semimembranosus) muscle activity increased during the braking (BF: +44 ± 18%, p < 0.001) and push-off (BF: +49 ± 12% and STSM: +123 ± 14%, p < 0.001 for both) phases, and even more so for ANX⁺ than for ANX^-. During the braking phase, only ANX⁺ showed significant increases in BF (+41 ± 15%, p < 0.001) and STSM (+53 ± 27%, p < 0.001) activities. During the push-off phase, ANX⁺ showed a more than twofold increase in STSM activity compared to ANX^- (+119 ± 10% vs. +48 ± 27, p < 0.001 for both). Conclusion: The increase in hamstring activity during the braking and push-off phases may have accelerated the subsequent swing of the free-leg, likely counteracting the unweighting-induced slowing of stride frequency. This was even more pronounced in ANX⁺ than in ANX^-, in an increased attempt not to deviate from their preferred running pattern. These results highlight the importance of individualizing LBPPT training and rehabilitation protocols, with particular attention to individuals with weak or injured hamstrings.

Acute effects of conventional versus wide-pulse neuromuscular electrical stimulation on quadriceps evoked torque and neuromuscular function

PURPOSE

The effectiveness of a neuromuscular electrical stimulation (NMES) program is proportional to the level of evoked torque, which can be achieved with either conventional or wide-pulse stimulations. The aim of this study was to compare evoked torque, objective fatigability, and related peripheral and central alterations, as well as changes in central nervous system (CNS) excitability induced by an acute session of conventional versus wide-pulse NMES.

METHODS

Seventeen young men underwent three 20-min NMES sessions: conventional (0.2 ms/50 Hz), wide-pulse at 50 Hz (1 ms/50 Hz), and wide-pulse at 100 Hz (1 ms/100 Hz). Neuromuscular measurements (i.e., maximal voluntary contraction, voluntary activation, evoked responses to femoral nerve stimulation, and CNS excitability) were performed on the right quadriceps femoris muscle before and after each NMES session. CNS excitability was measured using transcranial magnetic, thoracic, and transcutaneous spinal cord stimulations.

RESULTS

The level of evoked torque was not significantly different between conventional and wide-pulse protocols applied at the maximal tolerable current intensity. All NMES protocols induced objective fatigability (~14% decrease in maximal voluntary contraction torque, p < 0.001) associated with peripheral (decrease in doublet torque and potentiated M-wave amplitude, p = 0.002 and p < 0.001, respectively) but not central (unchanged voluntary activation, p = 0.79) alterations. However, these acute changes did not differ between NMES protocols and none of the NMES protocols modified markers of CNS excitability.

CONCLUSION

These results may allow to conjecture that chronic effects and treatment effectiveness could be comparable between conventional and wide-pulse NMES.

Acute passive stretching has no effect on gastrocnemius medialis stiffness in children with unilateral cerebral palsy

PURPOSE

The aim of this study was to investigate the effects of an acute high-intensity, long-duration passive stretching session of the plantar flexor muscles, on maximal dorsiflexion (DF) angle and passive stiffness at both ankle joint and gastrocnemius medialis (GM) muscle levels in children with unilateral cerebral palsy (CP).

METHODS

13 children [mean age: 10 years 6 months, gross motor function classification system (GMFCS): I] with unilateral CP underwent a 5 min passive stretching session at 80% of maximal DF angle. Changes in maximal DF angle, slack angle, passive ankle joint and GM muscle stiffness from PRE- to POST-intervention were determined during passive ankle mobilization performed on a dynamometer coupled with shear wave elastography measurements (i.e., ultrasound) of the GM muscle.

RESULTS

Maximal DF angle and maximal passive torque were increased by 6.3° (P < 0.001; + 50.4%; 95% CI 59.9, 49.9) and 4.2 Nm (P < 0.01; + 38.9%; 95% CI 47.7, 30.1), respectively. Passive ankle joint stiffness remained unchanged (P = 0.9; 0%; 95% CI 10.6, - 10.6). GM muscle shear modulus was unchanged at maximal DF angle (P = 0.1; + 34.5%; 95% CI 44.7, 24.7) and at maximal common torque (P = 0.5; - 4%; 95% CI - 3.7, - 4.3), while it was decreased at maximal common angle (P = 0.021; - 35%; 95% CI - 11.4, - 58.5). GM slack angle was shifted in a more dorsiflexed position (P = 0.02; + 20.3%; 95% CI 22.6, 18).

CONCLUSION

Increased maximal DF angle can be obtained in the paretic leg in children with unilateral CP after an acute bout of stretching using controlled parameters without changes in passive stiffness at joint and GM muscle levels.

CLINICAL TRIAL NUMBER

NCT03714269.

Can local vibration alter the contribution of persistent inward currents to human motoneuron firing?

The response of spinal motoneurons to synaptic input greatly depends on the activation of persistent inward currents (PICs), which in turn are enhanced by the neuromodulators serotonin and noradrenaline. Local vibration (LV) induces excitatory Ia input onto motoneurons and may alter neuromodulatory inputs. Therefore, we investigated whether LV influences the contribution of PICs to motoneuron firing. This was assessed in voluntary contractions with concurrent, ongoing LV, as well as after a bout of prolonged LV. High-density surface electromyograms (HD-EMG) of the tibialis anterior were recorded with a 64-electrode matrix. Twenty males performed isometric, triangular, dorsiflexion contractions to 20% and 50% of maximal torque at baseline, during LV of the tibialis anterior muscle, and after 30-min of LV. HD-EMG signals were decomposed, and motor units tracked across time points to estimate PICs through a paired motor unit analysis, which quantifies motor unit recruitment-derecruitment hysteresis (ΔF). During ongoing LV, ΔF was lower for both 20% and 50% ramps. Although significant changes in ΔF were not observed after prolonged LV, a differential effect across the motoneuron pool was observed. This study demonstrates that PICs can be non-pharmacologically modulated by LV. Given that LV leads to reflexive motor unit activation, it is postulated that lower PIC contribution to motoneuron firing during ongoing LV results from decreased neuromodulatory inputs associated with lower descending corticospinal drive. A differential effect in motoneurons of different recruitment thresholds after prolonged LV is provocative, challenging the interpretation of previous observations and motivating future investigations. KEY POINTS: Neuromodulatory inputs from the brainstem influence motoneuron intrinsic excitability through activation of persistent inward currents (PICs). PICs make motoneurons more responsive to excitatory input. We demonstrate that vibration applied on the muscle modulates the contribution of PICs to motoneuron firing, as observed through analysis of the firing of single motor units. The effects of PICs on motoneuron firing were lower when vibration was concurrently applied during voluntary ramp contractions, likely due to lower levels of neuromodulation. Additionally, prolonged exposure to vibration led to differential effects of lower- vs. higher-threshold motor units on PICs, with lower-threshold motor units tending to present an increased and higher-threshold motor units a decreased contribution of PICs to motoneuron firing. These results demonstrate that muscle vibration has the potential to influence the effects of neuromodulation on motoneuron firing. The potential of using vibration as a non-pharmacological neuromodulatory intervention should be further investigated.

