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

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.

Thermal perceptions and skin temperatures during continuous and intermittent ventilation of the torso throughout and after exercise in the heat.

OBJECTIVE: This study tested the hypothesis that intermittent cooling in air-perfused vests (APV) will not only maintain thermal balance but, due to cyclical activations of cutaneous thermoreceptors, also enhance thermal perceptions. METHOD: Ten physically active males completed four conditions where they exercised (walking: 5 km h(-1), 2 % gradient) in a hot environment (~34.0 °C, 50 % RH) for 72 min, followed by a 33-min period of rest. They wore an APV throughout. The four conditions differed in respect to the profile of ambient air that was perfused through the APV: continuous perfusion (CP); intermittent perfusion of 6 min ON/OFF periods (IPonoff); a steady increase and decrease in flow rate in equal increments (IPramp); and an initial step-increase in the flow rate followed by an incremental decrease to zero flow rate (IPtriang). Whole body and torso thermal comfort (TC, TTC), whole body and torso temperature sensation (TS, TTS), whole body and torso skin temperature ([Formula: see text], [Formula: see text]), local relative humidity ([Formula: see text]) and rectal temperature (T re) were measured. RESULTS: There were no significant differences in T re, absolute whole body and local [Formula: see text], TC, TTC and TS between the cooling profiles. However, TTS was cooler in CP and IPramp than IPonoff and IPtriang. Even though intermittent cooling did not significantly enhance thermal perceptions in CP, a trend existed for TC (P = 0.063) to become less favourable over time. CONCLUSION: To reduce the power consumption and extend the battery life of an APV, it is recommended that an intermittent cooling profile should be adopted.