musculotendon length calculations

[Tim Dorn]

Hi.

I was wondering how OpenSim calculates musculotendon lengths for a given pose. My line of thinking is something like:

1) convert the origin, insertion, and any via points to a common coordinate system

2) add the straight line segments of each point to determine the mt length (neglecting wrapping surfaces).

What I would like to do is to be able to input a model posture (joint angles) into a script of somesort, and output the musculotendon length at that posture. For example if I input knee_angle, ankle_angle, and subtalar_angle, I would like to retrieve the gastrocnemius musculotendon length (since this muscle-tendon spans both the knee and ankle joint). There is a unique mt length for a given set of joint angles, but somehow I need to be able to extract this information.

I can see the relationship between gastroc mt length and knee angle for instance by plotting the relationship using the plotter, but this is only the relationship for the knee_angle, and not all of the degrees of freedom about the ankle and knee. Any ideas?

Thanks,

Tim

[Ayman Habib]

Hi Tim,

If you make a motion file with one frame per pose that contains a column for each coordinate and load it, you can use the plotter to create the curves for muscle tendon lengths thru the motion (for the muscles you care about). Then you can export the data to a storage file using the "Export Data" option off the context menu in the plotter. The data files generated are text files that can be inspected later in the plotter or opened in Excel.

Hope this helps,
-Ayman