Identification of a Threshold for Skeletal Muscle Injury

This study was designed to detect the first evidence of injury to muscle induced by passive stretching. Rabbit extensor digitorum longus and tibialis anterior skeletal muscles were subjected to passive stretching 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 compared with their con tralateral control specimens. Maximum contractile force was decreased after stretching. Tibialis anterior and ex tensor digitorum muscles that were stretched to 20% of control failure force showed no decrement in the tensile parameters or maximum contractile force. Histology of the extensor digitorum longus muscles stretched to 30% of failure force showed small focal areas of muscle fiber rupture and hemorrhage near the distal myoten dinous 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 dec rement in contractile ability; therefore, a threshold for passive muscle stretch injury has been found. Further more, changes in contractile properties and histologic assessment appear to be more sensitive predictors of injury than measurement of structural failure properties

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 in jury "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 lon gus 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 con tractile force, muscle fiber disruption, edema, hemor rhage, and decreased electromyographic maximal volt age amplitude. The 90% group also showed alterations in tensile properties at failure along with connective tis sue 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 volt age correlated well as indices of damage. This study shows that a threshold and continuum for active stretch- induced injury exist, with muscle fiber disruption occur ring initially and connective tissue disruption occurring only with larger muscle displacements

Thermal effects on skeletal muscle tensile behavior

The effect of temperature on the mechanical failure properties of rabbit skeletal muscle (tibialis anterior and extensor digitorum longus) was examined. For all tests, one leg was maintained at 25°C and the contralateral leg at 40°C. Muscles were pulled to failure according to assignment into one of three groups: 1) passive failure at 10 cm/sec, 2) passive failure at 1 cm/sec, or 3) active (muscle is stimulated to contract as it is pulled) failure at 10 cm/sec. Load to failure was higher in the cold muscle for all groups tested. Total deformation was the same except in Group 1, when the warm muscle had a greater deformation. Energy absorbed before failure was greater in the cold muscle in Groups 2 and 3. Stiffness was higher in cold muscles for all muscles except the extensor digitorum longus in Group 1. In this study, temperature had a significant effect on the tensile properties; these thermal effects were de pendent on both loading rate and contractile state. Comparing loading rates, warm muscle tested at 10 cm/sec had higher failure loads than that tested at 1 cm/sec. Comparing stimulated versus unstimulated muscle (Group 1 versus Group 3), the stimulated tibialis anterior muscle absorbed more energy than unstimu lated ones. Stimulated extensor digitorum longus mus cles had higher failure loads, absorbed more energy, and were stiffer than nonstimulated muscles. This study offers experimental data to support the theory that warming muscles can aid in injury prevention and im provement in athletic performance

The role of warm-up in muscular injury prevention

This study is an attempt to provide biomechanical sup-port for the athletic practice of warming up prior to an exercise task to reduce the incidence of injury. Tears in isometrically preconditioned (stimulated before stretching) muscle were compared to tears in control (nonstimulated) muscle by examining four parameters: 1) force and 2) change of length required to tear the muscle, 3) site of failure, and 4) length-tension defor-mation. The tibialis anterior (TA), the extensor digitorum longus (EDL), and flexor digitorum longus (EDL) mus-cles from both hindlimbs of rabbits comprised our ex-perimental model. Isometrically preconditioned TA (P < 0.001), EDL (P < 0.005), and FDL (P < 0.01) muscles required mor