Assessment of passive knee stiffness and set point

The purpose of this study was to identify values of the nonlinear passive knee joint stiffness, K_p, and the passive knee set point angle, θ_p, that allow inclusion of a term in the equation of motion that is often overlooked in dynamic models. Five healthy subjects were tested using the "Push-Pull Test" in which the leg shank was allowed to come to static equilibrium at its resting angle, θ_FR, pushed to flexion, allowed to return to θ_FR, and then pulled to extension. The shank was held motionless for several seconds at 0.2 rad. increments, with the holding angles, θ_h, seen in the data as plateaus following each change in angle. Data were analyzed to determine the moment due to passive stiffness at each holding angle, the nonlinear stiffness coefficient over the range of movement, and the unique set point of the passive stiffness.