Hello Kevin and hardcore biomechanists,
Kevin said:
>However, Bart, what if you have one externally applied force
>affecting three independent axes of rotations (i.e. MTJ, STJ, ankle
>joint) and each of those joints will displace a different amount in
>response to that force. Then could we predict that the joint with
>the most compliance would displace more than the joint with the
>least compliance over a certain period of time when the force is
>applied simultaneously across all three joint axes? The greater
>displacement over time would mean greater angular acceleration of
>that particular joint? You are correct that the slope of the load
>vs. displacement curve will be the stiffness of that joint and it is
>simply determined by displacement and loading force without any need
>to consider a time parameter or a velocity (displacement/time)
>parameter. But if the relative displacements of each joint is
>different under a given load (i.e. stiffnesses are different), then
>couldn't we also assume that the velocity and/or acceleration of
>movement would increase in the joint with less stiffness?
Bart replies,
This is not a simple problem Kevin. What you ask is a free body
analysis of a 4-segment model, but not a common one, because the
interaction at the hinges is complicated by an extra joint moment
that is affected by the varying angle between the *two* adjacent
segments. It is therefore certain that the inertia moments of the
segments will influence the angular acceleration of the segment, an
thus also the relative angular acceleration between the segments. So
no answers are readily available on your questions. One first has to
solve the free body equations !
And honestly, I am not in the mood to solve it tonight.
But I 'll put it on the podiatric gold list. When I finish the hallux
limitus paper I am working on now, it may get a chance, especially if
you can convince me with a few excellent brews at the bar in Boston.
Cheers,
Bart
--
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Bart Van Gheluwe
Laboratory of Biomechanics
Vrije Universiteit Brussel -Fac. LK
Vakgroep BIOM
Pleinlaan 2, 1050 Brussel, Belgium
Tel.: 02/629.27.33 (31)
Fax: 02/629.27.36
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