Heat storage regulation in exercise during thermal transients

Area of Science:

• Human physiology
• Thermoregulation
• Calorimetry

Background:

• Accurate measurement of body heat storage (S) is crucial for understanding thermoregulation.
• Traditional thermometry methods for assessing changes in mean body temperature (delta Tb) can exhibit delays during dynamic thermal conditions.

Purpose of the Study:

• To simultaneously measure the rate of heat storage (S) using direct and indirect calorimetry.
• To investigate the body's thermal responses during rest and exercise across varying ambient temperatures and work intensities.
• To compare the accuracy and timeliness of calorimetric measurements versus traditional thermometry for assessing body temperature changes.

Main Methods:

• Simultaneous direct and indirect calorimetry were employed in 11 subjects.
• Measurements were conducted during rest and exercise (40 and 90 W) at three ambient temperatures (20, 25, and 30°C).
• Heat storage (S) and total heat losses (Radiation + Convection + Evaporation) were calculated.

Main Results:

• At rest, heat storage varied significantly with ambient temperature, ranging from negative values (heat loss) at cooler temperatures to positive values (heat gain) at warmer temperatures.
• During prolonged exercise (50 min at 40 or 90 W), heat storage approached zero, indicating thermal equilibrium was achieved.
• Total heat losses during exercise were independent of ambient temperature.
• Calorimetry provided immediate S measurements, while thermometry showed a 5-10 minute delay in detecting changes in mean body temperature (delta Tb).

Conclusions:

• Calorimetry is a precise method for immediate assessment of body heat balance, especially during dynamic thermal transients.
• Thermometry-based delta Tb measurements require adjustments for their inherent delay during rapid temperature changes.
• The findings support the hypothesis that mean body temperature (Tb) is the regulated variable in the human thermoregulatory system, evidenced by a consistent correlation between skin temperature (Esk) and delta Tb.