knee center, and the thigh-wand mar

knee center, and the thigh-wand marker in an orientation
perpendicular to the unit vector K and
points to the subject’s left side. The third vector I is
calculated from the cross product of J and K. The
construction of the shank unit vectors is identical to
the thigh unit vectors with the knee center, ankle
center, and shank wand, replacing the hip center,
knee center, and thigh wand. Since only two markers
are used on the foot, only two angular motions
can be derived for the ankle. Therefore, only one
unit vector is required to compute the foot orientation.
This is calculated from the line segment joining
the ankle center and the marker at the foot (between
the second and third metatarsal heads)