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What happens to muscle length when scaling?

Posted: Tue Aug 31, 2010 3:26 am

by marc_carc

Hi, I have a question which has been on my mind for a while.

Most muscle models use the optimal fibre length to calculate a normalized fibre length, which in turn is used with a force-length function (along with other parameters) to estimate muscle tension.

What happens when the model is scaled? Are the optimal fibre lengths for the muscles scaled accordingly? How is this done? If the optimal fibre length is not scaled, then doesn't this effectively tighten or loosen the muscles on the model's skeletal structure?

Cheers,

Marc

RE: What happens to muscle length when scaling?

Posted: Tue Aug 31, 2010 1:57 pm

by aymanh

Hi Marc,

When scaling a model, the following steps are taken:
1- Model is put in fixed configuration (default)
2- (Pre-Scaled) muscle length is computed and saved.
3- Scaling of Bodies/Joints and attachments is done.
4- (Post-scaled) muscle length is computed
5- optimalFiberLength and tendonSlackLength are scaled by the ratio between the lengths computed in steps 4 & 2 above.

This way no tightening or loosening of the muscles is introduced by scaling.

Hope this explains, and sorry for the lack of documentation.

Cheers,
-Ayman