FITNESS IS A FACTOR WHEN IT COMES TO HEAT STRESS

The American College of Sports Medicine (ACSM) recently published a study in its official monthly journal, Medicine & Science in Sports & Exercise´┐Ż, that probes what processes work, and how they work, in regulation of body heat during exercise conducted in high temperatures.

The team of researchers, led by Matthew J. Watt, Ph.D., of Deakin University in Burwood, Australia, reviewed other literature that evaluated the onset of hyperthermia and dehydration during heat stress. They focused on plasma volume expansion, seeking information on whether this expansion of blood volume influences body heat regulation during exercise in the heat. “The studies already in existence had measured cardiovascular stability and core temperature regulation,” said Watt. “We thought plasma volume expansion might improve exercise performance in the heat, so we set out to compare cycling performance and internal temperature control in individuals.”

Participants in the study were six moderately fit cyclists in their late twenties who were just beginning to prepare for competition in triathlons or cycling events. Two trials, a week apart, were conducted on all group members. One trial called for participants to exercise under controlled conditions without pretreatment (CON), and the other for exercise preceded by acute plasma volume expansion (PVE), achieved by infusion.

The tests were performed during the winter months to preclude heat acclimatization; food and water intake were tightly controlled, and all trials were performed in the morning with no alcohol, caffeine or exercise beforehand. Participants exercised for 40 minutes to a submaximal level at 35 degrees C and 40 percent humidity in an environmental chamber, during which time their skin blood flow, core temperature and sweat loss were measured.

Watt and his fellow researchers found that although 1) core temperature increased progressively and did not differ between trials, and 2) blood flow also increased progressively in both trials, the increase in blood volume had no effect on total sweat volume. Nor did the expansion of blood volume affect power output or performance times in the absence of volume loss or dehydration.

The primary mechanisms of heat loss during exercise are increased skin blood flow and sweating.

The research group found no reduction in core temperature even with plasma volume expansion, nor did they note that PVE affected the volume of sweat lost during exercise, even though previous studies had. They suggest that this absence may be attributable to the level of fitness in their study participants. (Possibly a higher volume of plasma exists in somewhat fit individuals.) The results of this study suggest that increased blood flow, or acute plasma volume expansion, does not improve the moderately trained individual’s performance under hot conditions.

Summer sports and conditioning activities can quickly evaporate in the hot summer sun if athletes do not properly hydrate themselves before, during and after a workout. Without proper hydration, the body can lose water and essential elements and run the risk of kidney problems or even death.

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