Selecting the correct exercise intensity for unbiased comparisons of thermoregulatory responses between groups of different mass and surface area

Abstract

We assessed whether comparisons of thermoregulatory responses between groups unmatched for body mass and surface area (BSA) should be performed using a metabolic heat production (prod) in Watts or Watts per kilogram for changes in rectal temperature (ΔTre), and an evaporative heat balance requirement (Ereq) in Watts or Watts per square meter for local sweat rates (LSR). Two groups with vastly different mass and BSA [large (LG): 91.5 ± 6.8 kg, 2.12 ± 0.09 m(2), n = 8; small (SM): 67.6 ± 5.6 kg, 1.80 ± 0.09 m(2), n = 8; P < 0.001], but matched for heat acclimation status, sex, age, and with the same onset threshold esophageal temperatures (LG: +0.37 ± 0.12°C; SM: +0.41 ± 0.17°C; P = 0.364) and thermosensitivities (LG: 1.02 ± 0.54, SM: 1.00 ± 0.38 mg·cm(-2)·min(-1)·°C(-1); P = 0.918) for sweating, cycled for 60 min in 25°C at different levels of prod (500 W, 600 W, 6.5 W/kg, 9.0 W/kg) and Ereq (340 W, 400 W, 165 W/m(2), 190 W/m(2)). ΔTre was different between groups at a prod of 500 W (LG: 0.52 ± 0.15°C, SM: 0.92 ± 0.24°C; P < 0.001) and 600 W (LG: 0.78 ± 0.19°C, SM: 1.14 ± 0.24°C; P = 0.007), but similar at 6.5 W/kg (LG: 0.79 ± 0.21°C, SM: 0.85 ± 0.14°C; P = 0.433) and 9.0 W/kg (LG: 1.02 ± 0.22°C, SM: 1.14 ± 0.24°C; P = 0.303). Furthermore, ΔTre was the same at 9.0 W/kg in a 35°C environment (LG: 1.12 ± 0.30°C, SM: 1.14 ± 0.25°C) as at 25°C (P > 0.230). End-exercise LSR was different at Ereq of 400 W (LG: 0.41 ± 0.18, SM: 0.57 ± 0.13 mg·cm(-2)·min(-1); P = 0.043) with a trend toward higher LSR in SM at 340 W (LG: 0.28 ± 0.06, SM: 0.37 ± 0.15 mg·cm(-2)·min(-1); P = 0.057), but similar at 165 W/m(2) (LG: 0.28 ± 0.06, SM: 0.28 ± 0.12 mg·cm(-2)·min(-1); P = 0.988) and 190 W/m(2) (LG: 0.41 ± 0.18, SM: 0.37 ± 0.15 mg·cm(-2)·min(-1); P = 0.902). In conclusion, when comparing groups unmatched for mass and BSA, future experiments can avoid systematic differences in ΔTre and LSR by using a fixed prod in Watts per kilogram and Ereq in Watts per square meter, respectively.

Physiological and psychological responses in Fire Instructors to heat exposures

“…Therefore, longer or more intense thermal exposures would be expected to induce a greater strain, through larger rates of metabolic heat production and higher T re (Eglin and Tipton 2005). Varied thermoregulatory responses would also be expected within FSI whilst working at similar relative intensities due to individual biophysical characteristics (Cramer and Jay 2014).…”

Morphological dependency of cutaneous blood flow and sweating during compensable heat stress when heat-loss requirements are matched across participants

“…Indeed, although our ability to explain interindividual variations in thermoeffector responses is comprehensive (25,39), we know relatively little about the contribution of body morphology to that modulation. Although this information must represent a cornerstone for interpreting group comparisons (e.g., gender and age), as well as providing a foundation upon which physiological adaptations develop, systematic research of this fundamental relationship remains surprisingly sparse and often incomplete (7,8,20,21). Therefore, the focus of this project was to provide a detailed evaluation of the body-size dependency of cutaneous vascular and sweating responses within individuals of widely varying size.…”

“…While some investigators have compared forearm blood flow (16,20,21) and whole body sweating (14,34) in people of varying size, participants in those investigations performed exercise at either the same absolute or relative work rates. Unfortunately, such workloads frequently represent unequal requirements for heat loss in those who differ significantly in size (7,8). Furthermore, those investigations typically involved individuals who differed not only in body size, but also in secondary factors that may independently influence thermoeffector function (e.g., age and aerobic fitness).…”

“…Furthermore, those investigations typically involved individuals who differed not only in body size, but also in secondary factors that may independently influence thermoeffector function (e.g., age and aerobic fitness). Although one group of researchers recently compared sweating responses in smaller and larger individuals matched for some of those secondary factors (7), that project did not consider cutaneous vasomotor function, nor did it involve the examination of individuals spanning the body size range required to fully elucidate the morphological dependency. Moreover, neither that study (7), nor its recent iteration (8), were designed to provide an unequivocal assessment of the relationship between body morphology and the autonomically mediated heat-loss responses.…”

“…Although one group of researchers recently compared sweating responses in smaller and larger individuals matched for some of those secondary factors (7), that project did not consider cutaneous vasomotor function, nor did it involve the examination of individuals spanning the body size range required to fully elucidate the morphological dependency. Moreover, neither that study (7), nor its recent iteration (8), were designed to provide an unequivocal assessment of the relationship between body morphology and the autonomically mediated heat-loss responses. To achieve that, thermal clamping is required so that both thermoeffector functions can be simultaneously assessed in the presence of equivalent thermoafferent drive across participants; Cramer and Jay (8) reported a fivefold variation in deep-body temperature across subjects.…”