Protocols Targeting Afferent Pathways via Neuromuscular Electrical Stimulation for the Plantar Flexors: A Systematic Review

This systematic review documents the protocol characteristics of studies that used neuromuscular electrical stimulation protocols (NMES) on the plantar flexors [through triceps surae (TS) or tibial nerve (TN) stimulation] to stimulate afferent pathways. The review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement, was registered to PROSPERO (ID: CRD42022345194) and was funded by the Greek General Secretariat for Research and Technology (ERA-NET NEURON JTC 2020). Included were original research articles on healthy adults, with NMES interventions applied on TN or TS or both. Four databases (Cochrane Library, PubMed, Scopus, and Web of Science) were systematically searched, in addition to a manual search using the citations of included studies. Quality assessment was conducted on 32 eligible studies by estimating the risk of bias with the checklist of the Effective Public Health Practice Project Quality Assessment Tool. Eighty-seven protocols were analyzed, with descriptive statistics. Compared to TS, TN stimulation has been reported in a wider range of frequencies (5-100, vs. 20-200 Hz) and normalization methods for the contraction intensity. The pulse duration ranged from 0.2 to 1 ms for both TS and TN protocols. It is concluded that with increasing popularity of NMES protocols in intervention and rehabilitation, future studies may use a wider range of stimulation attributes, to stimulate motor neurons via afferent pathways, but, on the other hand, additional studies may explore new protocols, targeting for more optimal effectiveness. Furthermore, future studies should consider methodological issues, such as stimulation efficacy (e.g., positioning over the motor point) and reporting of level of discomfort during the application of NMES protocols to reduce the inherent variability of the results.

Effects of prolonged local vibration superimposed to muscle contraction on motoneuronal and cortical excitability

Introduction: Acute effects of prolonged local vibration (LV) at the central nervous system level have been well investigated demonstrating an altered motoneuronal excitability with a concomitant increase in cortical excitability. While applying LV during isometric voluntary contraction is thought to optimize the effects of LV, this has never been addressed considering the acute changes in central nervous system excitability. Materials and Methods: In the present study, nineteen healthy participants were engaged in four randomized sessions. LV was applied for 30 min to the relaxed flexor carpi radialis muscle (VIB_RELAXED) or during wrist flexions (i.e. intermittent contractions at 10% of the maximal voluntary contraction: 15 s ON and 15 s OFF; VIB_CONTRACT). A control condition and a condition where participants only performed repeated low-contractions at 10% maximal force (CONTRACT) were also performed. For each condition, motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation and cervicomedullary evoked potentials (CMEPs) elicited by corticospinal tract electrical stimulation were measured before (PRE) and immediately after prolonged LV (POST) to investigate motoneuronal and corticospinal excitability, respectively. We further calculated the MEP/CMEP ratio as a proxy of cortical excitability. Results: No changes were observed in the control nor CONTRACT condition. At POST, CMEP decreased similarly in VIB_RELAXED (-32% ± 42%, p < .001) and VIB_CONTRACT (-41% ± 32%, p < .001). MEP/CMEP increased by 110% ± 140% (p = .01) for VIB_RELAXED and by 120% ± 208% (p = .02) for VIB_CONTRACT without differences between those conditions. Discussion: Our results suggest that LV to the flexor carpi radialis muscle, either relaxed or contracted, acutely decreases motoneuronal excitability and induces some priming of cortical excitability.

Measuring objective fatigability and autonomic dysfunction in clinical populations: How and why?

Fatigue is a major symptom in many diseases, often among the most common and severe ones and may last for an extremely long period. Chronic fatigue impacts quality of life, reduces the capacity to perform activities of daily living, and has socioeconomical consequences such as impairing return to work. Despite the high prevalence and deleterious consequences of fatigue, little is known about its etiology. Numerous causes have been proposed to explain chronic fatigue. They encompass psychosocial and behavioral aspects (e.g., sleep disorders) and biological (e.g., inflammation), hematological (e.g., anemia) as well as physiological origins. Among the potential causes of chronic fatigue is the role of altered acute fatigue resistance, i.e. an increased fatigability for a given exercise, that is related to physical deconditioning. For instance, we and others have recently evidenced that relationships between chronic fatigue and increased objective fatigability, defined as an abnormal deterioration of functional capacity (maximal force or power), provided objective fatigability is appropriately measured. Indeed, in most studies in the field of chronic diseases, objective fatigability is measured during single-joint, isometric exercises. While those studies are valuable from a fundamental science point of view, they do not allow to test the patients in ecological situations when the purpose is to search for a link with chronic fatigue. As a complementary measure to the evaluation of neuromuscular function (i.e., fatigability), studying the dysfunction of the autonomic nervous system (ANS) is also of great interest in the context of fatigue. The challenge of evaluating objective fatigability and ANS dysfunction appropriately (i.e.,. how?) will be discussed in the first part of the present article. New tools recently developed to measure objective fatigability and muscle function will be presented. In the second part of the paper, we will discuss the interest of measuring objective fatigability and ANS (i.e. why?). Despite the beneficial effects of physical activity in attenuating chronic fatigue have been demonstrated, a better evaluation of fatigue etiology will allow to personalize the training intervention. We believe this is key in order to account for the complex, multifactorial nature of chronic fatigue.

On-field low-frequency fatigue measurement after repeated drop jumps

Purpose: Monitoring fatigue is now commonly performed in athletes as it can directly impact performance and may further increase the risk of injury or overtraining syndrome. Among the exercise-induced peripheral alterations, low-frequency fatigue (LFF) assessment is commonly restricted to in-lab studies. Measuring LFF on-field would allow athletes and coaches to assess muscle fatigability on a regular basis. The aim of the present study was therefore to validate a new portable device allowing quadriceps LFF assessment in the field.

Methods: LFF was assessed in 15 active and healthy participants before (PRE) and after (POST) a series of drop jumps. LFF was assessed, thanks to a dedicated device recording evoked force to muscle submaximal electrical low- and high-frequency stimulation. Changes in the low- to high-frequency force ratio (further referred to as Powerdex® value) were compared to the changes in the ratio of evoked force induced by paired-pulse femoral nerve electrical stimulation at 10 and 100 Hz (i.e., DB10/DB100 ratio). Maximal voluntary contraction (MVC) and voluntary activation (VA) were also measured.

Results: MVC decreased (p &lt; 0.001), whereas VA was not affected by the fatiguing task (p = 0.14). There was a decrease in the DB10/DB100 ratio (from 96.4% to 67.3%, p &lt; 0.001) as well as in the Powerdex value (from 74.0% to 55.7%, p &lt; 0.001). There was no significant difference between POST values (expressed in percentage of PRE values) of the DB10/DB100 ratio and Powerdex (p = 0.44), and there was a significant correlation between the changes in Powerdex® and DB10/DB100 (r = 0.82, p &lt; 0.001).

