– Written by Mariem Labidi, Qatar and Sebastien Racinais, France

INTRODUCTION

Significant international sports competitions, including the Olympic Games are held in hot and possibly humid environmental conditions. Environmental conditions at sporting events are commonly estimated using the Wet-Bulb-Globe-Temperature (WBGT) index. The Wet-Bulb-Globe-Temperature (WBGT) index is derived by integrating measurements from three sources: the dry temperature (obtained via a standard thermometer), the wet-bulb temperature (which reflects the air’s efficacy in evaporating water, contingent on its relative humidity and air movement), and the globe temperature (a measure of solar radiation).

The environmental conditions are important as they will determine the ability to dissipate the heat produced by the muscle contraction into the environment. Indeed, in humans, the core body temperature typically stands at around 37°C, with muscle and skin temperatures averaging approximately 35°C and 31°C, respectively, under resting conditions in a temperate environment. Though, during activities like running or race-walking, muscular contractions generate significant heat, leading to a substantial rise in muscle temperature¹, which in turn elevates core body temperature². This heat is expelled into the environment through the skin via sensible (e.g., convection and radiation) and insensible (evaporation) heat loss mechanisms³ (Figure 1). However, in hot ambient conditions, the temperature differential between the skin and the environment is reduced, or may even be reversed, making sweat evaporation the primary mode of heat dissipation. Consequently, in hot and humid conditions, the body’s capacity to dissipate heat during exercise is compromised, resulting in a greater increase in core body temperature compared to cooler environments for the same level of physical activity.

An elevation in muscle temperature, achievable through warm-up, confers various performance advantages in explosive athletic disciplines like sprints, jumps, or throws⁴. In contrast, during prolonged physical activities, the increases in skin blood flow and perspiration required to thermoregulate^5,6 leads to an increased cardiovascular load⁷, necessitating a reduction in absolute work rate, such as speed, to sustain a given relative exercise intensity⁸. In essence, while warmer ambient conditions may enhance performance in short, explosive events, they can progressively impair performance in events of longer duration⁹. Nonetheless, the negative impacts of heat stress in endurance events can be mitigated through strategies like heat acclimation. The following recommendations aim to optimize performance and minimize heat illness risk during athletic events in hot conditions.

BEFORE THE EVENT

Essential Takeaway: In the weeks or months leading up to an endurance race in hot conditions, the primary emphasis should be on heat acclimatization. This is accomplished through repeated exercise-heat exposures that elevate both body core and skin temperatures, and induce substantial sweating.

Methods

There exists a diverse array of techniques to raise core and skin temperature and provoke sweating. Natural methods, such as conducting a training camp in hot ambient conditions, fall under the category of heat acclimatization¹⁰ (Figure 2). On the other hand, artificial methods are termed heat acclimation, which can be active (like training in a hot room) or passive (such as taking a hot bath). The optimal approach is to train in an environment similar to that of the forthcoming competition. However, this might not always be feasible. In cases where an athlete is unable to train in hot conditions prior to a competition in similar conditions, they can resort to artificial heat exposure methods. These include training in a hot room, sauna bathing, or taking a hot bath following training in thermoneutral conditions.

The key objective for inducing acclimation is to increase core and skin temperatures, stimulate extensive sweating, and boost skin blood flow¹¹. The most specific adaptations are achieved by training in heat, with daily heat exposure facilitating the quickest adaptations. Each heat training session should span 60-90 minutes¹², while other sessions can occur in temperate conditions. It’s crucial for sleep and recovery periods to be in cool environments. During heat training sessions, relative training intensity can be regulated by heart rate to maintain the appropriate training stimulus throughout the acclimation process. Initially, absolute training intensity may be lower but should progressively increase over a two-week period. If high-intensity, neuromuscular-focused work is included in the heat training sessions, it should be performed at the start of the session, before athletes experience elevated temperatures.

Timeline

Athletes typically possess a certain level of heat acclimatization from their regular training, even in cooler conditions. Nonetheless, a dedicated period of heat acclimatization is beneficial. The duration needed to achieve optimal acclimatization can vary, but most adaptations occur within 7-10 days, with 14 days being best for most athletes¹². Therefore, it is recommended for athletes to train in an environment similar to the competition conditions for two weeks prior to participating in events in hot and/or humid environments. It’s crucial to understand that most heat acclimatization adaptations diminish after 1-2 weeks, although some benefits can persist for up to a month¹³. The rate at which these adaptations are lost might be reduced through continued training and regular heat exposures after the initial acclimation phase. Notably, if re-acclimation is needed during this period, it tends to occur more quickly than the initial acclimatization process¹³.

