Exercise in the heat and pre-cooling techniques

Pre-cooling techniques increase performance in the heat

Research has tended to focus on how body cooling can aid the performance of endurance athletes competing in hot, humid environments. But there is growing evidence that pre-cooling can offer performance advantages in a range of temperatures, during training as well as competition, and in non-continuous sports.

Exercise causes your body temperature to rise, and the harder you work the more rapid this rise will be. This rise in core temperature (Tc) can be modified by increased fitness but is exacerbated in hot and humid conditions. However, regardless of training state or climate, it is apparent that there is a critical limiting Tc, at which point athletes are forced to either reduce exercsie intensity or risk heat-related illness.

Because the amount of heat stored in the body will limit the duration of exercise at a given intensity, it obviously makes sense to start exercising with as cool a body temperature as possible – ie by pre-cooling. In essence, the purpose of body cooling techniques is to increase the margin between your starting Tc and the Tc that will force you to reduce your pace.

Numerous studies have shown that pre cooling is advisable before prolonged exercise in hot temperatures, with evidence that it helps to sustain intensity and speed. In one study requiring subjects to run at 82% of max VO2 in a heat chamber – 24°C and 51% relative humidity (RH) – pre-cooling by cold air exposure (5°C) was shown to boost performance by a massive 16%⁽¹⁾. In another study, following a 20-minute cold water bath (23-24°C) the distance achieved during a 30-minute self-paced running test in hot humid conditions (32°C, 62%RH) increased by an average of 4%⁽²⁾.

The effect of pre-cooling on self-paced performance was investigated further by Kay and his research group (3)/sup>. In this study, pre-cooling the skin alone (by 24°C water immersion) stretched 30 minutes of self-paced cycling in 31.4°C and 60.2%RH ambient conditions by 0.9km. Kay suggested that, although there was no reduction in Tc, pre-cooling was effective in reducing thermal strain, thereby enabling the subjects to increase exercise intensity towards the end of the trial.

Other researchers have also suggested that the main benefit of pre-cooling is in enabling athletes to draw on reserves later in a performance due to reduced thermal strain, allowing for different pacing strategies and increased intensity in the later stages.

But what about those of us who don’t get to compete in hot and humid conditions? Well, performance has also been shown to improve in lower ambient temperatures (Ta). A group led by Olshewski employed a cold air method to cool their subjects and reported a whopping 12% improvement in a subsequent cycle test to exhaustion, performed in a Ta of 18°C ⁽⁴⁾. Using a similar technique, another group of scientists reported significant work rate improvements and increases in absolute work performed during a cycle test performed in a Ta of 18°C ⁽⁵⁾. A Ta of 18°C is commonplace for both indoor competition and typical UK summer months. So from this evidence we can surmise that body cooling has a real role to play in improving performance on home turf.

Cooling between bouts

Few studies have focused on whether body cooling between bouts of exercise can also improve performance. Undoubtedly, increases in Tc create a problem in exercise of this nature, but the rest periods allow for some degree of cooling down. In the real world, there is unlikely to be enough time to reduce Tc to resting levels, and the aim of any cooling strategy must be to gain a performance advantage by offsetting as much thermal strain as possible before the next bout.

A group of Texan scientists investigated the effects of body cooling during a 12-minute rest period between two bouts of exercise in a hot environment (38°C). They reported that pre-cooling (fan cooling with water spraying) resulted in a lower Tc and a reduced loss of body water throughout the second bout⁽⁶⁾. However, another study reported no significant physiological benefit from pre-cooling completed prior to the intermittent activity of a football game (2x45 minute periods, 15 minute intermission)⁽⁷⁾.

More studies are needed to investigate fully the potential benefits of body cooling for intermittent exercise performance, whether cooling is carried out pre-exercise or during a rest period. In my work as an applied sports scientist, I have used body cooling techniques with racket sport players to great effect; despite the relatively short rest periods between games, these sports lend themselves particularly well to cooling. The majority of evidence I have to support this claim is anecdotal, but if the athletes ‘feel’ and perform better, with no medical ill effects, I am more than happy to continue suggesting body cooling as an aid to performance in such sports.

Many of the ‘take home messages’ from the studies mentioned above can be transferred to the training arena, particularly if you are lucky enough to have access to indoor and/or warm-weather training facilities. Periods of reduced training volume as athletes acclimatise to warm weather training are expensive in terms of both money and time. And many of the athletes I work with employ cooling techniques before and during training sessions in such conditions, so allowing volume to be maintained despite a high Ta.

It should be noted, however, that metabolic and cardiovascular responses can be affected during the initial 15 minutes of exercise after pre-cooling. These ‘differences’, including reduced heart rate and perceived exertion, are, in essence, the benefits of body cooling. During this period, it is advisable to use pacing as the best indication of intensity; otherwise there is a good chance that you will be working above your target zones.

A practical cooling method

By this point you may be beginning to think what a great idea body cooling is. But you’ve probably also noticed that cold air exposure or water immersion have been the predominant methods used, which may not be practicable in the real world. Exactly these thoughts occurred to scientists at the Australian Institute of Sport (AIS), who created a highly practical cooling jacket for their athletes, made from wet suit material and designed to be packed with ice.

Using these jackets the AIS carried out maximal cycling trials in a heat chamber (32°C, 60%RH). Subjects wore the jackets for the first nine minutes of the protocol, leading to an average drop in skin temperature from 33.5°C to 12°C. On average, the subjects cycled for 1.1 minutes longer during the cooling trial than they were able to without the jackets and reported a lower perception of effort and thermal discomfort. Despite these differences in skin temperature and perceptions, however, the ice jacket did not affect Tc, heart rate or blood lactate⁽⁸⁾.

In a further AIS heat chamber study reported to a conference I attended, the same jacket was worn during a 30-minute ‘warm-up’ prior to a maximal 2,000m rowing test and the researchers found that average rowing times decreased by 2.8 seconds. These jackets became commercially available following their use in the 1996 Olympic Games in Atlanta.

The AIS has also demonstrated exercise improvement with an even simpler approach. Rowers completed a heat chamber (30°C, 30% RH) protocol involving a 10-minute warm-up and a six-minute all-out effort, separated by an eight-minute rest period. During the rest period, ice packed in damp towels was intermittently applied to the rowers’ heads, faces, necks, arms and thighs – and the improved ergometer performance was equivalent to a four-second margin over a 2,000m race.

To summarise, the current body of evidence suggests that pre-cooling can increase performance capacity in various ambient temperatures and exercise circumstances. I should point out, as a note of caution, that cooling the body by too great a degree will not only hamper performance but also pose a health risk. However, it was never the aim of this article to advocate drastic techniques which can only be carried out in a laboratory setting.

Here, in summary, are some pre-cooling tips derived from the scientific research:

• Ideally use a cooling jacket or, failing that, ice-packed damp towels;
• Make a reduction in skin temperature your major goal;
• Aim to pre-cool for 8-30 minutes during warm-ups and/or the intervals between warm-ups and competition;
• Be sure to practise your chosen pre-cooling technique before using it in a key event.

References

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3. J Sports Sci 1999; 17: 937-44
4. J Appl Physiol 1988; 68: 803-11
5. J Appl Physiol 1984; 57: 1731-7
6. J Strength & Cond Res 2001; 15(2): 247-254
7. Eur J Appl Physiol 2000; 81: 11-17
8. Med Sci Sports Exerc 1997; 29(5): S263