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 (TREC) 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 TREC 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 TREC 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 TREC (1.7 ± 0.1 vs. 1.3 ± 0.3°C) but also the final TREC 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 TREC than do the untrained ones which renders them more hyperthermic after high-intensity exercise.