Combined environmental stress and physiological strain indices for physical training guidelines

The purpose of this study was to develop guidelines based on the previously described environmental stress index (ESI) and physiological strain index (PSI) for work-rest cycles (WRC) during training, especially in the military. The ESI was introduced as a potential substitute for the Wet Bulb Globe Temperature Index because of the very high correlation coefficients between them. The ESI is constructed from the fast-reading meteorological response sensors ambient temperature (T_a), relative humidity (RH), and global radiation (GR), which require only a few seconds to reach equilibrium. The ESI, the first stress index using direct measurements of solar radiation (SR), is calculated as follows: ESI = 0.63T_a-0.03RH + 0.002SR + 0.0054 (T_aRH)-0.073(0.1 + SR)^-1. The PSI is based on heart rate (HR) and rectal temperature (T_re.) and can indicate heat strain online and analyze databases. The PSI is constructed as follows: PSI = 5(T_ret-T_re0)-(39.5-T_re0)^-1+ 5(HR_t-HR₀)-(I80-HR₀)^-1, where T_re0) and HR₀) are the initial T_re) and HR, and T_ret) and HR_tare simultaneous measurements taken at any time. The PSI is scaled from 0 to 10, whereby the respective variables. PSI_HR) and PSI_Tre), representing the cardiovascular and thermoregulation systems, can contribute up to five units to the overall strain assessment. To integrate the PSI and ESI, we decided to use only the PSI_HRcomponent, which represents the metabolic rate and the strain reflected by the cardiovascular system. Furthermore, PSI_HRis easier to measure, is easier to implement, and simplifies the integration with ESI. Concomitantly, PSI_HRcategorizes the strain between 0 and 5, the higher the value, the higher the strain. We believe that the use of the PSI WRC values will help in decreasing the risk of future heat injuries.