Adaptations

Exposure to heat stress can significantly impair endurance performance; however, this impairment can be progressively mitigated through proper heat acclimatization¹⁰. Notably, the efficacy of heat acclimatization in enhancing performance in hot conditions surpasses other strategies, such as high-altitude training. Additionally, heat acclimatization may confer a protective effect against heat-related pathologies. Therefore, it is imperative to prioritize heat acclimatization in preparation for competitions in potentially hot and humid conditions, regardless of the anticipated level of heat stress. It is important to note that acclimatization to heat does not hinder performance in cooler climates and may, under specific conditions, actually enhance it¹⁴. The most apparent physiological adaptations to consistent heat training include an elevated rate of perspiration, a reduction in heart rate for a given level of exertion, improved electrolyte conservation, and a lowered core body temperature.

THE DAYS BEFORE THE EVENT

Essential Takeaway: Central to the preparation for an endurance race in hot conditions are two essential concepts: taper and hydration.

Taper

Event preparation in hot conditions adheres to standard protocols, with an added focus on heat acclimatization. Athletes typically reduce training intensity before a competition, a process known as tapering. This necessitates a strategic approach to heat acclimatization, making it impractical to solely rely on a two-week intensive acclimatization immediately before the event. A more effective timeline includes an initial acclimatization phase a few weeks prior, with maintenance throughout the tapering period by using passive heat acclimation methods such as hot water immersion or sauna sessions for 30-40 minutes either before or after training for a couple of days per week. These methods are particularly beneficial as they capitalize on the elevated core body temperature induced by training, potentially enhanced by wearing extra clothing.

The stimulus for heat acclimatization is further amplified by training with additional layers of clothing. For optimal adaptation, the water temperature for immersion techniques should be maintained at approximately 40ºC, which can be comfortably endured and easily monitored using a floating pool thermometer. While not as directly targeted as exercise-induced heat acclimatization, these heat acclimation methods can be effectively integrated into an athlete’s schedule, accommodating both the tapering phase and travel arrangements¹⁵.

Hydration

Efficient heat dissipation primarily depends on the evaporation of sweat¹⁶. Excessive sweating, however, can lead to progressive dehydration unless fluid loss is adequately replenished. Severe dehydration can exacerbate the increase in overall body temperature and hinder the performance of prolonged exercise. This effect is partly due to dehydration’s impact on cardiac function, as it becomes more challenging to sustain blood pressure and ensure adequate blood flow to both the working muscles and skin for heat loss. Consequently, maintaining hydration before, during, and after exercise is crucial for optimal performance and safety in hot conditions, particularly during the period of heat acclimatization, which is characterized by an elevated sweat rate. Additionally, it is important to recognize that a sudden increase in fluid intake may lead to increased urine output, and the body may require a few days to adjust to this change.

For individuals who experience heavy or ‘salty’ sweating, sodium (salt) supplementation is advised during exercise that exceeds one hour in duration. It is beneficial to increase sodium intake both before and after training or competing in hot weather. Athletes who can tolerate it might use electrolyte tablets or a small amount of salt during events. Additionally, it is recommended to incorporate 30–60 grams of carbohydrates per hour in drinks for training sessions lasting over an hour, and up to 90 grams per hour for events exceeding 2.5 hours. This carbohydrate intake can be managed through a mix of fluids and solid foods. During the acclimatization phase, recovery drinks should include a combination of sodium, carbohydrates, and protein to facilitate optimal recovery. Milk is notably effective as a recovery drink. The most effective method of rehydration involves consuming fluids alongside food, particularly salty foods.

THE HOURS BEFORE THE EVENT

Essential Takeaway: Athletes must adapt their warm-up procedures and hydration strategies prior to participating in an endurance event in hot conditions.

Warm-up and Pre-cooling

Athletes need to reduce unnecessary heat exposure and accumulation before the commencement of an event. It’s advisable for them to conduct their warm-up routines in shaded areas if feasible. They should also consider combining external pre-cooling methods like ice vests, cold towels, or fanning with internal methods such as consuming cold fluids or ice slurries. An effective and practical method is using commercially available ice-cooling vests during warm-up. These vests provide efficient cooling without adversely impacting muscle temperature and functionality. During the event, athletes should protect their eyes by wearing UV ray-blocking sunglasses with a dark tint, such as grade 3, and safeguard their skin with non-greasy, preferably water-based, sunscreen as oil-based sunscreens may inhibit sweating. Light-colored clothing can also help minimize the impact of the sun’s radiation, but it’s important that the clothing does not hinder the evaporation of sweat. Techniques like self-dousing with water or other cooling strategies often rely more on an individual’s perceived benefit than on scientifically proven effectiveness. It’s crucial that any cooling method be trialed and tailored during training rather than during competition, to ensure it does not disrupt the athlete’s performance.

