At some point in their athletic training, almost all athletes will be exposed to hot and/or humid conditions. Intense exercise events such as running, cycling, football, lacrosse and soccer in elevated temperatures and humidity can result in significant decreases in athletic performance and place the athlete at risk for potential life-threatening heat strain.
For example, the 2014 World Cup that was held in Brazil was played in temperatures that were consistently in the high 80’s with a humidity levels in the upper 70%. Numerous well-conditioned players collapsed late in the games due to cramping. You may also remember when LeBron James was sidelined with severe cramps during the 2014 NBA finals in San Antonio. The air conditioning went out in the arena causing the temperature to exceed 90 degrees and many players succumbed to heat strain.
In order to reduce the incidents of heat strain, it is important to understand the key physiological responses to exercise in the heat and humidity:
- Changes to blood flow and sweating. During exercise in hot conditions, heat produced by exercising muscles is transported by the circulating blood to the surface of the skin and released to the outside environment by radiation, convection or evaporation of sweat. If outside temperatures are equal to or greater than the body’s temperature, the body will rely solely upon evaporative cooling as the primary mechanism for heat loss. Studies have shown that sweat losses of 1-3 liters per hour have been recorded in athletes training or competing in the heat. If humidity is high, evaporative cooling ceases, resulting in the athlete retaining heat, causing the core temperature to rise.
- Greater demand on the heart and blood vessels. During sustained exercise, the heart pumps oxygen rich blood during each beat to the working muscles in order to sustain maximum performance. Exercise in the heat compromises muscle blood flow by shunting a higher percentage of blood to the periphery. This redirection of blood will reduce cardiac filling (flow to the heart) and stroke volume (blood pumped per beat). To compensate, heart rate will increase to try and maintain blood flow to the muscles. Thus, the relative intensity of exercise for the same workload increases, leading to earlier onset of fatigue and a reduced performance capacity. Additionally, blood volume losses due to dehydration can result in increases in blood viscosity, forcing the heart to work harder, placing greater stress on the heart and blood vessels.
- Negative effects on muscle energy availability. Muscles become the victim when the blood volume is split between competing interests during exercise in the heat. When blood is shunted away from the muscles to the periphery, muscle oxygen availability decreases. For endurance athletes, oxygen is vital, as it is responsible for breaking down fats and carbohydrates, producing substantial amounts of energy for muscular contraction. A decrease in the flow of oxygen can shift the athlete’s body from an aerobic to anaerobic state. The switch to anaerobic energy production leads to a higher total energy expenditure, blood lactate accumulation, greater carbohydrate utilization and rapid exhaustion.
- Reduction in aerobic capacity (VO2). The athlete’s aerobic capacity (VO2 max) diminishes over time in the heat as blood and oxygen is shunted away from the working muscles to the surface of the skin. Preserving a high VO2 max during exercise is essential to maintaining maximum endurance capacity and sustaining performance. Because VO2 max decreases in the heat, the athlete will be working at a higher percentage of their VO2 max; in other words, relative stress increases at a given workload due to decreased efficiency in the circulatory system.
- Humidity impedes cooling. Relative humidity is the amount of water vapor in the air and directly impacts sweating and cooling. The principal mechanism for cooling the athlete in the heat is the evaporation of sweat. The higher the humidity level, the more saturated the air becomes with water and the harder it becomes to evaporate sweat from the skins surface. As sweat evaporation decreases, the body struggles to stay cool resulting in a reduction in endurance capacity and an increased susceptibility to heat stress.
Understanding the impact that heat and humidity can have on the body, athletes will better equipped to prevent its many threats by implementing the following recommendations:
- Heat acclimatization - Athletes should integrate sport-specific drills such as shuttle runs, agility drills, and short sprints at a high intensity, as an effective way to adapt aerobic and anaerobic activities to the heat. Thirty to forty five minute sessions of high intensity activity at (approximately 75% VO2 max) over the course of 10-14 days in hot conditions are sufficient to see improvements.
- Protective equipment - Sports requiring protective equipment such as football and lacrosse should avoid wearing equipment for the first 5 days in a hot environment and gradually add equipment to subsequent practices. Single practice sessions for the first 5 days with succeeding practices shifting to double practice days are recommended.
- Clothing - Athletes should wear loose fitting, cotton, light and reflective colored clothing to increase air flow to the skin and help reduce the amount of heat storage in the body during exercise in the heat and humidity. Additionally, wearing tight -fitting wicking material can improve heat dissipation. Wear as little clothing as required during exercise. Maximizing skin exposure enhances the cooling surface area for evaporation. Remove equipment such as helmets during breaks and instruction.
- Cooling - Athletes should take frequent breaks during exercise. Seek shelter from the heat, drink plenty of fluids, and use cold-water techniques such as cold wet towels, misting fans, or forearm and face immersion in ice water buckets to enhance cooling.
Exercising indoors or outdoors in hot and humidity conditions produces additional strain on athlete’s cardiovascular and temperature regulation systems. Using the techniques provided, there are a variety of ways athletes can improve performance and enhance safety. Maintaining hydration levels throughout exercise and progressing through a heat acclimatization protocol are two of the best ways to ensure optimal performance and prevent heat illness.
More information can be found in 100 Questions and Answers About Sports Nutrition and Exercise by Lilah Al-Masri, MS, RD, CSSD, LD and Simon Bartlett, PhD, CSCS, ATC.
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