The accuracy of the Physiological Heat Strain Index to identify individuals at risk of heat induced fatigue

The physiological heat strain index (PSI) was developed to assess individuals’ heat strain within various settings, yet evidence supporting its use to accurately identify individuals at potential risk is limited. The aim of this study was to assess whether PSI can identify individuals at risk of heat induced fatigue (HIF). Fifteen females (24.2 ± 5.7 years; 1.69 ± 0.09 m2) and 21 males (26.9 ± 4.7 years; 1.95 ± 0.13 m2) completed nine trials, each consisting of two 40-60 mins of treadmill walking separated by ~20-mins rest, wearing permeable or impermeable clothing, either in 25 °C, 50% relative humidity (rh), 35 °C, 35% rh , or 40 °C, 25% rh, with or without solar radiation (~530 W m−2). Heart rate (HR), skin temperature (Tsk), rectal temperature (Tre), temperature sensation (TS) and thermal comfort (TC) were measured throughout. PSI was calculated using the equation by Moran (1998) with initial Tre and HR set at 37.0 °C and 70 bpm. End of trial PSI, HR, Tre, Tsk, rate of change (ROC) in Tsk and Tre and TS and TC were compared between the trials completed (C, 117 trials) and those terminated prematurely (HIF, 35 trials) using independent t-tests. End PSI did not significantly differ between HIF (6.98 ± 1.67) and C (6.71 ± 1.56). However, there was a significant difference between HIF and C in Tre-Tsk (0.9 ± 0.86 °C vs 1.29 ± 0.86 °C, respectively, P = 0.009) and in TS (18.03 ± 2.22 vs 15.70 ± 4.33, respectively, P = 0.008). In HIF, 42.24% of the cases had a PSI > 7.5 (considered ‘at risk’ from HIF). These results suggest PSI alone does not distinguish between individuals at risk of HIF, however, using additional measures of Tsk and TS may provide a better prediction of heat tolerance.