Figure 2.

The set of relative angular velocity vectors ${\overrightarrow{m}}_k$ projected onto knee axis $\overrightarrow{r}$ with resulting residuals $e_k$. Note that for a perfect hinge $m_k\Vert \overrightarrow{r}\forall k$ however, due to imperfect hinge kinematics of the human knee and soft tissue perturbation of the gyroscopes mounted to the skin of the leg, the set of vectors ${\overrightarrow{m}}_k$ takes this characteristic double cone shape.