Hydration

For exercise sessions lasting less than 1-2 hours in cool environments, drinking according to thirst is generally sufficient. However, for activities exceeding 90 minutes, particularly in hot conditions or during high-intensity exercises with significant sweat rates, a pre-planned hydration strategy can be crucial for optimizing performance. This is especially relevant when there’s a need for carbohydrate intake at a rate of about 1 gram per minute. Individuals with high sweat rates, or those particularly focused on exercise performance, should assess their sweat rates under specific exercise conditions (such as intensity, duration, and environmental factors) to effectively tailor their hydration strategies. This assessment helps in determining the optimal fluid and electrolyte replacement necessary to maintain performance and prevent dehydration. It’s important for these individuals to customize their hydration strategy to prevent substantial body mass losses, such as those exceeding 2-3%. However, it’s crucial to keep in mind that individual hydration plans should not exceed the body’s maximum fluid absorption capacity, which is approximately 1.2 liters per hour. Additionally, athletes must avoid over-hydration, which can lead to hyponatremia - a potentially severe condition that can be dangerous and may even be fatal. Simple methods like weighing oneself before and after exercise or evaluating the color of the morning urine (first void) can be effective in assessing fluid losses through sweating. These techniques help athletes estimate their hydration needs and status, aiding in the development of an effective hydration regimen.

FROM PREVENTION TO REACTION

Key Takeaway: Athletes experiencing exertional heat stroke can generally recover without lasting effects if their extremely elevated body temperature is reduced to a core temperature below 40°C (104°F) within 30 minutes.

However, athletes may face permanent disability or even risk death if treatment for exertional heat stroke (EHS) is delayed beyond an hour. Adhering to the following four clinical management principles for EHS is crucial for improving patient outcomes:

Principle 1 - Early Recognition:

Competent medical personnel are essential for the immediate identification of exertional heat stroke (EHS) in athletes who collapse or struggle during intense exercise in hot conditions. This prompt recognition is vital to minimize the duration of extreme hyperthermia. Additionally, these professionals must differentiate EHS from other medical conditions that can present with mental impairment during intense heat exposure. This includes ruling out issues such as cardiac conditions, asthma, exertional hyponatremia, other heat-related illnesses, exertional sickling, and diabetes. Such thorough assessment is crucial for accurate diagnosis and effective treatment.

Principle 2 - Early Diagnosis:

For the swift diagnosis of potential exertional heat stroke (EHS), it’s crucial to assess the core body temperature using a reliable device. When athletes have been engaging in intense physical activity in hot conditions, measuring the rectal temperature is essential for accurately determining severe hyperthermia. The presence of Central Nervous System (CNS) dysfunction, which can manifest as confusion, altered consciousness, coma, convulsions, agitation, combativeness, or disorientation, coupled with a rectal temperature exceeding 40.5°C (>105°F), is indicative of an EHS episode. Such instances require immediate attention and intervention.

Principle 3 - Rapid Cooling:

The prompt cooling of patients with exertional heat stroke (EHS) is vital; the critical factor influencing the outcome of EHS is the duration for which the individual’s core temperature remains above 40.5°C. This makes the immediate recognition and accurate diagnosis (using rectal temperature measurement) of EHS crucial for initiating rapid cooling procedures. Cold Water Immersion (CWI) is the most effective method for achieving the quickest cooling rates and should be the preferred method for treating EHS¹⁷. It is important that CWI facilities are readily available at controlled athletic venues, including training sites, endurance sports events, and competitions in warm or hot climates. Here are a few practical tips to enhance the effectiveness of rapid cooling strategies:

Stirring the Water: Consistently stir the water during the cooling process. This action helps maintain a uniform temperature throughout the water, enhancing the cooling effect on the body.

Maximizing Skin Surface Area Exposure: Try to cover as much of the patient’s skin surface area as possible with the cooling water. The more skin that is exposed to the cold water, the more effective the cooling.

Stabilizing the Patient: Drape a sheet under the patient’s armpits to stabilize them in the tub. This ensures safety and prevents any potential injuries during the cooling process.

Continuous Temperature Monitoring: Utilize a rectal thermistor to accurately monitor the patient’s core temperature throughout the cooling process. This allows for precise adjustments to be made as needed.

Optimal Water Temperature: Maintain water temperatures in the range of 10 to 15°C. While a variety of water temperatures can be effective for cooling, this range is generally considered optimal for rapid reduction of core body temperature without causing discomfort or shock to the patient.

Principle 4 - On-Site Cooling: Cool First and Transport Second:

A critical aspect of effectively managing exertional heat stroke (EHS) is the principle of “cool first and transport second¹⁸.” This approach greatly enhances the chances of a successful outcome. Delaying on-site cooling to wait for an ambulance and subsequent transport to a hospital can result in the loss of crucial time for aggressive cooling management, often exceeding the recommended 30-minute window for reducing hyperthermia.

It is therefore imperative for event and team medical staff, as well as local ambulance and hospital services, to understand, agree upon, and practice this “cool-first and transport second” strategy prior to any sporting event in hot conditions. This coordinated approach ensures that EHS patients receive immediate and effective cooling treatment on-site, which is essential for minimizing the risks associated with prolonged hyperthermia.

Mariem Labidi Ph.D.

CME/CPD Coordinator

Aspetar Orthopaedic and Sports Medicine Hospital

Doha, Qatar

Sebastien Racinais  Ph.D

Research Engineer in performance and  environmental stress

CREPS

Montpellier, France

Contact: mariem.labidi@aspetar.com

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Header image by RUN 4 FFWPU (Cropped)