Tetanic responses of electrically stimulated paralyzed muscle at varying interpulse intervals

The influence of stimulus interpulse interval (IPI) on torque output during electrically-evoked contractions for the knee extensor muscles of paralyzed subjects is discussed. The parameters measured were the risetime, magnitude, and relaxation time of the contraction at stimulus IPIs ranging from 62 to 7 ms. Torque output increased as IPIs were decreased from 62 to 15 ms. Peak torques were recorded at IPIs of 12-15 ms; IPIs lower than these resulted in an insignificant loss of torque. Risetimes decreased as IPIs were decreased. Relaxation time generally increased as IPIs were decreased with the longest relaxation times occurring with stimulation at an IPI of 12 ms. To demonstrate the influence of IPI on muscle fatigue, the effects of prolonged stimulation at short (12-ms) and long (50-ms) IPIs were compared. After 30 s of stimulation with an IPI of 12 ms, mean torque had declined to 5% +or-3% and after 30 s of stimulation with an IPI of 50 ms, mean torque had declined to 82% +or-4% of the initial value.