Conclusion: The on-field device we tested is a valid tool to assess LFF after a strenuous exercise consisting of repeated drop jumps as it evidences the presence of LFF similarly to a lab technique. Such a device can be used to monitor muscle fatigability related to excitation–contraction in athletes.

Acute effects of quadriceps muscle versus tendon prolonged local vibration on force production capacities and central nervous system excitability

PURPOSE

The present study aimed to directly compare the effects of 30 min muscle (VIB_muscle) vs. tendon (VIB_tendon) local vibration (LV) to the quadriceps on maximal voluntary isometric contraction (MVIC) and rate of torque development (RTD) as well as on central nervous system excitability (i.e. motoneuron and cortical excitability).

METHODS

Before (PRE) and immediately after (POST) LV applied to the quadriceps muscle or its tendon, we investigated MVIC and RTD (STUDY #1; n = 20) or vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) electromyography responses to thoracic electrical stimulation (TMEPs; motoneuron excitability) and transcranial magnetic stimulation (MEPs; corticospinal excitability) (STUDY #2; n = 17). MEP/TMEP ratios were further calculated to quantify changes in cortical excitability.

RESULTS

MVIC decreased at POST (P = 0.017) without any difference between VIB_tendon and VIB_muscle, while RTD decreased for VIB_tendon (P = 0.013) but not VIB_muscle. TMEP amplitudes were significantly decreased for all muscles (P = 0.014, P < 0.001 and P = 0.004 for VL, VM and RF, respectively) for both LV sites. While no changes were observed for MEP amplitude, MEP/TMEP ratios increased at POST for VM and RF muscles (P = 0.009 and P = 0.013, respectively) for both VIB_tendon and VIB_muscle.

CONCLUSION

The present results suggest that prolonged muscle and tendon LV are similarly effective in modulating central nervous system excitability and decreasing maximal force. Yet, altered explosive performance after tendon but not muscle LV suggests greater neural alterations when tendons are vibrated.

Fatigue, physical activity and quality of life in people self-reporting symptoms of chronic venous disease

OBJECTIVE

The aim of this study was to quantify fatigue and quality of life (QoL) in people self-reporting chronic venous disease (CVD) symptoms or at risk of CVD within a large cohort representative of the French population. The relationship between self-reported physical activity and both fatigue and QoL was also investigated. We hypothesised that a greater fatigue and impaired QoL would exist in participants self-reporting CVD symptoms, with the impairments being attenuated in those with greater level of physical activity.

METHODS

Using a web-based, custom and adaptive survey, 3,008 participants were asked to self-report the presence of common symptoms and risk factors of CVD. Fatigue, QoL and physical activity were assessed using the Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F) scale, the Chronic Venous Insufficiency Quality of Life Questionnaire (CIVIQ-14) and the Godin-Shepard Leisure-Time Physical Activity Questionnaire (GSLTPAQ), respectively.

RESULTS

32% of the participants were categorized as having CVD symptoms whereas 50% were categorized as at risk of CVD. Fatigue was greater in participants with CVD symptoms than non-CVD participants (p < 0.001), with the score of participants at risk of CVD being intermediate (p ≤ 0.001). QoL was more impaired in participants with CVD symptoms compared to participants at risk of CVD (p < 0.001). In participants with CVD symptoms, there were relationships between fatigue and QoL (p < 0.001) and between physical activity and fatigue (p < 0.001). Despite the relationship between physical activity and QoL not reaching significance (p = 0.067), a lower QoL was found in insufficiently active as compared to active (p < 0.001) and moderately active (p < 0.001) participants with CVD symptoms.

CONCLUSIONS

Participants self-reporting CVD symptoms suffer from greater fatigue and impaired QoL. In this population, a higher level of physical activity is associated with reduced fatigue and a tendency toward improved QoL.

Prevalence of self-reported fatigue in intensive care unit survivors 6 months-5 years after discharge

Prolonged stays in intensive care units (ICU) are responsible for long-lasting consequences, fatigue being one of the more debilitating. Yet, fatigue prevalence for patients that have experienced ICU stays remains poorly investigated. This study aimed to evaluate fatigue prevalence and the level of physical activity in ICU survivors from 6 months to 5 years after ICU discharge using the Functional Assessment of Chronic Illness Therapy Fatigue (FACIT-F) and Godin questionnaires, respectively. Data from 351 ICU survivors (out of 1583 contacted) showed that 199 (57%) and 152 (43%) were considered as fatigued and non-fatigued, respectively. The median FACIT-F scores for fatigued versus non-fatigued ICU survivors were 21 (14–27) and 45 (41–48), respectively (p < 0.001). Time from discharge had no significant effect on fatigue prevalence (p = 0.30) and fatigued ICU survivors are less active (p < 0.001). In multivariate analysis, the only risk factor of being fatigued that was identified was being female. We reported a high prevalence of fatigue among ICU survivors. Sex was the only independent risk factor of being fatigued, with females being more prone to this symptom. Further studies should consider experimental approaches that help us understand the objective causes of fatigue, and to build targeted fatigue management interventions.

Neuromuscular fatigability during repeated sprints assessed with an innovative cycle ergometer

PURPOSE

Repeated sprint ability is an integral component of team sports. This study aimed to evaluate fatigability development and its aetiology during and immediately after a cycle repeated sprint exercise performed until a given fatigability threshold.

METHODS

On an innovative cycle ergometer, 16 healthy males completed an RSE (10-s sprint/28-s recovery) until task failure (TF): a 30% decrease in sprint mean power (Pmean). Isometric maximum voluntary contraction of the quadriceps (IMVC), central alterations [voluntary activation (VA)], and peripheral alterations [twitch (Pt)] were evaluated before (pre), immediately after each sprint (post), at TF and 3 min after. Sprints were expressed as a percentage of the total number of sprints to TF (TS_TF). Individual data were extrapolated at 20, 40, 60, and 80% TS_TF.

RESULTS

Participants completed 9.7 ± 4.2 sprints before reaching a 30% decrease in Pmean. Post-sprint IMVCs were decreased from pre to 60% TS_TF and then plateaued (pre: 345 ± 56 N, 60% 247 ± 55 N, TF: 233 ± 57 N, p < 0.001). Pt decreased from 20% and plateaued after 40% TS_TF (p < 0.001, pre-TF = - 45 ± 13%). VA was not significantly affected by repeated sprints until 60% TS_TF (pre-TF = - 6.5 ± 8.2%, p = 0.036). Unlike peripheral parameters, VA recovered within 3 min (p = 0.042).

CONCLUSION

During an RSE, Pmean and IMVC decreases were first concomitant to peripheral alterations up to 40% TS_TF and central alterations was only observed in the second part of the test, while peripheral alterations plateaued. The distinct recovery kinetics in central versus peripheral components of fatigability further confirm the necessity to reduce traditional delays in neuromuscular fatigue assessment post-exercise.

Local vibration training improves the recovery of quadriceps strength in early rehabilitation after anterior cruciate ligament reconstruction: A feasibility randomised controlled trial

BACKGROUND

After anterior cruciate ligament reconstruction (ACLR), quadriceps strength must be maximised as early as possible.

