Heat and the Athlete

Heat and the Athlete

The top elite athletes from around the world recently competed in the 2024 summer Olympics. This year, athletes contended with extreme heat in addition to world class levels of competition. Temperatures in Paris during the games reached 97oF, while events in southern France experienced conditions more extreme- up to 105oF.  The extreme heat prompted safety modifications during some events, with tennis players getting extended breaks between sets and dressage riders forced to cut short warmup routines to protect their horses. Almost universally, athletic performance drops when temperatures get hot; performance can be up to 20% worse than an athlete's personal best.[1] Seeking advantage, some athletes in sailing races wore ice vests during competition while beach volleyball players cooled off with bags of ice between play. In nearly all levels of sports, from the Olympics to high school, organizers are finding it necessary to innovate new cooling methods to keep participants safe from heat injury.

Most of us will never compete at a world-class level, but recreational athletes can learn lessons from how Olympians train and compete- and stay safe- in the heat. International Olympic Committee (IOC) scientists produce an official guide to optimize performance and reduce the risk from heat injury. These recommendations are often adopted in heat policies promulgated by professional leagues, collegiate conferences, recreational, and high school athletics programs.

This article explores risk factors that increase the likelihood of athlete heat injury, referring to the Olympics to illustrate how heat affects competition and athlete safety even at the highest levels. First, we consider the individual factors that may place the athlete at greater risk of heat injury. In the second part, we’ll take a brief look at some of factors outside the athlete’s control.

Individual Risk Factors in Athletes

Every athlete is unique; not all people respond similarly to all risk factors. Studies conducted under the same environmental conditions (such as different people competing in the same race) find athletes body core temperature can vary widely. Individual risk factors for heat injury in athletes include obvious ones such as being overweight, dehydrated, unacclimatized to the heat, or poor physical condition (such as at the start of pre-season training). There are also some surprising risk factors- motivation, medication, and sickness or infection are also key individual risk factors.

Body composition: Body composition risk factors come in two forms. First, anyone with higher body mass index (BMI), i.e., overweight athletes, have a harder time dissipating heat compared to leaner counterparts. This holds true even in generally fit populations. For example, studies of Marine Corps recruits show BMI directly correlates to risk of heat injury during training. BMI aside, individuals with greater body mass-to-surface ratios have higher rates of internal heat storage, particularly in hot and humid conditions. All else equal, a taller, leaner individual has an advantage compared to a heavyset person, even if both are equally physically fit, because the first individual has more skin surface area for body heat to dissipate from. In sports like football, body composition is the primary reasons why incredibly strong but heavyset linemen are more likely to experience heat injuries than players in other positions.

Dehydration: Most athletes know proper hydration is important, but in hot conditions it is essential for performance and safety. Athletic performance decreases faster in hot conditions compared to cooler conditions even at the same level of dehydration; a dehydrated athlete’s aerobic performance has been estimated to be reduced up to 1.6% for each 1.8 degrees above 84oF when compared to the same athlete in cooler conditions. Dehydration also limits blood volume, reducing how fast heat can be pumped away from working muscles and to the skin surface to be dissipated; dehydrated athletes have higher heart rates with less blood pumped by each heartbeat. By some accounts, the volume of blood pumped with each heartbeat is reduced by up to 30% in dehydrated athletes.

Lack of Acclimatization: Heat acclimatization is the single best way for athletes to prepare for competition in hot environments. Studies have found more than half elite endurance athletes have dedicated heat acclimatization training plans; those who incorporate heat acclimatization often outperform competitors who do not. (We’ll explore heat acclimatization in detail in a later blog.)

Motivation: The American College of Sports Medicine notes the most motivated individuals tend to train and compete the hardest, maintaining high rates of metabolic heat production and “pushing through” warning signs of heat injury. Athletes respond to coaching and teammate pressure to compete harder or run faster, and most have an innate desire to excel. This makes these athletes a higher risk for heat related injury. During competition in hot conditions, even superbly conditioned, elite athletes can experience heat injury despite no other obvious risk factors. During the Tokyo Olympics, a stunning 24 marathon and race walk athletes dropped out mid-competition due to heat injury, and Spanish tennis player Paula Badosa, playing in extremely hot conditions, withdraw after a first match to in a wheel chair to seek medical treatment for heat stroke.

Medication: Some medications increase risk of heat injury. Many drugs known to increase heat injury risk aren’t common in elite athletes (e.g., drugs controlling high blood pressure), although might be used by the recreational athlete. One study found a class of drugs used to treat ADHD was associated with increased heat injuries in military recruits, and the Center for Disease Control lists 22 drug classes believed to increase risk of heat illness.

