Injury to Skeletal Muscle as a Result of Lengthening Contractions (Eccentric, Concentric, Damage, Mice).

Abstract

Activity-induced injury to skeletal muscle was studied with an in situ muscle preparation. Injury was assessed with changes in histological appearance and maximum isometric force (P(,o)). Three hypotheses were tested: (1) greater injury to muscle fibers results from lengthening than from isometric or shortening contractions; (2) injury increases with stimulation duration; and (3) injury increases with peak force of the contractions. Mice were anesthetized and the distal tendon of the extensor digitorum longus muscle was attached to a servomotor. The experimental protocol consisted of isometric, shortening, or lengthening contractions. Muscles were stimulated at 150 Hz for 300 ms every 2 s for 15 min. Length change was 20% of fiber length (L(,f)) at a velocity 1.0 L(,f)/s. Mice were allowed to recover and injury was assessed 1-30 days later. Histological appearance 3 days after isometric and shortening contractions and sham operations was normal and P(,o) was 80% of control value. Three days after lengthening contractions, 37 (+OR-) 4% of muscle fibers had degenerated and P(,o) was 22 (+OR-) 3% of control value. Muscle fiber regeneration occurred and after 30 days P(,o) was 84 (+OR-) 3% of control value. Effect of stimulation duration on the extent of injury was tested with lengthening contractions at 0.2, 0.5, or 1.0 L(,f)/s. Lengthening at 1.0 L(,f)/s resulted in injury with 0.5 min of stimulation. Injury increased with duration to 5 min. No further injury occurred between 5 and 15 min. Muscle fatigue appeared to prevent further injury after 5 min. Results for 0.2 and 0.5 L(,f)/s were similar but with a slower onset and less injury. A good correlation existed between histological appearance and P(,o) but decreases in P(,o) were greater than that predicted by the number of degenerating fibers. Extent of injury was compared to peak force of the initial contraction. Peak forces during lengthening at 0.2, 0.5, and 1.0 L(,f)/s were 115 (+OR-) 5%, 130 (+OR-) 5%, and 150 (+OR-) 12% of P(,o), respectively. To separate the effects of velocity and peak force, peak force during lengthening at 1.0 L(,f)/s was lowered by: (1) inducing fatigue with isometric contractions; (2) stimulating at 70-100 Hz; or (3) lengthening 10% of L(,f). Resulting peak forces were 88 (+OR-) 3%, 100 (+OR-) 3%, and 139 (+OR-) 3% of P(,o), respectively. Only lengthening contractions caused injury. Extent of injury increased with stimulation duration and was related to peak force during lengthening.