Heat Adaptation Could Prepare Us to Play in the Heat
Young Ju, Ph.D.
For many, September is another month of heat and heavy rain. Playing competitive tennis in the heat is challenging for every player. Excessive sweating may cause body fluid and electrolyte deficits (1). Prolonged sun exposure and reflective solar energy off the court surface can contribute further to heat accumulation. However, it is possible to train yourself to adapt to excessive heat and improve your performance in adverse conditions.
Body Temperature, Core Temperature, and Thermoreceptors (2, 3)
A normal human body temperature ranges between 97.0-99.5°F. Your body has tight mechanisms to self-regulate temperatures between ~95.0-105.8°F. Your core body temperature is the temperature of internal organs which can be measured by inserting a thermometer into a body cavity, such as rectum or internal organs. Core temperature is the sum of heat production from metabolism, mechanical work, and heat exchange with the environment. Your external body temperature is the temperature of the body’s surface which can be measured from skin, mouth, or ear, etc. Usually, your core temperature is 1-2°F higher than your body temperature.
When external temperature and humidity rise, the body’s cooling down mechanism (called heat dissipation) becomes ineffective, and the body needs additional energy to regulate body temperature. Thermoreceptors, the sensory neurons present in skin, organs, and spinal cord that monitor the cooling down process, help manage body temperature. Heat-related dehydration reduces the sensitivity of these receptors and increases heat strain.
Body Responses During Exercise In Hot Temperature Conditions
Studies have shown that exercise performance in warm-to-hot temperatures (≥77°F) is impaired as compared to performance in cooler temperature conditions, especially when athletes are dehydrated (4, 5). The heat-related performance reduction could be due to a combination of physiological, psychological, and environmental factors as summarized below (Figure 1):
Your metabolic rate (the energy used to maintain body functions) is increased and generates heat, further contributing to increased core temperature and elevated skin blood flow and sweat rate. This transfers heat from the muscles to the body surface to keep body temperature within a normal range.
The body uses more oxygen leading to faster breathing to regulate core temperature.
Excessive sweating could occur leading to dehydration and decreased blood volume and blood flow. Lower blood flow could further impair oxygen and nutrient supply and heat dissipation. The heart rate could be increased to compensate for the decreased blood volume and blood flow. As blood volume decreases, the body may initially compensate by constricting blood vessels to maintain blood pressure to ensure blood supply to vital organs. Over time, it could put extra strain on the heart, and further lower blood volume and decrease blood pressure.
Excessive sweating also leads to loss of electrolytes, which affects muscle function and the central nervous system.
Elevated heat also affects muscle metabolism by depleting stored energy (glycogen) and accumulating lactic acid, which leads to impaired muscle contraction/relaxation (6).
In the brain, heat stress increases perceived exertion (how hard you feel an exercise is for you) and impaired cognitive function.
Figure 1. Physiological and psychological changes during exercise in hot conditions.
Hyperthermia (Also Known as Heat-Related Injury or Illness)
When the core temperature rises above 100.4°F, the result is hyperthermia, which is an abnormally high body temperature that can impair physiological functions and the central nervous system. There are several types of hyperthermia listed in terms of severity (7):
Heat edema: swelling in the extremities due to widening of blood vessels
Heat cramps: muscle cramping caused by loss of electrolytes
Heat exhaustion: your core temperature may be between 101.3 - 104°F. Symptoms include blurred vision, dizziness or fainting, fatigue/weakness, low blood pressure, fast breathing and heart rate, headache, muscle aches or cramps, and nausea/vomiting.
Heat rash: skin irritations due to obstruction of sweat glands, usually in elbow crease, under breasts, near groin or upper chest and neck.
Heat syncope: temporary dizziness, weakness, or loss of consciousness; usually self-limiting.
Heat stress: reduced metabolic rates when the cardiovascular system can’t maintain.
Heatstroke: the most severe form of hyperthermia, which can develop shock and lead to coma. This is a life-threatening condition that causes core temperature to rise > 104°F. Symptoms include dry skin, hot, flushed, or pale skin, balance problems, confusion, disorientation, hallucination, abnormal blood pressure, seizures, and coma.
Heat Adaptation
Heat acclimation and heat acclimatization are adaptation methods that mitigate physiological strain during exercise-heat stress. Heat acclimation refers to training in a hot artificial environment; heat acclimatization refers to training in an outdoor natural hot environment (8).
Some studies reveal that repeated exposure to exercise in the heat induces heat acclimation/heat acclimatization, which improves heat dissipation, skeletal muscle metabolism, heart stability, and heat tolerance. (6).
Adaptations following acclimation and acclimatization could counteract some of the negative effects of exercise in heat and support exercise performance by reducing your heart rate, lowering core temperature, lowering skin temperature, increasing blood flow, improving sweat rate, reducing fluid and electrolyte loss, increasing blood volume, increasing glycogen stores in skeletal muscles, and reducing lactic acid levels.
There is no standard heat adaptation strategy applicable to all athletes. Individualized heat adaptation should be based on the athlete’s training status, exercise type and intensity, hydration status, mental adaptation, and adequate rest and recovery.
