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Van Den Bogert, Ton
07-11-2007, 12:48 PM
Steve Piazza wrote:

> We studied cadaver specimens with
> markers mounted on bone pins, so skin movement was not an
> issue. We still saw errors in axis orientation that were
> between 20 and 40 degrees, errors that did not result when we
> tested an anthropomorphic mechanical linkage. This suggested
> to us that the cadaver joints exhibited deviation from their
> mechanical analogues that made the problem ill-posed.

Could it be that the cadaver joints are somewhat "loose" when you move
them passively? I did my in vivo measurements with the subject moving
their foot with their own muscles, without ground contact. The slight
compression of the joint may have caused the motion to be more
consistent with the two-hinge model.

With in vivo data I estimated the error being much less than 20-40
degrees. Yes, the minimum was shallow but quite robust, given enough
data over the entire range of motion. With weightbearing though, the
results were all over the place.

Ton van den Bogert

--

A.J. (Ton) van den Bogert, PhD
Department of Biomedical Engineering
Cleveland Clinic Foundation
http://www.lerner.ccf.org/bme/bogert/

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