High-intensity interval training via electrical stimulation enhances endurance by preserving sarcoplasmic reticulum Ca2+ transients in mouse skeletal muscle.

This study investigated the effects of high/low-intensity interval training (HIIT/LIIT) via electrical stimulation (ES) on muscle performance, specifically focusing on fatigue resistance and cytoplasmic free calcium ([Ca2+]i) dynamics in mouse skeletal muscle. Thirty 9-week-old female C57BL6 mice underwent four weeks of involuntary IT-ES in hindlimb plantar flexor muscles in vivo, with stimulation frequencies set at either 20 Hz (LIIT) or 100 Hz (HIIT). Our results showed that IT-ES significantly enhanced fatigue resistance, particularly with HIIT, evidenced by improved muscle torque output and better preservation of tetanic [Ca2+]i levels during repeated contractions. Additionally, IT-ES led to increases in sarcoplasmic reticulum (SR) Ca2+ handling proteins, such as SR Ca2+ ATPase 1 (SERCA1) and the ryanodine receptor 1 (RyR1), as well as mitochondrial respiratory complex proteins, indicating enhanced metabolic adaptations. The results suggest that HIIT-ES is an effective intervention for improving muscle function, intracellular Ca2+ management, and mitochondrial content, providing a foundation for future consideration in rehabilitation and clinical settings.