The Effect of Heat and Humidity on Athletic Performance

By Intermountain Healthcare

When coupled with high exercise intensity, high heat and humidity can lead to decreased athletic performance, particularly for longer duration athletic events like running or cycling events, football games and, of course, World Cup soccer matches!  

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Games in Manaus, Brazil, a city along the famous Amazon River, have been played in temperatures in the high 80’s and humidity levels approaching 70%.   This makes for very difficult playing conditions, and TV images of highly conditioned athletes cramping late in a game have been plentiful. 

Even NBA basketball fans have seen the effects of high heat and humidity.  During the NBA Finals in San Antonio the arena’s air conditioning went out and temperatures neared 90 degrees.  By the fourth quarter, at a very critical point of the game, Miami Heat star LeBron James was forced to the sidelines by debilitating cramps. 

High heat and humidity lead to two problems in the exercising body: 1) increased core body temperature and 2) dehydration.  Increased body temperature (hyperthermia) leads to decreased muscle endurance, which means the muscle’s ability to contract repeatedly or in a sustained manner over long periods of time.  High core temps also cause a shift in energy production from aerobic to anaerobic mechanisms, which means the body has to use up its muscle energy stores more rapidly.  Unfortunately, during a longer athletic event, the rate of adding energy (sports drinks, energy bars, gels, etc) can’t keep up with the rate of losing energy when heat and humidity are high.  Finally, high body temperature causes a decrease in blood flow to the heart as blood pools in the limbs.  If the heart doesn’t get as much blood, it can’t pump as much oxygenated blood back to the muscles.

Dehydration often occurs long before some athletes realize it or before cramps set in.  Athletes can lose as much as 2 to 8 % of their body weight during high intensity exercise, and the rate of fluid absorption from the gut just can’t keep up with that rate of loss.  Dehydration causes a decrease in VO2max, which means the body can’t utilize oxygen as efficiently to provide energy.  Dehydration also contributes to the decrease of heart blood pumping mentioned above.

So what can be done to combat or prevent the effects of high heat and humidity? 

If you paid attention to news reports building up to the start of the World Cup, you heard about teams from all over the world flying to places with higher heat and humidity to train.  Several teams trained in Miami, some in Latin America, and a few went right to Brazil.  Acclimatization to higher temps and humidity can occur fairly quickly, as quickly as 7-10 days.

Another prevention for heat/humidity effects is conditioning. Better-conditioned athletes suffer less performance loss in high heat and humidity because they have a higher blood volume, better VO2max, sweat rate and more efficient use/replacement of energy stores.  World Cup athletes have been training for several months (in addition to playing in their normal club/team roles) in preparation for this major event.

Fluid replacement is critical for events in high heat and humidity.  Fluid replacement starts before an event, continues during it, and doesn’t stop until long afterwards.  Drinking to thirst and keeping urine clear (not dark yellow) are good measures.  Wearing light-weight, light-colored clothes of open-weave natural fibers (cotton, wool) or fluid-wicking fibers help increase evaporation and cool the body. 

Even here in Utah where the humidity is low, we’re susceptible to heat-related performance drop or even heat illness if we’re not careful.  Be smart and enjoy the sun. Then, after a great, hot workout, find a cool drink and a TV positioned right below a soothing fan to enjoy some incredible World Cup matches!