A physiological strain index to evaluate heat stress

Abstract

A physiological strain index (PSI), based on rectal temperature (T_re) and heart rate (HR), capable of indicating heat strain online and analyzing existing databases, has been developed. The index rates the physiological strain on a universal scale of 0–10. It was assumed that the maximal T_re and HR rise during exposure to exercise heat stress from normothermia to hyperthermia was 3°C (36.5–39.5°C) and 120 beats/min (60–180 beats/min), respectively. T_re and HR were assigned the same weight functions as follows: PSI = 5(T_{re t} − T_re0) ⋅ (39.5 − T_re0)⁻¹+ 5(HR _t − HR₀) ⋅ (180 − HR₀)⁻¹, where T_{re t} and HR _t are simultaneous measurements taken at any time during the exposure and T_re0 and HR₀ are the initial measurements. PSI was applied to data obtained from 100 men performing exercise in the heat (40°C, 40% relative humidity; 1.34 m/s at a 2% grade) for 120 min. A separate database representing seven men wearing protective clothing and exercising in hot-dry and hot-wet environmental conditions was applied to test the validity of the present index. PSI differentiated significantly ( P < 0.05) between the two climates. This index has the potential to be widely accepted and to serve universally after extending its validity to women and other age groups.