Contractile properties of human thenar muscles paralyzed by spinal cord injury.

The electrical and mechanical properties of paralyzed human thenar muscles were measured in response to supramaximal stimulation of the median nerve in individuals with chronic cervical spinal cord injury. These data were compared to those recorded from control muscles. Spontaneous motor unit activity was common in paralyzed muscles. There was significantly more variance in the twitch and tetanic forces, twitch/tetanus force ratios, twitch and tetanic half-relaxation times, and the stimulus frequencies which generated half-maximal force in paralyzed versus control muscles. Approximately half the paralyzed thenar muscles were significantly weaker than control muscles and their compound action potential amplitudes were reduced significantly. Paralyzed muscles had significantly higher twitch/tetanus force ratios. The mean stimulus frequency which generated half-maximal force was also reduced significantly. Thus for rehabilitation purposes, lower stimulation rates are required to elicit any given submaximal force from chronically paralyzed thenar muscles.