OBJECTIVES

We tested whether local vibration training (LVT) during the early post-ACLR period (i.e., ∼10 weeks) could improve strength recovery.

METHODS

This was a multicentric, open, parallel-group, randomised controlled trial. Thirty individuals attending ACLR were randomised by use of a dedicated Web application to 2 groups: vibration (standardised rehabilitation plus LVT, n=16) or control (standardised rehabilitation alone, n=14). Experimenters, physiotherapists and participants were not blinded. Both groups received 24 sessions of standardised rehabilitation over ∼10 weeks. In addition, the vibration group received 1 hour of vibration applied to the relaxed quadriceps of the injured leg at the end of each rehabilitation session. The primary outcome - maximal isometric strength of both injured and non-injured legs (i.e., allowing for limb asymmetry measurement) - was evaluated before ACLR (PRE) and after the 10-week rehabilitation (POST).

RESULTS

Seven participants were lost to follow-up, so data for 23 participants were used in the complete-case analysis. For the injured leg, the mean (SD) decrease in maximal strength from PRE to POST was significantly lower for the vibration than control group (n=11, -16% [10] vs. n=12, -30% [11]; P=0.0045, Cohen's d effect size=1.33). Mean PRE-POST change in limb symmetry was lower for the vibration than control group (-19% [11] vs. -29% [13]) but not significantly (P=0.051, Cohen's d effect size=0.85).

CONCLUSION

LVT improved strength recovery after ACLR. This feasibility study suggests that LVT applied to relaxed muscles is a promising modality of vibration therapy that could be implemented early in ACLR.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT02929004.

Sex Differences in Neuromuscular Fatigue and Changes in Cost of Running after Mountain Trail Races of Various Distances

INTRODUCTION

Women have been shown to experience less neuromuscular fatigue than men in knee extensors (KE) and less peripheral fatigue in plantar flexors (PF) after ultratrail running, but it is unknown if these differences exist for shorter trail running races and whether this may impact running economy. The purpose of this study was to characterize sex differences in fatigability over a range of running distances and to examine possible differences in the postrace alteration of the cost of running (Cr).

METHODS

Eighteen pairs of men and women were matched by performance after completing different races ranging from 40 to 171 km, divided into SHORT versus LONG races (<60 and >100 km, respectively). Neuromuscular function and Cr were tested before and after each race. Neuromuscular function was evaluated on both KE and PF with voluntary and evoked contractions using electrical nerve (KE and PF) and transcranial magnetic (KE) stimulation. Oxygen uptake, respiratory exchange ratio, and ventilation were measured on a treadmill and used to calculate Cr.

RESULTS

Compared with men, women displayed a smaller decrease in maximal strength in KE (-36% vs -27%, respectively, P < 0.01), independent of race distance. In SHORT only, women displayed less peripheral fatigue in PF compared with men (Δ peak twitch: -10% vs -24%, respectively, P < 0.05). Cr increased similarly in men and women.

CONCLUSIONS

Women experience less neuromuscular fatigue than men after both "classic" and "extreme" prolonged running exercises but this does not impact the degradation of the energy Cr.

Acute Effects of Whole-Body Vibration Warm-up on Leg and Vertical Stiffness During Running

ABSTRACT

Paradisis, GP, Pappas, P, Dallas, G, Zacharogiannis, E, Rossi, J, and Lapole, T. Acute effects of whole-body vibration warm-up on leg and vertical stiffness during running. J Strength Cond Res 35(9): 2433-2438, 2021-Although whole-body vibration (WBV) has been suggested as a suitable and efficient alternative to the classic warm-up routines, it is still unknown how this may impact running mechanics. Therefore, the aim of this study was to investigate the effect of a WBV warm-up procedure on lower-limb stiffness and other spatiotemporal variables during running at submaximal speed. Twenty-two males performed 30-second running bouts at 4.44 m·s-1 on a treadmill before and after a WBV and control warm-up protocols. The WBV protocol (vibration frequency: 50 Hz, peak-to-peak displacement: 4 mm) consisted of 10 sets of 30-second dynamic squatting exercises with 30-second rest periods within sets. Leg and vertical stiffness values were calculated using the spring mass model. The results indicated significant increases only after the WBV protocol for leg stiffness (3.4%), maximal ground reaction force (1.9%), and flight time (4.7%). Consequently, the WBV warm-up protocol produced a change in running mechanics, suggesting a shift toward a more aerial pattern. The functional significance of such WBV-induced changes needs further investigation to clearly determine whether it may influence running economy and peak velocity.

Association between physical activity, quadriceps muscle performance and biological characteristics of very old men and women

The present study aimed to evaluate the association between physical activity, knee extensors (KE) performance (i.e. isometric strength and fatigability) and biological parameters (i.e. muscle structural, microvascular and metabolic properties) in healthy very old men and women. Thirty very old adults (82±1 yr, 15 women) performed an isometric quadriceps intermittent fatigue (QIF) test for the assessment of KE maximal force, total work (index of absolute performance) and fatigability. Muscle biopsies from the vastus lateralis muscle were collected to assess muscle fibers type and morphology, microvasculature and enzymes activity. Correlation analyses were used to investigate the relationships between physical activity (steps.day -1, actimetry), KE performance, and biological data for each sex separately.Men, compared to women, showed greater total work at the QIF test (44497±8629 N.s vs. 26946±4707 N.s; P<0.001). Steps.day -1 were correlated with total work only for women (r=0.73, P=0.011). In men, steps.day -1 were correlated with the percentage (r=0.57, P=0.033), shape factor (r=0.75, P = 0.002) and capillary tortuosity of type IIX fibers (r=0.59, P=0.035). No other relevant correlations were observed for men or women between steps.day -1 and biological parameters. Physical activity level was positively associated with the capacity of very old women to perform a fatiguing test, but not maximal force production capacity of the KE. Physical activity of very old men was not correlated with muscle performance. We suggest that very old women could be at higher risk of autonomy loss and increasing the steps.day -1 count could provide a sufficient stimulus for adaptations in less active women.

Effect of race distance on performance fatigability in male trail and ultra-trail runners

The etiology of changes in lower-limb neuromuscular function, especially to the central nervous system, may be affected by exercise duration. Direct evidence is lacking as few studies have directly compared different race distances. This study aimed to investigate the etiology of deficits in neuromuscular function following short versus long trail-running races. Thirty-two male trail runners completed one of five trail-running races as LONG (>100 km) or SHORT (<60 km). Pre- and post-race, maximal voluntary contraction (MVC) torque and evoked responses to electrical nerve stimulation during MVCs and at rest were used to assess voluntary activation and muscle contractile properties of knee-extensor (KE) and plantar-flexor (PF) muscles. Transcranial magnetic stimulation (TMS) was used to assess evoked responses and corticospinal excitability in maximal and submaximal KE contractions. Race distance correlated with KE MVC (ρ = -0.556) and twitch (ρ = -0.521) torque decreases (p ≤ .003). KE twitch torque decreased more in LONG (-28 ± 14%) than SHORT (-14 ± 10%, p = .005); however, KE MVC time × distance interaction was not significant (p = .073). No differences between LONG and SHORT for PF MVC or twitch torque were observed. Maximal voluntary activation decreased similarly in LONG and SHORT in both muscle groups (p ≥ .637). TMS-elicited silent period decreased in LONG (p = .021) but not SHORT (p = .912). Greater muscle contractile property impairment in longer races, not central perturbations, contributed to the correlation between KE MVC loss and race distance. Conversely, PF fatigability was unaffected by race distance.

