Adjustments in the force-frequency relationship during passive and exercise-induced hyperthermia

Abstract

Introduction: We examined the extent to which fatiguing cycling exercise in the heat influences contractile function in modulating the force-frequency relationship. Methods: Before ({reversed tilde}37.0°C) and after ({reversed tilde}38.5°C) exercise (ExH) and passive (PaH) hyperthermia, an 8-s train of stimulation at 10, 20, 50, and 100 Hz (2 s per frequency) and a potentiated twitch were evoked on the relaxed knee extensors using percutaneous stimulation. Results: ExH and PaH produced a decrease in the 20:50 Hz force ratio, indicative of low-frequency fatigue (P<0.01). This adjustment was more pronounced after ExH than PaH (P<0.01). A rightward displacement in the force-frequency relationship occurred after ExH and PaH (P<0.05) and was exacerbated by ExH (P<0.05). Peak twitch force also decreased after ExH (P<0.05). Conclusions: ExH reduces force summation due to development of skeletal muscle fatigue, exacerbating the shift in force-frequency to the right relative to PaH.