Introducing CORE’s new Heat Strain Score!

CORE’s new Heat Strain Score helps you better understand how heat affects your body. It is displayed in the updated version of the CORE mobile app.

Significance of heat strain

We’ve all become more heat aware by monitoring core temperature during training and races. We know that as core temp climbs, the body’s thermoregulatory system works to keep us cool by sweating, diverting blood from the muscles to the skin, and other processes. These all require resources from the body, reducing sports performance and possibly leading to heat exhaustion.

Thus, heat strain is the sum of the physiological processes used to dissipate heat from the body’s core. The higher the heat strain, the greater the performance decline. The Heat Strain Score objectively quantifies heat strain in an easy-to-understand metric.

Some technical background

Core body temp is an important metric for assessing an athlete’s thermal state and is a widely accepted proxy to estimate heat strain. However, core temp alone does not sufficiently estimate the body’s performance reserve. This is because at a given core temp, the exhaustion point is reached earlier when skin temperature is higher (due to an increase in heart rate and blood flow to the skin).

Extensive research has shown that mean body temperature is one of the top indicators for assessing heat strain. Mean body temperature is the average temp of the torso and all appendages, weighted based on the mass of each. A long-established formula estimates mean body temperature by using core body temp and mean skin temp. While the CORE sensor measures skin temp in only one location, extensive laboratory testing by CORE experts has identified a revised formula that allows the CORE sensor to accurately estimate mean body temperature.

Heat Strain Score

CORE’s Heat Strain Score measures the cumulative amount of heat strain experienced during a workout or entire day. It is a function of the amount of time that both core and skin temperatures are elevated, and it indicates how hard the thermoregulatory system is working. The longer that both temperatures are elevated, the higher the Heat Strain Score will be. The higher the Heat Strain Score, the more performance degradation an athlete can expect.

For example, if an athlete does two 60-minute workouts at the same core temperature, the Heat Strain Score will be higher for the workout where skin temperature was higher. This is because it is harder for the body to shed heat when skin temperature is higher.

Skin temperature is highly correlated to the ambient air temperature and to the amount of clothing worn. When training outdoors in winter conditions, it’s not unusual to have a Heat Strain Score of zero, even if core temp is elevated. This is because the body is not working hard to dissipate heat.

While a long workout in the heat may result in a score of several thousand, most users will find that their highest daily scores occur when sick with a fever. A daily Heat Strain Score of over 10,000 is not atypical during a strong fever, and accurately represents the extreme stresses on the body (and helps to quantify the reason why exercise avoidance is recommended while experiencing a fever).

Applications for training

The Heat Strain Score is valuable for analysing training and race performance. It adds another dimension to core body temp to explain the condition and physiology of the athlete.

In the CORE app, you will find a Heat Strain Score calculated for all prior workouts recorded with CORE. Examining the Heat Strain Score in various races and training sessions will likely reveal patterns and thresholds where athletes started to experience performance decline. In particular, look at long endurance efforts in the heat, races in any conditions, and longer high-intensity efforts.

Notice if there is a threshold Heat Strain Score associated with performance decline. Also note if there was an ambient air temperature that was associated with little or no Heat Strain Score, even during a race where core temp was quite elevated. More tips and training advice coming soon!