Lengthening contractions are not required to induce protection from contraction-induced muscle injury

We tested the hypothesis that lengthening contractions and subsequent muscl e fiber degeneration and/or regeneration are required to induce exercise-as sociated protection from lengthening contraction-induced muscle injury. Ext ensor digitorum longus muscles in anesthetized mice were exposed in situ to repeated lengthening contractions, isometric contractions, or passive stre tches. Three days after lengthening contractions, maximum isometric force p roduction was decreased by 55%, and muscle cross sections contained a signi ficant percentage (18%) of injured fibers. Neither isometric contractions n or passive stretches induced a deficit in maximum isometric force or a sign ificant number of injured fibers at 3 days. Two weeks after an initial bout of lengthening contractions, a second identical bout produced a force defi cit (19%) and a percentage of injured fibers (5%) that was smaller than tho se for the initial bout. Isometric contractions and passive stretches also provided protection from lengthening contraction-induced injury 2 wk later (force deficits = 35 and 36%, percentage of injured fibers = 12 and 10%, re spectively), although the protection was less than that provided by lengthe ning contractions. These data indicate that lengthening contractions and fi ber degeneration and/or regeneration are not required to induce protection from lengthening contraction-induced injury.