Indirect skeletal muscle injuries have been found to occur exclusively near the myotendinous junction (MTJ). Although muscle cells are syncytial and extend far into the muscle belly, the response to this injury has been shown to be limited to a focal area near the MTJ. This study examined single muscle fibers near their site of rupture in order to document structural changes that may help to explain this limited injury response. A partial, nondisruptive strain injury was created in a New Zealand white rabbit extensor digitorum longus muscle. The muscles were left in vivo for 60 min or 6 h before harvesting. The specimens were divided into four groups of single fibers: ruptured fibers (60 min) attached to muscle belly (group I); ruptured fibers (60 min) attached to tendon (group II); ruptured fibers (6 h) attached to muscle belly (group III); and normal, unstretched fibers (60 min) at the MTJ (group IV) (N = 10 for each group). Sarcomeres closest to the site of fiber rupture in the three injured groups (I, II, and III) were hypercontracted, with lengths well below the physiologic range (I:0.86 ± 0.08 µm, II: 0.96 ± 0.09/µm, III: 0.96 ± 0.21 µm). There was a progressive increase in sarcomere length, which became normal by 300-500 µm away from the site of rupture (I: 2.17 ± 0.48 µm, II: 2.69 ± 0.42 µm, III: 2.08 ± 0.48 µm). The 6 h fibers in group III showed evidence of necrosis before the transition to normal sarcomere spacing occurred. The presence of severe sarcomere hypercontraction in the 60 min muscle fibers and eventual necrosis in the 6-h fibers suggests a progressive response to cell rupture, consistent with well-known effects of calcium entry. However, in all three injured groups, sarcomere length abruptly normalized within 300-500 µm away from the site of rupture. This suggests that following an acute muscle strain an intracellular barrier effectively restricts the injury response to less than 500 µm away from the initial site of rupture.
- Injury response
- Muscle fiber
- Muscle injury
ASJC Scopus subject areas
- Orthopedics and Sports Medicine
- Physical Therapy, Sports Therapy and Rehabilitation