INJURY TO MUSCLE-FIBERS AFTER SINGLE STRETCHES OF PASSIVE AND MAXIMALLY STIMULATED MUSCLES IN MICE

1. Our purpose was to investigate the initial mechanisms responsible f or contraction-induced injury. Most studies of mechanisms of contracti on-induced injury have been based on observations made either shortly after many repeated contractions at the peak of fatigue, or days after , at the peak of delayed onset injury. As a result, conclusions based on these studies are complicated by interactions of mechanical and bio chemical events, as well as the passage of time. We studied the initia l mechanical events associated with contraction-induced injury immedia tely following single stretches of whole skeletal muscles of mice in s itu. 2. We tested the hypothesis that immediately following a single s tretch, the severity of contraction-induced injury is a function of bo th strain and average force. Consequently, the work done to stretch th e muscle would be the best predictor of the magnitude of injury. Exten sor digitorum longus muscles were adjusted to optimum length for force (L(o)). Passive (not stimulated) and maximally activated muscles were exposed to single stretches of 10, 20, 30, 50 or 60% strain, relative to muscle fibre length (L(f)), at a rate of 2 L(f) s(-1). 3. The magn itude of injury was represented by the force deficit 1 min after the s tretch expressed as a percentage of the maximum force prior to the str etch. The occurrence of injury was confirmed directly by electron micr oscopic analysis of the ultrastructure of muscle fibres that were fixe d immediately following single stretches. 4. For active muscles, a sin gle stretch of only 30% strain produced a significant force deficit, w hereas for passive muscles, a larger strain was required. Stretches of greater than 50% strain resulted in greater force deficits for passiv e than for maximally activated muscles. For either condition, the work done to stretch the muscle was the best predictor of the magnitude of injury, accounting for 76% of the variability in the force deficit fo r maximally activated muscles, and 85% for passive muscles.