Thermal Threshold

by Arnie Baker

The human body is about 25 percent efficient. Said differently, roughly three-quarters of the energy used to power muscles is lost as heat.

To avoid overheating, the body must get rid of this excess heat. As the body heats, more blood flows to the skin to improve heat loss.

The body cools through evaporation, conduction, and radiation.

At low levels of intensity, relatively little heat is produced, and cooling is usually not a problem. At high levels of exercise intensity or at moderate levels of intensity in the heat, cooling in the body can be a problem. Overheating can limit performance. Overheating can result in medical illness or death.

At low levels of intensity, heart rate parallels work intensity and stays constant. At high levels of intensity, or moderate levels of intensity in the heat, heart rate rises with steady effort—an effect known as cardiac drift.

These points are illustrated in Figure 10.

When heart rate levels approach steady state or time trial threshold, intensity cannot be increased.

Evaporation is the most important method of heat loss during road riding. The faster you ride on the road, the greater the wind speed and the more heat is reduced. In the mountains, riders often become chilled on descents. Conduction losses can be substantial during rain.

Trainer workouts have many advantages, but heat accumulation can limit training effectiveness. Use one or two fans to help evaporation keep the body cool. A fan lowers the thermal threshold. With a fan, either work can be done more easily or more work can be performed.