Does exercising in the heat improve our endurance?

Dubai, UAE- Environmental conditions have a profound impact on even the best-conditioned athletes. In the UAE the heat and the hot summer months provide a very real challenge/stress especially when it comes to aerobic endurance training. With a sensible approach there are many benefits that translate to endurance based events such as marathons or cycle races. Some would argue that the hot weather may even provide a performance advantage that is similar to altitude training. The main physiological changes that occur to facilitate heat tolerance include an expansion of plasma volume, a decrease in heart rate and core temperature, an increase in sweat rate during exercise as well as a reduced threshold to initiate sweating (sweating starts at a lower core temperature). The athletes sweat rate increase combines with a decrease in the sweat salt concentration. To break this down further the increased plasma volume typically occurs over the first one to three days of acclimation. This early process is important because an increased plasma volume leads to increased stroke volume (blood ejected from the heart per beat), this allows the maintenance of adequate cardiac output (blood ejected from the heart per minute) while additional physiological adjustments can be made.

The decreased heart rate and core temperature experienced with acclimation allow the body to perform more work before the onset of fatigue or exhaustion. The increased sweat rate occurs up to 10 days or more after initiating a hot training schedule and allows the body to dissipate heat more effectively. Sweating starts earlier also and improves heat tolerance sooner. Lastly the sweat that is produced is more dilute (has a lower concentration of electrolytes) which acts to conserve sodium in the cells. According to the ACSM Guidelines the best method of inducing heat acclimation is to progressively increase the duration and/or intensity of exercise for 10-14 days. Those who have an exercise prescription for a specific target heart rate (THR) should maintain the same exercise heart rate even with the added stress of heat. Practically speaking, training on a treadmill inside is going to be very different to running outdoors down Jumeirah beach road in the sun and humidity.

You see in the hot and humid weather, a reduced speed or resistance achieves the THR, even though the work of the heart remains unchanged. As acclimatization occurs, progressively higher exercise intensity is tolerated and required to elicit the THR. It is worthy to note that recently a research paper published 8th March 2016 in the Frontiers in Physiology Journal that presented Cross Acclimation between Heat and Hypoxia: Heat acclimation improves cellular tolerance and exercise in Acute Normobaric Hypoxia. In a nutshell heat acclimation improves our ability to tolerate steady state exercise similar to that of going to a moderate level of altitude where we would have less O2 available in the air. Studies like the one above and responses of athletes have geared towards adopting heat training as opposed to the more traditional approach of altitude training.

The reason for this shift in training environments is probably down to a few physiological changes that occur under hot conditions: The most beneficial of these is our ability to acclimatize to heat and this causes an increase in blood plasma volume. Just as altitude stimulates your body to produce more red blood cells, heat stress stimulates your body to produce more blood plasma. A higher plasma volume enhances circulation, which improves the delivery of oxygen to muscles. The result is a greater cardiac output, and higher VO2max at a given effort level, enhancing endurance performance.

The general consensus is that it can take about 10 days of gradually progressive heat training to acclimatize. With altitude it can take around 2 weeks to make these changes. So the overall time taken to adapt is significantly reduced through heat training.

The increased sweat rate occurs up to 10 days or more after initiating a hot training schedule and allows the body to dissipate heat more effectively
Phil Elder, Strength and Conditioning Coach and Lead Sport Scientist