The Association Between The Evaporative Requirements For Heat Balance And Whole-body Sweat Rate During Outdoor Cycling

PURPOSE: It has been demonstrated that whole-body sweat rate (WBSR) during cycling in a laboratory setting is closely associated with the evaporative requirements for heat balance (Ereq). The two main drivers of Ereq are metabolic heat production (Hprod) and dry heat transfer between the body and environment (Hdry). In the laboratory, power output (PO) and environmental conditions can be carefully regulated. When outdoors, PO and naturally fluctuating environmental conditions must be measured and combined to derive Ereq. Our aim was to assess 1) the association between Ereq (W) and WBSR (L·h-1) during outdoor cycling outdoor; and 2) how this association changes when PO is estimated using a popular physical activity social network (Strava) compared to a direct measure of PO. METHODS: 46 participants (11 women); cycled at a self-selected intensity (PO: 103-291 W) in a range of environmental conditions (Ta, 18.1-29.2 °C; absolute humidity, 0.75-2.82 kPa; Tr, 22.5-63.3 °C; wind speed, 0.4-2.1 m·s-1) for 60 minutes on an outdoor 3.78-km circuit. Participants completed up to 7 trials performed in a randomized order, to total of 108 trials. Hdry was estimated using partitional calorimetry with Ereq derived by subtracting Hdry from Hprod estimated using PO 1) measured with Garmin Rally RS100 pedals; and 2) estimated from Strava. RESULTS: Compared to the association between Ereq and WBSR previously reported indoors (r > 0.90), the association is diminished when Ereq is estimated outdoors even when PO is measured (r = 0.680). With PO estimated using Strava, the association is further weakened (r = 0.629). The integration of body mass as an additional independent variable increased the association between WBSR and Ereq for measured (r = 0.740) and estimated (r = 0.779) PO. CONCLUSIONS: These preliminary data indicate that in an outdoor environment, the strength of association between Ereq and WBSR is weaker than indoors. Estimates of PO for ultimately deriving Ereq can be improved by integrating body size. Standard equations for estimating dry heat transfer and sweating efficiency in highly convective outdoor environments are likely limited. These findings indicate the calculation of Ereq outside the laboratory, requires additional research and further refinement. Funding: This study was supported by The Coca-Cola Company (USA)