A reassessment of center-of-mass dynamics as a determinate of the metabolic inefficiency of above-knee amputee ambulation.

Above-knee (AK) amputation substantially increases the metabolic cost of ambulation. Although the biomechanical mechanisms contributing to the increase have not been well studied, it has been assumed that altered center-of-mass (COM) kinematics and increased mechanical work account for the adverse effect on oxygen consumption. To understand better the relationship between mechanical work and the metabolic cost, 8 normal subjects and 8 traumatic AK amputees were studied during overground ambulation. Using segmental energy and inverse dynamics analyses, trunk COM excursion, the mechanical work associated with movement of the COM, and the work done by the major lower limb sagittal plane muscle groups were determined. Oxygen consumption was measured using the Douglas bag technique. The metabolic cost was 27% higher in amputees compared with normal subjects (P > 0.01), but no significant differences were noted in the excursion of or the work done on the trunk COM. The muscle work needed to complete a stride was significantly greater in normal subjects compared with amputees. These results indicate that the role of abnormal kinematics and their effect on the mechanical work of walking are complex and incompletely understood, but may be overemphasized as a cause of the increased metabolic cost. Alternative mechanisms for the metabolic inefficiency must be considered.