A THRESHOLD AND CONTINUUM OF INJURY DURING ACTIVE STRETCH OF RABBIT SKELETAL-MUSCLE

Previous studies of acute muscle injury with active stretch used cycli c stretching or stretching the muscle to complete muscle-tendon dissoc iation. This study tried to determine minimal force required for skele tal muscle injury with one active stretch to establish an injury ''thr eshold.'' Tibialis anterior and extensor digitorum longus rabbit muscl es were actively stretched at 10 cml sec to 60%, 70%, 80%, or 90% of t he force required to passively fail tibialis anterior and extensor dig itorum longus muscles of the control (contralateral) limb. Maximal iso metric contractile force, tensile properties, histology, and electromy ography were measures of injury. Both muscles of the 60% group showed no abnormalities in maximal isometric contractile force, tensile prope rties, histology, or electromyographic activity; 70%, 80%, and 90% gro ups showed diminished maximal isometric contractile force, muscle fibe r disruption, edema, hemorrhage, and decreased electromyographic maxim al voltage amplitude. The 90% group also showed alterations in tensile properties at failure along with connective tissue damage. Injury sit e included fiber disruption both at the distal myotendinous junction a nd muscle belly, with injury noted initially at the distal myotendinou s junction in the 70% group. Electromyographic studies showed maximal isometric contractile force and maximal voltage correlated well as ind ices of damage. This study shows that a threshold and continuum for ac tive stretch-induced injury exist, with muscle fiber disruption occurr ing initially and connective tissue disruption occurring only with lar ger muscle displacements.