What is Heat Training?

Heat training involves altering environmental conditions during training to induce physiological adaptations that enhance performance in hot conditions. The physical changes caused by heat training help the body work more efficiently in hot conditions, minimizing thermal strain and maximizing performance. These adaptations also reduce the risk of exertional heat illness, which poses a significant safety threat to athletes.

The physiological adaptations achieved through heat training include increased blood volume, improved cardiac efficiency, lower core body temperature, higher sweat rate, and enhanced thermal comfort. Increased blood volume and improved cardiac efficiency mean that an athlete can provide more oxygen to their muscles without raising their heart rate. A lower core body temperature results in athletes working more efficiently. A higher sweat rate and lower sweat threshold improve an athlete’s ability to maintain a lower body temperature through heat dissipation as sweat pulls heat off the body when it evaporates. Ultimately, improved thermal comfort enables athletes to maintain high performance levels in higher temperatures. These changes enable athletes to maintain a higher work rate while maximizing aerobic performance in hotter environments. 

Heat Training Methods

Multiple methods exist for inducing thermal strain surrounding exercise to induce the physiologic adaptations necessary for maximized athletic performance. The best supported methods are environmental training, sauna suit training, and post-training sauna utilization. The ideal method is constant exposure to increased heat during training and outside of training sessions, i.e., temporarily relocating to an environment with conditions as close to competition conditions as possible. Alternatively, using a sauna suit during training sessions or post-exercise sauna use may provide a viable option for athletes who cannot travel or for maintenance of adaptations following an initial heat training cycle. Sauna suits are made of materials that trap heat, raising the body temperature. They are typically loose-fitting and can be worn while performing nearly any form of exercise. 

A 2014 conference of sports medicine experts concluded that the optimal heat training regimen would consist of repeated heat exposure during exercise over 2 weeks. The timeline of adaptations is also an important aspect to understand. During the first week of heat training, athletes can induce a lower resting heart rate, lower core body temperature, increased sweat rate, and increased work capacity. A second week of heat training further enhances those adaptations but also induces greater aerobic performance, increased plasma volume, and increased skin blood flow (allowing athletes to cool down faster). 

Following a heat training cycle, the adaptations were found to last between 2-4 weeks, with one study estimating a 2.5% decline per day without repeated heat exposure. The first changes to occur in the heat training cycle are also the first to decay. Continued exposure to thermal strain following an initial induction period can slow the decay of adaptations, and re-acclimatization occurs more quickly than in an initial cycle. The 2014 conference ultimately recommended that athletes undergo a heat training cycle months before competition in order to determine the individual’s rate of decay and a “refresher” cycle 1-2 weeks immediately before the competition for greatest physiological advantage. 

In addition to temperature considerations, the humidity,  elevation of the competition venue, and heat index should be considered. Some evidence has shown that training at greater humidity can further increase the thermal strain and therefore physiologic advantage, as athletes are unable to dissipate heat as efficiently in humid environments. Training at altitude, as often seen in endurance athletes who undergo altitude training camps, increases aerobic capacity through increased blood volume, although these camps must be completed in the “pre-season” period for maximum effect. 

Safety Considerations for Heat Training and Competition

Signs of Heat Illness

When training and competing in hot environments, it is imperative to know the signs of exertional heat illness. These signs include a core body temperature of over 40 degrees Celsius (104 F), lightheadedness, confusion, delirium, or balance issues. Confusion, Delirium, or balance issues can indicate heat stroke, which is a medical emergency. Athletes with any concerns should evaluate their temperature via a rectal thermometer. If found to be above the threshold, the most evidence-based rapid cooling method is full-body cold water immersion. 

Environmental Monitoring

Environmental factors play a large role in the risk of heat illness in athletes. In addition to temperature, factors such as the humidity, wind speed, and solar radiant heat play into the thermal strain experienced by athletes. The American College of Sports Medicine recommends that, at a minimum, event organizers should consider dry bulb temperature (the temperature measured by regular thermometers) in addition to the humidity. This can be easily quantified through the heat index using tables such as one found here. The heat index provides insight into the temperature for shaded areas, however, athletes are often competing in the sunlight. 

The current gold standard for most sporting organizations is the wet-bulb temperature, which provides a more comprehensive tool for measuring heat stress. The wet bulb temperature measures temperature, humidity, wind speed, and solar radiation to provide a more accurate assessment of heat strain experienced by athletes competing in the sunlight. Wet bulb thermometers can be purchased online for use by event leaders, coaches, and athletes to ensure safe training and competition conditions. Guidelines from other athletic associations based on wet-bulb temperature can be found here for further guidance on competing in the heat. 

