Knee elasticity influenced by joint angle and perturbation intensity.

The responses of the human knee joint system to small rotational displacements were studied in the horizontal plane to eliminate the effect of gravity. Band-limited noise was used to produce up to +/-3 degrees angular perturbations to the knee joint. Under these restricted conditions, the knee joint system could be described by a linear, second-order model with the assumption that the mechanical properties of the knee were constant over these small displacements. The elasticity about the knee was influenced not only by the static joint angle but also by the perturbation intensity. Realistic models of the knee joint system should be modeled by a position-dependent, nonlinear system to remain valid over a large range of rotation.