A threshold and continuum of injury during active stretch of rabbit skeletal muscle.

Previous studies of acute muscle injury with active stretch used cyclic stretching or stretching the muscle to complete muscle-tendon dissociation. This study tried to determine minimal force required for skeletal muscle injury with one active stretch to establish an injury "threshold." Tibialis anterior and extensor digitorum longus rabbit muscles were actively stretched at 10 cm/sec to 60%, 70%, 80%, or 90% of the force required to passively fail tibialis anterior and extensor digitorum longus muscles of the control (contralateral) limb. Maximal isometric contractile force, tensile properties, histology, and electromyography were measures of injury. Both muscles of the 60% group showed no abnormalities in maximal isometric contractile force, tensile properties, histology, or electromyographic activity; 70%, 80%, and 90% groups showed diminished maximal isometric contractile force, muscle fiber disruption, edema, hemorrhage, and decreased electromyographic maximal voltage amplitude. The 90% group also showed alterations in tensile properties at failure along with connective tissue damage. Injury site included fiber disruption both at the distal myotendinous junction and muscle belly, with injury noted initially at the distal myotendinous junction in the 70% group. Electromyographic studies showed maximal isometric contractile force and maximal voltage correlated well as indices of damage. This study shows that a threshold and continuum for active stretch-induced injury exist, with muscle fiber disruption occurring initially and connective tissue disruption occurring only with larger muscle displacements.