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PURPOSE: The purpose of this study was to investigate the influence of climatic conditions on performance in a ~50 km mountainous trail running race. Anecdotal evidence suggests that increased humidity in forested race courses is associated with increased heat stress and decreased performance in ultra-marathoners. We hypothesized that slower climb times would be associated with elevated ambient vapour pressure (VP) and temperature during an ultra-marathon course with heavy tree cover. The rationale follows from body heat loss and cooling being largely dependent on evaporation of eccrine sweat, which is directly proportional to the VP gradient between the ambient air and that on the skin surface. METHODS: Eight healthy males and one female volunteered for the study. Their mean height was 178.3±5.5 cm (mean±SD), weight 71.0±8.3 kg, Body Mass Index 22.4±2.1 ml/kg2, and age 49.7±8.2 years. The office of research ethics at SFU approved the study. Dry bulb (DBT) and wet bulb temperatures were assessed as well as relative humidity (RH) with an electronic humidity sensor. From these ambient VP was determined from a psychometric chart. Wind velocity was assessed with an anemometer. Globe temperature was determined using a Vernon globe thermometer. Remote Omega sensors were used to measure temperature, RH and dew points at three course locations. The GPS coordinates of measurement sites were determined using a handheld instrument. First order linear regression analysis was employed for the analyses and the P value was set at 0.05. RESULTS: Over the race duration there were progressive increases in DB temperature, RH and ambient VP. The mean DBT at the 3 maximum elevations in the race was 21.52±0.57°C and this was positively correlated to mean climb time of 72.26±9.76 min with an R2 = 0.92, p<0.05. The mean RH of 55.39±1.39% was negatively correlated to mean climb time with R2 = 0.47, p<0.05. The mean ambient vapour pressure of 10.73±0.28 mmHg, however, was positively correlated to mean climb time with R2 = 0.40, p<0.10. CONCLUSION: In conclusion, these preliminary results support the hypothesis that increased temperature and ambient vapour pressure are associated with a decrease in performance during strenuous climbs during an ultramarathon on a tree covered course.

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