Influence of temperature and performance level on pacing a 161 km trail ultramarathon.

BACKGROUND Even pacing has been recommended for optimal performances in running distances up to 100 km. Trail ultramarathons traverse varied terrain, which does not allow for even pacing. PURPOSE This study examined differences in how runners of various abilities paced their efforts in the Western States Endurance Run (WSER), a 161 km trail ultramarathon in North America, under hot vs cooler temperatures. METHOD Temperatures in 2006 (hot) and 2007 (cooler) ranged from 7-38°C and 2-30°C, respectively. Arrival times at 13 checkpoints were recorded for 50 runners who finished the race in both years. After stratification into three groups based on finish time in 2007 (<22, 22-24, 24-30 h), paired t tests were used to compare the difference in pace across checkpoints between the years within each group. The χ2 test was used to compare differences between the groups on the number of segments run slower in the hot vs cooler years. RESULTS For all groups, mean pace across the entire 161 km race was slower in 2006 than in 2007 (9:23 ± 1:13 min/km vs 8:42 ± 1:15 min/km, P < .001) and the pace was slower from the start of the race when temperatures were still relatively cool. Overall, the <22 h cohort ran slower in 2006 than 2007 over 12 of the 14 segments examined, the 22-24 h cohort was slower across 10 of the segments, and the >24 h cohort was slower across only 6 of the segments χ(2)2 = 6.00, P = .050). Comparable pacing between the 2 y corresponded with onset of nighttime and cooling temperatures. CONCLUSIONS Extreme heat impairs all runners' ability to perform in 161 km ultramarathons, but faster runners are at a greater disadvantage compared with slower competitors because they complete a greater proportion of the race in the hotter conditions.