Free radical injury to skeletal muscles of young, adult, and old mice.

We tested the hypotheses that 1) muscles of old mice are more susceptible to injury than muscles of young and adult mice, and 2) secondary or delayed onset injury results from free radical damage. Extensor digitorum longus muscles were injured in situ by lengthening contractions. Injury was assessed by measurement of maximum isometric tetanic force (Po) expressed as a percentage of the control value and by morphological damage. Mice were treated with a free radical scavenger, polyethylene glycol-superoxide dismutase (PEG-SOD). Three days postinjury, the Po of 44% for muscles of nontreated old mice was significantly lower than the Po of 58 and 61% for those of young and adult mice. In each group, the secondary injury at 3 days was alleviated by treatment with PEG-SOD. For treated muscles of young, adult, and old mice, values for Po were 88, 80, and 70%, respectively. We conclude that muscles of old mice are more susceptible to injury than muscles of young or adult mice and that free radicals contribute to the secondary or delayed onset injury.