Endurance performance: feel the heat!

Athletes know that endurance training brings about favorable adaptations in the muscles involved, allowing them to perform harder for longer as their fitness gains accrue. But what exactly happens within exercising muscles to bring about these positive changes? We know that the physiological and chemical stress of exercise can activate the production of muscle cell signalling molecules and help ‘activate’ genes that are involved with the production of enzymes needed to release energy in the presence of oxygen (aerobic metabolism)⁽¹⁾. We also know that this stress can switch on signalling to increase number of mitochondria (aerobic energy factories) within muscle cells, increasing their ability to use oxygen to produce energy for exercise⁽²⁾. But did you know that recent evidence points to a role for heat?

Hot topic for training

In previous and highly recommended SPB article by Rick Lovett, Rick looked at some of the early evidence suggesting that training in a hot environment seems to increase endurance training adaptations compared to the same training stimulus in cold conditions. In summary, this article highlighted research showing that cyclists who worked out for 100 minutes a day for ten days on stationary bicycles in a very hot room (40°C, 50% relative humidity) increased their maximum aerobic power measures by 5% compared to a control group who continued training in normal heat⁽³⁾. Likewise, elite rowers who trained for 90 minutes a day at 40°C improved their 2000m performance times compared to a control group who performed the same training at 20°C⁽⁴⁾.

Around this time, one theory for these observations was that exercising in the heat might produce extra training adaptations via an effect known as ‘arterial flow-mediated dilation’ (see figure 1)⁽⁵⁾. This effect is known to increase blood flow (always a good thing for muscles!). Evidence that flow-mediated dilation might be playing a key role in improving training adaptation came from a 2015 study suggesting that time spent in a sauna can drastically reduce the risk of a fatal heart attack⁽⁶⁾. The study tracked 2315 middle-aged Finnish men for an average of 21 years, and found that those who took saunas four or more times a week had only about a 40% the risk of dying of heart disease than those who took one or fewer saunas per week.

Figure 1: Flow-mediated dilation effect