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Van Den Bogert, Ton
10-06-2004, 11:59 PM
Trampas,

Dynamics of Human Gait (by Vaughan, Davis, O'Connor, 1992) defines segment local coordinate systems using markers at anatomical sites, and there is no guarantee that joint angles will be zero (i.e. segment coordinate axes aligned) during standing. In fact, it is important in clinical gait analysis that you do not define standing as zero, because then you no longer "see" that a patient has abnormal neutral posture. Nevertheless, I think that Vaughan intended that his method would give joint angles that are close to zero during standing in a healthy person.

We have used the book to define joint centers, and found that the ankle joint center (equation 3.13) may be too sensitive to where the markers are placed on the foot. Specifically, the unit vector Wfoot is defined to be perpendicular to the plane defined by the three foot markers. If these markers form a narrow triangle, the plane is not well defined. We replaced equation (3.13) by simply the midpoint of the medial and lateral malleolus, which seemed to be more robust. This is no longer dependent on where the heel and metatarsal markers are placed.

Similarly, the eversion angle may also be sensitive to this issue. I suggest trying to place the foot markers exactly as Vaughan did, especially marker 1, and the height of marker 2 (Figure 3.6). If you already have collected the data, redefining the local coordinate axes (U,V,W) according to your own marker placement seems to be the best alternative. If your application is not clinical, you could define (U,V,W) to be along the global XYZ axes during standing. If you do that for all body segments, your joint angles will be exactly zero during neutral standing.

And, of course, there may be an error in your Matlab code. But if similar code worked for the other joints, and you followed the recipe in the book to define (U,V,W), your code is probably OK.

Ton van den Bogert


--

A.J. (Ton) van den Bogert, PhD
Department of Biomedical Engineering
Cleveland Clinic Foundation
9500 Euclid Avenue (ND-20)
Cleveland, OH 44195, USA
Phone: (216)444-5566
Fax: (501)665-1506


> -----Original Message-----
> From: Trampas TenBroek @ ASU Biomechanics
>
> I am using Dynamic of Human Gait (1st addition I think) to
> compute ankle angles
> per the 15 marker system. I am getting about 30 degrees of
> eversion in a
> neutral or anatomical position, and through full range of
> inversion and
> eversion am not approaching 0. I am wondering if anyone has
> suggestions as to
> an error in the book or a possible place I could have an
> error in my Matlab
> code. I realize this is a very difficult question and am
> just wondering if
> anyone else has dealt with a similar issue. I am also
> wondering if anyone has
> a working program using this marker system I could use to
> compare some data
> against to validate my program. Thanks in advance for any
> help you can give.
>
> Trampas TenBroek
> Arizona State University
> Department of Kinesiology
> Biomechanics Graduate Student
> trampas.tenbroek@asu.edu
> Lab # (480)-965-7528
> Office # (480)-965-3875
>

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