What is exertion?

Exertion is a metric, measured as a percentage, that determines how high an individual's heart rate and core body temperature are above their resting heart rate and resting core body temperature.

• Below 70%: low or moderate exertion.
• Between 70% and 90%: high exertion, the user may need attention if this exertion rate is sustained for a long period of time.
• Above 90%: very high exertion, the user is in danger of overexertion.

What is the formula?

Exertion is based on the Physiological Strain Index (PSI), but Exertion is normalized onto a 0-100% scale. Physiological Strain Index is used by the US Military to assess heat strain. For more on Physiological Strain Index, you can reference this research paper.

Where:

• CT(0) represents the resting core body temperature
• CT(t) represents the current core body temperature
• HR(0) represents the resting heart rate
• HR(t) represents the current heart rate