can not see changes in fiber_length after scaling

[RoZi A]

Hi all,

I am trying to use a manual scaling for arm26.osim model. Actually, I have two questions.
I scaled the model by changing the total mass and I could see how the OpenSim automatically scaled the mass of each body. I have read that after scaling, OpenSim automatically will scale each muscle optimal_fiber_length and tendon_slack_lenght based on the target weight. But these factors (fiber length & tendon slack length) remained the same as the original model after scaling. Do you know if my scaling process was wrong or there is another issue?

Instead of scaling based on mass, can I scale my model based on each body length via GUI? I know some people did that based on normative and anthropometric measures database but I don't know how I can implement these data in manual scaling. I will appreciate if anybody can help me.
(I have OpenSim 3.3)

Any help is appreciated

RoZi

[Thomas Uchida]

I have read that after scaling, OpenSim automatically will scale each muscle optimal_fiber_length and tendon_slack_lenght based on the target weight.

Optimal fiber lengths and tendon slack lengths are not scaled as a function of mass; the documentation should be corrected if it says otherwise. Where did you find that? Also, how would you expect these parameters to scale with subject mass?

Instead of scaling based on mass, can I scale my model based on each body length via GUI?

Yes. Please refer to the documentation and videos in the "Scaling" section of the User's Guide on Confluence (https://simtk-confluence.stanford.edu/d ... im/Scaling) and listed on Forum Topic 8425 (viewtopicPhpbb.php?f=91&t=8425; see, in particular, the webinar on YouTube).

[RoZi A]

Hi Thomas,

Thanks for your reply. You are right, fiber length has nothing to do with mass. I read documentation again and I have noticed it was my misunderstanding. Sorry.
I have one more question and I will be thankful if you could answer that. Is there any way that I can see the length of each bodies in the original model? Let say in arm26.osim file, I want to see the length of the r_humerus. Should I subtract the dimensions of the r_acromion and r_humerus_epicondyle markers to come up with the actual length of that in the model?


Thanks,
RoZi

[Thomas Uchida]

There are a few strategies. In the arm26 model, the length of the "r_humerus" body could be described as the distance between the shoulder and elbow joint centers. The "r_elbow" joint's parent_body property is "r_humerus", so the length of the humerus is simply the Euclidean norm of the "r_elbow" joint's location_in_parent property. Another strategy is to place markers at points of interest in the GUI and read their location property (which will be expressed in the global frame if their body property is "ground").

[RoZi A]

Thanks a lot