One cuff is positioned on the midthigh and another
on the midshank sufficiently distal to the hip and
knee joints to avoid interference during walking.
Wands, 7 cm long, with markers at the tip are attached
to these cuffs. The cuffs are aligned laterally
with the long axis of bones to reflect the neutral
rotation angles while standing in a normal position.
The axes of the wands are also aligned such that
they are in line with the flexion-extension axis of
the corresponding distal segment.
An empirical relation, based on a pelvic radiograph
study (J. Gage, s. Tashman, personal communication,
1985), is used to estimate the location
of the hip joint center relative to the ASIS location
and pelvic orientations. In this method, the X, Y,Z
coordinate distances of the hip center from the
ASIS marker are calculated as a function of the leg
length. The location of the hip joint center can also
be computed using the distance between the two
ASISs as the independent variable (3). The knee
center is assumed to lie in the plane defined by the
knee marker, thigh-wand marker, and hip joint center,
halfway between the femoral condyles. In a
similar way, the ankle center is assumed to fall in
the plane defined by the ankle marker, the knee
center, and the shank-wand marker, and located
halfway between the malleoli.