Protection from contraction-induced injury provided to skeletal muscles of young and old mice by passive stretch is not due to a decrease in initial mechanical damage.

Contraction-induced injury occurs when muscles are stretched while activated (lengthening contractions). The injury is initiated by mechanical damage followed by an inflammatory response. Old animals are particularly susceptible to contraction-induced injury, yet exposure to stretches without activation (passive stretches) before lengthening contractions lessens the injury. We hypothesized that, for muscles of both young and old mice, prior exposure to passive stretches reduces the initial mechanical damage induced by lengthening contractions. Compared with unconditioned muscles in both age groups, administration of passive stretches 1 hour before lengthening contractions decreased the force deficit at 3 days by one half, but did not affect the force deficit at 10 minutes. Force deficits immediately after two lengthening contractions were also not different for passive stretch-conditioned and unconditioned muscles. The similarity in force deficits immediately following lengthening contractions for conditioned and unconditioned muscle indicates that passive stretch conditioning does not decrease initial mechanical damage in young or old mice.