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.