The Determination of Changes in Body Heat Content during Exercise Using Calorimetry and Thermometry

Abstract

Following the onset of exercise there is a sudden increase in metabolic heat production that cannot be initially offset by the body's heat loss mechanisms. During this thermal imbalance a change in body heat content (ΔHb) occurs. The only way the value for ΔHb can be truly determined is by performing simultaneous direct measurements of the rates of heat production and heat loss. There are several calorimetric methods that can be used to quantify these values. The most accurate and simple method for measuring the rate of heat production is by separately calculating the energy released from the oxidation of fats and carbohydrates using indirect calorimetry, while direct air flow calorimetry provides the most accurate and responsive measurement of total body heat loss. Thermometry is often used as a surrogate for calorimetry by estimating mean body temperature and subsequently deriving a value for ΔHb. The most common thermometry approach, the 2-compartment model of “core” and “shell” temperature, has been demonstrated to greatly underestimate ΔHb. The inclusion of a third compartment representing the thermal influences of muscle tissue has been demonstrated to provide a better thermometric estimation of ΔHb; however this requires the invasive measurement of intramuscular temperature. A modified 2-compartment thermometry model incorporating an “adjustment factor” correcting for the underestimation of the traditional model for ΔHb appears to be the most practical alternative. Further research is required to develop thermometry model for the better estimation of ΔHb under various conditions.