Fatigue in human thenar muscles paralysed by spinal cord injury

Muscle fatigue (force loss) induced by constant frequency stimulation (36 Hz) and variable rate stimulation (36 Hz to 18 Hz over 60 s) were compared in six individuals with thenar muscles which were paralysed by chronic cervical spinal cord injury (SCI), and in six volunteers with no known neurological disorder. The variable stimulation rate pattern represented the general decline in thenar motor unit firing rates recorded during 60 s maximum voluntary contractions performed by the able-bodied (AB) subjects. Constant and variable rate stimulation produced similar resultant force declines, as measured from abduction and flexion force components. However, significant force loss always occurred earlier and was of greater magnitude in SCI subjects, irrespective of the stimulation pattern (all, P < 0.01). Because more force was generally lost in one force component versus the other, the direction of the resultant force could also change with fatigue. The recordings from SCI participants were also contaminated by spontaneous motor unit activity, spasms and F responses. The stimulation frequency needed to produce half-maximum tetanic force increased for SCI subjects after fatigue, so higher, not lower stimulation frequencies were needed to produce any given submaximal force. Therefore, to match stimulation rate to changes in muscle contractile properties, these parameters have to be monitored and controlled on-line. The fatigue during each stimulation protocol, and for each subject population, was attributed primarily to contractile failure because any decrements in M-wave amplitude or area recovered completely within the first minute whereas twitch and tetanic forces remained somewhat depressed.