COMPOUND MOTOR ACTION-POTENTIALS AND MECHANICAL FAILURE DURING SUSTAINED CONTRACTIONS BY ELECTRICAL-STIMULATION IN PARAPLEGIC PATIENTS

In nine paraplegic patients we recorded the torque output and compound motor action potentials (CMAPs) produced by the quadriceps muscle dur ing an isometric contraction elicited by electrical stimulation. The t orque, the peak to peak amplitude, the latency, the peak to peak durat ion and the total surface of the rectified CMAPs were computed over a period of 126 s. After a brief increase the mechanical output rapidly decreased and reached a stable minimum level by the end of 126 s. The final torque output values ranged from 7.1 to 54% of initial values. T his torque decrease was related neither to length of time between inju ry and testing, nor to the thoracic level of the spinal cord injury. T he peak to peak amplitude of the CMAPs changed over the course of stim ulation. It was noted to increase over a period of time after which it decreased to a minimum level. The latency from the onset of stimulati on to the onset of the CMAP varied to a relatively small extent compar ed to the peak to peak duration. Therefore, the conduction velocity al ong the muscle fibres appeared to be more affected by the test than by the conduction velocity along the nerve fibres and the transmission a cross the neuromuscular junction. The mechanisms involved in the chang es in CMAPs and the change in torque output over the time course of st imulation are discussed.