Muscle strain injuries: clinical and basic aspects.

Indirect or strain injury to muscle is a common cause of athletic disability. Strain injuries often occur during powerful muscle eccentric contractions. Clinical studies suggest that most injuries cause partial disruption of certain characteristic muscles. Diagnostic imaging studies can demonstrate the location of many injuries. Laboratory studies show that partial and complete injuries exhibit disruption of muscle fibers near the muscle-tendon junction. Healing of partial injuries is characterized by an initial inflammatory response followed by a healing phase marked by fibrosis. Biomechanical studies show that muscle failure occurs at forces much larger than maximal isometric force, and stretch is necessary to create injury. Compared to the passively stretched muscle, muscle activated by nerve contraction and stretched to failure attains a small increase in force at failure, no change in strain to failure, and a large increase in energy absorbed prior to failure. These studies emphasize the ability of muscles to function as energy absorbers in preventing injury to themselves and to bones and joints. Experimental muscle stretching protocols show significant stress relaxation and reduction of stiffness in muscle due to inherent viscoelastic properties of muscle rather than to reflex-mediated effects. These viscoelastic properties may be useful in understanding how muscle injury might be prevented.