The physiological strain index does not reliably identify individuals at risk of reaching a thermal tolerance limit

Sarah L Davey, Victoria Downie, Katy Griggs, George Havenith, Sarah L Davey, Victoria Downie, Katy Griggs, George Havenith

Abstract

Purpose: The physiological strain index (PSI) was developed to assess individuals' heat strain, yet evidence supporting its use to identify individuals at potential risk of reaching a thermal tolerance limit (TTL) is limited. The aim of this study was to assess whether PSI can identify individuals at risk of reaching a TTL.

Methods: Fifteen females and 21 males undertook a total of 136 trials, each consisting of two 40-60 minute periods of treadmill walking separated by ~ 15 minutes rest, wearing permeable or impermeable clothing, in a range of climatic conditions. Heart rate (HR), skin temperature (Tsk), rectal temperature (Tre), temperature sensation (TS) and thermal comfort (TC) were measured throughout. Various forms of the PSI-index were assessed including the original PSI, PSIfixed, adaptive-PSI (aPSI) and a version comprised of a measure of heat storage (PSIHS). Final physiological and PSI values and their rate of change (ROC) over a trial and in the last 10 minutes of a trial were compared between trials completed (C, 101 trials) and those terminated prematurely (TTL, 35 trials).

Results: Final PSIoriginal, PSIfixed, aPSI, PSIHS did not differ between TTL and C (p > 0.05). However, differences between TTL and C occurred in final Tsk, Tre-Tsk, TS, TC and ROC in PSIfixed, Tre, Tsk and HR (p < 0.05).

Conclusion: These results suggest the PSI, in the various forms, does not reliably identify individuals at imminent risk of reaching their TTL and its validity as a physiological safety index is therefore questionable. However, a physiological-perceptual strain index may provide a more valid measure.

Keywords: Heat illness; Heat stress; Heat stress indices; Hyperthermia-induced fatigue; Physiological strain index; Thermal tolerance limit.

Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Fig. 1
Fig. 1
Two of the types of protective clothing worn by the participants: a impermeable clothing and b permeable clothing, plus localised thermal radiation directed onto the back of the participants
Fig. 2
Fig. 2
Cumulative frequency showing % of participants dropping out at each Physiological Strain Index (PSIfixed) value for group TTL and group C. The grey dashed line shows the PSIfixed value when 50% of participants dropped out or completed the trial. The black dashed line represents Buller’s (2008) ‘at risk’ classification of 7.5. The black solid line represents the threshold in PSIfixed required to protect 95% of the TTL group. The numbers associated to the cases in group TTL is the participant identifier corresponding to that case
Fig. 3
Fig. 3
Cumulative frequency showing % of participants dropping out at each rate of change (ROC) in the Physiological Strain Index (PSIfixed) for group TTL and group C. The grey dashed line shows the ROC in PSIfixed when 50% of participants dropped out or completed the trial. The black dashed line represents the upper limit in ROC in PSIfixed to complete a trial. The black solid line represents the threshold in PSIfixed required to protect 95% of the TTL group. The numbers associated to the cases in group TTL is the participant identifier corresponding to that case

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