Change in muscle force following electrical stimulation. Dependence on stimulation waveform and frequency.

Change in muscle force in healthy subjects due to electrical stimulation was accomplished with rectangular and sinusoidal currents. The pulse width of rectangular stimuli was 0.3 ms and repetition frequency was 25 Hz. The frequency of sinusoidal stimuli was 2 500 Hz, chopped by a 25 Hz rectangular signal. Thirteen healthy subjects were involved in the study and divided into three groups. The first group (A) had stimulation with rectangular impulses, the second (B) with sinusoidal impulses and the third (C) was a control group. The quadriceps muscle was stimulated daily for 10 minutes for 3 weeks. The maximal voluntary isometric torque increased for 25% in group A and 13% in group B, while there was no significant difference in group C. Different patterns of fatigue occurred with different stimuli. The presence of fatigue during the high frequency sinusoidal stimulation diminished the strengthening effects.