Independent neural drives and distinct motor unit discharge characteristics in hamstring muscles during isometric knee flexion
File(s)
Published version
Author(s)
Sahinis, Chrysostomos
Amiridis, Ioannis G
Farina, Dario
Enoka, Roger M
Kellis, Eleftherios
Type
Journal Article
Abstract
Purpose
Our study investigated the discharge characteristics of motor units (MUs) in the semitendinosus (ST) and biceps femoris (BF) at three knee-joint angles that varied muscle length.
Methods
Fifteen males (21.1 ± 2.8 years) performed steady isometric contractions with the knee flexors at four target torques (10%, 20%, 40%, and 60% of maximal voluntary contraction, MVC) at each of the three knee-joint angles (0°: long, 45°: intermediate, and 90°: short length). High-density electromyographic signals were recorded and decomposed into MU discharge times. We calculated mean discharge rate (MDR), the coefficient of variation for interspike interval (CoV ISI), and the standard deviation of the filtered cumulative spike train (SD of fCST). In addition, the neural drive within and between muscles was estimated from the cross-correlation of the fCST.
Results
Analysis of variance indicated that MVC was greatest at the long length and torque steadiness was worst at the intermediate length (p < 0.05). Linear mixed models revealed that BF exhibited greater MDR and variability in neural drive (SD of fCST), whereas the MUs in ST displayed greater discharge rate variability (CoV ISI) (p < 0.05). Cross-correlation of the estimated neural drives to ST and BF was relatively low, suggesting independent neural control of the two muscles. Moreover, the variability in neural drive for ST was more strongly correlated with torque steadiness (CoV torque) than that for BF.
Conclusion
The findings indicate that MU discharge characteristics differed for ST and BF across knee-joint angles, with each muscle receiving a distinct neural drive highlighting the importance of muscle-specific training strategies.
Our study investigated the discharge characteristics of motor units (MUs) in the semitendinosus (ST) and biceps femoris (BF) at three knee-joint angles that varied muscle length.
Methods
Fifteen males (21.1 ± 2.8 years) performed steady isometric contractions with the knee flexors at four target torques (10%, 20%, 40%, and 60% of maximal voluntary contraction, MVC) at each of the three knee-joint angles (0°: long, 45°: intermediate, and 90°: short length). High-density electromyographic signals were recorded and decomposed into MU discharge times. We calculated mean discharge rate (MDR), the coefficient of variation for interspike interval (CoV ISI), and the standard deviation of the filtered cumulative spike train (SD of fCST). In addition, the neural drive within and between muscles was estimated from the cross-correlation of the fCST.
Results
Analysis of variance indicated that MVC was greatest at the long length and torque steadiness was worst at the intermediate length (p < 0.05). Linear mixed models revealed that BF exhibited greater MDR and variability in neural drive (SD of fCST), whereas the MUs in ST displayed greater discharge rate variability (CoV ISI) (p < 0.05). Cross-correlation of the estimated neural drives to ST and BF was relatively low, suggesting independent neural control of the two muscles. Moreover, the variability in neural drive for ST was more strongly correlated with torque steadiness (CoV torque) than that for BF.
Conclusion
The findings indicate that MU discharge characteristics differed for ST and BF across knee-joint angles, with each muscle receiving a distinct neural drive highlighting the importance of muscle-specific training strategies.
Date Issued
2026-02-01
Date Acceptance
2025-08-14
Citation
European Journal of Applied Physiology, 2026, 126 (2), pp.839-852
ISSN
1439-6319
Publisher
Springer
Start Page
839
End Page
852
Journal / Book Title
European Journal of Applied Physiology
Volume
126
Issue
2
Copyright Statement
© The Author(s) 2025 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
License URL
Identifier
https://www.ncbi.nlm.nih.gov/pubmed/40905988
PII: 10.1007/s00421-025-05953-5
Subjects
Biceps femoris
BICEPS FEMORIS
DORSIFLEXION
EMG
FORCE
Hamstrings
INJURY
Life Sciences & Biomedicine
Motor unit
Muscle length
Physiology
POOLS
Science & Technology
Semitendinosus
SEMITENDINOSUS
Sport Sciences
T-2 RELAXATION
THIGH MUSCLES
Publication Status
Published
Coverage Spatial
Germany
Date Publish Online
2025-09-04