Neuromuscular responses to fatiguing locomotor exercise

Over the last two decades, an abundance of research has explored the impact of fatiguing locomotor exercise on the neuromuscular system. Neurostimulation techniques have been implemented prior to and following locomotor exercise tasks of a wide variety of intensities, durations, and modes. These techniques have allowed for the assessment of alterations occurring within the central nervous system and the muscle, while techniques such as transcranial magnetic stimulation and spinal electrical stimulation have permitted further segmentalization of locomotor exercise-induced changes along the motor pathway. To this end, the present review provides a comprehensive synopsis of the literature pertaining to neuromuscular responses to locomotor exercise. Sections of the review were divided to discuss neuromuscular responses to maximal, severe, heavy and moderate intensity, high-intensity intermittent exercise, and differences in neuromuscular responses between exercise modalities. During maximal and severe intensity exercise, alterations in neuromuscular function reside primarily within the muscle. Although post-exercise reductions in voluntary activation following maximal and severe intensity exercise are generally modest, several studies have observed alterations occurring at the cortical and/or spinal level. During prolonged heavy and moderate intensity exercise, impairments in contractile function are attenuated with respect to severe intensity exercise, but are still widely observed. While reductions in voluntary activation are greater during heavy and moderate intensity exercise, the specific alterations occurring within the central nervous system remain unclear. Further work utilizing stimulation techniques during exercise and integrating new and emerging techniques such as high-density electromyography is warranted to provide further insight into neuromuscular responses to locomotor exercise

French Translation and Validation of the Rating-of-Fatigue Scale

Background The Rating of Fatigue (ROF) scale can measure changes in perceived fatigue in a variety of contexts. Objective The aim of the present study was to translate and subsequently validate the ROF scale in the French language. Methods The study was composed of three phases. Phase 1 involved a comprehensive translation, back-translation, and consolidation process in order to produce the French ROF scale. During phase 2, the face validity of the French ROF scale was assessed. A cohort of 60 native French speaking participants responded to a range of Likert scale items which probed the purposes of the ROF scale and what it is intended to measure. During phase 3, the convergent and divergent validity of the ROF scale was assessed during ramped cycling to exhaustion and 10 min of resting recovery. Results The results from phase 1 demonstrated comparability and interpretability between the original and back-translated ROF scale. In phase 2, participants reported a high face validity, with a score of 3.48 ± 0.70 out of 4 when given the item probing whether the scale “measures fatigue”. This score further improved (3.67 ± 0.57, P = 0.01) after participants read the accompanying instructions. Participants were able to distinguish the purposes of the scale for measuring fatigue rather than exertion. In phase 3, strong correlations were found between ROF and heart rate (HR) both during exercise (r = 0.91, P < 0.01) and recovery (r = 0.92, P < 0.01), while discriminant validity between ROF and rating of perceived exertion (RPE) was found during recovery. Conclusion The present study permits the applications of the ROF scale in the French language

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% Mmax 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% Mmax, 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% Mmax, 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

Motor cortical and corticospinal function differ during an isometric squat compared with isometric knee extension

It has been suggested that task‐specific changes in neurophysiological function (neuroplasticity) should be assessed using testing modalities that replicate the characteristics of the intervention. The squat is a commonly prescribed resistance exercise that has been shown to elicit changes in CNS function. However, previous studies have assessed squat‐induced neuroplasticity using isometric knee extension, potentially confounding the results. The aim of the present study was to assess the agreement between corticospinal and intracortical activity relating to the knee extensors during isometric knee extension compared with an isometric squat task. Eleven males completed a neurophysiological assessment in an isometric squat (IS) and knee‐extension (KE) task matched for joint angles (hip, knee and ankle). Single‐ and paired‐pulse transcranial magnetic stimulation was delivered during isometric contractions at a range of intensities to assess short‐interval cortical inhibition (SICI) and corticospinal excitability. Group mean values for SICI (70 ± 14 versus 63 ± 12% of unconditioned motor evoked potential during IS and KE, respectively) and corticospinal excitability (mean differences 2–5% of the maximal compound muscle action potential at 25, 50, 75 and 100% maximal voluntary contraction between the IS and KE) were not different between the two tasks (P > 0.05) in the vastus lateralis. However, limits of agreement were wide, with poor‐to‐moderate average intraclass correlation coefficients (ICCs) (SICI, ICC3,1 = 0.15; corticospinal excitability, average ICC3,1 range = 0.0–0.63), indicating disparate corticospinal and intracortical activity between the IS and KE. These data highlight the importance of task specificity when assessing the modulation of corticospinal excitability and SICI in response to interventions resulting in neuroplastic change

Etiology and Recovery of Neuromuscular Fatigue following Competitive Soccer Match-Play

