Effect of alterations in prosthetic shank mass on the metabolic costs of ambulation in above-knee amputees.

The metabolic costs of above-knee amputee ambulation are significantly greater than normal. The role of prosthetic mass and mass distribution on the metabolic costs of walking has received limited study. The metabolic costs of eight unilateral traumatic above-knee amputees were therefore studied under varying mass conditions. All of the subjects were active young amputees with a common prosthetic prescription, which included a total contact ischial containment, suction suspension socket with a graphlite knee assembly and a hydraulic unit, with a Seattle Light Foot (Model and Instrument Development, Seattle, WA). Expired gases were collected during over-ground ambulation at their self-selected walking speed and at three control speeds (.6, 1.0 and 1.5 m/s). The expired volumes and gas concentrations were measured, and the metabolic cost (ml/kg/m) at each ambulation speed was calculated. Data were collected on two further sessions with the addition of 0.68 and 1.34 kg to the center of mass of the shank, after a 1 wk acclimatization period. Results show that in the unweighted condition, the self-selected walking speed had the lowest metabolic energy expenditure and that the addition of weight to the shank did not significantly alter the metabolic costs of ambulation at any of the speeds studied. The common request for lighter weight prostheses does not appear to be based on the metabolic costs of ambulation.