No announcement yet.

Instantaneous Helical Axis

This topic is closed.
  • Filter
  • Time
  • Show
Clear All
new posts

  • Instantaneous Helical Axis


    Dear Biomech-l Colleagues,

    I am currently conducting research in kinematically defining leg motion
    in chiropractic patients who have exhibited symptoms of leg length inequality
    (LLI). This LLI is physiological in nature, hypothesized to be caused by
    musculature imbalance somewhere in the back. It has been reported that
    chiropractic adjustment results in alleviation in pain related to the LLI
    phenomenon. My research partners and I are in the process of quantitatively
    assessing apparent LLI cases. Since the LLI is not a structural problem,
    adjustments which cause realligment of the legs should result in one of 3

    1) One leg moves inferior while the other remains stationary
    2) One leg moves superior while the other remains stationary
    3) One leg moves superior while the other moves inferior

    Regardless of which case is occuring, there must exist some translation, and
    probably rotation, of one leg with respct to the other. This probably means
    that some sort of pelvic rotation is involved. Nevertheless, we are
    interested in determining the location of the center of rotation of one leg
    with respect to the other. This is where my question arises.

    I have conducted a literature review concerning the calculation of the
    Instantaneous Helical Axis of rotation. Description of this variable lies in
    the equation:

    v = w X r

    where v is the linear velocity of a point, w is the angular velocity of that
    point, and r is the radius of rotation. McFayden et al. (1988) show how this
    equation may be used to find the hip and ankle joint centers. There work was
    conducted in 2D. Woltring expands upon this in a complex manner to find IHAs
    in 3D. It seems, however, that the previous equation could be used, since
    all are vector quantities. If a reference frame attached to one of the legs
    is considered to be the global reference frame and the reference frame
    attached to the other leg is the local reference frame, then the point of the
    ICR should be able to be determined using information from both reference

    My question concerns the determination of w. If a vector is constructed
    from the origin of frame 1 to frame 2, movement of frame 2 wrt frame 1 will
    result in some w. But frame 2 might also be "spinning" about itself. Does
    this "spinning" come into play when determining w? If it does not, the the
    previous equation can be utilized. If it does, then how do I calculate w?
    Any responses or thoughts would be greatly appreciated.


    John DeWitt
    Graduate Assistant
    Exercise and Sport Research Institute
    Arizona State University
    Tempe, Arizona USA 85287-0404
    (602) 965-7528