Response of normal and dystrophic muscles to increased functional demand.

Immature fast muscles appear to be unfavorably influenced by excessive activity and there is evidence suggesting that the maturation of muscles from animal models of muscular dystrophy and patients suffering from Duchenne dystrophy is impaired. We therefore examined the effects of chronic functional overload applied at different stages of postnatal life on a fast muscle in normal and dystrophic mice (C57B1/6J dy2j/dy2j). "Overload" of tibialis anterior muscle was produced by removal of its synergist extensor digitorum longus in one hind limb. Our results suggest that increased functional demand can be damaging to immature muscles and that in dystrophic animals, the inability to adjust to overload persists into adult life.