Fig. 1.

Fixed heart rate protocol measures how climatic heat impacts work capacity. Field studies demonstrate that individuals work at a similar heart rate despite changes in climatic heat due to adjustments in pacing. Although the heart rate profile is not different between conditions, heat stress causes a substantial increase in skin temperature, causing dramatic reductions in overall energy generated. Core (rectal) temperature responses in the early phase of the exposures are driven primarily by metabolic heat production (work rate). Changes to cumulative energy expenditure were used to calculate physical work capacity (PWC). Example data is shown for a representative participant for the reference condition (15°C, 50% humidity, solid blue line) and a hot condition (40°C, 20% humidity, dashed red line)