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 during an isometric contraction elicited by electrical stimulation. The torque, the peak to peak amplitude, the latency, the peak to peak duration 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. This torque decrease was related neither to length of time between injury and testing, nor to the thoracic level of the spinal cord injury. The peak to peak amplitude of the CMAPs changed over the course of stimulation. It was noted to increase over a period of time after which it decreased to a minimum level. The latency from the onset of stimulation to the onset of the CMAP varied to a relatively small extent compared to the peak to peak duration. Therefore, the conduction velocity along the muscle fibres appeared to be more affected by the test than by the conduction velocity along the nerve fibres and the transmission across the neuromuscular junction. The mechanisms involved in the changes in CMAPs and the change in torque output over the time course of stimulation are discussed.