Separate and combined effects of exposure to heat stress and mental fatigue on endurance exercise capacity in the heat

This study investigated the effects of exposure to pre-exercise heat stress and mental fatigue on endurance exercise capacity in a hot environment. Eight volunteers completed four cycle exercise trials at 80% maximum oxygen uptake until exhaustion in an environmental chamber maintained at 30 A degrees C and 50% relative humidity. The four trials required them to complete a 90 min pre-exercise routine of either a seated rest (CON), a prolonged demanding cognitive task to induce mental fatigue (MF), warm water immersion at 40 A degrees C during the last 30 min to induce increasing core temperature (WI), or a prolonged demanding cognitive task and warm water immersion at 40 A degrees C during the last 30 min (MF + WI). Core temperature when starting exercise was higher following warm water immersion (similar to 38 A degrees C; WI and MF + WI) than with no water immersion (similar to 36.8 A degrees C; CON and MF, P < 0.001). Self-reported mental fatigue when commencing exercise was higher following cognitive task (MF and MF + WI) than with no cognitive task (CON and WI; P < 0.05). Exercise time to exhaustion was reduced by warm water immersion (P < 0.001) and cognitive task (P < 0.05). Compared with CON (18 +/- 7 min), exercise duration reduced 0.8, 26.6 and 46.3% in MF (17 +/- 7 min), WI (12 +/- 5 min) and MF + WI (9 +/- 3 min), respectively. This study demonstrates that endurance exercise capacity in a hot environment is impaired by either exposure to pre-exercise heat stress or mental fatigue, and this response is synergistically increased during combined exposure to them.