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Acute adaptation of central and peripheral motor unit features to exercise‐induced fatigue differs with concentric and eccentric loading

Jones, Eleanor J.; Guo, Yuxiao; Martinez‐Valdes, Eduardo; Negro, Francesco; Stashuk, Daniel W.; Atherton, Philip J.; Phillips, Bethan E.; Piasecki, Mathew

Acute adaptation of central and peripheral motor unit features to exercise‐induced fatigue differs with concentric and eccentric loading Thumbnail


Eleanor J. Jones

Yuxiao Guo

Eduardo Martinez‐Valdes

Francesco Negro

Daniel W. Stashuk

Philip J. Atherton

Bethan E. Phillips


New Findings: What is the central question of this study? Conflicting evidence exists on motor unit (MU) firing rate in response to exercise‐induced fatigue, possibly due to the contraction modality used: Do MU properties adapt similarly following concentric and eccentric loading? What is the main finding and its importance? MU firing rate increased following eccentric loading only despite a decline in absolute force. Force steadiness deteriorated following both loading methods. Central and peripheral MU features are altered in a contraction type‐dependant manner, which is an important consideration for training interventions. Abstract: Force output of muscle is partly mediated by the adjustment of motor unit (MU) firing rate (FR). Disparities in MU features in response to fatigue may be influenced by contraction type, as concentric (CON) and eccentric (ECC) contractions demand variable amounts of neural input, which alters the response to fatigue. This study aimed to determine the effects of fatigue following CON and ECC loading on MU features of the vastus lateralis (VL). High‐density surface (HD‐sEMG) and intramuscular (iEMG) electromyography were used to record MU potentials (MUPs) from bilateral VLs of 12 young volunteers (six females) during sustained isometric contractions at 25% and 40% of the maximum voluntary contraction (MVC), before and after completing CON and ECC weighted stepping exercise. Multi‐level mixed effects linear regression models were performed with significance assumed as P < 0.05. MVC decreased in both CON and ECC legs post‐exercise (P < 0.0001), as did force steadiness at both 25% and 40% MVC (P < 0.004). MU FR increased in ECC at both contraction levels (P < 0.001) but did not change in CON. FR variability increased in both legs at 25% and 40% MVC following fatigue (P < 0.01). From iEMG measures at 25% MVC, MUP shape did not change (P > 0.1) but neuromuscular junction transmission instability increased in both legs (P < 0.04), and markers of fibre membrane excitability increased following CON only (P = 0.018). These data demonstrate that central and peripheral MU features are altered following exercise‐induced fatigue and differ according to exercise modality. This is important when considering interventional strategies targeting MU function.

Journal Article Type Article
Acceptance Date Mar 14, 2023
Online Publication Date Apr 5, 2023
Publication Date Jun 1, 2023
Deposit Date Apr 9, 2023
Publicly Available Date Apr 20, 2023
Journal Experimental Physiology
Print ISSN 0958-0670
Electronic ISSN 1469-445X
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 108
Issue 6
Pages 827-837
Keywords Physiology; Physiology (medical); Nutrition and Dietetics; Physiology; Physiology (medical); Nutrition and Dietetics
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