Exercise‐induced hyperthermia exacerbates the rightward displacement in the force‐frequency relationship compared with passive hyperthermia (1104.27)

Abstract

This study examined the extent to which exercise‐induced hyperthermia (ExH) influences contractile function and modulates the force‐frequency relationship, compared with passive hyperthermia (PaH). ExH was induced in ten males by cycling at 60% VO2max in 38°C conditions and PaH by sitting in a 48°C chamber. Prior to and following ExH and PaH, an 8 s train of stimulation (10, 20, 50 and 100 Hz; 2 s per frequency) and a potentiated twitch were evoked on the relaxed knee extensors via percutaneous stimulation. Core (~37.0 to ~38.5ºC) and muscle (~34.7 to ~38.5ºC) temperature were increased similarly during both trials (P<0.01). The development of ExH and PaH resulted in a decrease in the 20:50 Hz force ratio, indicative of low‐frequency‐fatigue (P<0.01). This alteration 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), but was exacerbated by ExH (P<0.05). Peak twitch force decreased following ExH (P<0.05) and was maintained after PaH. The exacerbated rightward shift in the force‐frequency relationship during ExH appears to stem from adjustments influencing force summation at low frequencies of stimulation. This is likely due to the development of peripheral fatigue, evidenced by a reduction in peak twitch force production.