Evaluation of different levels of hydration using a new physiological strain index

A physiological strain index (PSI), based on rectal temperature (T(re)) and heart rate (HR), was recently suggested for evaluating heat stress. The purpose of this study was to evaluate the PSI for different combinations of hydration level and exercise intensity. This index was applied to two databases. The first database was obtained from eight endurance-trained men dehydrated to four different levels (1.1, 2.3, 3.4, and 4.2% of body wt) during 120 min of cycling at a power output of 62-67% maximum O₂ consumption (V̇O(2max)) in the heat [33°C and 50% relative humidity (RH)]. The second database was obtained from nine men performing exercise in the heat (30°C and 50% RH) for 50 min. These subjects completed a matrix of nine trials of exercise on a treadmill at three exercise intensities (25, 45, and 65% V̇O(2max)) and three hydration levels (euhydration and hypohydration at 3 and 5% of body wt). T(re), HR, esophageal temperature (T(es)), and local sweating rate were measured. PSI (obtained from either T(re) or T(es)) significantly (P < 0.05) differentiated among all exposures in both databases categorized by exercise intensity and hydration level, and we assessed the strain on a scale ranging from 0 to 10. Therefore, PSI applicability was extended for heat strain associated with hypohydration and continues to provide the potential to be universally accepted.