Climate and physiological heat strain during exercise.

A body-atmosphere energy exchange model (BIODEX) using heat transfer theory and empirical relationships is described which predicts the change in body core temperature during exercise as an index of thermal strain. Index values may be interpreted as the length of the period of activity before the heat load on the body causes internal body temperature to rise to critical levels. The performance of the model tested under controlled laboratory conditions using human subjects was found to be reliable. BIODEX is used to show the thermal significance of midsummer climatic conditions in New Zealand for those jogging outdoors.