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 ((V) over dot(O2 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 (Delta T-re), whole body sweat loss (WBSL), and steady-state local sweat rate (LSRss) independently described by 1) biophysical factors associated with metabolic heat production (H-prod) and evaporative heat balance requirements (E-req) relative to body size and 2) factors independently related to (V) over dot(O2 max) and body fatness. In a total of 69 trials, 28 males of wide-ranging morphological traits and (V) over dot(O2 max) values cycled at workloads corresponding to a range of absolute H-prod (410-898 W) and relative intensities (32.2-82.0% (V) over dot(O2 max)) for 60 min in 24.8 +/- 0.7 degrees C and 33.4 +/- 12.2% relative humidity. H-prod (in W/kg total body mass) alone described similar to 50% of the variability in Delta T-re (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, E-req (in W) alone explained similar to 71% of variance (adjusted to r(2) = 0.713, P < 0.001), and the inclusion of BF% explained an additional 1.3%. Similarly, E-req (in W/m(2)) correlated significantly with LSRss (adjusted to r(2) = 0.603, P < 0.001), whereas %(V) over dot(O2 max) described an additional similar to 4% of total variance. In conclusion, biophysical parameters related to H-prod, E-req, and body size explain 54-71% of the individual variability in Delta T-re, WBSL, and LSRss, and only 1-4% of additional variance is explained by factors related to fitness or fatness.