Analysis of surface EMG of human soleus muscle subjected to vibration.

When vibration is applied to a muscle, the discharges of its motor units (MUs) become correlated with vibratory pulses. This effect has been extensively studied by single MU recording techniques. We suggested the surface EMG to be useful in this context to reveal a generalized pattern of MU firing. The surface EMG of human soleus muscle subjected to vibration during a voluntary contraction was recorded with precautions to suppress vibratory artifacts. Spectral analysis of the EMG and of the rectified EMG (REMG) revealed characteristic peaks, present in the power spectra at the vibration frequency (VF) and its harmonics. Vibratory artifacts were negligible, since during the ischemic block of the muscle, the peaks disappeared similar to the T-reflex extinction. If the VF was low compared with the major frequency domain of the EMG, the peaks were better expressed in the spectra of the REMG, while at higher VFs they were more pronounced in the EMG spectra. With the increase in the force of muscle contraction, the peaks, normalized to the total power of the spectrum, diminished. During a prolonged contraction in the presence of vibration, the peaks augmented, probably due to synaptic potentiation. Mathematical modeling of the surface EMG was performed to interpret the experimental data. The simulated patterns were consistent with the experimental results. Since the model predicted an increase in the normalized peaks with increasing number of MUs, it was suggested that at larger forces of contraction the correlation of MU discharges with the vibratory stimuli, on average, deteriorated.