Ankle fixation need not increase the energetic cost of human walking

We tested whether the metabolic energy cost of walking with the ankles immobilized can be comparable to normal walking. Immobilization of any lower extremity joint usually causes greater energy expenditure. Fixation of the ankle might be expected to eliminate the work it normally performs, to detrimental effect. But fixation using lightweight boots with curved rocker bottoms can also bring some benefits, so that the overall energetic effect might be quite small. We measured oxygen consumption, kinematics, and ground reaction forces in six (N=6) able-bodied human volunteers walking at 1.25 m/s in three conditions: normal walking in street shoes, walking with ankles immobilized by walking boots, and normally with ankles free but also weighted to match the mass of the walking boots. We estimated metabolic energy expenditure, joint work, and overall work performed on the body center of mass as a function of ankle fixation. Ankle fixation with walking boots caused the total rate of energy expenditure for walking to increase by 4.1% compared to normal (P=0.003), but differed by an insignificant amount (0.4% less, P=0.78) compared to walking with equivalent ankle weight. Compared to normal walking, ankle fixation can reduce ankle torque and work during the stance phase, most notably during late stance. This apparently makes up for the loss of ability to push-off as normal. With a suitably lightweight apparatus and curved rocker bottom surface, loss of ankle motion need not increase energy expenditure for walking.