Pliometric contraction-induced injury of mouse skeletal muscle: effect of initial length.

For single pliometric (lengthening) contractions initiated from optimal fiber length (Lf), the most important factor determining the subsequent force deficit is the work input during the stretch. We tested the hypothesis that regardless of the initial length, the force deficit is primarily a function of the work input. Extensor digitorum longus muscles of mice were maximally activated in situ and lengthened at 2 Lf/s from one of three initial fiber lengths (90, 100, or 120% of Lf) to one of three final fiber lengths (150, 160, or 170% of Lf). Maximal isometric force production was assessed before and after the pliometric contraction. No single mechanical factor, including the work input (r2 = 0.34), was sufficient to explain the differences in force deficits observed among groups. Therefore, the force deficit appears to 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 (r2 = 0.97-0.99). Consequently, differences in force deficits among groups were best explained on the basis of the initial fiber length and the work input during the stretch.