J.T. Strang, C.J. Alfiero, C.L. Dumke FACSM, B.C. Ruby FACSM, M.W. Bundle

University of Montana, Missoula, MT

PURPOSE: Despite extensive and ongoing scientific study into the metabolic requirements of load carriage, an understanding quantifying the effect of speed, load, sex and body mass has yet to come forth and the extent to which established models predict these requirements is largely untested. Specifically, because existing experimental efforts have typically focused on relatively modest walking speeds using loads representing a fixed portion of the subject’s mass, extending the available predictions to applications where individuals complete a common task carrying an identical absoluteload provides estimates of unknown accuracy. Here, we measured the energy use in a large subject group walking at speeds surrounding the 1.8 m s-1necessary to successfully complete the 4.83 km USFS wildland firefighter arduous pack (20.5kg) test, and compared these results to estimates available from the prevailing models. METHODS: We measured VO2from 61 young (age = 22.8±3.2 yrs) adults (36 males, Mb = 79.5±8.8kg; 25 females, Mb = 67.5±13.5kg; study range: 55.4-119.6 kg) as they performed four, 5min trials, with a 20.5kg pack, on a level treadmill at 1.7, 1.8, 1.9 m s-1, and their individual average speed from a previously administered pack test. We used the methods of Pandolf et al. 1977 and Ludlow & Weyand 2017 to generate VO2estimates for the 217 individual trials we administered. RESULTS: Measured values of VO2increased from 22.5±3.3 and 24.2±4.1 ml kg-1 min-1at 1.7 m s-1,to 31.3±5.2 and 30.5±4.3 ml kg-1 min-1at the fastest speed administered for males and females respectively. In contrast, the accuracy of the predictive models decreased with speed and yielded prediction errors of -12.1 and -22.6% at 1.7 m s-1 for the Pandolf and Ludlow & Weyand methods respectively, these errors were -17.2 and -31.0% at the fastest speeds administered. When evaluated at the speed subjects used in the field trial the prediction models underestimated energy expenditure by 5.7±4.6 and 9.8±4.8 ml kg-1 min-1respectively. CONCLUSION: We conclude that existing predictive models do not retain their internal accuracy, and substantially underestimate measured values when applied to a group of male and female subjects undertaking relatively fast walking speeds with a heavy load.

Supported by US Forest Service Agreement # 16-CR-11138200-005, to CD, BR and MB

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