It’s important for all coaches to have a general understanding of the physiological responses to exercise and how these may differ based on climate and weather conditions. Today’s blog will serve as a brief primer on the physiological effects of training in temperate versus hotter temperatures.
During exercise training, our athletes will experience a progressive increase in body temperature. This is because metabolism is inefficient and much of its energy production is lost as heat. This activates thermoreceptors located centrally and peripherally within the body that stimulate responses that facilitate heat loss. In temperate conditions, the body loses heat via radiation and convection when air temperature is cooler than body temperature. With increases in exercise intensity and duration (and thus greater heat production) the body responds by increasing perspiration from sweat glands that release heat via evaporation. In addition, there is enhanced skin blood flow to dissipate heat via convective heat loss. Blood serves as a storage place for heat and when it reaches the undersurface of the skin, it can be released into the environment. This places a mild strain on the cardiovascular system that is dramatically increased in warmer environments.
The same physiological responses that occur in temperate conditions to facilitate blood distribution to active muscle also occur during exercise in hotter conditions. The difference however, is in the rate at which the body heats up and thus how it responds to try and accommodate greater heat loss. In hot conditions, the temperature gradient between the body and the environment is drastically reduced and thus significantly less heat loss will occur via radiation and convection. Evaporation of sweat can also be limited by high humidity levels. The response to facilitate heat loss is to enhance skin blood flow and sweat rates both sooner and to a greater extent than when exercise is performed in cooler conditions. This can have a large impact on exercise performance and cardiovascular strain. Excess perspiration in attempt to release heat via evaporation progressively increases fluid and electrolyte loss and thus decreases plasma volume. Reduced plasma volume will result in reduced venous return, a lower stroke volume and increased total peripheral resistance (i.e., vasoconstriction). Heart rate will increase via sympathetic stimulation to try and maintain cardiac output. The enhanced skin blood flow can be problematic as there is now more distribution of blood throughout the periphery in attempt to dissipate heat and thus will further reduce venous return and stroke volume. This in turn results in a compensatory increase in heart rate to try and maintain cardiac output. This is referred to as cardiac drift, a progressive increase in heart rate without an increase in stroke volume as exercise persists.
Thus, coaches should always be aware of weather conditions during training and competition and have strategies in place to ensure athletes remain adequately cooled and hydrated. Strategies should include pre and post practice/competition hydration in addition to during training. Pre and post-training weigh-ins can be useful to estimate fluid loss and thus fluid replenishment. A general rule of thumb is to ingest 500 ml of water for every pound of weight loss.