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Precooling and Sport Performance

This is an excerpt from Physiology of Sport and Exercise 9th Edition With HKPropel Access by W. Larry Kenney,Jack H. Wilmore & David L. Costill.

Many athletes now artificially lower their body core temperature before exercise, a practice known as precooling. Methods include immersing the body in cold water, sitting in a cold room, taking a cold shower, applying cooling packs or vests, and ingesting a cold drink or ice slurry. If an athlete starts at a lower core temperature and the core temperature subsequently rises at the same rate, it will be lower at any given point in the exercise bout or competition. In laboratory studies, this often allows subjects to exercise longer at a given intensity under hot conditions. But does this practice improve sport performance?

Two meta-analyses (statistically based reviews of existing studies) addressed that important question, concurring that precooling does effectively enhance sport performance, particularly in hot environments. As expected, precooling had a larger effect during endurance events like cycling time trials, but improvement was also seen for intermittent running or cycling sprints. The effectiveness of precooling on short sprints was minimal. Of the methods compared, ingesting cold drinks was the most promising procedure. Further, the best results occurred in athletes with the highest VO_2max.

The second analysis concluded that precooling impaired sprint performance, which might be expected if active muscle temperature fell. Intermittent activity and endurance performance were both improved with precooling, however. Similarly, cooling maneuvers used during exercise were also successful at improving performance. Whether used before exercise or during exercise, the effectiveness of body cooling procedures depends on both (1) the amount of cooling (intensity and duration) and (2) how much thermal strain is involved in the exercise.

American football players are the athletes perhaps most at risk for exertional heat illness, the most severe of which is exertional heatstroke. From 2005 to 2015, 36 American football players died from heatstroke. To mitigate the risk of heatstroke, current guidelines recommend gradual heat acclimation, adequate hydration during physical activity, appropriate rest-to-work ratios for exercise in the heat, and emergency preparedness. More recently, precooling to lower core temperature before exercise, often achieved via whole-body cold-water immersion, has been proposed as an effective countermeasure to prevent exertional heatstroke. By reducing core temperature, precooling improves body heat storage, enhances the core-to-skin gradient, and delays the onset of hyperthermia-induced fatigue. However, precooling also causes cutaneous vasoconstriction, lowers skin temperature, delays the onset of sweating, reduces sweat rate, and may alter thermal comfort and sensation, all of which may influence how hot an athlete feels during exercise, potentially leading to the underreporting of heat illness symptoms. Therefore, before precooling can be adopted as a prevention strategy, it is imperative to more fully understand how it affects thermal perception and cold-water immersion cooling rates after an athlete becomes hyperthermic.

To begin to address these gaps in knowledge, investigators performed a randomized crossover counterbalanced research study. On the precooling experimental visit, participants completed a 15 min bout of water immersion up to the neck in ~10 °C (50 °F) water. They then put on a full American football uniform and exercised in the heat, without rest, until rectal temperature increased to 39.5 °C (103.1 °F). Immediately upon cessation of exercise, while still wearing all equipment, they again underwent cold water immersion until rectal temperature was reduced to 38 °C (100.4 °F).

Rectal temperature was ~0.5 °C (0.9 °F) lower during exercise in the heat after the precooling intervention, and participants exercised 18 min longer compared with no cooling. Because precooling delayed the onset of hyperthermia without affecting postexercise cooling rates, it appears that a relatively short bout of precooling may be a useful strategy for delaying, or possibly even preventing, exertional heat illness.

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