Effects of sustained stimulation on the excitability of motoneurons innervating paralyzed and control muscles

The excitability of thenar motoneurons (reflected by F-wave persistence and amplitude) and thenar muscle force were measured during a stimulation protocol (90 s of 18-Hz supramaximal electrical stimulation of the median nerve) designed to induce muscle fatigue (force decline). Data from muscles (n = 15) paralyzed by chronic cervical spinal cord injury were compared with those obtained from control muscles (n = 6). The persistence of F waves in both paralyzed and control muscles increased from similar to60 to similar to76% during the first 10 s of the fatigue protocol. Persistence then declined progressively to similar to33% at 90 s. These changes in F-wave persistence suggest that similar reductions occur in the excitability of the motoneurons to paralyzed and control motor units after sustained antidromic activation. Despite this, significantly larger force declines occurred in the paralyzed muscles of spinal cord-injured subjects (similar to60%) than in the muscles of control subjects (similar to15%). These data suggest that the decreases in motoneuron excitability for both the spinal cord-injured and control subjects are a result of activity-dependent changes in motoneuron properties that are independent of fatigue-related processes in the muscles.