PLIOMETRIC CONTRACTION-INDUCED INJURY OF MOUSE SKELETAL-MUSCLE - EFFECT OF INITIAL LENGTH

For single pliometric (lengthening) contractions initiated from optima l fiber length (L(f)), the most important factor determining the subse quent force deficit is the work input during the stretch. We tested th e hypothesis that regardless of the initial length, the force deficit is primarily a function of the work input. Extensor digitorum longus m uscles of mice were maximally activated in situ and lengthened at 2 L( f)/s from one of three initial fiber lengths (90, 100, or 120% of L(f) ) to one of three final fiber lengths (150, 160, or 170% of L(f)). Max imal isometric force production was assessed before and after the plio metric contraction. No single mechanical factor, including the work in put (r(2) = 0.34), was sufficient to explain the differences in force deficits observed among groups. Therefore, the force deficit appears t o arise from a complex interaction of mechanical events. With the data grouped by initial fiber length, the correlation between the average work and the average force deficit was high (r(2) = 0.97-0.99). Conseq uently, differences in force deficits among groups were best explained on the basis of the initial fiber length and the work input during th e stretch.