Can gender differences during exercise-heat stress be assessed by the physiological strain index?

A physiological strain index (PSI) based on rectal temperature (T(re)) and heart rate (HR) was recently suggested to evaluate exercise-heat stress. The purpose of this study was to evaluate PSI for gender differences under various combinations of exercise intensity and climate. Two groups of eight men each were formed according to maximal rate of O₂ consumption (V̇O₂(max)). The first group of men (M) was matched to a group of nine women (W) with similar (P > 0.001) V̇O₂(max) (46.1 ± 2.0 and 43.6 ± 2.9 ml · kg^-1 · min^-1, respectively). The second group of men (MF) was significantly (P < 0.001) more fit than M or W with V̇O₂(max) of 59.1 ± 1.8 ml · kg^-1 · min^-1. Subjects completed a matrix of nine experimental combinations consisting of three different exercise intensities for 60 min [low, moderate, and high (300, 500, and 650 W, respectively)] each at three climates {comfortable, hot wet, and hot dry [20°C 50% relative humidity (RH), 35°C 70% RH, and 40°C 35% RH, respectively]}. No significant differences (P > 0.05) were found between matched genders (M and W) at the same exposure for sweat rate, relative V̇O₂(max) (%V̇O₂(max)), and PSI. However, MF had significantly (P < 0.05) lower strain than M and W as reflected by %V̇O₂(max) and PSI. In summary, PSI applicability was extended for exercise-heat stress and gender. This index continues to show potential for wide acceptance and application.