Scaling Problem

[J.J. Mordang]

Hi,


I have a bit of a problem with scaling the lower limb. I edited the lower limb model such that it has a 7 segmented foot. However when I want to scale it with my marker setting and lab data something goes wrong. The virtual markers are not on the same places any more (e.g. the markers who initially where on the foot float around the shin after scaling)and the experimental markers are no where to be seen. Is there an easy way to fix this problem?


Kind regards,

Jan-Jurre

[Edith Arnold]

The scaling step is tricky and it is critical to get right before IK. The markers are supposed to move according to their weights. Markers with large weights will move less (these should be markers you trust are the same in the model and in the experiment, bony landmarks etc.). Markers with small weights, like the intersegmental ones, will move more so they match the experiment. The markers used for scaling measurements should be the trusted, bony landmark ones.

I recommend using "Preview static pose (no marker movement)" in the scale tool to check the virtual markers against the experimental ones. This is a checkbox in the Scale Tool GUI. You may need to move your trusted virtual markers in the model to match the locations of the experiment. It helps to have a picture of the experimental set up.

Other things to check:
segments in the measurements used
static pose weights
spelling of markers in trc file

More details about scaling should be available in the Opensim user guides.