CORE’s Heat Strain Index measures the overall heat stress your body is experiencing – it analyses both your core temperature and how easily you are cooling yourself (as influenced by skin temperature). Many coaches and athletes are finding that it is a good indication of performance decline.

Photo: @brazodehierro

Elevated Heat Strain Index

When the Heat Strain Index is elevated (usually greater than 3.0 or 4.0), the body must divert a lot more blood from the muscles to the skin to be cooled. If you want to maintain power/pace, your heart needs to pump faster to maintain oxygen supply to the muscles. Thus, heart rate increases at a steady power/pace. This performance decline is called heartrate decoupling.

In contrast, in cool-weather conditions you can have a quite-elevated core temperature with little performance decline. For example, during intense efforts you could generate a lot of heat, but it is dispersed easily if your skin temperature is very cool – meaning minimal blood diverted from the muscle. In such cases, your Heat Strain Index will be quite low.

Core temp is still important

Athletes monitoring Heat Strain Index during a race also monitor core temp. Core temp is still the early warning signal that you’re starting to heat up. Even if your Heat Strain Index is still 0.0, a rising core temp can tell you that it’s time to shed layers. Also, it reminds you to consider course and weather conditions.

Perhaps you’ll be leaving a shaded part of the course onto a sun-exposed climb? Or maybe the day is warming? Elevated skin temperatures can quickly change your Heat Strain Index from a comfortable 1.0 to a performance-draining 4.5. In such situations, keeping core temp in check guards against the problems caused by upcoming skin-temp increases.

But if you’re expecting cool-ish air temperatures all day? Maintaining a low Heat Strain Index may be the simplest way to avoid performance loss.

To see Heat Strain Index live on your Garmin watch, see this article.