Wrapping: muscle force computation

[Joseph Mainberg]

Hi,

I ve been trying to find out how the wrapping of the muscle path affects the calculation of moment arm and the muscle force using the OpenSim documentation. How the force of the muscle is calculated in an inverse dynamics approach, where static optimization is used? Is the “arc” on the wrapping-surface also considered in muscle force calculation?

My assumption: If Wrapping occurs, the muscle between insertion and origin is spitted into 3 parts: 2 linear segments and the “arc” on the Wrapping surface. The 2 linear segments are considered as muscle elements with a force between the 2 muscle-points. The arc-element is ignored.

Is this correct, or is the “arc”-element an additional force? If it is an additional force, how the curvature of this arc is considered?

[Bryce Killen]

Hi Jospeh,

From my understanding when a muscle wraps over a surface you are able to view the muscle as three segments if you desire (see OpenSim API PathWrap. However I do not think this would directly affect the muscle force generation via static optimization.

When a muscle does wrap on a surface, it does indeed change the muscle length and moment arm which would change the resulting moment produced by the muscle for a given force.

To sum up I don't think the "arc segment" is "ignored" or thought of as an additional force as you suggested during static optimization.

Hopefully that makes sense

Bryce

[Joseph Mainberg]

Hi,
thank you for your reply.

The issue is that according to Charlton 2001: “Application of spherical and cylindrical wrapping algorithms in a musculoskeletal model of the upper limb” the moment arm of a muscle is computed from the change of length of the muscle for each timestep. Muscles, which action lines are interrupted by wrapping structures, are exerting Forces at Origin, Effective Insertion, Effective Origin and the insertion point. This means forces are acting between Origin and Effective Insertion and between Effective Origin and the insertion point.

My question is: results this also in a force between Effective Insertion and between Effective Origin?

[Bryce Killen]

Hi Joseph,

I think I understand what you are saying. However I am afraid I do not know the answer that specific question.

Sorry

[Gabor Piskoty]

Hi Joseph, please consider that there is no friction between the muscle and the wrapping surface. The wrapping only influences the level arm of the muscle force in respect to the center of rotation, i.e. the "effectiveness" of the muscle to produce a momentum. In addition, the wrapping and also affects the muscle length (the "arc" is included in the total length), therefore the wrapping also affects the required activation for a given muscle force. I hope, I could help you.
Best regards
Gabor