Differential fatigue of paralyzed thenar muscles by stimuli of different intensities.

Muscles paralyzed by injury or disease fatigue excessively when stimulated. This study examined whether the first few paralyzed thenar motor units recruited by electrical stimulation of the median nerve were more fatigue resistant than the total thenar motor unit population. The paralyzed thenar muscles of four subjects with chronic cervical spinal cord injury were fatigued by a 2-min intermittent 40-HZ protocol on 2 days. One experiment involved submaximal stimulation, the other supramaximal stimulation. These stimuli resulted in activation of part and all of the thenar muscles, respectively. Relative force loss, force-time integral decline, and slowing of half-relaxation time were always significantly less when only part rather than all of the muscles was fatigued. The part of the paralyzed muscles that was activated was also relatively fatigue resistant compared with control single thenar motor units. Thus, a reversal of recruitment order from fatigable to fatigue-resistant units cannot explain the extreme fatigability of paralyzed muscles. Use of submaximal stimulation during functional electrical stimulation may therefore help to reduce muscle fatigue because it recruits the more fatigue-resistant units.