Failure of neuromuscular propagation during human maximal voluntary contraction

The mechanism for fatigue of the adductor pollicis was studied in normal subjects during maximal voluntary contractions (MVC) sustained for 90-100 s, by comparing the force and electrical response of this muscle to voluntary motor drive with that obtainable with artificial stimulation of the ulnar nerve. The adequacy of nerve stimulation was checked by recording simultaneously the electrical response of a nonfatiguing muscle, the abductor of the small finger. The decrease in force and in the natural electrical activity with fatigue was accompanied by a parallel decrease in the amplitude of synchronous muscle action potentials (M waves) evoked by artificial stimulation of the ulnar nerve at different frequencies. The decline in M-wave amplitude in the adductor pollicis was not due to a submaximal nerve stimulation, since the amplitudes recorded simultaneously from the nonfatiguing abductor digiti minimi remained unchanged. The force and the electrical responses from the adductor pollicis recovered in parallel with a half time of approximately 1 min. These results suggest that the loss of force of the adductor pollicis with fatigue and its subsequent recovery are largely determined by the extent of neuromuscular propagation failure. The slow recovery of the M-wave amplitude during repetitive stimulation suggests that it may be related to some aspect of muscle metabolism.