Backpack Moment of Inertia Affects Oxygen Consumption During Walking

Abstract

Prior research on the metabolic energy cost of carrying backpack loads has focused primarily on the effects of weight carried on the rate of oxygen consumption during walking. However, less research has been done on the effects of manipulating backpack mass center location or moment of inertia (MOI) on oxygen consumption. PURPOSE: The purpose of this study was to examine the effects of manipulating the MOI (distribution of mass about an axis of rotation) of a backpack load on the rate of oxygen consumption during walking. We hypothesized that varying the relationships among MOI values for the principal axes would differentially affect oxygen consumption. METHODS: Fourteen male volunteers participated in the study. Steel weights (26 kg total mass) were positioned in each of three military backpacks to produce different MO Is, but with the same mass (35 kg, inclusive of the backpack) and mass center locations (0.3 m posterior to the pack frame and 0.3 m superior to the bottom of the pack). The three packs differed in that each had the MOI of a unique principal axis (Ixx, Iyy, Izz) set at a small value (< 1.0 kg.m2), while the other two principal axes had large moments of inertia (> 2.0 kg.m2). Relative to the load carrier's body, Ixx, Iyy, and Izz defined the MOI about the anterior-posterior axis, the medial-lateral axis, and the vertical axis, respectively. The volunteers walked on a level treadmill at 4.8 km/h for 10 min carrying each of the packs. The rate of oxygen consumption (VO2) was recorded (COSMED K4b2, COSMED, Rome, Italy) during the last 90 sec of the walking period and was scaled to body plus load mass. A repeated measures ANO VA with three levels of MOI and follow-up comparisons were used to assess differences in the mean mass specific VO2. RESULTS: A significant difference (p < .05) in mean mass specific VO2 was found between the small Ixx value pack (15.04 ml/kg/min (SD 1.96)) and the small Iyy value pack (16.27 ml/kg/min (SD 2.09)). Mean differences approached statistical significance (p = .061) between the small Ixx pack and the small Izz pack (16.15 ml/kg/min (SD 2.23)). The small Iyy value and small Izz value packs did not statistically differ. CONCLUSION: The mean mass specific rate of oxygen consumption was 7.6% lower when Ixx was minimized relative to Iyy. Therefore, the metabolic energy cost of carrying backpack loads is not solely dependent on the amount of mass carried. The manner in which items are stowed in the backpack also affects metabolic energy expenditure and thus the physical endurance of the load carrier.