How to Ride Safely in the Heat

We’re well into summer now and as enjoyable as it is, continuous heat and humidity don’t come without problems.  We need to be aware of the changes that occur in our bodies when we exercise in the heat, how they affect our performance and how to take measures to keep our bodies performing well. 

Air Conditioning

Our bodies are elaborate finely tuned air conditioning units attempting to maintain status quo 24-7.  Unbeknown to us, they try 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 the status quo.  If heat production is greater than heat loss (such as is 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.   As the body becomes dehydrated, sweat rate decreases, further increasing body temperature and increasing heat stress.

Effect on Performance

During exercise an essential function of the cardiovascular system is to deliver blood and oxygen to the working muscles.  As you dehydrate through sweating you lose blood volume and the volume of blood pumped per beat (stroke volume). 

To maintain the same exercise intensity and therefore the same cardiac output your 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 your heart rate will gradually increase when cycling at the same speed.  This can make heart rate training zones useless as they can change dramatically.

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.   

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’s possible to adjust to it through acclimation and the resulting adaptions listed below.  These adaptations can be 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 include:
 

  • Increased plasma and blood volume
  • Increased venous return and cardiac output
  • Decreased sub-maximal heart rate
  • Earlier onset of sweating and more sustained sweat response
  • Increased capacity for and improved evaporative cooling
  • Electrolyte conservation (mainly sodium)
  • Decreased muscle glycogen use and likelihood of 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.  Note 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 conditions 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 or training ride 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.
 

Amy Taylor is an Auckland based Exercise Physiologist with a MSc(Hons) in Sports Science. One of the founders of Kinetic Edge Training Technology (www.kecycling.com) she coaches recreational to World Champion cyclists and has been cycling herself for 16 years and can be contacted on +64-9-368-7819.