Neuromuscular propagation after fatiguing contractions of the paralyzed soleus muscle in humans

We analyzed the M wave and torque after repetitive activation and recovery of the human soleus muscle in individuals with spinal cord injury. Fifteen individuals with complete paralysis had the tibial nerve activated for 330 ms every second with a 20-Hz train. The M wave and torque were analyzed before fatigue, immediately after fatigue, and during recovery. The torque and three M-wave measurements (amplitude, duration, median frequency) changed significantly after fatigue in the chronic group, but the M-wave area was not changed. The M wave was completely recovered after 5 min of rest, even though the torque remained depressed during recovery. The M-wave changes appeared to contribute minimally to the reduced torque in individuals with chronic paralysis. The disassociation in the M-wave-torque relationship during fatigue and recovery suggests, that electrical stimulation under electromyography control is not an ideal method to optimize torque in paralyzed muscle.