Site and mechanical conditions for failure of skeletal muscle in experimental strain injuries

Failure in muscle strain injuries has been reported to occur within the muscle belly, at the myotendinous junction, or within muscle near the myotendinous junction. The goal of this investigation was to determine by electron-microscopic examination the site of lesion in whole muscle strained to failure. In addition, site and conditions for failure of stimulated and unstimulated muscle were compared. Frog semitendinosus myotendinous units with intact tendon-bone junctions were strained at physiological strain rates to failure. All failures occurred at or near the proximal myotendinous junction in both stimulated and unstimulated muscle. Stimulated muscle required approximately 30% more force and approximately 110% more energy to reach failure. Electron-microscopic examination of longitudinal sections of small bundles of fibers showed that unstimulated muscle failed within the muscle near the myotendinous junction. Failure occurred in a single transverse plane of each cell within Z disks. Other Z disks near the failure site displayed strains of several hundred percent. Stimulated muscle failed within the lamina lucida at the myotendinous junction in most fibers. No Z-disk strain was observed in those fibers. We conclude that the site of failure in muscle strain injuries varies with the state of activation of the cell at the time of injury. Furthermore, the data show that the breaking strength of the Z disk varied with muscle stimulation and indicate the existence of two load-bearing systems in parallel within Z disks.