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  Last updated: 6/13/2014

Heat Illnesses
Hyperthermia & Hypothermia

Although hyperthermia is generally ones first thought when discussing heat issues in athletics, hypothermia is the other end of the spectrum of what I will call "heat illnesses". The "heat" equation for an individual at any point in their activity is the sum of heat production from metabolism plus/minus the heat gained/lost from the environment. Heat energy is a byproduct of working muscles. Conversion of ATP to ADP and then into energy delivered to the pedals is a relatively inefficient process with ~70% of stored energy ultimately released a byproduct - heat. (And during exercise heat production is increased approximately 20 times over your basal level.)


When there is an imbalance and heat production > heat loss, hyperthermia results. Muscle cramps may be the first sign of hyperthermia. Next in the early spectrum of heat exhaustion is feeling hot, fatigued, and nauseated, but still sweating. And finally there is heat stroke when you stop sweating, your core temperature begins to rise (often >104 F), and nervous system symptoms (altered state of consciousness or confusion) appear. It is potentially life threatening and heat stress with exercise is associated with an increase sudden death as late as the next day. (In military recruits there is an increase in exercise related deaths the morning after intense exercise, especially in hot and humid conditions).

Heat gain from your surroundings is dependent on the ambient temperature. As problems during exercise are usually related to shedding excess heat produced, let's focus on the ways your body can eliminate excess heat energy in order to keep your temperature in a normal range. Heat "unloading" occurs via:

  1. radiation - an inefficient method
  2. convection - more efficient when the air is moving over your body as in cycling
  3. conduction - the way we lose heat while swimming
  4. sweating - by far the most effective way to shed heat

Sweating is the only effective way to cool your body once the ambient temperature tops 92 degrees Fahrenheit. And as it requires evaporation, its effectiveness drops off quickly at high humidities, especially above 90% humidity. You have anywhere from 2 to 4 million sweat glands and can lose up to 2 to 4 liters of fluid an hour. In fact Alberto Salazaar was estimated to have lost 1 pint every 11 minutes in the 1984 Olympics. The heat energy loss from sweating is in the neighborhood of 580 Calories per hour.

Another potential risk associated with heat illness is hyponatremia. When you are losing sodium in sweat (15 - 50, avg 20 meq per liter, depending on sweat rate and acclimatization) and replacing it with pure water, your serum sodium concentration can drop below normal (hyponatremia) with serious consequences. Recent studies have shown that there are "salt losers", those with high concentrations of salt in their sweat (those with salt crusted on their jerseys after an event), who may need even more than the normal salt replacement. It has been speculated that their tendency to salt loss may be the result of these athletes carrying a single gene for cystic fibrosis (present in 5% of Caucasians).

How do you protect yourself from hyperthermia? There are really just a few things you can do:

Here are a couple of riding tips if you are going to be in a competitive event.


Hypothermia is a decrease in your core body temperature. It is always a risk for the cyclist, especially in that combination of conditions that often occur with winter training.

What are the warning signs?

ADDITIONAL ENERGY REQUIREMENTS - (at rest) in a cold environment

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Cycling Performance Tips
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