issue with Gait 2392 marker placement

[farzaneh yazdani]

Dear All,
I am working on “Gait2392.simbody” to simulate walking trials. During scaling the model, in“ preview static pose” I find that there is too much difference between some of the experimental marker’s location and what is default for virtual markers placement in Opensim. Any clear guideline to exactly define the landmarks location for “Gait2392” markers placement?

Any information would be appreciated.

Regards
Farzaneh Yazdani
PhD Candidate,SUMS Center for Human Motion Sciences Research

[John O Connor]

Hi farzaneh,

Can you describe the experimental marker set you are using? The markers should be in the same relative positions on the model as they were on the subject during collection of your motion data

have you had a look at the videos on scaling?

http://simtk-confluence.stanford.edu:80 ... th+Scaling

Regards
John

[farzaneh yazdani]

Dear John,
Thank you for taking the time to answer my question.
i attach the markers as below:
acromium markers on Acromioclavicular joint (too much RMS error with this marker during scaling!)
Top.Head on Inion of the skall, Vsarcal on sacral base, RT & LT.ASIS on the ASIS bony landmark, Sternum on the mid point of sternal bone, knee.med & lat on medial and lateral femoral epicondyles, Ankle.med & lat on medial and lateral malleolus,cluster markers for shank and thigh segments.I am doubtful about the location of foot markers, but i attach the Midfoot.Lat on the 5th metatarsal base, Midfoot.Sup on talonavicular area on medial side, Toe.Lat on the 5th metatarsal head. Toe.Med on the 1st metatarsal head and finally Toe.Tip on the 2nd metatarsal head.
during scaling, the reported errors are: total error = 0.029, RMS= 0.037 for L.Acromium marker and I cant make these errors lessen in any way (by moving the experimental markers or changing the coordinates).

Regards,
Farzaneh

[farzaneh yazdani]

Dear John,
Thank you for taking the time to answer my question.
i attach the markers as below:
acromium markers on Acromioclavicular joint (too much RMS error with this marker during scaling!)
Top.Head on Inion of the skall, Vsarcal on sacral base, RT & LT.ASIS on the ASIS bony landmark, Sternum on the mid point of sternal bone, knee.med & lat on medial and lateral femoral epicondyles, Ankle.med & lat on medial and lateral malleolus,cluster markers for shank and thigh segments.I am doubtful about the location of foot markers, but i attach the Midfoot.Lat on the 5th metatarsal base, Midfoot.Sup on talonavicular area on medial side, Toe.Lat on the 5th metatarsal head. Toe.Med on the 1st metatarsal head and finally Toe.Tip on the 2nd metatarsal head.
during scaling, the reported errors are: total error = 0.029, RMS= 0.037 for L.Acromium marker and I cant make these errors lessen in any way (by moving the experimental markers or changing the coordinates).

Regards,
Farzaneh

[John O Connor]

Sorry for the late reply Farzaneh,

Have you weighted the marker's you're more confident in? If you're don't have much confidence in the foot markers, they should have a lower weighting

As suggested by the tutorial videos, It may be useful to look at static photos of the subject with the markers on and adjust the position if the marker is in a different location on the model (you can use ctrl + right-click to select a marker for movement in the GUI). Changing the location of a marker should certainly have some effect on the RMS error for that marker however they should always be located based on physical location in the photo.

Another thing you could look at is how you are scaling the segments. Some segments can be scaled uniformly in X-Y-Z and for others (e.g. Pelvis) it is better to have different scaling factors in the X-Y-Z directions

I hope this helps
Best of luck
John