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re: THREE DOF FROM TWO DOF

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  • re: THREE DOF FROM TWO DOF

    In response to Richard Smith's question about measuring the angular
    orientation of the upper arm from two markers on the upper arm and one
    on the forearm, may I suggest the following method:

    I do not use markers, rather I digitize the shoulder joint center, the elbow
    joint center, and the wrist joint center (in 3-D) from film or video. The
    shoulder flexion/extension angle and ab/adduction angle can be computed from the
    shoulder and elbow points relative to points on the trunk. To get
    internal/external rotation, however, you need the wrist point as well. As
    long as the elbow does not approach full extension, you can compute the cross
    product of the upper arm vector and the forearm vector to get the elbow joint
    flexion axis vector. The orientation of this axis in the transverse plane
    of the humerus defines its internal/external rotation angle. If you need to
    use reflective markers for an automatic digitizing system, however, you may
    introduce errors by not locating the true joint centers with this method.

    This elbow flexion axis is not defined when the forearm and upper arm are
    parallel because of the limitations of the cross product. In reality, because
    of the "carrying angle" of the forearm relative to the upper arm, the cross
    product will produce an elbow flexion axis pointing in the AP direction when
    the elbow is fully extended. I have not figured out a solution to this
    problem.

    I hope this was helpful. Other people's comments here would be appreciated.

    Sincerely,

    Rick Hinrichs
    Dept. of Exercise Science
    Arizona State University
    Tempe, AZ 85287-0404 USA
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