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Julie Wilson Steege
03-22-1996, 03:55 AM
BIOMCH-L'ers:

We are using an ELITE motion analysis system to collect whole-body, 3D
kinematic data, although we are performing 2D, sagittal plane analysis of
pulls made while standing. Because we study whole-body movements, we deal
with many joints including the ankle, knee, hip, lumbar spine, shoulder,
elbow, wrist, and neck. Currently, we palpate bony landmarks in order to
estimate the location of each joint's center of rotation and then place
reflective markers directly over these estimated joint centers. We are
concerned about the accuracy of this method, however, and are looking for a
more precise approach.

Recently, several questions have been posted to BIOMCH-L regarding the best
way of estimating specific joint center locations from anatomical landmarks.
It is clear from the responses to these postings, as well as from our search
of the literature, that there are several ways to approach the problem of 2D
joint center identification. These include:

(1) Putting markers directly on the estimated joint center locations;
(2) Putting markers on various anatomical landmarks and then using them to
infer a joint center location;
(3) Using an array (rigid or not) of at least 2 markers on each segment and
rigid body analysis to compute joint center locations from "calibration
movements", performed before the start of the experiment, that involve
substantial changes in joint angles;
(4) Using an array on each segment and rigid body analysis on the movements
in the experiment to determine the instantaneous center of rotation.

It seems that many labs use a combination of these methods due to the
limitations of the various techniques (solutions breaking down when changes
in joint angle are small, peculiarities of a particular joint, etc.).

While we realize that it is difficult to generalize across joints, we are
wondering if anyone has compared the errors in 2D joint center estimation
for these different techniques. We are aware of an article by Spiegelman
and Woo (1987) which compares a geometric (Reuleaux) approach with a rigid
body analysis approach and another by Crisco et al. (1994) who did a more
extensive error analysis of a similar rigid body method.


References:
Crisco, J., Chen, X., Panjabi, M., Wolfe, S. (1994). Optimal marker
placement for calculating the instantaneous center of rotation. Journal of
Biomechanics 27(9): 1183-1187.
Spiegelman, J., Woo, S. (1987). A rigid-body method for finding centers of
rotation and angular displacements of planar joint motion. Journal of
Biomechanics 20(7): 715-721.


Please send any responses to jwsteege@merle.acns.nwu.edu and we will post a
summary of responses. Thanks in advance.


Wynne Lee, Ph.D.
Jim Patton, M.S.
Julie Steege, M.S.

Motor Control and Learning group
Programs in Physical Therapy
Northwestern University
Chicago, Illinois, USA


************************************************** ********************
Julie Wilson Steege, Research Engineer
Programs in Physical Therapy
Northwestern University
Ph. (312)908-6785
jwsteege@merle.acns.nwu.edu