Recovery from contraction-induced injury is impaired in weight-bearing muscles of old male mice

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Abstract
With aging, the skeletal muscles of humans sustain decreases of ∼30% in mass and maximum force. Contraction-induced injury may contribute to these declines. When a 225 lengthening contraction protocol (LCP) was administered to small, non-weight-bearing muscles of mice, muscles of young/adult mice recovered completely, whereas those of old mice sustained permanent deficits of 20% in muscle mass and maximum force. Despite these observations, whether a large, frequently recruited, weight-bearing muscle sustains such permanent damage is not known. The hypothesis tested is that after a severe contraction-induced injury, large, weight-bearing muscles of old mice sustain permanent reductions in mass and force. The LCP was administered to plantar flexor muscles of adult and old, male C57BL/6 mice. At 3 days, 1 mo, and 2 mo after the LCP, maximum isometric forces were measured, anesthetized mice were euthanized, and muscles were removed and weighed. Two months after the LCP, the muscles of the adult mice regained control values of mass and force, whereas for muscles of old mice the mass decreased by 24% and the maximum force decreased by 32%. We conclude that a severe contraction-induced injury to large, weight-bearing muscles of old mice causes permanent deficits in mass and force.