Thankfully we are finally having a good summer this year!  As enjoyable as it is, continuous heat and humidity don't come without problems.  This month we are going to look at why we should be aware of the changes that occur in our bodies when we exercise in the heat and how they affect our performance. 

Our bodies are elaborate finely tuned air conditioning units attempting to maintain status quo 24-7.  Unbeknown to us, it tries to keep our internal body (core) temperature at 37.2°C, and skin temperature at 35°C.  When we exercise our metabolism increases and we generate heat that needs to be lost to keep status quo.  We lose heat to the environment, however if heat production is greater than heat loss (such as common in hot, humid environments) this causes increases in body and core temperature.  The change in core temperature is influenced by metabolic rate, and evaporative heat loss from breathing and through our skin.  Skin temperature rises with increased heat production, and this is dependent on environmental temperature, evaporative cooling, degree of dehydration and rate of blood flow to the skin.

During prolonged exercise in hot humid environments sweat rate can increase to 2-3L/hr.  Fluid losses come from blood plasma, interstitial fluid, and intracellular fluid.  As the body becomes dehydrated, sweat rate decreases, further increasing body temperature, and increasing heat stress.

Affect on Performance:

During exercise an essential function of the cardiovascular system is to deliver blood to the working muscles.  The heart does this by pumping blood out of the left ventricle to the muscles, and the volume of blood it pumps each beat is called "stroke volume."  During exercise, our muscles demand oxygen and therefore blood to deliver the oxygen.  This means that stroke volume must increase and it does to a certain extent.  However, the left ventricle is only so big and this limits the volume of blood pumped out of it per beat, and our heart rate rises to continue to increase blood flow to our working muscles.  This relationship is called "cardiac output" or formally the amount of blood pumped by each ventricle per minute.

Cardiac Output = Stroke Volume x Heart Rate

So the question is what has this have to do with exercising in the heat?  As you dehydrate through sweating you loose blood volume, which decreases stroke volume.  To maintain the same exercise intensity and therefore the same cardiac output, heart rate must increase.  This is easily identified on a long hot ride at this time of year when fluid intake and absorption is insufficient, as throughout the ride heart rate will gradually increase when cycling at the same speed.  This makes heart rate training zones useless as they can change dramatically, as shown in table 1.  This is where power monitors are an excellent tools for training zones.

Table 1: Change in Heart Rate According to Body Weight

Along with heart rate, muscle metabolism is affected.  Muscle glycogen breakdown increases so we accumulate lactate earlier than normal and recruit fatigueable fast twitch fibres earlier, limiting our performance.  Our maximal capacities are decreased as it only takes 3% dehydration to decrease our VO2max (this is only 2kg in a 70kg person).

Danger Zone:

When our bodies become overly heated we can experience heat stress, which not only affects our performance but can endanger our lives.   As we get dehydrated our neural reflexes respond to our decreasing blood volume by decreasing blood pressure, which decreases heat transport to the skin, and skin temperature can decrease (cold and clammy), despite our internal ovens cooking.

Excessive heat storage is called hyperthermia, and results in cardiovascular complications, central nervous system dysfunction and motor function impairment.    Hyperthermia can lead to a series of cellular changes that increase risk for organ failure and even cause death.  Hyperthermia can occur without dehydration, however this exacerbates it and symptoms to look out for include lethargy, dizziness, and lack of coordination. 

Heatstroke is the result of an increase in core temperature (40-44°C) which impairs central nervous system dysfunction, and damages body tissues.  Symptoms include disorientation, confusion and even coma. 

Heat Tolerance and Acclimation:

We all vary in our ability to tolerate the heat and it is possible to adjust to it through the adaptations listed below.  These adaptations are retained for about 3 weeks.  Typically improved heat tolerance can occur in 4-10 days of training in hot environments. 

Adaptations to Exercising in Hot Environments that Improve Exercise in the Heat:

• Increased plasma volume
• Increased blood volume
• Increased venous return
• Increased cardiac output
• Decreased submaximal heart rate
• More sustained sweat response
• Increased capacity for evaporative cooling
• Earlier onset of sweating
• Improved evaporative cooling.
• Decreased osmolality of sweat
• Electrolyte conservation (mainly sodium)
• Decreased muscle glycogen use
• Decreased likelihood for muscle fatigue during prolonged exercise.

 Recommendations for Training and Racing In the Heat:

1. Hydrate before training and racing with water or a diluted carbohydrate solution.

2. During exercise fluid ingestion is obviously helpful to maintain hydration levels for evaporative cooling.  Carbohydrate enriched drink solutions increase intestinal absorption and are therefore better than water alone.  Drinking too much water can cause voluntary dehydration (hyponatremia) which lowers serum sodium levels.

3. If you tend to have a high sweat rate and suffer in the heat, it is advisable to train in the heat (don't avoid it) to allow your body to adapt to exercising in such conditions.  If you only train in the morning or night when it is cool and then expect your body to race on the weekend in hot, humid condition you are more likely to experience heat stress.

4. Weighing before and after exercise (taking into account fluid intake during exercise), to determine re-hydration needed.  Current recommendations for replenishment are 1 to 1.5 times the fluid loss.  For example if you weigh 70kg before a race and then 68kgs after, and drank 2L of fluid throughout it, then you have in fact lost 4kgs of fluid.  You need to then replenish 4 to 6L of fluid after the race to restore your body fluid levels.

References:

Burke, E.R.  (2003).  Serious Cycling, Human Kinetics, Illinois.

Haymes, E.M., & Wells, C.L.  (1986).  Environment and Human Performance.  Human Kinetics Illinois.

Martini, F.H.  (1995).  Fundamentals of Anatomy and Physiology.  Prentice Hall, New Jersey.

Robergs, R.A., Roberts, S.O.  (1996).  Exercise Physiology.  Mosby Inc.