Influence of wide-pulse neuromuscular electrical stimulation frequency and superimposed tendon vibration on occurrence and magnitude of extra torque

Low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation (NMES) can generate extra torque (ET) via afferent pathways. Superimposing tendon vibration (TV) to NMES can increase the activation of these afferent pathways and favor ET generation. Knowledge of the characteristics of ET is essential to implement these stimulation paradigms in clinical practice. Thus, we aimed to investigate the effects of frequency and TV superimposition on the occurrence and magnitude of ET in response to wide-pulse NMES. NMES-induced isometric plantar flexion torque was recorded in 30 healthy individuals who performed five NMES protocols: wide-pulse low-frequency (1 ms; 20 Hz; WPLF) and wide-pulse high-frequency (1 ms; 100 Hz; WPHF) without and with superimposed TV (1 mm; 100 Hz) and conventional NMES (50 µs; 20 Hz; reference protocol). Each NMES protocol consisted of three 20-s trains interspersed by 90 s of rest, with NMES intensity being adjusted to reach 10% of maximal voluntary contraction. The ET occurrence was similar for WPLF and WPHF (P = 0.822). In the responders, the ET magnitude was greater for WPHF than WPLF (P < 0.001). There was no effect of superimposed TV on ET characteristics. This study reported an effect of NMES frequency on ET magnitude, whereas TV superimposition did not affect this parameter. In the context of our experimental design decisions, the present findings question the clinical use of wide-pulse NMES and its combination with superimposed TV. Yet, further research is needed to maximize force production through the occurrence and magnitude of ET.NEW & NOTEWORTHY This study is the first to assess the effect of stimulation frequency and superimposed tendon vibration on extra torque characteristics generated by wide-pulse neuromuscular electrical stimulation. The percentage of subjects showing extra torque (i.e., considered as responders) was similar for low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation. In the responders, the extra torque was greater for high-frequency than for low-frequency wide-pulse neuromuscular electrical stimulation. The superimposition of tendon vibration had no effect on extra torque occurrence or magnitude.

Bedside voluntary and evoked forces evaluation in intensive care unit patients: a narrative review

Around one third of intensive care unit (ICU) patients will develop severe neuromuscular alterations, known as intensive care unit-acquired weakness (ICUAW), during their stay. The diagnosis of ICUAW is difficult and often delayed as a result of sedation or delirium. Indeed, the clinical evaluation of both Medical Research Council score and maximal voluntary force (e.g., using handgrip and/or handheld dynamometers), two independent predictors of mortality, can be performed only in awake and cooperative patients. Transcutaneous electrical/magnetic stimulation applied over motor nerves combined with the development of dedicated ergometer have recently been introduced in ICU patients in order to propose an early and non-invasive measurement of evoked force. The aim of this narrative review is to summarize the different tools allowing bedside force evaluation in ICU patients and the related experimental protocols. We suggest that non-invasive electrical and/or magnetic evoked force measurements could be a relevant strategy to characterize muscle weakness in the early phase of ICU and diagnose ICUAW.

Reductions in motoneuron excitability during sustained isometric contractions are dependent on stimulus and contraction intensity

Cervicomedullary stimulation provides a means of assessing motoneuron excitability. Previous studies demonstrated that during low-intensity sustained contractions, small cervicomedullary evoked potentials (CMEPs) conditioned using transcranial magnetic stimulation (TMS-CMEPs) are reduced, whereas large TMS-CMEPs are less affected. As small TMS-CMEPs recruit motoneurons most active during low-intensity contractions whereas large TMS-CMEPs recruit a high proportion of motoneurons inactive during the task, these results suggest that reductions in motoneuron excitability could be dependent on repetitive activation. To further test this hypothesis, this study assessed changes in small and large TMS-CMEPs across low- and high-intensity contractions. Twelve participants performed a sustained isometric contraction of the elbow flexor for 4.5 min at the electromyography (EMG) level associated with 20% maximal voluntary contraction force (MVC; low intensity) and 70% MVC (high intensity). Small and large TMS-CMEPs with amplitudes of ∼15% and ∼50% M_max at baseline, respectively, were delivered every minute throughout the tasks. Recovery measures were taken at 1-, 2.5- and 4-min postexercise. During the low-intensity trial, small TMS-CMEPs were reduced at 2-4 min (P ≤ 0.049) by up to -10% M_max, whereas large TMS-CMEPs remained unchanged (P ≥ 0.16). During the high-intensity trial, small and large TMS-CMEPs were reduced at all time points (P < 0.01) by up to -14% and -33% M_max, respectively, and remained below baseline during all recovery measures (P ≤ 0.02). TMS-CMEPs were unchanged relative to baseline during recovery following the low-intensity trial (P ≥ 0.24). These results provide novel insight into motoneuron excitability during and following sustained contractions at different intensities and suggest that contraction-induced reductions in motoneuron excitability depend on repetitive activation.NEW & NOTEWORTHY This study measured motoneuron excitability using cervicomedullary evoked potentials conditioned using transcranial magnetic stimulation (TMS-CMEPs) of both small and large amplitudes during sustained low- and high-intensity contractions of the elbow flexors. During the low-intensity task, only the small TMS-CMEP was reduced. During the high-intensity task, both small and large TMS-CMEPs were substantially reduced. These results indicate that repetitively active motoneurons are specifically reduced in excitability compared with less active motoneurons in the same pool.

Relationship between intensive care unit-acquired weakness, fatigability and fatigue: What role for the central nervous system?

PURPOSE

To provide a comprehensive review of studies that have investigated fatigue in intensive care unit (ICU) survivors and questions the potential link between intensive care unit-acquired weakness (ICUAW), fatigability and fatigue. We also question whether the central nervous system (CNS) may be the link between these entities.

MATERIAL AND METHODS

A narrative review of the literature that investigated fatigue in ICU survivors and review of clinical trials enabling understanding of CNS alterations in response to ICU stays.

RESULTS

Fatigue is a pervasive and debilitating symptom in ICU survivors that can interfere with rehabilitation. Due to the complex pathophysiology of fatigue, more work is required to understand the roles of ICUAW and/or fatigability in fatigue to provide a more holistic understanding of this symptom. While muscle alterations have been well documented in ICU survivors, we believe that CNS alterations developing early during the ICU stay may play a role in fatigue.

CONCLUSIONS

Fatigue should be considered and treated in ICU survivors. The causes of fatigue are likely to be specific to the individual. Understanding the role that ICUAW and fatigability may have in fatigue would allow to tailor individual treatment to prevent this persistent symptom and improve quality of life.

