Explained variance in the thermoregulatory responses to exercise: the independent roles of biophysical and fitness/fatness-related factors.

Individual variation in the thermoregulatory responses to exercise is notoriously large. Although aerobic fitness (V̇o2 max) and body fatness are traditionally considered important predictors of individual core temperature and sweating responses, recent evidence indicates potentially important and independent roles for biophysical factors. Using stepwise regression, we examined the proportion of individual variability in rectal temperature changes (ΔTre), whole body sweat loss (WBSL), and steady-state local sweat rate (LSRss) independently described by 1) biophysical factors associated with metabolic heat production (Hprod) and evaporative heat balance requirements (Ereq) relative to body size and 2) factors independently related to V̇o2 max and body fatness. In a total of 69 trials, 28 males of wide-ranging morphological traits and V̇o2 max values cycled at workloads corresponding to a range of absolute Hprod (410-898 W) and relative intensities (32.2-82.0% V̇o2 max) for 60 min in 24.8 ± 0.7°C and 33.4 ± 12.2% relative humidity. Hprod (in W/kg total body mass) alone described ∼50% of the variability in ΔTre (adjusted to r(2) = 0.496; P < 0.001), whereas surface area-to-mass ratio and body fat percentage (BF%) explained an additional 4.3 and 2.3% of variability, respectively. For WBSL, Ereq (in W) alone explained ∼71% of variance (adjusted to r(2) = 0.713, P < 0.001), and the inclusion of BF% explained an additional 1.3%. Similarly, Ereq (in W/m(2)) correlated significantly with LSRss (adjusted to r(2) = 0.603, P < 0.001), whereas %V̇o2 max described an additional ∼4% of total variance. In conclusion, biophysical parameters related to Hprod, Ereq, and body size explain 54-71% of the individual variability in ΔTre, WBSL, and LSRss, and only 1-4% of additional variance is explained by factors related to fitness or fatness.