Spectral electromyographic changes during a muscular strengthening training based on electrical stimulation

The aim of this study is to describe the effects of neuromuscular electrical stimulation on electromyographic power spectrum evolution through a program of muscle strengthening. Ten healthy subjects received electrical stimulation of the quadriceps femoris muscle. Surface EMG was obtained from rectus femoris during maximum voluntary isometric contractions of quadriceps. Directly from EMG the Zero Crossing Rate and Root Mean Square (RMS) were calculated. Afterwards, the Mean Power Frequency and Median Frequency from EMG power spectrum were obtained using the Maximum Entropy method to estimate the spectrum. An analysis of variance for repeated measures revealed a statistical significance for RMS through the sessions and for spectral shifts towards higher frequencies between, before and after electrical stimulation. The evolution of power spectrum through sessions showed a curvilinear evolution, first decreasing towards lower frequencies and then increasing. These results suggest that electrical stimulation produces immediate changes on the motor unit: increasing the recruitment number and action potential velocity and a selective recruitment of type II fibres. During the strengthening program sessions two main phases were detected: the appearance and recovery from fatigue.