In space environments, there exists no free convection; therefore, only forced convection occurring by movement, such as cycle ergometer exercise, augments required heat dissipation necessary to regulate body temperature. With no free convection, increased skin wettedness (w) throughout the body surface (Esk/Emax) occurs during continuous exercise, which is an annoyance that induces thermal discomfort and is associated with increased mean body temperature. A rational effective temperature (ET*) can be derived by the heat balance equation that allows integration of thermal strain with heat exchange avenues in a zero-g atmosphere. In this paper, ET* (°C) is compared with the physiological strain index (PSI) constructed from core temperature (Tc) and heart rate (HR) in which PSI scaling from minimal to maximal (0-10) is evaluated by PSI = 5·(Tc,t-Tc,0)·(39.5-Tc,0)-1 + 5(HR,t –HR,0)·(180-HR,0)-1, where Tc,0 and HR,0 are initial values and Tc,t and HR,t describe simultaneous values at a given time point, Δt. Increasing thermal discomfort is shown by levels of ET* greater than 41.5°C which is accompanied by PSI values from 8-9. PSI and ET* can be used together to describe extent of environmental stress and physiological strain during various activity modes during long-term space shuttle cabin sojourns.