Reliability and agreement of a dynamic quadriceps incremental test for the assessment of neuromuscular function

The quadriceps-intermittent-fatigue (QIF) test assesses knee extensors strength, endurance and performance fatigability in isometric condition. We aimed to assess reliability and agreement for this test in dynamic conditions and with the use of transcranial magnetic stimulation. On two separate sessions, 20 young adults (25 ± 4 yr, 10 women) performed stages of 100 knee extensors concentric contractions at 120°/s (60° range-of-motion) with 10% increments of the initial maximal concentric torque until exhaustion. Performance fatigability across the test was quantified as maximal isometric and concentric torque loss, and its mechanisms were investigated through the responses to transcranial magnetic and electrical stimulations. Reliability and agreement were assessed using ANOVAs, coefficients of variation (CVs) and intra-class correlation coefficients (ICCs) with 95% CI. Good inter-session reliability and high agreement were found for number of contractions [489 ± 75 vs. 503 ± 95; P = 0.20; ICC = 0.85 (0.66; 0.94); CV = 5% (3; 7)] and total work [11,285 ± 4,932 vs. 11,792 ± 5838 Nm.s; P = 0.20; ICC = 0.95 (0.87; 0.98); CV = 8% (5; 11)]. Poor reliability but high agreement were observed for isometric [-33 ± 6 vs. -31 ± 7%; P = 0.13; ICC = 0.47 (0.05; 0.75); CV = 6% (4;8)] and concentric [-20 ± 11% vs. -19 ± 9%; P = 0.82; ICC = 0.26 (-0.22; 0.63); CV = 9% (6; 12)] torque loss. The dynamic QIF test represents a promising tool for neuromuscular evaluation in isokinetic mode.

Is Accelerometry an Effective Method to Assess Muscle Vibrations in Comparison to Ultrafast Ultrasonography?

OBJECTIVE

The purpose of this study was to assess whether accelerometry effectively reflects muscle vibrations measured with ultrafast ultrasonography.

METHODS

Vibration characteristics initiated on the vastus lateralis muscle by an impactor were compared when assessed with accelerometry and ultrasonography. Continuous wavelet transforms and statistical parametric mapping (SPM) were performed to identify discrepancies in vibration power over time and frequency between the two devices.

RESULTS

The SPM analysis revealed that the accelerometer underestimated the muscle vibration power above 50 Hz during the first 0.06 seconds post impact. Furthermore, the accelerometer overestimated the muscle vibration power under 20 Hz, from 0.1 seconds after the impact. Linear regression revealed that the thicker the subcutaneous fat localized under the accelerometer, the more the muscle vibration frequency and damping were underestimated by the accelerometer.

CONCLUSION

The skin and the fat tissues acted like a low-pass filter above 50 Hz and oscillated in a less damped manner than the muscle tissue under 20 Hz.

SIGNIFICANCE

To eliminate some artifacts caused by the superficial tissues and assess the muscle vibration characteristics with accelerometry, it is suggested to 1) high-pass filter the acceleration signal at a frequency of 20 Hz, under certain conditions, and 2) include participants with less fat thickness. Therefore, the subcutaneous thickness must be systematically quantified under each accelerometer location to clarify the differences between subjects and muscles.

Fatigue-induced changes in short-interval intracortical inhibition and the silent period with stimulus intensities evoking maximal versus submaximal responses

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.

Reduced Active Muscle Stiffness after Intermittent Submaximal Isometric Contractions

PURPOSE

Whether muscle stiffness is influenced by fatigue remains unclear. Classical methods used to assess muscle stiffness provide a global measure at the joint level. As fatigue may selectively affect specific muscles, a joint-level approach may not be sensitive enough to detect potential changes in muscle stiffness. Taking advantage of ultrasound shear wave elastography, this study aimed to determine the influence of a fatiguing protocol involving intermittent submaximal isometric contractions on muscle shear modulus (an index of stiffness).

METHODS

Shear modulus was measured on either the vastus lateralis (n = 9) or the abductor digiti minimi (n = 10) before and after 15 min of intermittent submaximal isometric contractions at 60% of maximal voluntary contraction (MVC) (4 s ON, 4 s OFF). An index of active muscle stiffness was estimated PRE- and POST-fatigue as the slope of the linear regression established between shear modulus and absolute joint force up to 60% MVC.

RESULTS

After the fatiguing exercise, MVC was significantly decreased by 22% ± 7% and 32% ± 15% for knee extension and little finger abduction, respectively (P < 0.001). When compared to PRE-fatigue, the index of active muscle stiffness was 12% ± 15% lower for the vastus lateralis (P < 0.031) and 44% ± 19% lower for the abductor digiti minimi (P < 0.001) POST-fatigue.

CONCLUSIONS

Although the present results cannot clearly determine the involved mechanisms, they demonstrate a decreased active muscle stiffness after a fatiguing task involving intermittent submaximal isometric contractions. Further studies should now determine whether this change in stiffness affects performance and risk of injury.

Effects of graduated compression stockings, local vibration and their combination on popliteal venous blood velocity

Objectives

The purpose of this pilot study was to examine and compare the effects of graduated compression stockings, local vibration, and combined graduated compression stockings and local vibration on popliteal venous blood velocity.

Method

Twenty-four healthy subjects received four 15 min interventions (control, graduated compression stockings alone, local vibration alone, and combined graduated compression stockings and local vibration), while resting inactive in the prone position. Popliteal vein blood velocity was investigated before (PRE) and at the end (POST) of each intervention using Doppler ultrasound.

Results

At POST, peak velocity was reported to be 26.3 ± 53.5% ( p <  0.05) greater for local vibration than control (CONT). Peak velocity was 46.2 ± 54.6% ( p <  0.001) and 21.1 ± 37.6% ( p <  0.01) higher for graduated compression stockings than CONT and local vibration, respectively. Graduated compression stockings + local vibration presented 64.1 ± 58.0% ( p <  0.001), 38.4 ± 52.4% ( p <  0.001) and 15.0 ± 31.6% ( p <  0.05) greater values than CONT, local vibration and graduated compression stockings, respectively.

Conclusions

This study demonstrated an increase in popliteal venous blood velocity after graduated compression stockings and local vibration application. Their combination provided the greatest effects.

Disparate kinetics of change in responses to electrical stimulation at the thoracic and lumbar level during fatiguing isometric knee extension

The present study compared the fatigue-induced change of matched-amplitude thoracic evoked potential (TMEP) and lumbar evoked potential (LEP) following electrical stimulation. Ten participants performed a 3 × 3 min isometric knee extension contraction separated by 4 min of recovery at the level of EMG required to produce 50% maximal voluntary contraction (MVC) force at baseline. The TMEP and LEP were evoked during the ongoing contraction at baseline and every minute into the fatiguing protocol and during recovery. Both responses were also assessed during a transcranial magnetic stimulation (TMS) evoked silent period to elicit a TMS-TMEP and TMS-LEP to assess responses without the confounding influence of descending drive. The results displayed disparate kinetics of the TMS-TMEP and TMS-LEP throughout the fatiguing protocol. The TMS-TMEP was reduced at all time points during exercise ( P < 0.001), whereas the TMS-LEP was reduced at 2 min into set 1 and 1 min into sets 2 and 3 ( P ≤ 0.04). TMS-LEPs were higher than the TMS-TMEPs at most time points ( P ≤ 0.04). No change was observed in the TMEP or LEP. When evoked during the silent period, the reduction in TMEP is greater than the LEP during fatiguing isometric exercise. The disparate kinetics of change suggest that differential mechanisms are responsible for evoked responses to thoracic and lumbar stimulation. More research is required to identify the mechanisms responsible for the TMEP and LEP before precise inferences can be made on what fatigue-induced changes in these variables reflect.

