Nonlinear Dynamic Behavior of the Human Knee Joint—Part I: Postmortem Frequency Domain Analyses

Characteristic results of postmortem experiments on five knee-joint specimens are reported. The experiments were performed to investigate the applicability of a local linearization technique that would make it possible to describe the dynamic behavior of the joint in terms of transfer functions. The results indicate that the stiffness of the bracing wires, attached to muscle tendons to create a static equilibrium position, can be accounted for when determining the stiffness of the joint. Besides the static equilibrium configuration, the magnitude of the dynamic load and the type of dynamic load applied to the joint can be shown to have their influence. As the influence of the dynamic load is significant, it has to be concluded that in essence the knee joint has to be regarded as a nonlinear system, making application of a Local Linearization Technique questionable. However, when the magnitude of the dynamic load is included as an additional measurement parameter, an indication can be obtained about the behavior of the joint and the degree of nonlinearity.