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Exercise in the Heat

Of all of the extreme places around the world in which we can perform physical activity, the hot and humid environments, including coastal, equatorial and jungle regions, are some of the most challenging, perhaps second only to very high altitude. The two greatest concerns for runners in hot and humid environments are temperature regulation (i.e. preventing hyperthermia) and hydration (i.e. preventing dehydration). 

Although these two conditions can occur independently, the underlying mechanisms for one are likely to contribute to the other, with increases in temperature leading to increased water losses through sweating, and dehydration reducing the body's ability to dissipate heat. Both conditions lead to decreases in performance and, ignored or inadequately addressed, can have disastrous consequences for health, ultimately leading to death if severe and left untreated.

Balancing Heat Gain & Loss

Heat gained during exercise must be matched by heat lost, in order to preserve the body's core temperature, which averages about 37 degrees Celsius at rest. If the core temperature rises above 38 degrees, then the athlete will likely begin to feel tired and performance will decline. If this rise is not corrected, temperature will increase above the individual's ability to tolerate it, and performance-inhibiting exhaustion will result.  If the runner ignores the internal warning signs of exhaustion, he or she will probably collapse, as the body elects to end the cause of heat rise regardless of the runner's intention to continue. 

Heat gain is affected by external factors, such as the ambient temperature, wind speed, humidity, solar radiation, ground thermal radiation, and clothing. Internal factors influencing heat gain include metabolism and muscle activity. The efficiency of sweating, which is our primary means of cooling, is compromised by the saturated air in a humid environment. 


Depending upon the intensity, physical activity increases metabolism 5-15 times above normal resting levels. Approximately 70-90% of the energy produced is heat, which needs to be dissipated to maintain normal functioning. The proteins and enzymes required for life processes within cells begin to slow down as temperature increases towards 40 degrees Celsius. 


Heat is usually lost in our breath and via the skin.  The heat in the core of the body moves into the blood and is transported to vessels near the surface of the skin, from where that heat is then transferred into the cooler outside air, via a radiator effect. The problem in a hot environment is that the outside air is hotter than our skin temperature, which means our body's initial mechanisms for cooling (via breath and the skin) do not work.  


The only means by which the body can cool itself during exercise in a hot environment is via sweating, and this mechanism is far less effective if the air is humid. Sweating works by moving water from the blood and glands out onto the skin. From there the water evaporates into the surrounding air, and the energy involved in this exchange has a cooling effect on the skin.  The problem is that if the air is already saturated there is nowhere for the sweat to evaporate to, and no cooling reaction takes place. 


If the body cannot cool itself, the core temperature will continue to rise unless deliberate action is taken. This might involve reducing exercise intensity, ceasing exercise, or by entering a cooler environment (such as into shade) or by soaking oneself in cold water. The temperature of cold water should not be so cold (relative to skin and body temperature), as to illicit cold-water shock, which can occur if the body is subjected to a much colder temperature, and can cause shivering to re-warm, potentially with unusual cardiac responses, and even resulting in a heart attack.


Sweat Composition

The body attempts to maximise cooling by increasing the rate of sweating. So, the body produces sweat in an attempt to cool itself, but in a humid environment there is a very limited effect. In this case, not only should the athlete be concerned about the amount of heat they are gaining through exercise, but he or she should also be concerned about the amount of water that they are losing in their sweat. 


The liquid component of blood is called plasma, which is more than 90% water, and contains various dissolved nutrients and waste products.  The remainder of blood is made up of the blood cells (red cells, white cells and platelets). When we sweat, we begin taking some of that plasma component away, and we have to compensate for it. 


The amount of blood in the body, and its concentration of sodium relative to water (osmolarity), is continuously monitored via numerous receptors. When changes occur that deviate from the body's optimal 'set-point' (i.e. blood pressure, osmolarity, etc), receptors detect this and send out signals to respond.  If plasma levels drop (or if the sodium concentration increases), we feel thirsty, our mouth becomes dry, we do not have a desire for salty foods, and we produce a more concentrated urine.  If plasma levels have increased (or if sodium levels drop), the opposite responses occur. 


If the responses to reduced plasma volume or increased osmolarity are insufficient to bring them back to within their normal range, the blood can be 'topped up' by water that exists in and around the cells of the body. When this happens over a short duration there is unlikely to be any ill-effects.  However, prolonged sweating at high rates will have the potential to negatively impact upon those cells, potentially compromising their ability to function (ultimately, this is the reason severe dehydration is life-threatening). 


Sodium, potassium, chloride and other electrolytes are all kept within very particular concentrations within and without the cells of the body. If water is lost from the blood, or the cells or spaces between the cells, this will affect the concentration of electrolytes in those areas. To help prevent imbalances, electrolytes are lost in the sweat to maintain internal balance (homeostasis).  That is, although water levels remain low (and may be continuing to fall), the concentration of that water and the electrolytes is preserved at close to normal levels. 


The kidneys play a key role in regulating both water and electrolyte balance, preserving one and eliminating the other in urine, as required. Again, there is a potential for prolonged exercise to cause such high losses of both that the kidneys fail to maintain balance. This may be further complicated by taking very high doses of sodium or other electrolytes relative to actual losses, and/or relative to the water that is lost. 


Consuming too much water, sodium or other electrolytes can alter the normal concentration of blood and cell fluids, and with prolonged exercise and too high an intake of electrolytes or water, health can become severely compromised. This might initially be noticed via a general feeling of ill-health, or by increased muscle cramping, and will lead to deterioration in performance and health.


Cardiovascular Effects of Dehydration

Unless lost water is replaced, the volume of blood in the body will decrease. This has serious repercussions for the whole cardiovascular system. Our heart rate is determined by how much blood our heart needs to pump around the body each minute. If there is less blood, the heart has to pump faster to compensate and ensure all cells, tissues and organs receive sufficient oxygen, nutrients and have the necessary removal of wastes. Aside from the potential to negatively impact on health, there is a clear mechanism here for how running performance will suffer, as muscles fail to receive sufficient oxygen and nutrients to maintain performance, or heart rate approaches levels sufficient for exhaustion.  


If we are used to exercising at a particular workload (i.e. pace), then the heart will now have to beat faster to be able to maintain that workload, or else we will have to slow down in order to maintain the usual exercising heart rate. Many runners are used to using a heart rate monitor to maintain exercising heart rate below whatever level causes exhaustion. This is their 'target' exercising heart rate, or heart rate range.  If dehydrated, that heart rate will be reached at a slower speed, or in a shorter period of time. 


So, we are at risk of exhaustion as our temperature rises above normal, and we are at risk of exhaustion due to an increasing heart rate.  Not only this, but more and more blood is being directed to the skin, so as to aid cooling. In doing so, blood flow to the organs and working muscles is reduced, which creates an additional limiting factor to performance. The increased blood flow to the skin also leads to a decrease in blood pressure close to the heart, resulting in a further increase in heart rate to compensate. 


The increased heart rate is an attempt to maximise cooling whilst still preserving other requirements, which in our case includes exercise performance. However, the attempt by the heart to preserve the total amount of blood pumping eventually fails, and our cardiac output decreases. Our heart is beating faster, but this becomes insufficient to meet our exercising requirements for exercise. As a general rule, heart rate will increase by 4 beats per minute, for every 1% increase in dehydration. 

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