Conditioning and physiology: how to train and compete in hot weather

Why hot-weather training is a delicate balance between staying cool and hydrated while performing at a competitive level

Alicia Filley explains why hot-weather training is a delicate balance between staying cool and hydrated while performing at a competitive level.

When training in hot weather, it’s likely that you’ll feel more sluggish. This is because your body regulates your activity level based on its ability to keep itself cool. However, whether this heat-based fatigue is a reactionary event once a critical core temperature is reached (somewhere around 40°C), or a feed-forward response where the body selects a pacing strategy that avoids the critical temperature, is a matter of debate among researchers. What everyone agrees upon is that training the body to take a longer time to reach this temperature will delay fatigue and provide a performance edge.

Excess heat generated during exercise is carried to the skin where it is lost via radiation, conduction, convection and evaporation. When nude and at rest, 60% of the body’s total heat loss comes from radiating heat in the form of infrared rays(1). Conduction is the transfer of heat from one object to another along a temperature gradient. In air, this accounts for only about 3% of the body’s heat loss, but becomes more important when exercising in water because water is a far more efficient heat conductor. Convection is what makes us feel cooler on a windy day. As air moves across the skin, heat is transferred to it.

Evaporative heat loss is the most important cooling mechanism during exercise and occurs primarily through sweating. Evaporation accounts for 25% of the heat loss while at rest in a comfortable room(1). While that percentage increases with exercise, the exact amount varies based on weather conditions and individual tendencies for sweating.

In the summer, your weather bureau probably forecasts an apparent temperature (AT) or ‘heat index’, along with the actual air temperature. The AT adjusts the ambient temperature to reflect the level of humidity. What this produces is a ‘feels like’ temperature that allows the athlete to make judgements about the activity level appropriate in that environment (see table 1). However, the AT does not take into account the effect of radiant heat or wind conditions. Common sense dictates that staying on the shady side of a course or field will reduce your radiant heat load.

Heat related illness

Heat cramps, heat exhaustion, and heat stroke are all signs of the body’s inability to cope with the heat stress of the environment. When the environmental heat load is great, the body attempts to cool itself further by sweating more. Sweat is composed primarily of water and sodium, but the exact composition of sweat varies between people and circumstances.

Doctors at the University of Oklahoma believe that those who suffer from heat cramps are ‘salty sweaters’(2). When they matched five National Collegiate Athletic Association football players with a history of heat cramps with five without a history of heat cramps, the composition of their sweat was the most significant difference between them. The players with a history of cramping showed more than twice the amount of sodium in their sweat than those who did not cramp. The researchers concluded that heat cramps are a result of the loss of sodium through increased sweating, and encourage players prone to cramping to consume healthy salty snacks or add salt to their food.

When the fluid lost from sweating is not replaced, dehydration results. This can trigger heat exhaustion, despite a core body temperature that is still within normal range. Signs of heat exhaustion include pallor, excessive sweating, and complaints of weakness, dizziness, thirst, nausea or fainting. Treatment in the field includes rest in a shaded area, cooling with water-soaked towels, and re-hydration, preferably with a sport drink to replace lost sodium. If the athlete is unable to keep fluids down, complains of other symptoms, or loses consciousness, emergency medical attention should be sought.

Heat stroke results when the body can no longer handle the heat load and the core temperature begins to rise above 40°C. Early signs of heat stroke include disorientation and memory loss. The athlete may not be sweating, and complains of chills, flushing, weakness and dizziness. Complete collapse usually follows and emergency medical care should be implemented immediately.

With heat stroke, the faster the body is cooled, the less damage occurs. To initiate cooling in the field, wrap the athlete in ice soaked sheets or towels. Place ice packs under the arms, in the groin, and around the neck and head. While heat stroke can be fatal, the prognosis is usually good if cooling is implemented immediately.
Optimising performance

To appreciate the delicate balance between the physiology of exercise and the physics of heat transfer to the environment, consider these three facts:

The resting body temperature is 37°C;
Optimal metabolic function occurs at around 39°C (hence the practice of ‘warming up’ prior to exercise);
Heatstroke occurs at body temperatures greater than 40°C(3).

The obvious question, therefore, is how do you maintain a safe body temperature while exerting your maximal effort in competition?
The first thing to do is to acclimatise yourself to the weather conditions where your competition is held. This may mean travelling to train in a similar climate. Acclimatisation requires 14 successive days of training at 50% or more of maximal effort in the heat, for 100 minutes each day(1). Simply being in the heat is not enough for the necessary physiological changes to take place and adaptations will be lost if the heat stress is no longer present (see table 2).
The second is to make sure that you rehydrate properly during activity. Dehydration increases core body temperature and negates any advantage of acclimatisation(1). Hydration should begin 20-30 minutes before the competition does so that you start your work with a positive fluid balance. For activity longer than 30 minutes you should drink at a rate that closely matches your fluid loss. Your sports drink should be composed of 6-7% carbohydrates to maximise fluid absorption and at least 50 mmol/L of sodium for optimal fluid retention(1).

The third way to make the most of your effort in the heat is by pre-cooling. Pre-cooling lowers your body temperature so that you are able to store more metabolic heat before reaching the critical temperature that signals fatigue. Although the theory has shown promise in the laboratory, practical applications are challenging due to the inability to reproduce outdoor environments in the lab, and the lack of indoor precooling mechanisms (full-body ice baths, cooling rooms, cooling jackets) in the field. Two recent studies, however, examined ‘real life’ techniques that any athlete can use.
Pre-cooling strategies

The first study was performed by researchers at Charles Sturt University in Australia who sought to bring precooling techniques to team sports players(4). Prior to a 30-minute session of training sprints in outdoor temperatures of 32.4°C with 44% relative humidity, seven male lacrosse players were either pre-cooled for 20 minutes using a cooling vest, cold towels to the neck, and ice packs to the quadriceps, or not pre-cooled. When the subjects were pre-cooled, they covered more distance during timed sprints at a moderate speed (7-15 km/h) and had a smaller rise in core body temperature with activity.

The second study, conducted at Edith Cowan University, also in Australia, evaluated the use of drinking slushy ice as a practical way to increase core temperature capacity and improve running times(5). The rectal temperatures of the runners were significantly lower after drinking the ice slurry than when they drank the cold water. Core temperatures remained lower during the first 30 minutes of running. However, at exhaustion, rectal temperatures were significantly higher after the ice slurry ingestion trial, suggesting a greater capacity for heat storage before fatigue. In addition, after ingesting the ice slurry, the runners ran longer than when they drank the cold water.

Considering the body of work conducted in the laboratory that shows the effectiveness of pre-cooling on performance, and the ease and safety of the techniques, there’s no harm in implementing these strategies into your own training regimen and monitoring your results.

For 20 minutes prior to activity in the heat, apply cold packs or towels soaked in ice water to your neck, underarms, and quadriceps. Make your own ice slurries by blending crushed ice with your favourite recovery drink in the blender, or pre-freezing your recovery drink for about one hour prior to ingestion. Remove drink from the freezer and shake vigorously to break up the ice crystals. The objective is to drink the actual ice crystals so that heat is required to melt them internally, thus making more room to store the metabolic heat generated during exercise.

Alicia Filley, PT, MS, PCS, lives in Houston, Texas and is vice president of Eubiotics: The Science of Healthy Living, which provides counselling for those seeking to improve their health, fitness or athletic performance through exercise and nutrition