Summer’s hot. And as you get hot, your performance drops. Here’s the how and why, and how to keep your cool.
Human temperature regulation processes maintain a core body temperature over a very narrow functional range despite elevated metabolic rates and exposure to very hot environments. During submaximal exercise, we can limit the effects of the increased heat production through sweating and evaporative cooling. But as intensity climbs, our ability to offload heat becomes limited; in conjunction with a hot/humid environment; we are most likely going to experience hyperthermia. Technically, heat stress and hyperthermia are the overarching umbrella terms for heat illness (heat exhaustion, heat stroke), and are identified as an elevated body temperature due to failed thermoregulation that occurs when the body produces or absorbs more heat than it can dissipate.
During exercise in the heat, the most significant physiological burden is supporting blood flow to the skin: high skin blood flow is needed for heat dissipation. (The average blood volume of an adult male is ~5 liters, and the average female ~4.4 liters, not much to go around!). We often think of a high core temperature as being the limiting factor for performance; eg the “critical core temperature”, but it is more the temperature of the skin that affects aerobic performance. Skin temperature is highly influenced by the ambient temperature and humidity, whereas the core temperature is influenced by the intensity of the exercise (which is partially why a power decline is seen with elevated internal temperatures). Warmer skin induces a greater amount of skin blood flow, decreasing blood volume available for circulation. For example, with a sustained elevated core temperature, an increase in skin temperature will have a concomitant increase in heart rate but the reverse is also true- cooling the skin even with a sustained elevated core temperature will reduce heart rate due to more blood shunting back into circulation(1).