Effects of repetition duration on skeletal muscle hypertrophy in a rat model of resistance exercise

This study aimed to elucidate the effects of repetition duration (contraction duration for each repetition) of resistance exercise on muscle hypertrophy and its underlying mechanisms using a rat exercise model. Male Sprague-Dawley rats were randomly assigned to three groups trained with short (S), medium (M), and long (L) repetition durations. During resistance exercise, the right gastrocnemius muscles were electrically stimulated to induce maximal tetanic contractions, each lasting for 1, 3, and 9 s in S, M, and L groups, respectively. The number of contractions in each set and the interset rest duration were the same across groups, whereas the number of sets was adjusted to match the total torque-time integral of plantar flexion. The left (untrained) and right (trained) gastrocnemius muscles were sampled 48 h after 12 exercise bouts. The average and peak torques during each exercise were consistently highest in S and lowest in L. The muscle mass and fiber cross-sectional area significantly increased in S and M but not in L. The concentrations of total RNA and 18S + 28S rRNA increased only in S and were correlated with muscle mass when the three groups were combined. We also sampled muscle tissues 6 h after a single exercise bout and found no significant difference in muscle protein synthesis, mTOR signaling activity, ribosome biogenesis, or protein degradation between the groups. These results suggest that a long repetition duration does not promote but rather diminishes the hypertrophic effects of resistance exercise, and that acute molecular responses to resistance exercise cannot predict such effects.<b>NEW & NOTEWORTHY</b> Using a rat model, we investigated the hypertrophic effects of resistance exercise with different repetition durations under conditions where the torque-time integrals were matched. The results suggest that exercise with short but not long repetition duration effectively induces muscle hypertrophy. The lack of hypertrophic response or ribosome biogenesis with long repetition duration may not be predicted by acute molecular responses to exercise but may be explained in part by attenuated torque production during exercise.