Mass and mass distribution of below-knee prostheses: effect on gait efficacy and self-selected walking speed.

Objective: To study mass and mass distribution effect on function of below-knee prostheses.

Design: Design modifications were done to produce proximal center of mass location versus distal center of mass location variations, and prosthesis weight was modified from 42% to 70% of normal limb weight. Work across joints of affected and unaffected extremities was compared to assess the ability of the prosthesis to substitute for function loss.

Setting: University biomechanics laboratory.

Participants: Fifteen volunteers with below-knee amputations, residual limb length greater than 8.3 cm, but excluding Syme amputations.

Interventions: Patients walked with all configurations at self-selected walking speeds and 120 m/min.

Main Outcome Measures: Self-selected walking speed and metabolic efficiency. Work across the joints of affected and unaffected sides was compared.

Results: Proximal center of mass location produced a more efficient gait. Weight change from 42% to 70% of normal had no significant effect. Mechanical studies show that the prosthesis is a relatively poor substitute for the normal limb; most work is done by the nonamputated side. Particularly, the prosthesis failed to produce effective forward impulses on the body, resulting from push-off and deceleration of the swing leg.

Conclusions: For a proximal center of mass, lightweight distal components (e.g., feet) should be used; it is questionable whether further expenditure to develop ultralightweight prostheses would be cost effective for level walking.