View Full Version : Re: Inverse dynamic data from an unstable surface on a GRFplatform?

Van Den Bogert, Ton
09-22-2006, 05:08 AM
Toran mentioned inverse dynamic analysis, which usually implies that motion data is also collected.

In that case, if you use standard inverse dynamic software, the results may be just fine. You would be assuming that the force acting on the foot is the same as the force vector that you measure on the ground. If (mg+ma) of the ball is much smaller than the ground reaction forces, this would be a good assumption. You can do some rough estimates to check this. Rotational inertia could also be a factor, here you need to consider the moment of inertia and the angular acceleration of the ball. Distance (and changes in distance) between foot and ground is definitely not a source of error. Nowhere in a normal inverse dynamic analysis is there an assumption that the foot is on the ground. The true position of the foot is measured with the motion capture system and therefore correctly incorporated in the analysis.

If the ball undergoes large accelerations, or has large mass, you would have to add it to the inverse dynamic model as an extra segment, and you would have to put markers on it to measure the motion. Whether this is feasible depends on the software you use for the analysis.

So, to answer Toran's specific question: motion and height of the ball are not a problem as long as its mass and acceleration are small enough.


Ton van den Bogert
Department of Biomedical Engineering
Cleveland Clinic Foundation


From: * Biomechanics and Movement Science listserver on behalf of David Smith
Sent: Mon 9/18/2006 4:14
Subject: Re: [BIOMCH-L] Inverse dynamic data from an unstable surface on a GRF platform?


Do you intend to use 3D video (eg vicom) aswell as force plate? If not then
i can't imaging how you will get any
further than deternining the forces on the force plate. You won't know the
location and accelerations of the limbs and joints so you can't calculate
the joint forces. You can only guess about the relationship of the force
time graphs to the body motion. I would think the only way this might be
useful is as comparative data.

Cheers Dave

----- Original Message -----
From: "Toran MacLeod"
Sent: Friday, September 15, 2006 9:34 PM
Subject: [BIOMCH-L] Inverse dynamic data from an unstable surface on a GRF

> Does anyone have a suggestion for how to acquire data for inverse
> dynamics, if the foot forces are going through an unstable surface before
> hitting the GRF platform?
> I'm interested in putting a subject on the flat surface of a bosu ball
> (half of a thera-ball with a solid surface http://www.bosu.com/ ) to do
> single leg squats. A GRF plate beneath the ball would be able to collect
> all the forces, and the center of pressure. My concern is that the motion
> and height of the bosu ball will require too many assumptions for accurate
> inverse dynamics data.
> Any thoughts or articles you can direct me to for help?
> Toran MacLeod
> California State University, Sacramento
> http://www.sacramentoroc.com
> http://www.hhs.csus.edu/PT/
> http://www.hhs.csus.edu/khs/Facilities/Facilities.htm#biomechanics
> http://www.ucdmc.ucdavis.edu/pmr/patient_services/therapeutic/physical.html
> http://www.lafootplus.com
> http://www.cuisineforfitness.com
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