During high-intensity interval training (HIIT), why does muscle fatigue eventually force a runner to reduce their pace, even when mentally willing to continue?

💡 Explanation

Muscle fatigue during HIIT is caused by a reduction in calcium release from the sarcoplasmic reticulum. Because the reduced Ca2+ impairs cross-bridge formation and force generation, the runner slows down. Therefore, pacing adjusts to match physiological capacity, rather than sustained intensity due to other energy sources or enhanced stimulation.

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