Heat Strain Index

CORE's Heat Strain Index measures how hard your thermoregulatory system is working to cool itself

Significance of Heat Strain

The human body aims to maintain core temperature within a narrow range (~36.0–37.5°C/ 96.8–99.5°F). This is achieved by balancing the body's heat production and heat loss. During exercise, the body's heat production rises. To balance this, the body activates its heat loss mechanisms.

It generates sweat for evaporative cooling of the skin, and it widens skin blood vessels and raises heart rate to increase blood flow to the skin. When cooling demands are high, the increased blood flow to the skin causes blood to be diverted away from the muscles.

When exercise intensity and/or environmental heat stress (high ambient temperature, high humidity) are high, the body's heat loss mechanisms can no longer compensate for its heat production. This will lead to an increase in core temperature, skin temperature, and heart rate. This physiological response to heat stress is called heat strain. During athletic endeavours, excessive heat strain causes performance loss. Repeated exposure to heat strain creates heat adaptations.

Moving Beyond Core Temperature

Core temperature is an important metric for assessing an athlete’s thermal state. However, core temperature alone does not sufficiently estimate thermal strain on the body. This is because at a given core temperature, performance loss occurs earlier when skin temperature is higher, due to an increase in heart rate and blood flow to the skin. In addition, a high skin temperature increases the feeling of discomfort during exercise in the heat.

Extensive research has shown that the concept of mean body temperature is one of the top indicators for assessing heat strain. Mean body temperature is the weighted average of the body’s core temperature and skin temperature. This calculation typically requires measurements from different locations of the body. As the CORE sensor collects data from only one location, extensive laboratory testing by CORE experts has resulted in a formula that allows the CORE sensor to accurately estimate mean body temperature.

CORE’s Heat Strain Index

CORE’s Heat Strain Index converts estimated mean body temperature into a value ranging from 0 to around 10. It gives a real-time, minute-by-minute measure of how hard your body is working to stay cool.

• A Heat Strain Index between 0 and 0.9 is measured when the thermoregulatory system is not strained. Core temperature may be elevated, but skin temperature is low (ie. when exercising in a cool climate).
• A value of 10 is exceptionally high and represents a core temperature of 40°C/104°F and a skin temperature of 37°C/98.6°F. Exercising at these temperatures for too long may have severe consequences for your health and may place you at risk for heat illness or even heat stroke. For more information, see the articles Heat Zones and Heat Safety.

Heat Strain Index is displayed live on the CORE app (convenient for indoor training) and also on Garmin devices. Heat Strain Index can also be reviewed post-workout on the CORE app.

Preventing Performance Loss and Guiding Heat Training

When high performance is desired, such as during competition and high-intensity training sessions, the Heat Strain Index can be used to limit performance loss.

Keeping Heat Strain Index low (for example, through cooling or pacing adjustments) ensures that no blood is diverted away from the muscles.

Heat Strain Index can also be used to guide heat training. Regular and repeated heat training sessions at an elevated Heat Strain Index for 45–80 minutes results in heat adaptation.

For more information, see the article CORE’s Heat Zones.

References

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