IDENTIFICATION OF A THRESHOLD FOR SKELETAL-MUSCLE INJURY

This study was designed to detect the first evidence of injury to musc le induced by passive stretching. Rabbit extensor digitorum longus and tibialis anterior skeletal muscles were subjected to passive stretchi ng at set force levels of 20% or 30% of load to failure. Both tibialis anterior and extensor digitorum longus muscles that were stretched to 30% exhibited no difference in the three tensile parameters when comp ared with their contralateral control specimens. Maximum contractile f orce was decreased after stretching. Tibialis anterior and extensor di gitorum muscles that were stretched to 20% of control failure force sh owed no decrement in the tensile parameters or maximum contractile for ce. Histology of the extensor digitorum longus muscles stretched to 30 % of failure force showed small focal areas of muscle fiber rupture an d hemorrhage near the distal myotendinous junction. Tibialis anterior and extensor digitorum longus muscle-tendon units stretched to 30% of failure force suffered functional injury as their contractile ability decreased after stretching. In contrast, muscle-tendon units stretched to 20% of failure force suffered no decrement in contractile ability; therefore, a threshold for passive muscle stretch injury has been fou nd. Furthermore, changes in contractile properties and histologic asse ssment appear to be more sensitive predictors of injury than measureme nt of structural failure properties.