Perceptual versus physiological heat strain during exercise-heat stress.

Purpose: The physiological strain index (PSI) has been proposed as a universally applicable measure of exercise-heat strain. Unknown is whether this index, based on normalized increases in core temperature and heart rate, is matched by its perceptual analog.

Methods: By using a similar mathematical construct to the PSI, the perceptions of thermal sensation and perceived exertion were combined, and the resultant index, PeSI, was compared with its physiological counterpart, denoted as PhSI, for the exercise-heat stress specific to this study. Twenty-six young and healthy subjects wore semi-impermeable clothing and walked (3.5 km.h(-1)) under hot conditions (40 degrees C and 30% RH) until exhaustion or when their core temperature reached 39.5 degrees C. Subjects were divided into two fitness groups [endurance trained (T) and untrained (U)] comprised of 10 men and 3 women each. U subjects had a higher level of body fatness (mean +/- SD 18.1 +/- 5.3 vs 12.6 +/- 4.5%; P=0.010) and a lower level of aerobic fitness ((.)VO(2max)= 43.6 +/- 3.8 +/- vs 59.0 +/- 6.2 mL.min(-1).kg(-1); P<0.001).

Results: During the first hour of exposure, there was no group difference in PhSI, yet T perceived their physiological strain (PeSI) lower than U (P=0.002). Further, the indices were not different for U whereas PhSI was higher than PeSI for T (P=0.008). At the end of the exposure, T had a higher value of PhSI than U (8.23 +/- 0.72 vs 6.74 +/- 1.47; = 0.002), but there was no group difference in PeSI. Although the indices were again not different for U, PhSI at the end was higher than PeSI for T (6.14 +/- 1.68; P<0.001).

Conclusion: T underestimated and U consistently perceived their physiological strain, as defined by PhSI, in accordance with the measured increases in core temperature and heart rate throughout an exposure to uncompensable exercise-heat stress.