Precooling and exercise capacity during heat stress

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Vigorous exercise in a hot and humid environment can substantially impair exercise performance and exercise capacity compared to moderate temperature conditions. Cycling requires high force generation and consequently utilizes a high rate of energy expenditure and often takes place during the hot summer months, which also contributes to heat storage and a rise in core temperature during exercise. There are several factors that could affect exercise performance, however, one explanation for the decrease in exercise performance while working in the heat is the strain put on the cardiovascular system. Part of this strain involves cardiovascular drift (CV drift), defined as a steady increase in heart rate (HR) and a decrease in stroke volume (SV) that occur during an extended bout of constant-rate, submaximal exercise [50-75% of maximal oxygen uptake (VO2max)]. Precooling has been shown in previous studies to diminish cardiovascular strain and thus, improve exercise performance during prolonged exercise lasting between 15 - 45 min in hot ambient temperatures. In order to evaluate the effects of precooling on cycling performance, two studies and a literature review were conducted. The first study evaluated the effects of 20 min of precooling the upper body (torso, head and neck) during an active warm-up, followed by a 15-km simulated time trial. The second study evaluated the effects on CV drift following 20 min of whole body (calve, thigh, torso, neck and head) precooling. Healthy and physically active male cyclists were recruited for both of the studies. A repeated measures, counterbalanced design was used for both of the studies. For both studies, the effect of pre-cooling on time trial performance and CV drift showed very limited differences between the cooling intervention and the control trials. The method of cooling used in both trials may not be the best way to truly decrease the core temperature enough to elicit a true ergogenic effect in time trial pace cycling and CV drift. More research looking at a practical and convenient way of cooling cyclists before exercising in the heat is warranted.

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