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Thomas M. Greiner
11-19-1992, 02:02 AM
I have encountered some unexpected problems with the calculation
of instantaneous centers of rotation. I am hoping that someone
out there might have encountered a similar problem, and
developed a solution.

Just so we all know where I'm coming from, I will outline how I
calculate the instantaneous center of rotation. At the knee,
for example, data is required that describes both the thigh and
the leg before and after knee flexion. These two data sets are
then superimposed so that the thighs match up exactly. Two
landmarks are then identified on the leg from the straight and
flexed knee data sets. Each homologous landmark is joined by a
line and a perpendicular bisector is constructed. The point of
intersection of the two resulting perpendicular bisectors is the
instantaneous center of rotation.

My data consists of three dimensional coordinates for a series
of skeletal landmarks ranging from the sacrum to the foot.
Landmark coordinate data was obtained for each individual in two
postures, one is basically a fully extended limb while the other
is a flexed limb. Using a computer, I am able to precisely
superimpose the coordinates describing any single bone from one
posture over the same bone in the other posture. I assumed that
I could make use of this procedure to examine the displacement
of landmarks associated with an adjacent bone, and thereby
calculate the coordinates for the instantaneous centers of
rotation in the appropriate joint in any given plane of action.

Unfortunately, the answers I get are not consistent and are
frequently extremely inappropriate. Although the joint centers
I have calculated for flexion/extension at the hip and knee
joints seem appropriate, the same methods do not work for the
ankle joint, nor for rotation or abduction at the hip. For one
individual, the calculated center of rotation for the ankle
joint was located near the sacrum. Obviously, this is the wrong