Estimating loaded, inclined walking energetics: No functional difference between added and body mass

We used external loading and surface inclination as experimental tools to: 1) test whether the metabolic cost of transporting body mass and torso mass are equal during walking, and 2) to develop an algorithm for estimating walking metabolism in the field. Rates of oxygen uptake were measured in ten physically active volunteers during constant-velocity treadmill trials (0.6-1.4 m·s-1) on four grades (0-9°) under three loading conditions (1.0-1.31 times body mass). Walking metabolic rates (Egross-Erest) increased systematically with speed, load, and grade to span values from 2.2 to 14.2 times measured resting metabolic rates. When walking metabolism was expressed in relation to the total mass carried, loaded and unloaded metabolic rates were nearly identical across all conditions. The equivalent costs of transporting one kg of body and external mass allowed formulation of a promising estimation algorithm requiring only total mass, speed, and grade as inputs (R2=0.98; SEE=0.37 W·kg-1; n=360 trials).