3D inverse dynamics in non-orthonormal segment coordinate system.

The net joint forces and moments may be computed by several 3D inverse dynamic methods. To do so, an orthonormal segment coordinate system (SCS) is generally mandatory. However, the segment axes ought to be selected following anatomical, functional, and inertial requirements that are hardly compatible with orthogonal axes. An alternative method based on generalized coordinates allows computing inverse dynamics using directly a set of basic points and unitary vectors. A segment definition is put forward in order to follow all of the anatomical, functional, and inertial requirements and the inverse dynamics is performed in a non orthonormal segment coordinate system (NSCS). The NSCS seems a convenient definition in biomechanics as far as anatomical, functional and inertial axes are concerned, but providing that the 3D joint forces and moments are still computable. The inverse dynamic method in NSCS is applied to the gait of a knee valgus subject and compared to a classical inverse dynamic method. The inverse dynamic method in NSCS shows comparable results but implies further clinical interpretations.