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Joint Moments (norm & Local Coord)

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  • Joint Moments (norm & Local Coord)

    To All,

    First Question - Recent papers reporting the net joint moments have
    taken two approaches to
    decreasing variability across subjects. Some studies (Eng and Winter, J
    Biomech, 1995) divide the net joint moment by body mass. Other papers
    (Berchuck, JBJS 1990, Holden and Stanhope, Gait & Posture, 7:1-6, 1998)
    the net joint moment by % body weight * height. The reasoning behind
    second approach is that both the ground reaction force (which is
    correlated with body weight)
    and segment length (which is correlated with height) are used to
    estimate net joint

    However, a recent abstract by Sum et al(Sum et al, Gait & Posture, 7,
    1998) suggests that using % body weight * height generally doesn't
    variability, except for the ankle dorsiflexion peak during walking. In
    height explained little of the variability in the frontal plane moments
    (< 11%) and
    explained no additional variability once the moments had been normalized
    to body mass.

    This appears to suggest that normalizing to % body weight * height
    does not
    significantly reduce variability, and therefore, dividing net joint
    moments by
    body mass such as in the Eng and Winter(Eng and Winter, J Biomech, 1995)
    is adequate. Yet current studies appear to be dividing the net joint
    moments by % Body weight * height (Holden and Stanhope, Gait & Posture,
    7:1-6, 1998).
    Are there other compelling arguments to make % body weight * height the
    over body mass?

    Second question -- When moving to reporting three dimensional joint
    moments there are three options when mapping the moments into the local
    coordinate system. The moments can be reported in the proximal segments
    system, distal segments coordinate system or mapped into the non -
    orthogonal axes of
    the joint coordinates used to estimate kinematics (JCS).

    Siegler and Liu (In Allard et al, Three Dimensional Analysis of Human
    Locomotion, Wiley and Sons, Ltd, 1997 page 203) suggest that the moments
    be mapped into the JCS. However, Andrews (Andrews, JG, J Biomech,
    17(2):155-158, 1984) raised the issue that the joint center is not
    common to all 3 axes when using the JCS. And therefore, "...the vector
    sum [of the three joint torques] does not represent the combined turning
    effect of all joint structures about a single point because the the
    three axes do intersect at a common point."

    Comparisons of mapping the net moments into the femoral coordinate
    system and
    tibial coordinate system during walking suggest very close
    approximations in
    the sagittal and frontal planes, however, the differences are > 100 % in
    transverse plane(Pilot data). In terms of the transverse plane it
    appears to make sense for interpreting muscle function to map the
    moments into the local coordinate system of the distal segment. For
    example the hamstrings are considered internal/external rotators because
    of their relationship to the tibial long axis not the femoral long

    What should the standard be for reporting local joint moments?

    I will post all replies.


    Jeff Houck, PhD, PT
    Ithaca College - Rochester Campus
    300 East River Road Suite 1-102
    Rochester, NY 14623

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