PSI Calculator — Physiological Strain Index

Формула

How this is calculated

The Physiological Strain Index (PSI) was developed by Moran et al. (1998) to provide a single, easy-to-interpret measure of physiological strain during heat stress. It combines two components that each contribute up to 5 points: a thermal component based on the rise in core body temperature (Tc) above its resting value (Tc₀ ≈ 36.5 °C), scaled to a maximum expected core temperature of 39.5 °C; and a cardiovascular component based on the rise in heart rate (HR) above resting (HR₀), scaled to a maximum expected HR of 180 bpm. PSI therefore ranges from 0 (no strain relative to rest) to 10 (maximal strain at both limits simultaneously).

Core temperature is most accurately measured rectally or with an oesophageal probe in research settings; ingestible gut-temperature pills are a practical field alternative. Tympanic (ear) and axillary (armpit) measurements are less accurate and may underestimate true core temperature, especially during exercise. PSI assumes a typical healthy adult; athlete populations may have lower resting values and higher exercise tolerances, shifting the practical interpretation of each PSI band.

PSI levels of 7–8 signal high physiological strain requiring attention, while 9–10 indicate potential medical risk; guidelines from occupational health bodies generally recommend work-rest schedules that keep workers below PSI 7. The index is validated for moderate-intensity exercise in temperate and hot environments and has been used in military, athletic and industrial settings. It is descriptive, not diagnostic — always combine it with clinical judgment and direct observation.