Cool it: use your head!

Here in the northern hemisphere, summer has arrived, which means that athletes will be training and competing in much warmer or even hotter conditions than the rest of the year. But while summer sunshine and warmth can be very uplifting mentally, it comes with a potential own penalty – environmental heat stress.

The stress of heat stress

Performing strenuous exercise in warm or hot conditions puts extra physiological strain on the body. That’s because during exercise, only a portion of the energy liberated in muscles is converted to mechanical energy for movement. The rest is produced as heat – heat that needs to be lost to the environment. Unfortunately, the same heat production that is welcome - because it keeps you toasty warm on a winter’s day - becomes more of a problem when the mercury rises.

In an effort to keep core temperatures within the body fairly constant during exercise, the human body needs to discard the excess heat energy. While relatively easy in the winter when the surrounding air is cool, this can be much more challenging in the summer when the air is warm. The response is to send more blood (carrying this heat) to surface of the body in order to help dump the unwanted heat energy. Sweating also takes place where the evaporation of water from the skin’s surface greatly accelerates heat loss (thanks to the high ‘latent heat of evaporation’ of water).

While effective for maintaining core temperatures in a safe range, these physiological responses to hot weather exercise come at a cost; firstly, increasing blood flow to the surface of the body is harder for the cardiovascular system when the muscles are simultaneously demanding a copious blood supply to deliver oxygen to the working muscles. Secondly, when sweating is profuse or prolonged, hydration levels in the body may be adversely affected. As we have discussed at length in previous articles, when dehydration levels of more than 2% of bodyweight occur, performance can be adversely affected, especially in longer events^(1-4).

Reducing heat stress to boost performance

Leaving aside the issue of fluid and dehydration, it’s important to understand that simply performing in the heat is enough to blunt performance. In other words, simply completing an exercise task in very hot conditions can impact performance, even when fully hydrated. Research on cyclists performing time trials at temperatures of 17°C (63F), 22°C (72F), 27°C (81F), and 32°C (90F) found that performance declined with increasing ambient temperature⁽⁵⁾. A similar pattern has also been noted in runners performing treadmill time trials⁽⁶⁾.

“A 2017 study concluded that the brain and the central nervous system actually play an important role in determining the performance benefits of cooling.”

Given these facts, it’s hardly surprising that practical strategies which can alleviate increases in body temperature, and maintain or improve athletic performance are of great interest to athletes, coaches and sports scientists. Invariably, these strategies focus mainly on cooling different regions of the body, either before or during exercise, and research does indeed support the use of such strategies to help augment endurance exercise performance in the heat (or at least prevent its decline)⁽⁷⁾. Some of the cooling strategies used to help to support hot-weather endurance performance include:

• Torso cooling using an ice vest⁽⁸⁾. 
• The intermittent ingestion of cold fluid and ice slurries^(9,10).
• The use of menthol mouth rinses (see this article)⁽¹¹⁾.
• Palm cooling⁽¹²⁾.
• Neck cooling (using a cooling collar)^(13,14).

Role of the brain in cooling

Given the findings above, you might assume that the most effective mode of cooling to combat heat stress is the one that results in the greatest loss of heat. However, a 2017 study on this topic looked at a range of cooling strategies and concluded that the brain and the central nervous system actually play an important role in determining the performance benefits of cooling⁽¹⁵⁾. In particular, it seems that performance enhancements arise through a combination of central nervous system effects (for example, reduced activity of neurotransmitters and changes in muscle activation), cardiovascular effects such as a decreased heart rate and psychological effects such as improved thermal comfort and a reduction in the perceived effort levels.

What’s also intriguing is that when cooling takes place during exercise resulting in performance gains, it’s quite unusual to observe a drop in core temperature as might be expected. In other words, there must be something else going on other than just heat loss. Because of these factors, researchers have wondered whether cooling strategies for the head might be especially effective because they are more able to impact the brain and central nervous system. Also in its favor, head cooling has another advantage in that the head region is one of high alliesthesial thermosensitivity⁽¹⁶⁾. In plain English, when you are overheated, head cooling is especially effective in producing pleasant feelings of thermal comfort!

There’s good evidence for head cooling for reducing effort perception and improving performance; continuous facial fanning combined with intermittent water spraying has been shown to increase cycling time to exhaustion⁽¹⁷⁾, while spraying the face at regular intervals during a 5km run has been shown to enhance running performance⁽¹⁸⁾. Moreover, another 2017 study found that intermittently cooling the head improved the peak power output during a maximal graded exercise test on a cycle ergometer⁽¹⁹⁾.