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Heat and Physical Performance*

By Dr. Lawrence Armstrong

The question “What effects do heat and humidity have on exercise performance?” has no single answer for all types of exercise. Endurance and strength activities are affected differently because each involves a unique blend of heat exposure, exercise intensity, and opportunity for drinking fluids. In fact, from a physiological viewpoint, it is best to consider the influences of elevated body temperature (hyperthermia) and dehydration as the critical factors, rather than heat and humidity per se.

Hyperthermia
If you review figures 2.3 and 2.4, you will see the three most important causes of hyperthermia in athletes, soldiers, and laborers: high exercise intensity, high air temperature, and high relative humidity. This explains why any athlete, female or male, can experience performance-degrading hyperthermia if they push themselves too fast, for too long a time, in hot-humid conditions. There are three ways in which such hyperthermia might degrade physical performance:

  1. Hyperthermia reduces muscular endurance (the ability to sustain muscular contractions for several minutes to hours). It may alter performance in the long-distance events of cycling, track and field, and soccer.10, 11, 12 However, the peak force (maximal strength) exerted by muscles is virtually unaffected.10, 12
  2. Hyperthermia shifts metabolism from primarily the aerobic (with oxygen) to the anaerobic (without oxygen) form. Unfortunately, this means that the body’s stores of carbohydrate (e.g., glycogen) in skeletal muscles and liver will be consumed at a faster rate.13 This may partly explain why exercise in the heat cannot be maintained as long as in a cool environment.14, 15 This metabolic response will affect events that rely heavily on carbohydrate stores in the body, such as endurance road cycling and marathon running.7 In these contests, the body’s limited carbohydrate stores can sustain no more than a couple of hours of moderate to intense exercise.
  3. As noted previously (see the section titled “Cardiovascular Responses to Heat and Humidity”), hyperthermia causes the dilation of blood vessels in the skin and pooling of blood in the limbs. This reduces the volume of blood that returns to the heart, reduces cardiac output, and increases circulatory strain. These effects are perceived as increased fatigue16 because the capacity to deliver oxygen to muscles is reduced.17 All forms of prolonged labor and exercise may be affected by these thermoregulatory responses.



Dehydration
Although mild to moderate work and routine physical training typically result in whole-body sweat losses of 0.8-1.4 L/h, the highest sweating rate ever reported for an athlete was 3.7 L/h, in conjunction with the marathon of the 1984 Summer Olympic Games.18 Further, we know that the maximum amount of fluid that empties from the stomach during exercise is 0.8-1.2 L/h in most athletes.19 This explains why athletes routinely experience a 2-8% loss in body weight during competition and training.

Previous publications have examined the impact that dehydration has on muscular strength. These articles, written by respected physiologists, indicate that it is unlikely that small or moderate reductions in body weight due to dehydration (–1% to –3%) alter strength.19, 20 In fact, dehydration to –5% or more can be tolerated without a loss of maximal strength. However, sustained or repeated exercise that lasts longer than 30 seconds deteriorates when moderate to severe dehydration exists (–6% or more).21 This probably results from reduced muscle blood flow, waste removal, and heat dissipation—all of which are necessary for high-powered, sustained muscle action in events such as boxing, judo, and pursuit races in velodrome cycling.

In contrast, there is little doubt that acute dehydration, or its long-term counterpart chronic hypohydration (lasting 4 or more hours without rehydration), degrades endurance performance, regardless of the environmental temperature or whole-body hyperthermia. For example, maximal oxygen uptake ( .VO2max), a critical component of successful endurance performance, is reduced significantly following moderate body water losses (–3% of body weight) in a cool environment. In a hot environment, small to moderate levels of dehydration (–2% to –4%) result in a large .VO2max decline. Similarly, endurance capacity (i.e., exercise time to exhaustion) is reduced more in a hot environment than in a cool or mild one.20 Table 2.1 demonstrates the impact that dehydration has on endurance performance.7 This summary of previous investigations shows that increasing dehydration (column 1) interacts with air temperature (column 2) to reduce both .VO2max (a physiological measurement) and endurance capacity (a performance variable). You will recall from the previous section that weight losses of –2% to –5% had little effect on strength output. These observations coincide with the fact that decreases of plasma volume during prolonged exercise are greater in a hot (versus a mild) environment.19 They also suggest that changes in cardiovascular function (e.g., decreased cardiac output) contribute to the decline in endurance capacity shown in table 2.1.

* To reprint this excerpt with permission from Human Kinetics Publishers, Inc., please contact the publicity department at 1-800-747-4457 or publicity@hkusa.com.


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