Hot conditions improve power output during repeated cycling sprints without modifying neuromuscular fatigue characteristics

Abstract

This study investigated the effect of hot conditions on repeated sprint cycling performance and post-exercise alterations in isometric knee extension function. Twelve physically active participants performed 10×6-s "all-out" sprints on a cycle ergometer (recovery=30s), followed 6min later by 5×6-s sprints (recovery=30s) in either a neutral (24°C/30%rH) or a hot (35°C/40%rH) environment. Neuromuscular tests including voluntary and electrically evoked isometric contractions of the knee extensors were performed before and after exercise. Average core temperature during exercise was higher (38.0±0.1 vs. 37.7±0.1°C, respectively; P<0.05) in hot versus neutral environments. Peak power output decreased (-17.9% from sprint 1 to sprint 10 and -17.0% from sprint 11 to sprint 15; P<0.001) across repetitions. Average peak power output during the first ten sprints was higher (+3.1%; P<0.01) in the hot ambient temperature condition. Maximal strength (-12%) and rate of force development (-15 to -26%, 30-200ms from the onset of contraction) decreased (P<0.001) during brief contractions after exercise, irrespectively of the ambient temperature. During brief maximal contractions, changes in voluntary activation (~80%) were not affected by exercise or temperature. Voluntary activation declined (P<0.01) during the sustained contraction, with these reductions being more pronounced (P<0.05) after exercise but not affected by the ambient temperature. Resting twitch amplitude declined (P<0.001) by ~42%, independently of the ambient temperature. In conclusion, heat exposure has no effect on the pattern and the extent of isometric knee extensor fatigue following repeated cycling sprints in the absence of hyperthermia.