scaling mass error

[Peter Staab]

Hello,

I´m using opensim 3.1. In the scaling step, mass properties of the 'upscaled' segments are not calculated correctly. So e.g. calcaneus in my model is scaled with factor 1.119, but mass of this segment changes from 1.25 to 3.24 kg. When I use version 3.01 with the same model and setupfile everything seems to be correct. Perhaps you could reproduce this error.

Best,
Peter

[jimmy d]

Hi Peter,

I tried to replicate the issue. Unchecking 'Preserve mass distribution during scale', setting all other other bodies to 1 and setting calcaneus to 1.119, I scaled the mass from 1.25 to 1.69. Perhaps the mass of the subject is high and 'Preserve mass distribution during scale' was checked.

If you are still having the issues, could you attached the files you are using so we can replicate the problem

cheers,
-james

[Peter Staab]

Hi James,

thanks for the reply.

I also unchecked the 'preserve mass' option before scaling. But after the scaling step the sum of the masses of all segments far exceeds the total mass in the scaling setup file. Indeed the subject is obese, but I don´t understand the distribution of the masses for the single bodies. As I said before, in version 3.01 this problem doesn´t occur. In this version all masses are distributed correctly, concerning the relative proportion.
I can send you the data via email.

Cheers,
Peter

[jimmy d]

Hi Peter,

If the 'preserve mass' option is unchecked then you wouldn't expect the scaled model mass to equal the mass in the setup file. The masses are getting scaled relative to the size of the segments, not the input mass.

3.0 had a bug that caused 'preserve mass' to be persistent when unchecked (discussed here: https://simtk.org/forums/viewtopic.php? ... ling#p9024).

Hope that helps,
-james