The effect of cycling in the heat on gastrointestinal-induced damage and neuromuscular fatigue

This study investigated the effect of exercise in the heat on neuromuscular function, gastrointestinal damage, endotoxemia and inflammatory cytokines.

Eight male cyclists completed two 60 min cycling trials in both hot (HOT 34.5 ± 0.1 °C and 53 ± 1% relative humidity) and temperate environments (CON 20.2 ± 0.3 °C and 55 ± 3% relative humidity). The cycling task comprised of alternating 3 min intervals at a moderate-vigorous intensity (50% and 70% of maximum power output; P_max) for 30 min, followed by 30 min at moderate intensity (40–50% P_max). Neuromuscular function was assessed at pre-, post-exercise and 60 min post-exercise. Circulating levels of endotoxins, inflammatory cytokines and markers of gut permeability and damage were also collected at these time points. Heart rate, core temperature, skin temperature, perceived exertion, thermal sensation and comfort were also measured.

Post-exercise voluntary activation of HOT (87.9% [85.2, 90.8]) was statistically lower (mean difference − 2.5% [− 4.5, − 0.5], d = 2.50) than that of CON (90.5% [87.8, 93.2]). The HOT trial resulted in statistically elevated (+ 69%) markers of gastrointestinal damage compared to CON (mean difference 0.424 ng mL⁻¹ [0.163, 0.684, d = − 3.26]), although this was not observed for endotoxin, other inflammatory markers, or gastrointestinal permeability.

This research provides evidence that short-duration cycling in the heat results in sub-optimal neuromuscular activation and increased expression of gastrointestinal damage markers, without a simultaneous elevation in circulating endotoxins or pro-inflammatory cytokines.