Hydration

Hydration also plays an important role in heat training. Heat training in a dehydrated state has not shown any benefit for future performance. Body water deficits of greater than 2% of body mass have been shown to impair aerobic adaptations in warm-hot conditions. Drinking for thirst has not proven to provide adequate hydration, with athletes generally exceeding body water deficits of 2-3%. The general recommendation is to consume 6mL of water per kg of body mass over a 2-3 hour period before and during exercise. The addition of 0.5-0.7 g/L of sodium for exercise over 1 hour duration, 1.5 g/L if experiencing cramping, or 90g/h for any session over 2.5 hours is also recommended. 

Exercise-induced hyponatremia is more commonly seen in endurance athletes with prolonged events (>4 hours) and overhydration without electrolytes before, during, or after competition. The amount of water consumed by the athlete must surpass the amount of fluid loss due to sweat. Symptoms include headache, vomiting, swollen hands/feet, restlessness, confusion, disorientation, or wheezing. The risk of developing exercise-induced hyponatremia due to water consumption during athletic events is low, as a high-level athlete would be unlikely to consume such volume during an event. Ideally, both electrolyte beverages and water should be available during events. However, fluid replacement plays a vital role in reducing cardiovascular strain due to dehydration, especially in hot environments. Therefore, if electrolytes are unable to be provided, water should still be made available to athletes. 

Athlete hydration can be objectively evaluated through urine specific gravity or by pre-training and post-training weight measurements. Testing strips (which can be found on Amazon) provide athletes and coaches with accurate, real-time feedback on their hydration levels during training and competition. An alternative method is to monitor athletes for changes in body weight during exercise, with the assumption that any drop in weight is due to water loss by sweat. An important aspect of this method is to weigh the athlete with minimal clothing to prevent inaccurate measurements due to sweat trapped in the clothing. A gram of weight loss is equivalent to 1 mL of sweat/water loss. For example, if an athlete were to lose 1lb of water weight during a training session, they should plan to rehydrate with 450mL of water and 272 mg of sodium to replete their electrolytes. 

To plan ahead for sweat loss in competition, pre & post-training athlete weight, fluid intake, and duration of exercise can be used to calculate sweat rate using online calculators. These can predict an athlete’s sweat rate but are limited in accuracy as training and competition conditions often differ. 

Cooling Strategies

Methods for reducing core body temperature during exercise include cooling vests, cold fluid ingestion, and cold water immersion. Cooling vests offer the advantage of reducing an athlete’s core body temperature without affecting the heat needed for optimal muscle performance. This may be an ideal option for athletes between events to limit the number of warm-up sessions needed during a competition with multiple events per day. Ice slurry brings down core body temperature to a greater degree than cold fluid ingestion. Electrolyte popsicles (Pedialyte pops) provide an option for athletes to reduce core body temperature while ingesting electrolytes. Cold water immersion provides the most evidence-based method for fast reduction of core body and total body temperature. However, the rapid cooling of muscles may limit athletic performance in events with quick interval breaks. 

Overall Key Points:

• Core body temperature over 104F by rectal thermometer indicates exertional heat illness, and athletes should undergo cold water immersion for rapid cooling

• Heat training offers athletes a physiological advantage. These adaptations include: 

• Increased hemoglobin and plasma blood volume

• Lower resting heart rate

• Lower resting core body temperature

• Lower sweat threshold & increased sweat rate 

• Greater cardiovascular efficiency & stability

• Improved thermal comfort 

• Ideal heat training occurs with a pre-season session and a “refresher” cycle 1-2 weeks immediately before competition. Methods include:

• Relocation to a hot environment comparable to competition conditions

• Creating a hot training environment

• Training in a sauna suit 

• Post-training sauna utilization

• Hydration is important for both safety and performance in the heat. Training in a dehydrated state has shown no future performance benefits but poses increased safety risks. 

• Electrolytes should be consumed especially when pre-hydrating or replenishing fluid loss surrounding training

• Fluid consumption during athletic events is important for reducing cardiovascular strain caused by dehydration

• Hydration should be objectively measured through urine testing or weight monitoring

• Drinking to thirst is an inaccurate gauge for adequate hydration 

For more information, we recommend reading the 2014 paper resulting from an expert panel on the subject of training and competing in the heat. The full paper can be read for free here. 

For specifics as to how to apply this information from the perspective of a seasoned athlete, see From the Field: Heat Prep by Brent Fikowski.