R.L. Baker, K.M. Berglund, H.E. Habighorst, J.T. Strang, C.J. Alfiero, M.W. Bundle

University of Montana, Missoula, MT

PURPOSE: The scientific understanding of energy use during load carriage suggests that the additional metabolic increment necessary to support an external load is determined by the load’s percentage of the subject’s body weight. Accordingly, for comparison purposes experimental undertakings often adjust the mass of an external load to represent a constant fraction of each subject’s mass. However, in occupational and applied settings, individuals are frequently asked to support similar absolute loads irrespective of their body weight. Here, we evaluated whether the energy requirements in male and female subjects differed during treadmill walking across a range of speeds, while supporting a common 20.5kg external load. METHODS: We measured VO2during three, 5min trials, administered with a 20.5kg pack, on a level treadmill at 1.7, 1.8, 1.9 m s-1, from 20 young adults(age = 22.1±2.4 yrs),who had been assigned as sex-matched pairs on the basis of mass (10 males, Mb= 72.6±6.3kg; 10 females, Mb = 72.8±6.2kg; range 63.6-82.7kg, difference between pairs = 0.6±0.5kg, max 1.4kg). RESULTS: Measured values of VO2in females were 24.7±4.2, 28.9±3.7, and 30.8±3.3ml kg-1 min-1at 1.7, 1.8, and 1.9 m s-1, respectively, whereas these values in males, although lower, were statistically(min p-value = 0.08) indistinguishable and were 23.1±3.3, 25.8±3.7, and 30.1±4.6 ml kg-1 min-1at the same speeds. Nonetheless, we note our data provide 27 points of comparison, with identical loads, at similar speeds (3 of 10 female subjects were unable to complete the 1.9 m s-1trial), across 10 males and females who are very similar in mass; in 8 of these 27 points of comparison females were more economical than their matched pair. CONCLUSION: Our data lends support to the presence of a sex based difference in load carriage economy, warranting further study. We note also that the similar rates of energy expenditure between the sexes observed here, translate to higher relative intensities for females due to their likely lower mass-specific aerobic capacities (i.e. VO2max).

This work was supported by a multi-investigator award, US Forest Service Agreement # 16-CR-11138200-005, to MB and colleagues.

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