Department of Kinesiology, Faculty of Applied Health Sciences, Brock University, St. Catharines, ON, L2S 3A1, Canada.
School of Education and Human Sciences, University of Kansas, Lawrence, KS, USA.
Eur J Appl Physiol. 2024 Oct;124(10):2965-2979. doi: 10.1007/s00421-024-05500-8. Epub 2024 May 19.
Motor unit (MU) activation during maximal contractions is lower in children compared with adults. Among adults, discrete MU activation differs, depending on the rate of contraction. We investigated the effect of contraction rate on discrete MU activation in boys and men.
Following a habituation session, 14 boys and 20 men completed two experimental sessions for knee extension and wrist flexion, in random order. Maximal voluntary isometric torque (MVIC) was determined before completing trapezoidal isometric contractions (70%MVIC) at low (10%MVIC/s) and high (35%MVIC/s) contraction rates. Surface electromyography was captured from the vastus lateralis (VL) and flexor carpi radialis (FCR) and decomposed into individual MU action potential (MUAP) trains.
In both groups and muscles, the initial MU firing rate (MUFR) was greater (p < 0.05) at high compared with low contraction rates. The increase in initial MUFR at the fast contraction in the VL was greater in men than boys (p < 0.05). Mean MUFR was significantly lower during fast contractions only in the FCR (p < 0.05). In both groups and muscles, the rate of decay of MUFR with increasing MUAP amplitude was less steep (p < 0.05) during fast compared with slow contractions.
In both groups and muscles, initial MUFRs, as well as MUFRs of large MUs were higher during fast compared with slow contractions. However, in the VL, the increase in initial MUFR was greater in men compared with boys. This suggests that in large muscles, men may rely more on increasing MUFR to generate torque at faster rates compared with boys.
与成年人相比，儿童在最大收缩时的运动单位（MU）激活较低。在成年人中，离散 MU 的激活取决于收缩率。我们研究了收缩率对男孩和男性离散 MU 激活的影响。
在适应阶段后，14 名男孩和 20 名男性以随机顺序完成了两次膝关节伸展和腕关节屈曲的实验。在以低（10%MVIC/s）和高（35%MVIC/s）收缩率完成梯形等长收缩（70%MVIC）之前，确定最大自主等长扭矩（MVIC）。从股外侧肌（VL）和桡侧腕屈肌（FCR）采集表面肌电图，并分解为单个 MU 动作电位（MUAP）列车。
在两组和肌肉中，高收缩率时的初始 MU 发射率（MUFR）更高（p<0.05）。与男孩相比，男性快速收缩时 VL 中初始 MUFR 的增加更大（p<0.05）。仅在 FCR 中，快速收缩时的平均 MUFR 明显较低（p<0.05）。在两组和肌肉中，MUAP 幅度增加时 MUFR 的衰减率较慢（p<0.05）。
在两组和肌肉中，与慢收缩相比，快收缩时初始 MUFR 以及大 MU 的 MUFR 更高。然而，在 VL 中，男性的初始 MUFR 增加大于男孩。这表明，在大肌肉中，男性可能比男孩更依赖于增加 MUFR 以更快的速度产生扭矩。
