Skin Calorimetry of the Vastus Lateralis During Incremental Exercise: Thermal Analysis and Modeling

In this study, we examined the thermal response of the vastus lateralis muscle using a skin calorimeter designed for localized measurements. Five healthy young male participants (20-23 years old) performed an incremental test on a cycle ergometer at 20 W·min^-1 until exhaustion. The calorimeter's thermostat was maintained at 30 °C. Ambient temperature was 23 ± 1 °C, and relative humidity ranged from 55% to 60%. The measured heat flow was modeled using functions relating mechanical power output to the thermal response. The model included the exercise phase, recovery, and sweat evaporation. The proposed model accurately reproduced the experimental data and supported a physiological interpretation of the main thermal effects. Two major contributions were identified and physiologically interpreted as muscle warming due to increased metabolic activity and a cooling effect likely linked to changes in blood perfusion. For an area of 2 × 2 cm² and an incremental exercise of 20 W·min^-1, the following values were obtained: (1) a resting heat loss of 150 ± 20 mW; (2) an exponential increase due to exercise of 0.4 ± 0.1 mW·W^-1, with a time constant of 1.0 ± 0.3 min; and (3) a negative blood-flow contribution described by a function that, in the steady state, has an amplitude of -20 ± 9 mW. Since only five participants were included, correlations with anthropometric variables were treated as exploratory consistency checks. These results demonstrate the usefulness of the calorimeter for decomposing and quantifying muscle thermal dynamics during exercise.