In Posturography we see an increasing tendency that people want to look

at movements of the CoM in addition to the CoP measured with a force

plate.

Calculating CoM seems to be a simple **double integration of the shear**

forces of the force plate. However this doesn't work in practice because

the double integration **enlarges the smallest errors** in your
measuring

chain - resulting in a CoM miles away of your starting point.

Even when you force your starting and your ending point to be the same

(by playing with the initial conditions) your CoM path is not

reasonable.

I have recently thought up a solution for the above problem:

* At the **instant when the shear
force is zero**, the CoM and CoP are

exactly above each other.

* This would allow the design of
a double integration algorithm

which only has to **integrate the short periods of time** between
these

instants.

Has anybody come across this method before? And if yes, is it verified?

Best Regards

Christian Calame

Mr. Christian Calame, Product Manager Biomechanics

Kistler Instrumente AG Winterthur, P.O.Box 304,

CH-8408 Winterthur, Switzerland

Tel: +41 52 224 11 11, Fax: +41 52 224 14 14

http://www.kistler.ch/biomech

I and my supervisor have been discussing the

would indeed be valuable to have an accurate method for estimation of

the position of CoM from force plate data. I believe Mr. Calame's

proposed method to estimate CoM with a double integration of the shear

forces between timepoints when the shear force is zero, will work fine

if the body behaves exactly like an

ground reactin force vector, starting at the CoP, is pointing at the

CoM, and the

However, in reality we are **not perfect inverted pendulums** and
I think

there are situations where the shear force can actually be zero but
the

projection of CoM and CoP do not neccesserily have to coincide. I hope

this example will make my train of thought easier to understand:

Imagine a man with a wooden leg, where his feet have **only a point**
**contact with the floor**. This man can produce a **shear force**,
for instance

by using hip flexors, which will cause his projection of CoM to move

backwards and away from the point support surface. To try **to prevent**
**falling** he then produces a shear force in the opposite direction
by

using hip extensors. Sometime between these two actions the shear force

must be zero, but the CoP (which can not move from the point support

surface) and CoM will not be exactly above each other, since CoM has

moved a little backwards.

I would also like to comment the idea of a normative data base. I think

the idea is splendid, and I **hope the data base will include postural**
**measurements as well**. I definitely would like to contribute
to such a

data base. By the way who will have the copyright of the data?

Best regards

Annica Karlsson

Annica Karlsson, Ph.D
student

Systems and Control, Uppsala University

PO Box 27

751 03 Uppsala, Sweden

Tel: +46 18 741 78 46

E-mail: Annica.Karlsson@SysCon.uu.se,

http://frej.teknikum.uu.se/Personnel/ack/ack.html