Infection: The role of infections, such as a cold, as risk factor for heat stroke may surprise some people but relies on a simple physiological principle. During an infection, body temperature is generally slight elevated to help fight off the infection. Core temperature can only increase slightly before humans potentially experience heat stroke; as a general rule, 104oF, or 5.4 degrees above normal body temperature of 98.6oF, is the threshold used to define clinical heat stroke. An athlete fighting a mild infection may start out with an elevated body temperature, diminishing the amount of additional heat the body can store before getting dangerously overheated. For example, a mild fever of 100oF may not keep a motivated runner home the day of a big race but decreases the range between starting body temperature and the threshold where heat stroke can occur. 

Risk Factors Outside the Athlete’s Control

Some factors increasing risk of heat injury are outside the athlete’s control. The environment obviously matters; while heat injury can occur in relatively cool temperatures, it is far more likely if training or competition is done in hot – and especially hot and humid – conditions. Even among similar events (such as running races), different distances carry different risk of heat injury, sometimes in an unintuitive way. The athletic season and training/conditioning plan influence athletic heat injury, as does the type and amount of athletic equipment required in a sport.

Role of the environment: This risk factor is intuitive– in hot conditions, dissipating body heat becomes difficult. Given the same physical effort, athletes typically perform better in cooler conditions, for both physiological reasons (e.g., less blood flow diverted to the skin to help dissipate heat) and psychological reasons (e.g., a natural protective instinct to reduce effort in the heat to avoid heat stroke). A plethora of medical studies explore the influence of high temperature on performance, finding run times increase for events longer than 400m and power output drops when cycling in the heat. Even World Cup soccer players run less overall, and sprint slower, in hot conditions.

Event: Athletes in some events are more pre-disposed to heat injury than others. During the Tokyo Olympic games, more than half of heat illnesses occurred during the marathon and race walking events. These events are long enough for competitors to sustain an increased core temperature and short enough to allow the continuous heavy physical exertion needed to increase core temperature above a critical threshold. Other events with high risk of heat injury have these characteristics, such as cycling, longer (>800m) track and field events, and tennis. Surprisingly, very long competitions often have lower risk of heat injury. Ultramarathon runners have lower rates of heat stroke than their marathon running counterparts because it is not physically possible to sustain the effort required to generate metabolic heat faster than it can be dissipated over very long durations. Setting aside very short duration events, it is exercise intensity, not duration, that is most correlated with heat injury.[2]

Athletic Season & Training Plan: Organizational planning is increasingly recognized as a key factor in mitigating the risk of athletic heat injury. The timing of the athletic season has an influence on athlete heat injury, but it is often during preseason training that heat injuries occur, especially if preseason workouts are held in the summer.  More than half of college football heat injuries occur during the first fifteen practice sessions, with the highest rates occurring on the second practice.  Football preseason occurs toward the end of summer when temperatures are hot, and athletes just starting the first few practices aren’t yet acclimatized to the heat. 

Athletic Equipment: Not unsurprisingly, wearing protective equipment interferes with heat dissipation. In sports like football, about 70% of the athlete’s skin surface is covered by padding or uniform. Studies have found a direct link between the amount of football equipment worn and the speed at which core temperature increases when exercising. Other outdoor sports with protective equipment requirements, such as lacrosse, are known to increase the core temperature of competitors. 

Heat Injury Policies in Sports

Athletic heat injury, and preventing it, is increasingly considered by sports organizations at all levels. As mentioned in the introduction, the IOC provides recommendations for competing in the heat to Olympic athletes. In some cases, elite level competition like Australian Open tennis matches are now suspended if conditions are too extreme. The National Football League also has heat related guidelines applying to practices. These guidelines are developed in partnership with the Korey Stringer institute, a research and outreach organization created following the death of Minnesota Vikings lineman Korey Stringer from heat stroke in 2001. The National Athletic Trainers’ Association has a position statement on heat illness, as does the American College of Sports Medicine, both recommending ways to prevent and treat athlete heat injury. Most U.S. States have some version of heat policy in place for high school athletics, although guidelines and enforcement can vary significantly.

Athletes face a variety of risk factors that contribute to the risk of heat injury when practicing or competing. This article provides a brief overview of some of those risk factors; not every athlete will deal with all (or even most) of them, but in an increasingly hotter world, understanding their contribution to heat injury and how to mitigate it can provide a competitive advantage.

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About the author: Erik is a doctoral candidate at Duke University where he conducts research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120oF) to Arctic conditions in central Alaska (-42oF).

[2] Yes, almost no one considers ultramarathons less “intense” than marathons except in a thermoregulatory context!

[1] There is evidence that for events requiring very short term, explosive effort (e.g., the 200m sprint), athletic performance benefits from warm conditions. However, the effect is usually determined based on an “above or below” temperature and not studied using extreme temperature; for example, the IOC uses above and below 77oF to illustrate that athletic performance usually decreases above this temperature.