Interpretations of Physiological Strain Index During Heat Tolerance Testing

Abstract

The heat tolerance test (HTT) assesses cardiorespiratory and thermoregulatory capacity during heat stress using core temperature (Trec) and heart rate (HR) as pass/fail criteria. Additional information regarding physiological stress and heat tolerance may be derived from the physiological strain index (PSI), which provides a representation of total physiological strain using Trec and HR measurements. Currently, there is minimal information available on the efficacy of various PSI interpretations, with respect to accurately identifying differences in physiological strain between those that have passed and failed an HTT. PURPOSE: To report different methods to evaluate physiological strain during HTT using PSI. METHODS: Eighty-two military personnel (age: 25 ± 5 yrs, height: 178.2 ± 7.2 cm, weight: 84.5 ± 9.9 kg) completed up to 120 min of continuous treadmill walking (3.3 mph; 4.0% grade) in 40°C and 40% relative humidity. PSI was calculated from Trec and HR measurements that were recorded every 5 min. PSI was then interpreted as trial mean, end of test (EOT), slope, and time-weighted (0 min - EOT) area under the curve (AUC) for subjects that passed and failed an HTT and compared using independent samples t-tests (p < .05). RESULTS: The trial mean PSI was significantly less in those that passed compared with those that failed the HTT (3.7 ± 0.7 vs. 4.8 ± 0.3; p < .001). Similarly, EOT PSI was significantly less for subjects that passed compared with those that failed (4.7 ± 1.1 vs. 6.8 ± 0.6; p < .001). Graphed as a function of time, the slope of the best-fit line for PSI was less in those that passed compared with those that failed (0.02 ± 0.01 vs. 0.07 ± 0.03; p < .001). PSI was also calculated as AUC (pass: 416 ± 141, fail: 453 ± 87; p = .299) and, when adjusted for trial time, resulted in a significantly lower PSI value for those that passed compared with those that failed (3.8 ± 0.7 vs. 4.8 ± 0.3; p < .001). CONCLUSION: Findings indicate all PSI parameters investigated appropriately differentiated between subjects that passed and failed the HTT, resulting from either cardiorespiratory stress, thermoregulatory stress, or a combination of the two. Further work on the use of this index and its evaluation is warranted, as there are no defined criteria for HTT fail using PSI as an integrative value of total thermal strain.