Should we avoid exercise in hot weather?

A colleague here at CERG spoke to me before she went on holiday and expressed concern about a running race in Denmark she would be competing in where temperatures could be up to 28º. The Met Office in Britain issuing a heatwave health warning with recent forecast temperatures of 28º to 32º over two days. These stories are hilarious to Australians like me, who have experienced temperatures like this continually for 4-5 months of the year, and have survived and continued to train through events like the March 2008 Adelaide heatwave where maximum temperatures were above 37.8º for 13 consecutive days. But should I be laughing? 300 mostly infant and elderly Britons died as a result of a 32º heatwave in 2009. And now, acclimatised to living in Norway and with a warm summer by Norwegian standards currently upon us, I do find myself struggling a little bit exercising in these conditions that I used to find so mild. Why is it so hard to exercise in the heat and what can we do to acclimatise and improve performance under these conditions?

Photo: iStock

Read also: How to deal with extremely hot weather?

Temperature regulation is critically important for a normally functioning body, and the body has a number of mechanisms to tightly maintain body temperature between 36.5º and 37.5º. If those mechanisms are overwhelmed and body temperature increases beyond these levels, heat exhaustion or even potentially deadly heat stroke symptoms begin to arise. Two of the main mechanisms we use to keep cool in hot conditions are sweating, and arteriolar vasodilation. Arteriolar vasodilation is the widening of blood vessels, which allows increased blood flow to the skin and extremities, allowing more heat to be lost by convection and conduction. The heart must work harder to supply that blood flow to the skin, and at maximal exercise intensities a compromise must be reached between maintaining blood supply to active muscles, and blood supply to the skin to lose heat, which is one of the main reasons for a reduction in exercise performance. These reductions in performance can also be accompanied by central fatigue as a result of heat sensitive neurological changes, and symptoms of dehydration as a result of increased sweating.

Read also: What is the energy cost of falling into cold water?

Of course, there are many commonsense things that we can do to either avoid the heat, or to minimize its effects, and still enjoy training or competing outside in warm conditions. For example, training early in the morning or later in the evening to avoid the hottest parts of the day, making sure that we drink enough, and wearing less clothes, and using tightly fitted sports clothes helps to increase evaporative cooling by sweating can all help you avoid heat exhaustion.

However, by exposing ourselves to heat, we can actually acclimatise to warm conditions and improve exercise performance. Adaptation to heat can occur over days to weeks of living in a hotter environment. Physiological changes occur to improve heat tolerance such as a reduction of heat production from fat and increases in sweating. Training in hot conditions has been found to give significant increases in athletic performances, and is now often used by elite athletes to give them an edge even if competing at normal temperatures. As an example, submaximal training of competitive cyclists in 40º heat for 90 minutes every day for 10 days was found to give an increase of 8 percent VO2max compared to the a 5 percent increase in the exact same training in 13º conditions. Subjects who had trained in the heat performed better 8 percent better in the heat, and 6 percent better in cool conditions in a time trial test, and had a higher lactate threshold than those who had trained in cool conditions.

While we should be aware of our limits and careful to avoid the negative effects of heat exhaustion, dehydration and sun overexposure, there are benefits to be had from training in warm conditions. Another reason to enjoy the fine sunny weather!

Nathan Scrimgeour, Senior engineer at CERG

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