Examples of the Heat Strain Index

CORE’s new Heat Strain Index is an advanced metric that considers both core temperature and skin temperature to quantify how hard the body is working to stay cool. For practical purposes the index ranges from 0 to 10 (which would be an extreme value).

To help understand the range of values you might see, let’s look at a few examples and discuss why they look this way.

Heat Strain Index: no value

Easy effort training ride

Easy efforts in cool temperatures will typically yield a Heat Strain Index of zero. Here, the athlete did a 45-minute indoor recovery spin at air temperature 19°C/66°F. Intensity and heart rate was low, so little heat was generated internally. And because air temperature was relatively cool, sweat evaporated easily. Thus, cooling was easily achieved by the body.

Heat Strain Index: 1–3

Indoor Bike Race

The next chart shows an indoor bike race – 20 minutes of warmup and 40 minutes of racing. The room temperature was 17°C/66°F and fans were used for cooling. The intensity of the effort brought core temperature into the athlete’s heat training zone, but the fans kept skin temperature low. Thus, the Heat Strain Index remained low and cooling was effective.

Half marathon

In a half marathon in cool conditions (13°C/55°F), the runner’s core temperature quickly rose into his heat training zone. The Heat Strain Index also quickly rose to above 3. However, the runner began pouring 4 cups of water over himself at every aid station. This lowered skin temperature (and the Heat Strain Index) substantially and kept core temp stable throughout the race. The runner reported high thermal comfort.

Indoor ride and outdoor run

The following chart shows an indoor ride followed by an outdoor run. The moderate-intensity ride raised both core temp and skin temp, resulting in a low Heat Strain Index and low-level thermal discomfort for the athlete. He then transitioned to an outdoor run, where the higher intensity caused an elevated core temperature. But because air temperature was cool (14°C/57°F), the Heat Strain Index dropped to zero. The athlete’s thermal comfort was high, even though core temperature was elevated more than during the indoor ride.

Heat Strain Index: 4–7

Heat ramp test

A heat ramp test often yields a Heat Strain Index of 5–6. The chart below is fairly typical – Heat Strain rises as both core temp and skin temp increase. Many athletes report substantial thermal discomfort when the Heat Strain Index is above 5. (Note that this athlete’s heat training zone is on the lower range of normal – however, this does not always influence self-described comfort levels).

Data to share?

Do you have any interesting Heat Strain Index data to share? If so, we’d enjoy seeing it! A screenshot like the ones above, plus a paragraph about the workout/race can help us all better understand athletes’ responses to various situations. You can always reach us though our website or social media channels.