Biomechanics of human quadriceps muscles during electrical stimulation.

The quadriceps muscles of neurologically intact and spinal cord injured (SCI) human subjects were stimulated with constant current pulses. Up to three, separately adjustable stimulating electrodes over the motor points for vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscles were used to maximize torque generation while minimizing discomfort. The torque generated by stimulation increased as the knee was slowly flexed to about 1 rad (50-60 degrees) and decreased beyond that point (a 'negative slope' on a torque-angle curve). Despite this region of negative slope the force generated by small oscillations remained positively correlated to the angle changes. When the knee was slowly extended again from a flexed position, the torque continued to decline and therefore showed a large degree of 'hysteresis'. Of the three heads studied, only stimulation of RF muscle generally produced this behavior. VL and VM had torques that increased monotonically with knee flexion over the range studied. The torques generated with electrical stimulation of normal subjects represented up to about 30% of maximum voluntary contraction. When subjects generated similar torques voluntarily, the negative slope region and substantial hysteresis were not observed. Thus, SCI subjects may be adversely affected by hysteresis during electrically-induced transitions from sitting to standing and vice versa, while normal subjects are not.