Background: The purpose of this study was to develop a predictive model for uphill and downhill load carriage. Relative to level walking, net energy costs increase with uphill movement and decrease moving downhill. To simulate load carriage over complex terrain, a model must estimate the cost of downhill movement. The net cost of downhill movement is expected to reach a minimum value, then increase as work is required to maintain stability. Thus, downhill costs cannot be simply extrapolated from a linear relationship for uphill work.
Method: Oxygen uptake (VO2) was measured for 16 subjects during test sessions which consisted of walking at 1.34 m x s(-1) on a single grade (-12%, -10%, -8%, -4%, -2%, 0%, +4%, +4%, +8% and +12%) with a 0, 9.1- or 18.1-kg load.
Results: No significant gender differences were found, therefore data were pooled. The minimum VO2 values occurred at -8% grade.
Conclusion: Our model assumes that the total energy requirement (WT) is the sum of the cost of level walking (W(L)) plus the cost of vertical displacement (Wv) for the total mass (body plus load). For uphill work, Wv was calculated by multiplying the cost of vertical displacement by an efficiency factor. For downhill work, the cost of vertical displacement was modified by an exponential function of the slope angle. Values for level and negative slope walking with no load were compared with estimated values derived from two published studies to partially validate the negative model.