Effects of excessive use of remaining muscle fibers in prior polio and LV lesion.

Twenty-three subjects with weakness of musculus tibialis anterior (TA) due to prior poliomyelitis (n = 19) or prior LV lesion (n = 4) were examined regarding the use of remaining TA motor units during walking and the consequences of long-term overuse for the TA muscle fibers. Subjects with an excessive overuse exhibited almost only type 1 muscle fibers and with marked hypertrophy. Subjects who did not use remaining TA fibers because of too-severe paralysis had a normal fiber type differentiation and fiber atrophy. The use of remaining TA motor units was correlated to the type 1 muscle fiber percentage (r = 0.69) and to the type 1 fiber size (r = 0.78). Identical changes were observed in subjects with prior poliomyelitis and in subjects with prior LV lesion and were, thus, not specific for poliomyelitis. The changes in fiber type composition may be explained by a transition of type 2 to type 1 muscle fibers in accordance with previously reported experimental transneuronal stimulation through implanted electrodes in animals.