Sahinis Chrysostomos, Amiridis Ioannis G, Farina Dario, Enoka Roger M, Kellis Eleftherios
Laboratory of Neuromechanics, Department of Physical Education and Sport Sciences at Serres, Aristotle University of Thessaloniki, Agios Ioannis, 62110, Serres, Greece.
Department of Bioengineering, Imperial College London, London, UK.
Eur J Appl Physiol. 2025 Sep 4. doi: 10.1007/s00421-025-05953-5.
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.
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.
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.
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.
我们的研究调查了在三个改变肌肉长度的膝关节角度下，半腱肌（ST）和股二头肌（BF）运动单位（MU）的放电特征。
15名男性（21.1±2.8岁）在三个膝关节角度（0°：长肌长度，45°：中间肌长度，90°：短肌长度）下，对膝关节屈肌进行四个目标扭矩（最大自主收缩的10%、20%、40%和60%，即MVC）的稳定等长收缩。记录高密度肌电信号并分解为运动单位放电时间。我们计算了平均放电率（MDR）、峰间期变异系数（CoV ISI）和滤波后的累积放电序列的标准差（fCST的SD）。此外，根据fCST的互相关估计肌肉内部和之间的神经驱动。
方差分析表明，在长肌长度时MVC最大，在中间肌长度时扭矩稳定性最差（p<0.05）。线性混合模型显示，BF表现出更大的MDR和神经驱动变异性（fCST的SD），而ST中的运动单位表现出更大的放电率变异性（CoV ISI）（p<0.05）。估计的ST和BF神经驱动的互相关相对较低，表明这两块肌肉有独立的神经控制。此外，ST的神经驱动变异性与扭矩稳定性（CoV扭矩）的相关性比BF更强。
研究结果表明，ST和BF在不同膝关节角度下的运动单位放电特征不同，每块肌肉接受不同的神经驱动，突出了肌肉特异性训练策略的重要性。
