Aerobic fitness and body fatness describe minimal variability in the thermoregulatory responses to exercise after accounting for heat production and body size
© Cramer and Jay; 2015
Published: 14 September 2015
Aerobic fitness (VO2max) and body fatness have been regularly suggested as important determinants of core temperature and sweating responses to exercise [3, 5], but recent studies suggest that biophysical factors related to heat production (Hprod), total body mass (TBM), and body surface area (BSA), predominantly influence rectal temperature changes (ΔTre) and sweating [1, 2, 4]. The present study tested the hypotheses that (i) individual variation in ΔTre, whole-body sweat loss (WBSL), and steady-state local sweat rate (LSRss) is determined primarily by Hprod (W.kg-1 TBM), evaporation required for heat balance (Ereq, W), and Ereq (W.m-2), respectively, and (ii) factors related to VO2max and body fat percentage (BF%) contribute minimally to the residual variance in these responses.
Twenty-eight male subjects [TBM: 78.2(11.3) kg, BSA: 1.96(0.15) m2, VO2max: 3.86(0.68) L.min-1)] performed exercise at external workloads corresponding to a wide range of %VO2max (32.2-80.0%), Hprod (5.2-12.1 W.kg-1 TBM), and Ereq (256-672 W) in 24.8(0.7) °C, 33.4(12.2) % RH, and 1.2(0.1) m.s-1 air velocity. Tre and forearm LSR were measured continuously; WBSL was estimated from changes in body mass. Forward stepwise multiple regression analysis was subsequently performed and partial contributions of each independent variable were determined using standardized regression coefficients.
Hprod (W.kg-1 TBM) alone described ~50% of the variance in ΔTre (adjusted R2 = 0.496, P < 0.001), while BSA-to-mass ratio and BF% added 4.3% and 2.3%, respectively, to the explained variance. For WBSL, Ereq (W) alone explained ~71% of the variance (adjusted R2 = 0.713, P < 0.001), and the inclusion of BF% explained an additional 2% of the variance in WBSL. Similarly, Ereq (W.m-2) correlated significantly with LSRss (adjusted R2 = 0.603, P < 0.001), while %VO2max contributed an additional ~4% to the total variance.
Previous findings that identified VO2max and body fatness as important modulators of core temperature and sweating may be confounded by collinearity between independent variables, since fitter individuals tend to be lighter and leaner and thus generate more heat (in W.kg-1 TBM) and have a higher Ereq (in W and W.m-2) at a fixed %VO2max, resulting in expectedly higher Tre and sweating rates. The relatively minor independent contribution of BF% and %VO2max to these responses warrants consideration.
Biophysical factors related to heat production and body size explained ~54-71% of the total variability in the core temperature and thermoregulatory sweating responses to exercise in a compensable environment, with only a minor contribution (<4%) to the explained variance in ΔTre and WBSL by BF%, and LSRss by %VO2max.
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