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-intensity interval training (HIIT) or low-intensity interval training (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 female C57BL6 mice (9 wk old) underwent 4 wk of involuntary interval training with electrical stimulation (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. In addition, 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.NEW & NOTEWORTHY We show that 4 wk of high-intensity interval training (HIIT) or low-intensity interval training (LIIT) via electrical stimulation (ES) improves fatigue resistance in mice via enhanced sarcoplasmic reticulum (SR) Ca²⁺ transients and increased mitochondrial complex protein content. This study demonstrates the mechanisms by which fatigue resistance is improved with chronic electrical stimulation interval training exercise.