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

• Published in: Journal of applied physiology (1985) Vol. 123; no. 5; pp. 1288 - 1302
• Main Authors: Ludlow, Lindsay W., Weyand, Peter G.
• Format: Journal Article
• Language: English
• Published: United States American Physiological Society 01.11.2017
• Subjects: Adult; Biomechanical Phenomena - physiology; Body weight; Economics; Energy metabolism; Exercise Test - methods; Exercise Test - standards; Female; Forecasting; Gravitation; Gravity; Human mechanics; Humans; Load distribution; Male; Mechanical properties; Mechanics (physics); Metabolic rate; Oxygen uptake; Quality; Torso; Vanadium oxides; Velocity; Walking; Walking - physiology; Walking - standards; Walking Speed - physiology; Weight-Bearing - physiology; generalized equation; load carriage; metabolism; wearable sensors; locomotion; algorithm
• ISSN: 8750-7587; 1522-1601; 1522-1601
• DOI: 10.1152/japplphysiol.00504.2017

The metabolic energy that 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 to 1.6 m/s on six gradients: −6, −3, 0, 3, 6, and 9°. 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 to 1.4 m/s 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 ml O 2 ·kg -body −1 ·min −1 , respectively). As theorized, the walking portion (V̇o 2-walk  =  V̇o 2-gross – V̇o 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 (V̇o 2-gross , ml O 2 ·kg -body + load −1 ·min −1 ) of all the remaining loaded and unloaded trials combined ( n = 1,412 trials from 90 speed/grade/load conditions). The accuracy of prediction achieved ( r 2  = 0.99, SEE = 1.06 ml O 2 ·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. NEW & NOTEWORTHY Introduced is a “minimum mechanics” model that predicts human walking economy across a broad range of conditions from only three variables: speed, surface grade, and body-plus-load mass. The derivation/validation data set includes steady-state loaded and unloaded walking trials ( n = 3,414) that span a fourfold range of walking speeds on each of six different surface gradients (−6 to +9°). The accuracy of our minimum mechanics model ( r 2  = 0.99; SEE = 1.06 ml O 2 ·kg −1 ·min −1 ) appreciably exceeds that of currently used standards.