EVALUATION OF A PHYSIOLOGICAL STRAIN INDEX FOR USE DURING INTERMITTENT EXERCISE IN THE HEAT

Abstract

Moran et al. (Am. J. Physiol. 44: R129, 1998) developed a physiological strain index (PSI) using rectal temperature and heart rate to monitor heat strain on a scale from 1 to 10. While this has been valuable during continuous, steady-state exercise in the heat, most work environments involve work-rest cycles. Therefore, we evaluated the PSI for use with intermittent exercise in the heat. Two groups of healthy subjects each, men and women, volunteered for the study. Ten subjects entered the continuous exercise protocol, and 6 subjects entered the intermittent protocol. Subjects randomly exercised in normal (20°C, 50%RH), hot-humid (40°C, 35%RH), hot-humid (35%C, 70%RH) climates. Continuous exercise was treadmill walking at 1.34m/s, 5% grade for 60 min. Intermittent exercise consisted of the same exercise but accomplished in 15-min work, 5-min rest cycles for the 60 min. Rectal temperature and heart rate were continuously recorded. PSI was calculated from the formula of Moran et al. The PSI discriminated among the 3 climatic conditions for continuous exercise, and for the hot vs. normal during intermittent. The intermittent exercise protocol produced less physiological strain than the continuous protocol, not significantly different between the two hot climates, and this fact was reflected by the PSI. This study has shown that the PSI is also valid in assessing heat strain during work-rest cycles.Table 1: Values (mean (SE)) for PSI