Wrapping Surfaces vs. Via Points for CMC and Forward Dynamic

[Colin Smith]

Hello All,

I am currently working on building subject-specific lower extremity models from MRIs for use in forward dynamic simulations. I am trying to decide whether to implement the muscle paths using only via points or with wrapping surfaces. Does anyone have experiences they can share with using wrapping surfaces with the CMC tool and forward dyanmic tool? I know I have read in the past that wrapping surfaces caused these tools to take extreme lengths of time to run, is this still the case?

Thanks for your input,
Colin

[jimmy d]

Hi Colin
The trade offs between wrapping surfaces and via points tend to be a speed accuracy tradeoff. Via points make your simulation faster but they create discontinuities in your muscle dynamics. Wrapping surfaces give a convex analytical surface for the muscle length to be expressed as, but the equations that are needed to express it add processing time to simulation.

As I understand it, ellipsoid shapes add large amounts of processing time and create especially slow simulations. If you are using wrapping surfaces and want a nice trade off between speed and accuracy, you can use cylinders or spheres as wrapping surfaces, they tend to be much faster.

Muscles that arent necessarily involved in answering your question can be done with via points to speed up the overall time of the simulation.

hope that helps,
-james

[Colin Smith]

James,

Thanks for your comments, they were very helpful. In my preliminary tests, I have found your thoughts on the speed of cylinders and spheres vs. ellipsoids to be right on.

Colin