Walking economy is predictably determined by speed, grade and gravitational load

The metabolic energy human walking requires can vary by more than 10-fold depending on the speed, surface gradient and load carried. Although the mechanical factors determining economy are generally considered to be numerous and complex, we tested a minimum-mechanics hypothesis that only three variables are needed for broad, accurate prediction: speed, surface grade, and total gravitational load. We first measured steady-state rates of oxygen uptake in 20 healthy adult subjects during unloaded treadmill trials from 0.4-1.6 m•s^-1 on six gradients: -6º, -3º, 0º, 3º, 6º, and 9º. Next, we tested a second set of 20 subjects under three torso-loading conditions (no-load, +18, and +31% body weight) at speeds from 0.6-1.4 m•s^-1 on the same six gradients. Metabolic rates spanned a 14-fold range from supine rest to the greatest single-trial, walking mean (3.1±0.1 to 43.3±0.5 mls O₂•kg_-body^-1•min^-1, respectively). As theorized, the walking portion (VO_2-walk=VO_2-gross-VO_{2-supine-rest}) of the body's gross metabolic rate increased in direct proportion to load and largely in accordance with support force requirements across both speed and grade. Consequently, a single minimum-mechanics equation was derived from the data of 10 unloaded-condition subjects to predict the pooled mass-specific economy (VO_2-gross, mls O₂•kg_-body+load^-1•min^-1) of all the remaining loaded and unloaded trials combined (n=1412 trials from 90 speed-grade-load conditions). The accuracy of prediction achieved (R₂=0.99, SEE=1.06 mls O₂•kg^-1•min^-1) leads us to conclude that human walking economy is predictably determined by the minimum mechanical requirements present across a broad range of conditions.

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