The effects of heat on exercise performance

The heat has a large impact on how you pace yourself during exercise. Your body temperature is kept within a narrow range by the body. If your body temperature rises too quickly during exercise, the brain increases your perception of fatigue and reduces its signal to the working muscles, in an attempt to slow you down and reduce your temperature. Because of this it’s no surprise that you can’t perform to the same level in the heat compared to a cooler day.

The interesting thing about the way this works is that you actually start pacing yourself more slowly BEFORE your body temperature get anywhere near a critical level. We still don’t understand exactly how the brain does this, but it seems to anticipate what’s coming based on the temperature of your skin, core, and knowledge of the exercise that’s being done. Jonothan Dugas and Ross Tucker (both originally from the University of Cape Town) did a large amount of work in this area – check out their excellent blog The Science of Sport for a layman’s overview.

Sports scientists have used several strategies over the years to try and reduce body temperature, before or during competitive sport. You’ve probably heard of ice vests being used both before and during breaks in sport. But in the lead up to the Beijing Olympics in 2008 some very clever sports scientists and dietitians went about a cheaper and easier alternative to lower the body’s temperature prior to competition, in an attempt to give Mick Rogers and Cadel Evans an advantage over their competition in the Individual Time Trial.

Using nutrition to lower body temperature

Sports scientists, coaches and athletes spent a huge amount of time and effort planning for the high temperatures and humidity expected for the Beijing Olympics. At the Australian Institute of Sport one strategy was studied for Mick and Cadel, aiming to “pre-cool” the body before competition. If you begin exercise with a lower body temperature, it is likely that you will perform better in the heat. This had already been shown at the time in studies using ice vests and cold water immersion (ice baths).

So why abandon the already established methods of pre-cooling? The issue in Beijing was a practical one, in that access to the necessary equipment at the competition venue was not an option. So instead AIS physiologist Dave Martin and head of sports nutrition Louise Burke set out to devise a cheaper and more practical alternative.

The AIS recruited 12 top level club cyclists to trial two new methods to reduce their body temperature. In an initial pilot study the cyclists drank around 700-1000mL of an ice slushie (made from Gatorade) 30 minutes before exercise. This is the equivalent of a large ice slushie or Frozen Coke in a convenience store. The slushie caused a drop in body temperature of about half a degree Celsius, and following a 30 minute warm up body temperature remained lower compared with drinking sips of water instead.

The researchers also tried using towels soaked in iced water, wrung out and wrapped around the legs and body.  This had minimal effect on its own, but combined with the ice slushie was a very effective in reducing body temperature.

Meanwhile the sports scientists had been doing their homework. At a test event on the Olympics TT course, they strapped a camera to Mick Rogers’ bike, and combined with his GPS and power meter data they completely recreated the Beijing TT course on computer in Canberra. This was used by Mick and Cadel to practice the exact replica of the course on an ergometer, placed in a purpose built heat chamber that replicated the weather conditions expected in Beijing (35C and 55% humidity). It was also used to test the effect of the ice slushies and wet towels in the study with club cyclists. The ABC program Catalyst ran a story on the AIS in the leadup to Beijing, which includes an overview of this set up and an interview with Mick Rogers. Click here for a link to the video. (**Please note that in the video (shot in 2008) Louise Burke adds glycerol to the slushie to enhance fluid retention. In 2010 WADA placed glycerol on the banned substances list. Using glycerol now would result in a positive doping outcome).

The result of the study of club cyclists is shown in the graphs below, but essentially there was an improvement in time trial performance with pre-cooling with the ice slushie and towels of 1 minute and 6 seconds, or 1.3% compared to the control group (sipping water of 4C rather than the ice slushie). A combination of cold water immersion combined with ice vests also improved performance, but statistically there was a 57% probability that this difference was due to chance.

Now consider the results of the ITT in Beijing – 1 minute and 6 seconds slower would’ve dropped Cadel from 5th to 8th, and Mick Rogers from 8th to 18th! For the record Fabian Cancellara won by 33 seconds over Gustav Larsson, with Levi Leipheimer 3rd.

Time Trial power output (left) and completion time (right) on the simulation of the 2008 Olympics ITT course, at 33-35oC and 55% humidity following Control (sips of 4oC water), Cold Water Immersion (CWI) & Ice Vest or Ice slushie & Iced water soaked towels 30min prior to warm up. Redrawn from Ross ML et al. Novel precooling strategy enhances time trial cycling in the heat. Med Sci Sports Exerc. 2011 Jan;43(1):123-33.

In the period since Beijing, other studies have been published using ice slushies without the towels, and have shown similar improvements in performance. Ice slushies have also been used to reduce body temperature after exercise in the heat, which is thought may improve recovery when the next training session or game occurs within 24 hours.

So how can you use this research to your advantage this summer?

A lot of bike races tend to take place off the beaten path, and there’s not usually a 7-Eleven on the nearest corner. But there are other places where you can pick up an ice slushie in a country town, such as a petrol station or even McDonalds (Frozen Coke). But other frozen foods and drinks may also help to reduce body temperature. Ice blocks, icy poles or those tubes of frozen fruit juice that school canteens sell are all good ways to bring your temperature down, and a couple of those before or after a training session or race on a hot day are a real treat. Icy poles are essentially carbohydrate (sugar) and fluid, so they also provide the fuel that you want for your riding, and help to replace it after a race.

There are also a small number of studies looking at the effect of drink temperature during exercise on performance, but sadly the design of the studies doesn’t reflect real world situations. Most compared a very cold drink (4C or 10C) with a very warm one (37C), probably an unrealistic comparison. However the research does suggest that there may be a benefit in having very cold drinks during a race if possible.

So how do you achieve that? Adding ice blocks, or partially freezing drink bottles that you know you don’t need to drink for several hours may be one way to do it. Or if you’ve got a support crew providing you with drinks along the course, they can keep them in an esky until they’re needed.

So whether it’s before or during a race, keep cool this summer. Your brain and muscles will thank you for it by working that little bit harder.