School of Psychiatry, University of New South Wales, Australia.
J Neurophysiol. 2012 Feb;107(3):796-807. doi: 10.1152/jn.00066.2011. Epub 2011 Nov 9.
Oscillatory activity plays a crucial role in corticospinal control of muscle synergies and is widely investigated using corticospinal and intermuscular synchronization. However, the neurophysiological mechanisms that translate these rhythmic patterns into surface electromyography (EMG) are not well understood. This is underscored by the ongoing debate on the rectification of surface EMG before spectral analysis. Whereas empirical studies commonly rectify surface EMG, computational approaches have argued against it. In the present study, we employ a computational model to investigate the role of the motor unit action potential (MAUP) on the translation of oscillatory activity. That is, diverse MUAP shapes may distort the transfer of common input into surface EMG. We test this in a computational model consisting of two motor unit pools receiving common input and compare it to empirical results of intermuscular coherence between bilateral leg muscles. The shape of the MUAP was parametrically varied, and power and coherence spectra were investigated with and without rectification. The model shows that the effect of EMG rectification depends on the uniformity of MUAP shapes. When output spikes of different motor units are convolved with identical MUAPs, oscillatory input is evident in both rectified and nonrectified EMG. In contrast, a heterogeneous MAUP distribution distorts common input and oscillatory components are only manifest as periodic amplitude modulations, i.e., in rectified EMG. The experimental data showed that intermuscular coherence was mainly discernable in rectified EMG, hence providing empirical support for a heterogeneous distribution of MUAPs. These findings implicate that the shape of MUAPs is an essential parameter to reconcile experimental and computational approaches.
振荡活动在肌肉协同的皮质脊髓控制中起着至关重要的作用，并且广泛使用皮质脊髓和肌肉间同步来研究。然而，将这些节律模式转化为表面肌电图（EMG）的神经生理机制尚未得到很好的理解。这一点在关于在进行频谱分析之前对表面 EMG 进行整流的持续争论中得到了强调。虽然实证研究通常对表面 EMG 进行整流，但计算方法却反对这样做。在本研究中，我们使用计算模型来研究运动单位动作电位（MAUP）对振荡活动的翻译作用。也就是说，不同的 MUAP 形状可能会扭曲常见输入向表面 EMG 的传递。我们在一个由两个接收共同输入的运动单位池组成的计算模型中对此进行了测试，并将其与双侧腿部肌肉之间的肌肉间相干性的实证结果进行了比较。MUAP 的形状是参数化变化的，并且在有和没有整流的情况下研究了功率和相干谱。该模型表明，EMG 整流的效果取决于 MUAP 形状的均匀性。当不同运动单位的输出尖峰与相同的 MUAP 卷积时，无论是在整流和非整流的 EMG 中都可以明显看出振荡输入。相比之下，异质的 MAUP 分布会扭曲共同输入，并且只有周期性幅度调制才会显示出振荡分量，即在整流的 EMG 中。实验数据表明，肌肉间相干性主要在整流的 EMG 中可辨，因此为 MUAP 异质分布提供了经验支持。这些发现表明，MUAP 的形状是协调实验和计算方法的重要参数。
