The Role of Friction in Swing-Phase Control of the Juvenile Above-Knee Prosthesis

R. HICKS, J. CARY, S. TASHMANAND J. CAGE*Newington, Connecticut


Traditionally it has been taught that, by modifying knee friction in a juvenile above-knee prosthesis, the rate of swing could be controlled. A review of the equation for the period of a compound pendulum suggests that this should not be the case, since:

T = 2* Pi (sqrt) I/Mgd where:

T = natural swing period of the shank/foot as a pendulum I = rotational inertia of shank/foot about the knee pivot

M = mass of the shank/foot

g = acceleration due to gravity d = distance from the knee pivot to the center of mass. Thus, friction is not a component of the equation.

The hypothesis was tested in the kinesiology laboratory of Newington Children's Hospital by varying the knee friction of five unilateral above-knee prostheses while collecting kinematic data.

In all cases the swing period of the prosthetic shank was equal or nearly equal to the natural swing period of the shank when bench tested off the patient. Varying the knee friction resulted in changes in the arc of the swing but not the swing time (period).

Efforts to make the prosthetic shank swing faster would be better directed to bringing the rotational inertia of the shank closer to the knee pivot, i.e., subtract weight distally and/or add weight proximally. An estimation of the swing time of the shank can be done easily during fabrication by swinging the limb on a small bolt or axle placed through the hinge bolt.

*Newington Children's Hospital, 181 East Cedar Street, Newington, CT 06111