Why Is It So Hard To Exercise In The Heat?
Muscles that are active during exercise demand more energy and, consequently, increased blood supply and delivery of oxygen. This means the heart has to work harder to re-circulate blood to the active muscles.
Exercising muscles also generate heat, as a by-product of chemical reactions inside cells. This increases core body temperature which, if not compensated for, can compromise the ongoing function of the central nervous system and/or muscle cells themselves.
To lose heat, blood needs to be sent to the small arteries under the skin surface, so that heat transfer can take place via radiation. Sweating alone is relatively ineffective if this re-distribution of blood does not occur concurrently.
So when exercise occurs under circumstances where heat loss is challenged (because the gradient between skin and environmental temperatures is narrow), more blood needs to be directed to the skin at the same time as this blood is needed in the muscles to increase workload.
A competition therefore develops between the skin and working muscles for the limited maximal blood flow that the heart can manage.
Different theories explain fatigue (the point at which you cannot maintain exercise duration and/or intensity) as either the inability to sustain oxygen delivery to the muscle in the face of thermoregulatory demand, or an inability to control body and brain temperature during exercise in hot environments.
The causes of fatigue are of great interest to exercise scientists concerned with sport and workplace performance. We know that acclimation (the process of repeated exercise in hot conditions) can enhance an individual’s capacity for heat loss and, therefore, improve exercise performance and delay fatigue.
Several steps in the exercise and thermoregulatory chain are amenable to adaptations that improve performance, including enhanced function and structure of the blood vessels, the function and size of the heart and even blood volume itself.