A physiological strain index to evaluate heat stress

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 degrees C (36.5-39.5 degrees 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-ret - T-re0).(39.5 - T-re0)(-1) + 5(HRt - HR0).(180 - HR0)(-1), where T-ret and HRt 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 degrees 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.