Records of muscular rhythms and action-currents obtained during voluntary effort showed a striking similarity. They presented a modulated wave, i.e., a wave having the amplitude of its oscillations varied periodically. This modulated wave is made up of an audible or principal frequency and an inaudible frequency. The inaudible frequency (10-12 per sec.) is composed of the periodic variations in intensity of the audible frequency (300-600 per sec.).