Conwit R A, Stashuk D, Tracy B, McHugh M, Brown W F, Metter E J
Department of Neurology, Johns Hopkins Bayview Medical Center, Baltimore, MD 21224, USA.
Clin Neurophysiol. 1999 Jul;110(7):1270-5. doi: 10.1016/s1388-2457(99)00054-1.
Using a clinical electromyographic (EMG) protocol, motor units were sampled from the quadriceps femoris during isometric contractions at fixed force levels to examine how average motor unit size and firing rate relate to force generation.
Mean firing rates (mFRs) and sizes (mean surface-detected motor unit action potential (mS-MUAP) area) of samples of active motor units were assessed at various force levels in 79 subjects.
MS-MUAP size increased linearly with increased force generation, while mFR remained relatively constant up to 30% of a maximal force and increased appreciably only at higher force levels. A relationship was found between muscle force and mS-MUAP area (r2 = 0.67), mFR (r2 = 0.38), and the product of mS-MUAP area and mFR (mS-MUAP x mFR) (r2 = 0.70).
The results support the hypothesis that motor units are recruited in an orderly manner during forceful contractions, and that in large muscles only at higher levels of contraction ( > 30% MVC) do mFRs increase appreciably. MS-MUAP and mFR can be assessed using clinical EMG techniques and they may provide a physiological basis for analyzing the role of motor units during muscle force generation.
采用临床肌电图（EMG）方案，在等长收缩时以固定力水平从股四头肌采集运动单位样本，以研究平均运动单位大小和放电频率与力产生之间的关系。
在79名受试者的不同力水平下，评估活跃运动单位样本的平均放电频率（mFRs）和大小（平均表面检测到的运动单位动作电位（mS-MUAP）面积）。
mS-MUAP大小随力产生的增加呈线性增加，而mFR在最大力的30%之前保持相对恒定，仅在更高力水平时才明显增加。发现肌肉力与mS-MUAP面积（r2 = 0.67）、mFR（r2 = 0.38）以及mS-MUAP面积与mFR的乘积（mS-MUAP×mFR）（r2 = 0.70）之间存在关系。
结果支持以下假设，即在强力收缩过程中运动单位按顺序募集，并且在大肌肉中仅在较高收缩水平（>30%最大随意收缩）时mFRs才会明显增加。mS-MUAP和mFR可使用临床EMG技术进行评估，它们可能为分析运动单位在肌肉力产生过程中的作用提供生理基础。
