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Revisiting classical experiments on the impact of air resistance on metabolic rate, we aimed to overcome limitations of previous research, notably: low participant numbers (n = 1–3), highly turbulent wind, and confounding effects of rising body temperature. In a custom-built wind tunnel with reduced turbulence, 14 participants (8 males, 6 females) walked (5 km·h−1) and ran on a treadmill (70%V̇ o 2max) at 0, 2, 4, and 6 m·s−1 headwind or tailwind in a counterbalanced design, with rest breaks between each exposure to avoid rises in body core temperature. Oxygen consumption (V̇ o 2) exhibited strong linear relationships versus wind direction, dynamic pressure, and air speed squared (Vwr2), lower in magnitude for headwind than tailwind. A moderate linear relationship was observed between heart rate, wind direction, dynamic pressure, and Vwr2. Below 4 m·s−1, the effect of wind was well within inter- and intraindividual variation and equipment uncertainty, and only at wind speeds ≥4 m·s−1 did the differences in physiological responses reach statistical significance. Our data indicate that at running speeds below 4 m·s−1 (14.4 km/h), indoor treadmills and outdoor running are comparable in terms of the metabolic impact of air movement relative to the person. However, this does not extend to the thermoregulatory effect of wind, with outdoor running providing a higher cooling rate due to the self-generated wind created during running. By removing the confounding impact of core temperature rises, the observed effects of headwind were lower and those of tailwind larger than observed previously. In the context of middle-distance running, the headwind created by running at 21.5 km·h−1 would result in a 2.2% increase of V̇ o 2. A relative tailwind of the same speed would lead to a 3.1% reduction. NEW & NOTEWORTHY: Revisiting classical work by Pugh and Davies on the metabolic effects of air speed and direction, shortcomings in the original studies were addressed. Using more participants, less turbulent wind, and avoiding confounding effects of work-induced core temperature increases, new equations describing the impact of air speed/direction were developed. This study observed a lower impact of headwind and a larger impact of tailwind in the absence of an exercise-induced core temperature increase. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate which climate/heat indices perform best in predicting heat-induced loss of physical work capacity (PWCloss). Integrating data from earlier studies, data from 982 exposures (75 conditions) exercising at a fixed cardiovascular load of 130 beats·min−1, in varying temperatures (15–50°C), humidities (20–80%), solar radiation (0–800 W·m−2), wind (0.2–3.5 m·s−1), and two clothing levels, were used to model the predictive power of ambient temperature, universal thermal climate index (UTCI), wet bulb globe temperature (WBGT), modified physiologically equivalent temperature (mPET), heat index, apparent temperature (AT), and wet bulb temperature (Twb) for the calculation of PWCloss, skin temperature (Tskin) and core-to-skin temperature gradient, and thermal perception (thermal sensation vote, TSV) in the heat. R2, RMSE, and Akaike information criterion were used indicating model performance. Indices not including wind/radiation in their calculation (Ta, heat index, AT, and Twb) struggled to provide consistent predictions across variables. For PWCloss and TSV, UTCI and WBGT had the highest predictive power. For Tskin, and core-to-skin temperature gradient, the physiological models UTCI and mPET worked best in seminude conditions, but clothed, AT, WBGT, and UTCI worked best. For all index predictions, Ta, vapor pressure, and Twb were shown to be the worst heat strain predictors. Although UTCI and WBGT had similar model performance using the full dataset, WBGT did not work appropriately in windy, hot-dry, conditions where WBGT predicted lower strain due to wind, whereas the empirical data, UTCI and mPET indicated that wind in fact increased the overall level of thermal strain. The findings of the current study highlight the advantages of using a physiological model-based index like UTCI when evaluating heat stress in dynamic thermal environments. NEW & NOTEWORTHY: There is an urgent need to determine the optimal heat stress metric when forecasting the impact of heat stress on human performance, physiological stress, and perception. We analyzed a wealth of laboratory data, simulating heart rate (HR)-paced work with wide variations in air temperature, humidity, wind speed, solar radiation, and clothing. We conclude that the universal thermal climate index (UTCI) [followed by wet-bulb globe temperature (WBGT)] is the optimal heat index to reliably predict reductions in performance, and elevations in physiological and perceptual stress. [ABSTRACT FROM AUTHOR]