Multisurface Wrapping

I’m trying to add an external band to the human model by defining a wrapping surface between the L1 and right thigh segments. I added the gluteus maximus as a wrapping surface for the lower part of the band, however for the upper part, I’m not sure how to use erector spinae as the other surface for the rest of wrapping.
AnySurface &srf = Main.HumanModel.BodyModel.Left.Leg.Seg.Pelvis.Muscles.GlueteusMaximusWrapSurfaces.Inferior2.cyl;
I’m wondering if it’s correct. I guess what I did is like adding the band to the same surface that the gluteus maximus is attached, since this surface has been defined previously in Anybody, however for erector spinae, as no wrapping surface is defined (it’s a via point muscle), I’m not sure how I can define it.



Hi Zeinab,

It seems reasonable to use the wrapping surface that is used for gluteus maximus for your external band.

For the spine, I suppose, you will need to define new wrapping surfaces. There are different types of surfaces available in AnyBody. I would suggest you use one of the analytical surfaces (e.g., AnySurfCylinder, AnySurfEllipsoid, AnySurfSphere, AnySurfTorus) as they are computationally more efficient. The surface must be defined on some reference frame (AnyRefNode, AnySeg, etc.). You can see the reference manual for more details.

You will need to figure out the best configuration for the wrapping surfaces based on the activities you want to simulate and the attachment of the band. One idea could be to have a surface on L5 in combination with the surface for gluteus maximus to ensure the band wraps over L5 and doesn't penetrate the lumbar segments.

I hope this helps.

Best regards,

Thank you Dave, it was very helpful and I changed my model accordingly.
I have two more questions.

  1. Is it possible to access the backpack model? I want to add a shoulder strap to my model, and I guess the backpack model would be an ideal point to start.

  2. In my model when manikin is in standing posture, there is no contact force between the band and the body, but, when the manikin leans forward (like lifting an object) the band completely sits over the body. By adding the band and wrapping it over a surface, AnyBody assumes that the band slides effortlessly on the surface, and there is no friction between. Therefore, there won't be any contact forces by default, but in reality, we have the contact forces. So, is there any contact elements or functionalities for modeling contact forces between the band and the wrapping surface? Now I think the band force is just based on the lengthening, but we need to also consider the contact forces.
    In the SeatedHuman model, some connections are defined between body and the seat. However, I guess it’s different, as I’m trying to model an elastic band and not a rigid segment to make the contacts between. The other option seems to simulate the force by FDK. Am I right?
    Also, I was thinking about adding two prismatic joints at the ending and starting points of the band to attach it to the body. However, I’m not sure how it can affect my model.


Hi Zeinab,

We don't have any backpack model that we can share. There is a ShoulderBag model in the AMMR Application\Examples folder. Maybe this can help.

Your case is a little bit different than the seated human model, of course. You would need to add some dummy segments to simulate the elastic band as a rigid segment for using the contact elements. Please be aware that if you want to try the contact elements, you should be careful about interpreting your results because sometimes adding these contact elements can introduce some unrealistic solutions. I am somewhat concerned about how the contact elements will interfere with the elastic force that you want to simulate in the band. The contact elements can share the elastic load over the strap and simulate more than just friction.

I would suggest that you avoid simulating friction. It can be really cumbersome with the addition of dummy segments and the interaction of all the loads and the contact elements. Please note that the tensile force over the band will also be distributed over the wrapping surfaces.

Regarding FDK, I don't think that's the right tool for simulating friction.

Is there any particular reason why you want to add the prismatic joints? I don't exactly understand what you want to simulate with the prismatic joints? If you add the joint, then you would also need to add a driver.

I hope this helps.

Best regards,

p.s: You can read more about contact elements over here.

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