Establishing a Physiological Strain Index Criterion During the Final Sixty Minutes of Heat Tolerance Testing: 2082 Board #238 May 30 2:00 PM - 3:30 PM

The heat tolerance test (HTT) assesses cardiorespiratory and thermoregulatory capacity during heat stress using heart rate (HR) and core temperature (Tcore) as pass/fail criteria, where HR and Tcore may not exceed 160 bpm and 38.6°C, respectively. Additional information regarding physiological stress and heat tolerance may be derived from the physiological strain index (PSI), which provides a singular physiological strain value using HR and Tcore measurements. To our knowledge, the change in PSI during the final 60 min of an HTT (ΔPSIF60) has not been evaluated as a potential criterion for classifying heat tolerance/intolerance during HTT. PURPOSE: The purpose of this study was to establish a criterion threshold for ΔPSIF60 and report on its ability to classify heat tolerance/intolerance during an HTT. METHODS: Seventy-seven US military men (age: 25 ± 5 yr, ht: 178.5 ± 7.1 cm, wt: 84.8 ± 10.1 kg) completed up to 120 min of continuous treadmill walking (3.3 mph, 4.0% grade) in 40°C and 40% relative humidity and were classified for their heat tolerance (n = 64) or heat intolerance (n = 13) using established Tcore and HR criteria. A ΔPSIF60 was calculated for, and compared between, heat tolerant (HT) and heat intolerant (HI) subjects using an independent sample t-test. A maximal normal accepted value for ΔPSIF60 was calculated by taking the mean value of ΔPSIF60 for HT subjects plus two standard deviations. RESULTS: ΔPSIF60 was significantly less in HT subjects than in HI subjects (0.54 ± 0.64 vs 1.95 ± 0.72; p < .001). For HT subjects, a ΔPSIF60 maximal normal accepted value was determined to be 1.82. CONCLUSION: Findings indicate that ΔPSIF60 appropriately differentiated HT from HI subjects during an HTT in this population of military personnel. Therefore, we report that exceeding a ΔPSIF60 of 1.82 may serve as an additional criterion for classifying heat intolerance during HTT. Further work on the validation of this maximal normal accepted value for ΔPSIF60 is needed.