Brereton L C, McGill S M
Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, Ontario, Canada.
J Electromyogr Kinesiol. 1998 Aug;8(4):227-32. doi: 10.1016/s1050-6411(98)00009-1.
During muscle contraction, electrical activity necessarily precedes force output, yet models that utilize processed electromyograms sometimes predict force as preceding EMG under rapid ballistic loading conditions. The purpose of this study was to define the frequency response transfer function of the upper and lower erector spinae musculature, at different lengths and tensions, using rectified, low pass filtered EMG. This would enable accurate estimates of force from the processed electromyogram, specifically during impulsive contractions. Abdominal and erector spinae EMG were measured in synchrony with impulsive low back moments in five men. EMG signals were rectified and low pass filtered repeatedly with cut-off frequencies from 1 to 3 Hz at 0.5 Hz increments in order to quantify the frequency response. It was found that EMG signals processed through a simple, Butterworth low pass filter could not produce the measured force output without an additional time shift. These shifts were quantified by cross-correlating EMG and force with increments of 1 ms. In order to define the transfer function of EMG to force, optimal cut-off frequencies were selected two ways: quantitatively by searching for maximum cross correlations coefficients, and qualitatively. Results indicated that the frequency response of both the upper and lower erector spinae can be modelled with a cut-off frequency between 2 and 2.5 Hz and that these values are not significantly modulated by changes in muscle length or tension.
在肌肉收缩过程中，电活动必然先于力的输出，但在快速弹道加载条件下，利用处理后的肌电图的模型有时会预测力先于肌电图出现。本研究的目的是使用经整流、低通滤波的肌电图来定义不同长度和张力下上、下竖脊肌的频率响应传递函数。这将能够从处理后的肌电图中准确估计力，特别是在脉冲性收缩期间。对五名男性在产生脉冲性下背部力矩时同步测量腹部和竖脊肌的肌电图。为了量化频率响应，对肌电图信号进行多次整流和低通滤波，截止频率从1Hz到3Hz，以0.5Hz的增量递增。结果发现，通过简单的巴特沃斯低通滤波器处理的肌电图信号，若没有额外的时间偏移，就无法产生测量到的力输出。这些偏移通过以1毫秒的增量对肌电图和力进行互相关来量化。为了定义肌电图到力的传递函数，通过两种方式选择最佳截止频率：一种是通过寻找最大互相关系数进行定量选择，另一种是定性选择。结果表明，上、下竖脊肌的频率响应都可以用2Hz到2.5Hz之间的截止频率来建模，并且这些值不会因肌肉长度或张力的变化而受到显著调制。
