Disintegrating The Metabolic Cost Of Walking: Propulsion, Leg Swing, And Lateral Stability Across Speed

Three of the major factors contributing to the net metabolic cost of walking include performing work to restore and redirect the center of mass velocity during step-to-step transitions, swinging the limbs, and maintaining lateral stability. PURPOSE: To determine if the proportions of the net cost of walking attributable to each of these factors are different at slow, normal, and fast walking speeds. METHODS: Using three different external assistive devices, we aided walking during slow (0.75 m/s), normal (1.25 m/s), and fast (1.75 m/s) treadmill speeds. These devices use pulling forces (forward propulsion, leg swing, lateral stability) that have previously been shown to reduce metabolic cost at normal walking speeds. RESULTS: The average gross metabolic rates at the slow, medium, and fast walking speeds were 3.21, 4.65, and 5.74 W/kg. Average standing metabolic rate was 1.64 W/kg. Propulsive assist alone decreased net (standing-gross) metabolic rate during slow, normal, and fast walking speeds (21.5%, 44.3%, and 40.7%, respectively) compared with unassisted walking (P<0.001). Leg swing assist, in combination with the aiding horizontal force, further decreased net metabolic rate for slow, normal, and fast walking speeds (11.2%, 9.8%, and 11.8%, respectively) compared with unassisted walking (P<0.001). The lateral stability device alone decreased net metabolic rate for slow, normal, and fast walking speeds (5.2%, 4.5%, and 1.9%, respectively) compared with unassisted walking (P<0.001). CONCLUSIONS: Our external assistive devices decreased net metabolic cost at slow, normal, and fast walking speeds. However, the proportional cost of the three major factors was not the same across walking speeds. The metabolic costs of each of the three factors were greater at faster speeds. The change in the relative proportional costs may be due to sub-optimal inverted-pendulum mechanics during nonpreferred walking speeds changes.