Aim: Previous research into the etiology of neuromuscular fatigue following competitive soccer match-play has primarily focused on peripheral perturbations, with limited research assessing central nervous system function in the days post-match. The aim of the present study was to examine the contribution and time-course of recovery of central and peripheral factors toward neuromuscular fatigue following competitive soccer match-play. Methods: Sixteen male semi-professional soccer players completed a 90-min soccer match. Pre-, post- and at 24, 48, and 72 h participants completed a battery of neuromuscular, physical, and perceptual tests. Maximal voluntary contraction force (MVC) and twitch responses to electrical (femoral nerve) and transcranial magnetic stimulation (TMS) of the motor cortex during isometric knee-extension and at rest were measured to assess central nervous system (voluntary activation, VA) and muscle contractile (potentiated twitch force, Qtw, pot) function. Electromyography responses of the rectus femoris to single- and paired-pulse TMS were used to assess corticospinal excitability and short-interval intracortical inhibition (SICI), respectively. Fatigue and perceptions of muscle soreness were assessed via visual analog scales, and physical function was assessed through measures of jump (countermovement jump height and reactive strength index) and sprint performance. Results: Competitive match-play elicited significant post-match declines in MVC force (−14%, P < 0.001) that persisted for 48 h (−4%, P = 0.01), before recovering by 72 h post-exercise. VA (motor point stimulation) was reduced immediately post-match (−8%, P < 0.001), and remained depressed at 24 h (−5%, P = 0.01) before recovering by 48 h post-exercise. Qtw,pot was reduced post-match (−14%, P < 0.001), remained depressed at 24 h (−6%, P = 0.01), before recovering by 48 h post-exercise. No changes were evident in corticospinal excitability or SICI. Jump performance took 48 h to recover, while perceptions of fatigue persisted at 72 h. Conclusion: Competitive soccer match-play elicits substantial impairments in central nervous system and muscle function, requiring up to 48 h to resolve. The results of the study could have important implications for fixture scheduling, the optimal management of the training process, squad rotation during congested competitive schedules, and the implementation of appropriate recovery interventions

Myths and Methodologies: how loud is the story told by the transcranial magnetic stimulation-evoked silent period?

New Findings: What is the topic of this review? The origin, interpretation and methodological constraints of the silent period induced by transcranial magnetic stimulation are reviewed. What advances does it highlight? The silent period is generated by both cortical and spinal mechanisms. Therefore, it seems inappropriate to preface silent period with ‘cortical’ unless additional measures are taken. Owing to many confounding variables, a standardized approach to the silent period measurement cannot be suggested. Rather, recommendations of best practice are provided based on the available evidence and the context of the research question. Abstract: Transcranial magnetic stimulation (TMS) of the motor cortex evokes a response in the muscle that can be recorded via electromyography (EMG). One component of this response, when elicited during a voluntary contraction, is a period of EMG silence, termed the silent period (SP), which follows a motor evoked potential (MEP). Modulation of SP duration was long thought to reflect the degree of intracortical inhibition. However, the evidence presented in this review suggests that both cortical and spinal mechanisms contribute to generation of the SP, which makes prefacing SP with ‘cortical’ misleading. Further investigations with multi-methodological approaches, such as TMS–EEG coupling or interaction of TMS with neuroactive drugs, are needed to make such inferences with greater confidence. A multitude of methodological factors can influence the SP and thus confound the interpretation of this measure; namely, background muscle activity, instructions given to the participant, stimulus intensity and the size of the MEP preceding the SP, and the approach to analysis. A systematic understanding of how the confounding factors influence the interpretation of SP is lacking, which makes standardization of the methodology difficult to conceptualize. Instead, the methodology should be guided through the lens of the research question and the population studied, ensuring greater reproducibility, repeatability and comparability of data sets. Recommendations are provided for the best practice within a given context of the experimental design

Etiology and recovery of neuromuscular fatigue following competitive soccer match-play

Aim: Previous research into the etiology of neuromuscular fatigue following competitive soccer match-play has primarily focused on peripheral perturbations, with limited research assessing central nervous system function in the days post-match. The aim of the present study was to examine the contribution and time-course of recovery of central and peripheral factors toward neuromuscular fatigue following competitive soccer match-play. Methods: Sixteen male semi-professional soccer players completed a 90-min soccer match. Pre-, post- and at 24, 48, and 72 h participants completed a battery of neuromuscular, physical, and perceptual tests. Maximal voluntary contraction force (MVC) and twitch responses to electrical (femoral nerve) and transcranial magnetic stimulation (TMS) of the motor cortex during isometric knee-extension and at rest were measured to assess central nervous system (voluntary activation, VA) and muscle contractile (potentiated twitch force, Qtw, pot) function. Electromyography responses of the rectus femoris to single- and paired-pulse TMS were used to assess corticospinal excitability and short-interval intracortical inhibition (SICI), respectively. Fatigue and perceptions of muscle soreness were assessed via visual analog scales, and physical function was assessed through measures of jump (countermovement jump height and reactive strength index) and sprint performance. Results: Competitive match-play elicited significant post-match declines in MVC force (−14%, P < 0.001) that persisted for 48 h (−4%, P = 0.01), before recovering by 72 h post-exercise. VA (motor point stimulation) was reduced immediately post-match (−8%, P < 0.001), and remained depressed at 24 h (−5%, P = 0.01) before recovering by 48 h post-exercise. Qtw,pot was reduced post-match (−14%, P < 0.001), remained depressed at 24 h (−6%, P = 0.01), before recovering by 48 h post-exercise. No changes were evident in corticospinal excitability or SICI. Jump performance took 48 h to recover, while perceptions of fatigue persisted at 72 h. Conclusion: Competitive soccer match-play elicits substantial impairments in central nervous system and muscle function, requiring up to 48 h to resolve. The results of the study could have important implications for fixture scheduling, the optimal management of the training process, squad rotation during congested competitive schedules, and the implementation of appropriate recovery intervention