Combined effects of mechanical overload and high-intensity interval training on skeletal muscle hypertrophy in male mice.

Athletes often perform concurrent training that combines different exercise modalities. It is believed that muscle hypertrophic adaptation is inhibited by endurance exercise; however, the molecular mechanisms underlying the effects of high-intensity, short-duration exercise on muscle hypertrophic responses remain unclear. In this study, we determined the effects of high-intensity interval training (HIIT) on mechanical overload-induced muscle hypertrophy in mice. Eight-week-old male mice were divided into the following three groups (n = 6-8): Sham surgery (Sham), myotenectomy-induced mechanical overload (OL), and OL with HIIT by forced swimming (OL + HIIT). After 4 weeks of intervention, the OL + HIIT group exhibited an increase in plantaris muscle weight and muscle fiber cross-sectional area as well as the OL group. The OL + HIIT group showed a similar increase in mTOR downstream proteins rpS6, S6K1, and 4E-BP1 phosphorylation compared with the OL group. AMPK was activated by HIIE and may play an inhibitory role by attenuating mTOR. To clarify its involvement, acute phase experiments were conducted to evaluate mTOR signaling immediately after a single round of HIIE. The increase of rpS6 and S6K1 phosphorylation was unchanged after a single HIIE exposure, despite AMPK upregulation. Our results suggest that interference effects induced by HIIE may not occur in mechanically overloaded mouse skeletal muscle.