The Dangers of Heat Stress for Athletes

A survey of studies examining the effects of uniforms and sports gear on heat stress in athletes.

Many athletes wear protective clothing and equipment that's highly insulated with low moisture permeability. This may cause uncompensable heat stress on the body, resulting in rapid heat storage and, often, a reduction in productivity and an increased risk of heat-related illness.

Uncompensable heat stress occurs when the evaporative cooling requirement of the body exceeds the environment's cooling capacity. This phenomenon happens when working or playing in excess clothing or protective uniforms or gear, often in hot and humid environments. Most importantly for our purposes, under these conditions individuals are unable to achieve thermal steady state and continue to store heat until exhaustion occurs, placing their health in danger. Between the years 2000 and 2007, twenty deaths that occurred during high school and collegiate football practice were attributed to heat stress.

The protective equipment and clothing worn during an athletic event or task establishes a microclimate above the skin surface but beneath the uniform, and this scenario reduces heat dissipation via radiation, convection and evaporation, causing exertional heat exhaustion or exercise-induced hyperthermia.

For example, the helmet and pads football players wear cover approximately 50 percent of their skin surface area, and other clothing covers an additional 20 percent. If a contest is played in a hot environment (>91°F), dry heat loss is reduced because the thermal gradient from skin to air is small or negative; in addition, high humidity decreases evaporative skin cooling, because the air has a greater water vapor content, causing a number of problems.

Impact on the body

Thermal stress typically elevates dehydration and impairs aerobic performance. Physiological changes include increases in both heart rate and core temperature along with impaired heart function. It has also been shown that dehydration up to 3.9 percent of body mass during moderately intense exercise in heat reduces blood flow to active muscles and elevates carbohydrate oxidation and lactate production, but does not impair the delivery of either glucose, free fatty acids or the removal of lactate. However, it has been shown that exhaustion still occurs in the absence of both circulatory failure and substrate utilization or availability during moderate exercise. It appears that a high core temperature induces a non-localized metabolic change that may be the main factor in limiting endurance in hot environments.

Hyperthermia affects muscle function, and it appears that muscles need to be at an optimal temperature for adequate performance. A muscle temperature of 77 to 86°F appears to be optimal for maintaining isometric contractions. A rise in temperature leads to shorter contractions and relaxation times, causing premature fatigue.

Understanding the physiological mechanisms behind how the body responds to excess heat can help us manage the effects and make jobs, exercise and activities safer. Keeping the body at equilibrium allows individuals to sustain activity safely without undue stress or harm to many of the body's complex systems and physiological reactions.

Preventative Measures and Treatment

From 1980 to 2009, there were 58 documented hyperthermia deaths of American-style football players in the United States. In a study that examined the geography, timing and meteorological conditions during the onset of heat illness, it was found that deaths were concentrated in the eastern quadrant of the United States, the most common month being August.

Over half the deaths occurred during morning practices, when humidity levels are highest; and all of deaths occurred under conditions defined as "high" or "extreme" by the American College of Sports Medicine using the wet bulb globe temperature (WBGT), but under lower threat levels using the heat index.

Most heat-stroke deaths in football occur on day 1 or 2 of two-a-day practices. If athletes are training the day after an exhausting and dehydrating day in the heat, they will be at an increased risk for heat illness. The most effective countermeasure is ensuring that the individual is adequately hydrated both before and throughout the exercise or work session.

In contrast, neither short-term aerobic training nor heat acclimation significantly improves exercise-heat tolerance. Wearing protective clothing in the heat contributes to the risk of uncompensable heat stress, when evaporative heat loss is limited and is less than what is needed to maintain thermal equilibrium.

In these situations, increased aerobic fitness or heat acclimation may be of limited effectiveness in reducing the risk of prolonging the ability to tolerate the heat. Because of limited water vapor permeability through the clothing, it is possible that increased sweat production in trained or heat-acclimated athletes may actually increase the physiological strain by accelerating dehydration rather than increasing evaporative heat loss.

In football, it would be wise for coaches and trainers to limit gear in the heat. A study demonstrated some success in acclimatizing players by suiting-up in stages when training in the heat. For example, players may begin in shorts and T-shirt the first day or two, and subsequently add helmets, shoulder pads and jerseys before finally suiting up in their full uniforms. Uniforms can also be varied depending on the activity. For example, helmets can be removed for general fitness training when they are not needed for protection.

Athletes considered at risk of heat stroke may benefit from pre-cooling before workouts. Pre-cooling has been shown to lower physiological demands during exercise heat stress. Intermittent cooling has also been shown to increase work capacity and decrease time to exhaustion.

It's a good idea to frequently remind athletes of the risks their gear and the warm environment impose and to promote frequent cooling breaks. By providing shade, ice water and misting fans for rest breaks, the warrior mentality can be de-emphasized and a healthier, safer climate can be encouraged. Coaches can also hold practices earlier or later in the day, and during two-a-days, allow for more time for recovery. As the temperature rises, consideration should be given to reducing practice pace and increasing rest breaks. Having players sit in cold tubs after practice may also reduce risk and accelerate recovery.


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