The effect of modeling the knee as a simple spherical or revolute joint during gait, cutting and hopping were evaluated. The results indicate that, even during gait, a revolute joint may be too restrictive in its representation of the knee, with flexion/extension errors as high as 9.4°. By imposing the spherical joint, joint angles showed consistent and strong correlations with the true joint angles for the functional tasks, although internal/external rotation angle were moderately affected (errors up to 4.2°). For both constraint types, the remaining Degrees of Freedom (DOF) were not consistent across subjects and must be considered unreliable, with translation errors of up to 11.0 mm with a spherical joint and 11.9 with a revolute joint.
In conclusion, the results of this study suggest that inclusion of a spherical joint produced reliable results only for joint angles, yet significant bone translations were eliminated from the recovered motion. Furthermore, the constraints imposed flexion/extension errors of up to 4.4° with a spherical joint (observed during hopping) and 9.4° with a revolute joint (observed during gait). Although the joint DOF errors are generally smaller than those associated with soft tissue artefacts, they may still contribute to a significant source of error in the models when used.