Influence Of Aerobic Fitness On The Thermophysiological Responses In Compensable And Uncompensable Heat Stress

Abstract

PURPOSE: To examine the influence of aerobic fitness in groups matched for anthropometric characteristics on the thermal and thermoeffector responses in compensable (CO) and uncompensable (UN) heat stress, during exercise at matched relative and absolute work rates. Previous work has been confounded by differences in body characteristics and the selection of external work rate. METHODS: Eight high (HI) (mean [SD] V˙O2max = 58.73[6.28] mL.kg-1.min-1) and eight low aerobic fitness (LO) (46.53[6.73] mL.kg-1.min-1) males, matched for body mass, surface area and % body fat volunteered. LO cycled at 60 W (ABS) in 34 °C, 20 % rh. HI completed the same condition, plus an additional test to match the relative intensity of the LO at ABS as a % of V˙O2max (REL). There were two phases in each trial: 1) Compensable phase - 60 minutes at the specified external work rate, CO was confirmed by a plateau in rectal temperature (Tre). 2) Uncompensable phase – immediately following the compensable phase, ambient temperature (Tdb) was increased by 1 °C every 5 minutes, UN was confirmed by an inflection in Tre (Treinfl). Tdb, Tre, change in Tre from rest (ΔTre), metabolic heat production (M) back sweat rate (SRBack) and laser Doppler forearm skin blood flow (SkBF) were measured at end of the compensable phase and Treinfl identified. Data were analysed by independent t-tests. RESULTS: 1) Compensable phase - there were no differences in Tdb, Tre, ΔTre or SkBF between fitness groups in ABS or REL (all p=>0.05) or M in ABS (p=0.633). HI had higher SRBack in both ABS (p=0.002) and REL (p<0.001) and M in REL (p=0.011). 2) Uncompensable phase – there were no differences in Tdb, Tre, ΔTre or SkBF between fitness groups in ABS or REL (all p=>0.05) or M in ABS (p=0.361) at Treinfl. HI had higher SRBack in both ABS (p=0.005) and REL (p<0.001) and M in REL (p=0.012) at Treinfl. CONCLUSION: When anthropometric characteristics are controlled between HI and LO, HI have greater SR than LO regardless of M. This greater SR is likely due to an earlier onset and greater sensitivity of sweating as an adaptation from aerobic training. This improved thermoregulatory function appears to allow the HI to offset the greater M at the matched REL and thermoregulate and transition into UN at a similar Tdb with a similar Tre, if the greater SR is able to evaporate freely into the environment.