One review examined heat acclimation in competitive athletes and sports. This review concluded that athletes can fully heat acclimate with 10 consecutive days of exercise for 90 minutes at 86°F (9). This review also pointed out that heat acclimation is lost in the absence of repeated exercise-heat stress within a month. The International Olympic Committee recommends daily 60-90 min heat training sessions for 2 weeks to acclimatize across sports (10). Further research is needed to determine the optimal protocol for different sports.
As mentioned in a previous NWTO article (Sports Drinks), managing fluid and energy levels during exercise in the heat is important to avoid performance decline. The rate of muscle glycogen depletion is accelerated during exercise in the heat. Therefore, restoring carbohydrates before and/during exercise would help replenish muscle glycogen stores.
Management of Hyperthermia (7, 11, 12)
If you experience heat edema, heat cramps, heat exhaustion, heat rash, and heat stress, you can usually manage these conditions yourself:
Stop physical activity and rest in a cool, well-ventilated environment.
Remove heavy or tight clothing.
Replenish water and electrolytes.
Apply a cool compress to skin.
Gently stretch cramping muscles.
Heatstroke may require treatment in a hospital in addition to immediate attention. It is recommended that prompt action is taken to:
Immerse the individual in cool water.
Mist with water and blow air.
Apply ice packs to the neck, groin, and armpits.
Avoid any medications (including aspirin and acetaminophen).
Prevention (7, 11, 12)
Your checklist for preventing hyperthermia includes these tips:
Avoid strenuous physical activity in hot, humid conditions.
If you must do activities in the heat, let the body gradually acclimate to the heat. Do appropriate training and practicing before the real activity.
Properly use sports drinks or fluids.
Stay in air-conditioned or well-ventilated areas during heat waves.
Wear lightweight, loose-fitting and light-colored clothing.
Maintain healthy weight: being overweight generates more heat and impairs the body’s ability to cool down.
USTA Extreme Heat Guidelines
USTA Mid-Atlantic League Championships follows the Extreme Heat Guidelines using a Heat Index (13). When Apparent Temperature (combined index of heat and humidity; what it feels like to the body) reaches the orange zone, the Chief Umpire will be notified, and the Tournament Director will suspend play until the temperature reading drops below the orange zone. You can find more information HERE.
Table 1. Heat Index
Table 2. Apparent Temperature and Hyperthermia
Conclusion
Everyone has a different susceptibility to heat, and players living and exercising in hot parts of the country probably have adapted better to high heat than those living in other parts of the country. But heat adaptation for all tennis players is crucial for maintaining your performance level and reducing the risk of heat-related illnesses during practices and matches in hot environments. Players should work toward developing an effective strategy for playing in heat situations that considers the duration, intensity, and frequency of exercise along with players’ own physical and environmental situations.
Dr. Young Ju is a Ph.D and Associate Professor of Human Nutrition, Foods, and Exercise at Virginia Tech.
References
Bergeron. Hydration and thermal strain during tennis in the heat. Br J Sports Med. 2014; 48(Suppl 1):i12.
Leiva and Church. Heat Illness. NIH StatPearls https://www.ncbi.nlm.nih.gov/books/NBK553117/
Periard et al. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiol Rev. 2021; 101(4):1873.
Racinais et al. Effect of heat and heat acclimatization on cycling time trial performance and pacing. Med Sci Sports Exerc. 2015; 47(3):601.
Junge et al. Prolonged self-paced exercise in the heat-environmental factors affecting performance. Temperature. 2016; 3(4):539.
Pryor et al. Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives. Temperature. 2019; 6(1):37.
Cleveland Clinic. Hyperthermia. https://my.clevelandclinic.org/health/diseases/22111-hyperthermia
Tyler et al. The effects of heat adaptation on physiology, perception and exercise performance in the heat: a Meta-analysis. Sports Med. 2016; 46:1699.
Periard et al. Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports. Scand J Med Sci Sports. 2015; 25(Supple 1):20.
Racinais et al. IOC consensus statement on recommendations and regulations for sports events in the heat. Br J Sports Med. 2023; 57:8.
Centers for Disease Control and Prevention. Extreme Heat: Heat and Athletes. https://www.cdc.gov/extreme-heat/risk-factors/extreme-heat-and-athletes.html#:~:text=If%20you%20plan%20to%20exercise%20while%20it's%20hot%20outside:&text=Wear%20and%20reapply%20sunscreen%20as,to%20prevent%20heat%2Drelated%20illness.
Johns Hopkins Medicine. Exercise-related heat exhaustion. https://www.hopkinsmedicine.org/health/conditions-and-diseases/exerciserelated-heat-exhaustion#:~:text=Get%20plenty%20of%20fluids%20while,signs%20of%20heat%2Drelated%20illness.
USTA League. Extreme Heat Guidelines for USTA Mid-Atlantic League Championshipshttps://www.usta.com/content/dam/usta/sections/mid-atlantic/pdfs/adult-league-championships/md-5-adult-18-regionals/Extreme%20Heat%20Policy.pdf
This information is provided for your reference and you use at your own risk; you should rely on your medical professional for medical advice.
