Repeated bouts of fast velocity eccentric contractions induce atrophy of gastrocnemius muscle in rats.

One bout of exercise consisting of fast velocity eccentric contractions has been shown to increase muscle protein degradation in rats. The present study tested the hypothesis that muscle atrophy would be induced after four bouts of fast velocity eccentric contractions, but not after four bouts of slow velocity eccentric contractions. Male Wistar rats were randomly placed into 3 groups; fast (180°/s) velocity (180EC, n = 7), slow (30°/s) velocity eccentric exercise (30EC, n = 7), or sham-treatment group (control, n = 7). The 180EC and 30EC groups received 4 sessions of 4 sets of 5 eccentric contractions of triceps surae muscles by extending the ankle joint during evoked electrical stimulation of the muscles, and the control group had torque measures, every 2 days, and all rats were sacrificed 1 day after the fourth session. Medial and lateral gastrocnemius wet mass were 4-6 % smaller, cross-sectional area of medial gastrocnemius was 6-7% smaller, and isometric tetanic torque of triceps surae muscles was 36 % smaller (p < 0.05) for 180EC than control at 1 day after the fourth session, but no such differences were evident between 30EC and control. The expressions of atrophy-related molecules such as FoxO1, FoxO3 and myostatin were upregulated (78-229 %) only for 180EC, but an increase in phosphorylated p70s6k (227%) was found only for 30EC at 1 day after the fourth session (p < 0.05). The level of Bax, a pro-apoptotic protein, was greater (p < 0.05) for 180EC than control. These results support the hypothesis that muscles are atrophied by repeated bouts of fast but not slow velocity eccentric contractions.