Frequency response of spine extensors during rapid isometric contractions: effects of muscle length and tension

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.