The aim of the present study was to examine skeletal muscle injury resulting from acute bouts of oscillatory contractions (strech-shortening contractions) in vivo. The injury is estimated by the changes in the mechanical and histological properties of the skeletal muscle before and after the exposures to oscillatory contractions. The dorsiflexor muscles of male Sprague Dawley rats (N=72) were selected for use in this study. In vivo testing of anesthetized animals was accomplished via a custom designed isokinetic rat dynamometer. The rats were assigned to either an oscillatory injury group or an isometric control group. The oscillatory testing consisted of fifteen sets of ten oscillatory contractions performed at a velocity of 8.72 radians/s from 1.22 radians to 2.09 radians ankle angle for a total of 150 oscillatory contractions. The fifteen sets of oscillatory contractions were administered at one-minute intervals. The isometric control group was exposed to isometric contractions at a neutral ankle position of 1.57 radians. Muscles in the isometric group were activated in the same manner as the oscillatory group to account for the metabolic impact of the oscillatory protocol. The oscillatory and isometric groups were divided into sub groups (N=6) according to the time mechanical and histological properties would be assessed (0, 6, 24, 48, 72, and 240 hours). The results indicate significant reductions in isometric and concentric forces after oscillatory contractions but not after isometric contractions. Muscle fiber damage and inflammatory cell proliferation were present in the tibialis anterior muscle at 24, 48, and 72 hours after exposure to oscillatory contractions, but not in the tibialis anterior muscle exposed to isometric contractions. This is indicative of delayed-onset muscle damage due to acute stretch-shortening muscle contractions. Complete mechanical and histological recovery of the muscle was evidenced at 240 hours in animals exposed to oscillatory contractions.