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Hi guys,

The new forum seems to allow more research and writing options. It seems to be a good idea. I’m sure that it is well appreciated by the AnyBody users community.

I would like to know if someone at AnyBody or elsewere is actually planning to include some sort of scapulohumeral rhythm for the abduction movement in the shoulder model. From your knowledge, do you think it would be possible to do so, without using an experimental kinematics datas and by using the actual scapula frames ?

Thank you very much in advance.

Best regards.

Pierre-Olivier

Hi Pierre-Olivier,

The scapular rythm is something we hope to implement in the model someday. If you don’t want to drive it directly with measured kinematic data you need to define the relative motion of the humerus and scapula with some sort of equtions.

There is a paper about a linear regression model of the scapular motion: JH de Groot, R Brand. A three-dimensional regression model of the shoulder rhythm. Clin Biomech 2001;16:735-743. You should find usefull information there.

Hi Sylvain,

Effectively, I was thinking to use some equations.

I see that the frames used in the AnyBody’ shoulder model are quite similar to those of Wu et al. (2005) ISB recommendation on deﬁnitions of joint coordinate systems of various joints for the reporting of human joint motion - Part II_ shoulder, elbow, wrist and hand. Are they based on this paper ? If yes, I think that it would be easier for us to use the new regression model available in the litterature to add the scapulohumeral rhythm.

Thank you again

Pierre-Olivier

Hi Pierre-Olivier,

the coordinate systems of the shoulder model are a bit different than the ISB recommendation. There are rotated -pi/2 around Y. This corresponds to the descrption of the Deutch Shoulder Group from where our shoulder model is based on.

But if you have equtions using the ISB coordinate system, you can easily create a new node with the corresponding orrientation and use it to drive the model.

Best regards,

Sylvain, AnyBody Support.

Hi again Sylvain,

I got it about the Deutch Shoulder group reference frames.

My other questions are about the 3 elements muscle model .

Q.1 :

In 1989, Zajac defined well this model, and from what I know, his theory about the Hill type model have been widely used in the past few years. The equations used need an activation signal A(t) in the active part of the muscle. AnyBody does not need any activation, why ? How the equations deal without this signal and are still considered as a Hill-Type model ?

Q.2 :

Lt0 is the tendon slack-length. Thus, can we consider Lfbar as the muscle slack length ? If not, is there a value for the muscle slack-length ?

Q.3:

How does the optimization process based on the min/max criterion used in AnyBody solves a problem involving only one muscle acting on a joint ? Is it like the dumbell curl example ?

Thank you very much for you advices.

Best regards

Pierre

Hi Pierre

These are good question:

A1: The muscle model does actually use the activations, it calculates the muscle model in two ways: one with the activation set to zero, this gives info about passive force and one analysis with activation set to one, this provides info about the maximum strength.

A2: Lfbar is the nominal fiber length, the neutral fiber length, i.e. the length of the contractile element at which the muscle has the strength of F0. Lfbar is measured in length units, i.e. meters.

A3: If the system has one dof and one muscle, there is only one solution, it is solved as any other solution by the optimizer, but there are no room for the optimizer to reduce the activation.

Please ask again if you have further questions.

Best regards

Søren, AnyBody Support