Conditioning of skeletal muscles in adult and old mice for protection from contraction-induced injury.

The purpose of this study was to design a conditioning program that protected muscles in both adult and old mice from a protocol of contractions that previously caused a significant number of damaged fibers and a deficit in force. Hind-limb dorsiflexor muscles of adult (7 months) and old (22 months) female B6D2F1 mice were exposed once a week to a protocol of repeated forced stretches while maximally activated in vivo. By week 4, muscles of adult, but not old, mice showed no force deficit. Conditioning was continued for 6 weeks, when both age groups showed no force deficit for two consecutive weeks. Three days after the sixth contraction protocol, when morphological damage and force deficits are most severe, the numbers of damaged fibers in muscles of adult and old mice were not different from those in uninjured control muscles and the force deficits were reduced dramatically compared with unconditioned muscles. We conclude that muscles of both adult and old mice conditioned successfully, but muscles of old mice conditioned more slowly than those of adult mice.