Distance between points across wrapping objects

[Brandon Brown]

Hello all,

Does anyone know of any papers or resources which might describe how the distance between points spanning a joint with a wrapping object is found? For example, if a cylindrical wrapping object is used at the elbow and the triceps long wraps over it, how is the length of that portion of the triceps long calculated?

Thanks,

Brandon

[Michael Sherman]

Hi, Brandon. The basic idea is that the path is a series of straight line segments and geodesic curves. So the distance between two points on the path is the sum of line segment and geodesic lengths. For simple shapes like spheres and cylinders the geodesics can be calculated analytically; for more complicated shapes they are approximated by discretization or numerical integration.

There are lots of papers on the general topic. Our most recent one is Sholz et al. 2015 (let me know if you have trouble getting the full text). That one discusses where we are going but not how things work now, but its references cover a lot of earlier work. Garner & Pandy 1999 is a good background.

Regards,
Sherm

[Brandon Brown]

Hey Michael,

Thanks for the paper recommendations. I'll look at them.

Brandon

[David John Saxby]

Hi Guys,

I just downloaded that new paper Sherm, I wanted to ask is there a way using the OpenSim API or potentially more lower-level in SimBody to specify the directionality of the wrapping of a muscle around a wrap object? I noticed in the Garder and Pandy paper and through experience with the OpenSim models that at a certain angle (90 or 180 depending of ref frame) the muscle "flips" side as the radius to the tangent changes polarity as the bounding points are at parity. Can this be a priori specified so that, for example, at large hip flexion angles during a squat motion the gluteals will still wrap on the on the + radius of wrap object?

Best,

David