EFFECTS OF ELECTRICALLY-INDUCED FATIGUE ON THE TWITCH AND TETANUS OF PARALYZED SOLEUS MUSCLE IN HUMANS

We analyzed the twitch and summated torque (tetanus) during repetitive activation and recovery of the human soleus muscle in individuals wit h spinal cord injury. Thirteen individuals with complete paralysis (9 chronic, 4 acute) had the tibial nerve activated every 1,500 ms with a 20-Hz train (7 stimuli) for 300 ms and a single pulse at 1,100 ms The stimulation protocol lasted 3 min and included 120 twitches and 120 t etani. Minimal changes were found for the acute group. The chronic gro up showed a significant reduction in the torque and a significant slow ing of the contractile speeds of both the twitch and tetanus The decre ase in the peak twitch torque was significantly greater than the decre ase in the peak tetanus torque early during the fatigue protocol for t he chronic group. The twitch time to peak and half relaxation time wer e prolonged during fatigue, which was associated with improved fusion of the tetanus torque At the end of the fatigue protocol, the decrease in the peak twitch torque was not significantly different from the de crease in the peak tetanus torque. After 5 min of rest, the contractil e speeds recovered causing the tetanus to become unfused, but the teta nus torque became less depressed than the twitch torque. for the twitc h and the tetanus The differential responses suggest an interplay betw een optimal fusion created from contractile speed slowing and excitati on contraction coupling compromise These issues make the optimal desig n of functional electrical stimulation systems a formidable task.