Jay, O, Gariépy LM, Reardon FD, Webb P, Ducharme MB, Ramsay T, Kenny GP. A three-compartment thermometry model for the improved estimation of changes in body heat content. Am J Physiol Regul Integr Comp Physiol 292: R167-R175, 2007. First published August 24, 2006; doi:10.1152/ajpregu.00338.2006.-The aim of this study was to use whole body calorimetry to directly measure the change in body heat content (⌬Hb) during steady-state exercise and compare these values with those estimated using thermometry. The thermometry models tested were the traditional twocompartment model of "core" and "shell" temperatures, and a threecompartment model of "core," "muscle," and "shell" temperatures; with individual compartments within each model weighted for their relative influence upon ⌬Hb by coefficients subject to a nonnegative and a sum-to-one constraint. Fifty-two participants performed 90 min of moderate-intensity exercise (40% of V O2 peak) on a cycle ergometer in the Snellen air calorimeter, at regulated air temperatures of 24°C or 30°C and a relative humidity of either 30% or 60%. The "core" compartment was represented by temperatures measured in the esophagus (Tes), rectum (Tre), and aural canal (Tau), while the "muscle" compartment was represented by regional muscle temperature measured in the vastus lateralis (Tvl), triceps brachii (Ttb), and upper trapezius (Tut). The "shell" compartment was represented by the weighted mean of 12 skin temperatures (T sk). The whole body calorimetry data were used to derive optimally fitting two-and three-compartment thermometry models. The traditional two-compartment model was found to be statistically biased, systematically underestimating ⌬Hb by 15.5% (SD 31.3) at 24°C and by 35.5% (SD 21.9) at 30°C. The three-compartment model showed no such bias, yielding a more precise estimate of ⌬Hb as evidenced by a mean estimation error of 1.1% (SD 29.5) at 24°C and 5.4% (SD 30.0) at 30°C with an adjusted R 2 of 0.48 and 0.51, respectively. It is concluded that a major source of error in the estimation of ⌬Hb using the traditional two-compartment thermometry model is the lack of an expression independently representing the heat storage in muscle during exercise. body heat storage; calorimetry; muscle temperature; thermoregulation THE DERIVATION OF THE CHANGE in body heat content (⌬H b ) is of fundamental importance to the physiologist assessing the exposure of the human body to environmental conditions that result in thermal imbalance. In theory, the measurement of body heat exchange using simultaneous measures of direct and indirect calorimetry is the only method whereby ⌬H b can be directly determined. Thus the difference between metabolic heat production using the stoichiometric relationship of the products and reactants of oxidative metabolism (indirect calorimetry) and the total heat lost from the body can be used to estimate ⌬H b . By definition, ⌬H b is the product of the change of the mean temperature of the tissues of the body (⌬T b ), the total body mass (b m ), and the average sp...
