Laboratorio di Ingegneria del Sistema Neuromuscolare e della Riabilitazione Motoria (LISiN), Dipartimento di Elettronica, Politecnico di Torino, Turin, Italy.
J Electromyogr Kinesiol. 2010 Aug;20(4):580-9. doi: 10.1016/j.jelekin.2009.11.008. Epub 2009 Dec 30.
This work investigated motor unit (MU) recruitment during transcutaneous electrical stimulation (TES) of the tibialis anterior (TA) muscle, using experimental and simulated data. Surface electromyogram (EMG) and torque were measured during electrically-elicited contractions at different current intensities, on eight healthy subjects. EMG detected during stimulation (M-wave) was simulated selecting the elicited MUs on the basis of: (a) the simulated current density distribution in the territory of each MU and (b) the excitation threshold characteristic of the MU. Exerted force was simulated by adding the contribution of each of the elicited MUs. The effects of different fat layer thickness (between 2 and 8mm), different distributions of excitation thresholds (random excitation threshold, higher threshold for larger MUs or smaller MUs), and different MU distributions within the muscle (random distribution, larger MU deeper in the muscle, smaller MU deeper) on EMG variables and torque were tested. Increase of the current intensity led to a first rapid increase of experimental M-wave amplitude, followed by a plateau. Further increases of the stimulation current determined an increase of the exerted force, without relevant changes of the M-wave. Similar results were obtained in simulations. Rate of change of conduction velocity (CV) and leading coefficient of the second order polynomial interpolating the force vs. stimulation level curve were estimated as a function of increasing current amplitudes. Experimental data showed an increase of estimated CV with increasing levels of the stimulation current (for all subjects) and a positive leading coefficient of force vs. stimulation current curve (for five of eight subjects). Simulations matched the experimental results only when larger MUs were preferably located deeper in the TA muscle (in line with a histochemical study). Marginal effect of MU excitation thresholds was observed, suggesting that MUs closer to the stimulation electrode are recruited first during TES regardless of their excitability.
本研究旨在利用实验和模拟数据，探讨经皮电刺激（TES）胫骨前肌时运动单位（MU）的募集情况。在 8 名健康受试者中，在不同电流强度下进行电诱发收缩时，测量了表面肌电图（EMG）和扭矩。通过选择基于以下两种方式诱发的 MU 来模拟 EMG 检测到的刺激期间（M 波）：（a）每个 MU 区域中的模拟电流密度分布和（b）MU 的激发阈值特性。通过添加每个诱发 MU 的贡献来模拟施加的力。测试了不同脂肪层厚度（2-8mm）、不同兴奋阈值分布（随机兴奋阈值、较大 MU 或较小 MU 具有较高阈值）以及肌肉内 MU 分布（随机分布、较大 MU 较深、较小 MU 较深）对 EMG 变量和扭矩的影响。随着电流强度的增加，实验性 M 波幅度首先迅速增加，然后达到平台期。进一步增加刺激电流会导致施加力增加，而 M 波没有明显变化。模拟结果也得到了类似的结果。作为增加电流幅度的函数，估计了传导速度（CV）的变化率和力与刺激水平曲线的二阶多项式的主导系数。实验数据显示，随着刺激电流水平的增加，估计的 CV 增加（所有受试者），力与刺激电流曲线的主导系数为正（8 名受试者中的 5 名）。只有当较大的 MU 更好地位于 TA 肌肉深处时（与组织化学研究一致），模拟结果才与实验结果相匹配。观察到 MU 兴奋阈值的边缘效应，这表明在 TES 期间，无论其兴奋性如何，靠近刺激电极的 MU 首先被募集。