NEW & NOTEWORTHY Assessing spinal excitability using lumbar stimulation when measuring responses in lower limbs has been suggested as an alternative method that could circumvent the issues associated with thoracic stimulation. The present study compared responses to the two types of stimuli throughout a fatiguing protocol and demonstrated that lumbar evoked responses differ substantially from thoracic responses when measured in the absence of voluntary drive. These findings suggest that different mechanisms are responsible for evoked responses to thoracic and lumbar stimuli.

Critical Peripheral Fatigue Thresholds Among Different Force-Velocity Conditions: An Individual-Based Model Approach

During high intensity exercise, metabosensitive muscle afferents are thought to inhibit the motor drive command to restrict the level of peripheral fatigue to an individual's critical threshold. No evidence exists of an individual relationship between peripheral fatigue and the decrease in voluntary activation reached after prolonged all-out exercise. Moreover, there is no explanation for the previously reported large decrease in voluntary activation despite low metabolic stress during high force contractions. Thirteen active men completed two maximal intensity isokinetic knee extension tests (160 contractions) under conditions of low force - high velocity and high force - low velocity. Neuromuscular testing including maximal torque, evoked torque and voluntary activation, was done every 20 contractions. The exponential modeling of these variables over time allowed us to predict the stable state (asymptote) and the rate of decrease (curvature constant). For both high and low force contractions the evoked torque and voluntary activation asymptotes were negatively correlated (R ² = 0.49 and R ² = 0.46, respectively). The evoked torque asymptotes of the high and low force conditions were positively correlated (R ² = 0.49). For the high force contractions, the evoked torque and voluntary activation curvature constant were negatively correlated (R ² = 0.43). These results support the idea that a restrained central motor drive keeps peripheral fatigue under this threshold. Furthermore, an individual would show similar fatigue sensibility regardless of the force generated. These data also suggest that the decrease in voluntary activation might not have been triggered by peripheral perturbations during the first high force contractions.

Rate of force development and rapid muscle activation characteristics of knee extensors in very old men

The age-related decrease in neuromuscular performance is usually accentuated in very old age (>80 yr) as evidenced by a marked reduction in maximal force production. However, little is known about the ability to rapidly produce force, which limits daily activities and increase the risk of falling. We aimed to assess rapid force production characteristics and rate of muscle activation of the knee extensors in 15 very old (82 ± 1 yr) vs 12 young (24 ± 4 yr) men. Maximal force (F_max) and maximal rate of force development (RFD_max) were determined during separated specific isometric contractions. EMG from the vastus lateralis was analyzed to assess the rate of EMG rise (RER). Finally, RFD and RER were examined at time intervals of 0-50, 50-100, 100-200, 0-200 ms and 0-30, 0-50, 0-75 ms, respectively. We reported lower F_max (414 ± 91 N vs. 661 ± 139 N) and absolute RFD_max (8720 ± 2056 N*s^-1vs. 5700 ± 2474 N*s^-1) in the very old men compared to young men (P < 0.01). When normalized to F_max, RFD_max was similar between groups. Normalized RFD at 0-50 and 0-200 ms were lower (-34% and - 46%, P = 0.04) for the very old men, while no difference was observed at 50-100 ms and 100-200 ms. RER values were higher (~346%, P < 0.01) for the young men at every time interval. These data suggest that the decline in RFD_max is associated with the decrease in F_max. Impairments in RFD were accentuated in the first phase of the contraction, which has been associated in the literature with impairments in neural factors associated with aging.

Intermediate Muscle Length and Tendon Vibration Optimize Corticospinal Excitability During Knee Extensors Local Vibration

While local vibration (LV) has been recently proposed as a potential modality for neuromuscular conditioning, no practical recommendations to optimize its effects have been published. Because changes in corticospinal excitability may reflect at which degree the neuromuscular function is modulated during LV exposure, this study investigated the effects of muscle length and vibration site on LV-induced on motor evoked potentials (MEPs) changes. Twenty-one subjects participated in a single session in which MEPs were evoked on the relaxed knee extensors (KE) during three conditions, i.e., no vibration (CON), muscle (VIBMU), and tendon vibration (VIBTD). Three muscle lengths were tested for each condition, i.e., short/intermediate/long KE muscle length. Both VIBMU and VIBTD significantly increase MEPs compared to CON. Higher increases (P < 0.001) were found for VIBTD compared to VIBMU for vastus lateralis (mean increases of the three angles: +241% vs.+ 148%), vastus medialis (+273% vs. + 180%) and rectus femoris muscles (+191% vs. +141%). The increase in MEPs amplitude was higher (p < 0.001) at an intermediate (mean pooled increase for VIBTD and VIBMU: +265%, +290%, and +212% for VL, VM, and RF, respectively) compared to short (+136%, + 144%, and + 127%) or long (+ 184%, + 246% and + 160%) muscle lengths. These results suggest that LV should be applied to the tendon at an intermediate muscle length to optimize the acute effects of LV on the KE neuromuscular function.

Neural adaptations in quadriceps muscle after 4 weeks of local vibration training in young versus older subjects

This study investigated the effects of a 4-week local vibration training (LVT) on the function of the knee extensors and corticospinal properties in healthy young and older subjects. Seventeen subjects (9 young and 8 older) performed 3 testing sessions: before (PRE₁) and after (PRE₂) a 4-week resting period to control the repeatability of the data as well as after the LVT (POST). Jump performance, maximal voluntary contraction (MVC) and electromyographic (EMG) activity on vastus lateralis and rectus femoris muscles were assessed. Single-pulse transcranial magnetic stimulation (TMS) allowed evaluation of cortical voluntary activation (VA_TMS), motor evoked potential (MEP) area, and silent period (SP) duration. All training adaptations were similar between young and older subjects (p > 0.05) and the following results reflect the pooled sample of subjects. MVC (+11.9% ± 8.0%, p < 0.001) and VA_TMS (+3.6% ± 5.2%, p = 0.004) were significantly increased at POST compared with PRE₂. Maximal vastus lateralis EMG was significantly increased at POST (+21.9% ± 33.7%, p = 0.03). No changes were reported for MEPs on both muscles (p > 0.05). SPs recorded during maximal and submaximal contractions decreased in both muscles at POST (p < 0.05). Vertical jump performance was increased at POST (p < 0.05). LVT seems as effective in young as in older subjects to improve maximal functional capacities through neural modulations occurring at least partly at the supra-spinal level. Local vibration may be used as an efficient alternative training method to improve muscular performance in both healthy young and older subjects.

