Why is My Heat Strain Zero?

Heat strain is the sum of the physiological processes used to dissipate heat from the body’s core. CORE’s Heat Strain Index quantifies heat strain in a real-time, easy-to-understand metric. It is calculated using both core temperature and skin temperature and uses a formula that estimates the well-researched mean body temperature.

The Heat Strain Index has a practical range of 0 to around 10. (While the index has no theoretical limit, a human could likely not survive a Heat Strain Index of 15.) Even 10 is an exceptionally high value, representing a core temperature of 40°C/104°F and a skin temperature of 37°C/98.6°F.

A Heat Strain Index of zero?

Many training sessions where the core temp is below the heat training zone will result in a Heat Strain Index of zero. The combination of air temperature, relative humidity, wind exposure, sun exposure, clothing worn, cooling measures, and training intensity will all influence this value. In particular, expect to see a value of zero in many situations when the air temperature is cool (<10°C/50°F). Low humidity conditions may also result in a zero, as cooling through sweat evaporation is extremely effective then.

Some may consider a Heat Strain Index value of zero to be optimal training conditions. In other words, there is very low strain on the thermoregulatory system, and more of the body’s resources can be dedicated to producing power. Personal thermal comfort levels are also generally quite high when the index value is at zero – heat is not a distraction.

Higher Heat Strain Index values

If your core temp is in the heat training zone, you will likely experience a greater-than-zero Heat Strain Index value. An exception may be very intense training (or racing) while wearing light clothing in very cool outdoor conditions (air temp <5°C/41°F).

Because the Heat Strain Index is new and research using it is limited, it is yet unknown if there is an optimal Heat Strain Index value for heat training. But we are confident that a core temperature in the athlete’s heat training zone will produce physiological adaptions, regardless of the Heat Strain Index value.

Athletes may find that their comfort level at a given core temp changes dramatically at different Heat Strain Index values. For example, a high intensity, cool-weather outdoor session can elevate the core temp without raising the Heat Strain Index value very high. Conversely, low intensity training in very hot air temperatures can both elevate the core temp and cause a higher Heat Strain Index value. Athletes and coaches can best decide how to mesh their heat training with the rest of their training schedule.

Sample scenarios

While it is difficult to predict a Heat Strain Index in each situation, here are some typical examples you may encounter.

Heat Strain Index Value

Sample Scenarios


Core temp below the heat training zone in very cool outdoor air temperatures (<5°C/41°F). Self-generated wind (ie. running/cycling speed) cools the skin and lowers the Index value


Core temp in or below the heat training zone in cool outdoor air temperatures (<15°C/59°F) while wearing light clothing


Core temp below the heat training zone in cool indoor air temperatures (<15°C/59°F). Lack of wind causes more heating. Index value highly influenced by clothing choices

Core temp in the heat training zone in very cool outdoor temperatures (<5°C/41°F)

A mild fever


Core temp in the heat training zone – either in warm air temperatures or while wearing heavy clothing in cool temperatures

During a heat ramp test

Many race situations, with the exception of very cool conditions

A moderate fever


Wearing your CORE sensor during a hot shower (skin temp is very high)

6.1 and

Intense training in hot ambient conditions or while wearing heavy clothing


Late stages of a race, especially in hot ambient conditions

A strong fever


Your feedback welcome!

Do you have observations about the Heat Strain Index in various situations? We’d appreciate hearing about them. A great place to share them is on our community forum.