We wonder whether the day ahead of us is going to be hot or cold in order to decide if we should do certain activities, like practice our favorite outdoor sports. We might think that no matter how hot or cold it gets, we should still go on our running routine because exercise is always good for us. However, we should always remind ourselves that excessive heat or cold can not only make us uncomfortable during exercise, but even put our health at risk.
Based on morphological characteristics, muscles can be classified into two major types, smooth muscle (e.g., the type of muscle found in the walls of hollow organs such as blood vessels) and striated muscle (including heart muscle and skeletal muscle). Skeletal muscles generate most of the heat that causes body temperature to rise during exercise, and thus, it is the main focus of this article.
Muscle contraction requires energy. In muscle cells, like in any other cells, adenosine triphosphate (ATP) is the molecule that stores energy. This energy is transformed into work by the muscles that we use during a specific activity. However, not all the energy is transformed into work. Both the production and hydrolysis (water-mediated cleavage of chemical bonds) of ATP release heat as a by-product, and it is this heat that contributes to the body’s temperature.
Since we are homeotherms (our body temperature is kept relatively constant with respect to ambient temperature), our basal metabolism is higher than that of non-homeotherm animals. When we engage in any type of activity, our body produces extra heat that is added to the heat generated by our basal metabolism and thus, our body temperature increases. If our bodies could not regulate internal temperature, we would store great amounts of heat compromising cell function.
We can regulate body temperature via behavioral and physiological means. For example, we can exercise in the shade to avoid direct sunlight (behavioral) and depending on ambient temperature, we can experience vasodilation (widening of blood vessels) and vasoconstriction (narrowing of blood vessels) to facilitate or restrict heat transfer from the skin to the surrounding air. Thus, thermoregulation can profoundly affect how we perform in different sports and under different ambient conditions.
Furthermore, we need to be extra careful when engaging in sports or other vigorous physical activities in hot and humid weather. Hot weather means that we may accumulate heat more rapidly than what our body can dissipate, and humid conditions imply that sweating (the only known mechanism for dissipating heat against a thermal gradient) may not be possible. Under these circumstances we are prone to suffer from heat stress. Heat stress can lead to cramps, syncopes, exhaustion, and even stroke.
All of these can be avoided by learning about the different strategies to reduce the risks of heat-illness, always hydrating appropriately, adjusting exercise activity levels according to current fitness status, having adequate recovery periods between bouts of exercise, and realizing when it is better to cancel athletics and stay home.
By Jose G. Crespo
Jose G. Crespo is a researcher in the field of animal physiology and behavior with an emphasis on insect thermoregulation and neuroscience. He is currently a Postdoctoral researcher at the University of Utah – Department of Biology.
Articles by Jose G. Crespo.