Aerobically trained individuals have greater increases in rectal temperature than untrained ones during exercise in the heat at similar relative intensities

To determine if the increases in rectal temperature (T _REC) during exercise in the heat at a given percent of \( \dot{V}\hbox{O}_{{2\,{\text{peak}}}} \) depend on a subject’s aerobic fitness level. On three occasions, 10 endurance-trained (Tr) and 10 untrained (UTr) subjects (\( \dot{V}\hbox{O}_{2\,{\rm peak}} \): 60 ± 6 vs. 44 ± 3 mL kg⁻¹ min⁻¹, P < 0.05) cycled in a hot-dry environment (36 ± 1°C; 25 ± 2% humidity, airflow 2.5 m s⁻¹) at three workloads (40, 60, and 80% \( \dot{V}\hbox{O}_{2\,{\rm peak}} \)). At the same percent of \( \dot{V}\hbox{O}_{2\,{\rm peak}} \), on average, Tr had 28 ± 5% higher heat production but also higher skin blood flow (29 ± 3%) and sweat rate (20 ± 7%; P = 0.07) and lower skin temperature (0.5°C; P < 0.05). Pre-exercise T _REC was lower in the Tr subjects (37.4 ± 0.2 vs. 37.6 ± 0.2; P < 0.05) but similar to the UTr at the end of 40 and 60% \( \dot{V}\hbox{O}_{2\,{\rm peak}} \) trials. Thus, exercise T _REC increased more in the Tr group than in the UTr group (0.6 ± 0.1 vs. 0.3 ± 0.1°C at 40% \( \dot{V}\hbox{O}_{2\,{\rm peak}} \) and 1.0 ± 0.1 vs. 0.6 ± 0.3°C at 60% \( \dot{V}\hbox{O}_{2\,{\rm peak}} \); P < 0.05). At 80% \( \dot{V}\hbox{O}_{2\,{\rm peak}} \) not only the increase in T _REC (1.7 ± 0.1 vs. 1.3 ± 0.3°C) but also the final T _REC was larger in Tr than in UTr subjects (39.15 ± 0.1 vs. 38.85 ± 0.1°C; P < 0.05). During exercise in the heat at the same relative intensity, aerobically trained individuals have a larger rise in T _REC than do the untrained ones which renders them more hyperthermic after high-intensity exercise.