An Acute Exposure to Muscle Vibration Decreases Knee Extensors Force Production and Modulates Associated Central Nervous System Excitability

Local vibration (LV) has been recently validated as an efficient training method to improve muscle strength. Understanding the acute effects may help elucidate the mechanism(s). This study aimed to investigate the effects of a single bout of prolonged LV on knee extensor force production and corticospinal responsiveness of vastus lateralis (VL) and rectus femoris (RF) muscles in healthy young and old adults. Across two visits, 23 adult subjects (20-75 years old) performed pre- and post-test measurements, separated by 30-min of either rest (control; CON) or LV. Maximal voluntary contraction (MVC) force was assessed and transcranial magnetic stimulation (TMS) was used to evaluate cortical voluntary activation (VATMS) as well as the motor evoked potential (MEP) and silent period (SP). In 11 young adults, thoracic electrical stimulation was used to assess the thoracic motor evoked potential (TMEP). Although MVC decreased after both CON (-6.3 ± 4.4%, p = 0.01) and LV (-12.9 ± 7.7%, p < 0.001), the MVC loss was greater after LV (p = 0.001). Normalized maximal electromyographic (EMG) activity decreased after LV for both VL (-25.1 ± 10.7%) and RF (-20.9 ± 16.5%; p < 0.001), while it was unchanged after CON (p = 0.32). For RF, the TMEP and MEP/TMEP ratio decreased (p = 0.01) and increased (p = 0.01) after LV, respectively. Both measures were unchanged for VL (p = 0.27 and p = 0.15, respectively). No changes were reported for TMS-related parameters. These results confirm our hypothesis that modulations within the central nervous system would accompany the significant reduction of maximal voluntary force. A reduced motoneuron excitability seems to explain the decreased MVC after prolonged LV, as suggested by reductions in maximal EMG (all subjects) and TMEP area (data from 11 young subjects). A concomitant increased cortical excitability seems to compensate for lower excitability at the spinal level.

Sleep Extension before Sleep Loss: Effects on Performance and Neuromuscular Function

PURPOSE

This study aimed to investigate the effects of six nights of sleep extension on motor performance and associated neuromuscular function before and after one night of total sleep deprivation (TSD).

METHODS

Twelve healthy men participated in two experimental conditions (randomized crossover design): extended sleep (EXT, 9.8 ± 0.1 h time in bed) and habitual sleep (HAB, 8.2 ± 0.1 h time in bed). In each condition, subjects performed six nights of either EXT or HAB at home followed by an assessment of motor performance and neuromuscular function at baseline (D0) and after one night of TSD, i.e., 34-37 h of continuous wakefulness (D1). Maximal voluntary contractions with superimposed femoral nerve electrical and transcranial magnetic stimulations and stimulations on relaxed muscles were investigated before and after submaximal isometric knee extensor exercises performed until task failure.

RESULTS

Time to exhaustion was longer in EXT compared with HAB (+3.9% ± 7.7% and +8.1% ± 12.3% at D0 and D1, respectively). Performance at D1 decreased from D0 similarly between conditions (-7.2% ± 5.6% and -3.7% ± 7.3% in HAB and EXT, respectively). At D1, the RPE during exercise was lower in EXT compared with HAB (-7.2% ± 7.5%) with no difference at D0. No difference was observed in voluntary activation between the two conditions.

CONCLUSIONS

Six nights of sleep extension improved sustained contraction time to exhaustion, and this result cannot be explained by smaller reductions in voluntary activation, measured by both nerve and transcranial magnetic stimulation. The beneficial effect on motor performance in the EXT condition was likely due to reduced RPE after TSD.

Eight weeks of local vibration training increases dorsiflexor muscle cortical voluntary activation

The aim of this study was to evaluate the effects of an 8-wk local vibration training (LVT) program on functional and corticospinal properties of dorsiflexor muscles. Forty-four young subjects were allocated to a training (VIB, n = 22) or control (CON, n = 22) group. The VIB group performed twenty-four 1-h sessions (3 sessions/wk) of 100-Hz vibration applied to the right tibialis anterior. Both legs were tested in each group before training (PRE), after 4 (MID) and 8 (POST) wk of training, and 2 wk after training (POST_2W). Maximal voluntary contraction (MVC) torque was assessed, and transcranial magnetic stimulation (TMS) was used to evaluate cortical voluntary activation (VA_TMS), motor evoked potential (MEP), cortical silent period (CSP), and input-output curve parameters. MVC was significantly increased for VIB at MID for right and left legs [+7.4% (P = 0.001) and +6.2% (P < 0.01), respectively] and remained significantly greater than PRE at POST [+12.0% (P < 0.001) and +10.1% (P < 0.001), respectively]. VA_TMS was significantly increased for right and left legs at MID [+4.4% (P < 0.01) and +4.7% (P < 0.01), respectively] and at POST [+4.9% (P = 0.001) and +6.2% (P = 0.001), respectively]. These parameters remained enhanced in both legs at POST_2W MEP and CSP recorded during MVC and input-output curve parameters did not change at any time point for either leg. Despite no changes in excitability or inhibition being observed, LVT seems to be a promising method to improve strength through an increase of maximal voluntary activation, i.e., neural adaptations. Local vibration may thus be further considered for clinical or aging populations.NEW & NOTEWORTHY The effects of a local vibration training program on cortical voluntary activation measured with transcranial magnetic stimulation were assessed for the first time in dorsiflexors, a functionally important muscle group. We observed that training increased maximal voluntary strength likely because of the strong and repeated activation of Ia spindle afferents during vibration training that led to changes in the cortico-motoneuronal pathway, as demonstrated by the increase in cortical voluntary activation.

Effects of compression stockings on ankle muscle H-reflexes during standing

INTRODUCTION

Wearing compression stockings (CS) may improve postural stability through additional cutaneous feedback. The aim of this study was to further determine how wearing CS could influence spinal excitability by investigating ankle muscle H-reflexes.

METHODS

Fifteen subjects were asked to stand barefoot on a rigid floor with their eyes open. H-reflex amplitude was measured in the soleus (SOL), fibularis longus (FL), and tibialis anterior (TA) muscles, with and without CS. Concomitant M-waves and baseline electromyographic activity (EMG) were monitored.

RESULTS

Baseline EMG activity and concomitant M-wave amplitude remained stable across conditions in all tested muscles. Although CS did not affect the H-reflex in the SOL (+0.8 ± 19.2%; P = 0.77) and FL (-10.0± 33.2%; P = 0.28) muscles, the TA H-reflex was significantly depressed (-21.9% ± 24.0%; P = 0.03).

CONCLUSIONS

These results suggest decreased spinal motoneuron excitability and/or increased presynaptic inhibition of Ia-afferent terminals through increased cutaneous inputs provided by CS while standing. Muscle Nerve 55: 596-598, 2017.