IK marker weights

[Radhakrishnan Vignesh]

Hello,

I would like to understand how to set IK marker weights and which marker to use in more detail. I have followed the tutorial and I have set greater weights for markers like L.ASIS and R.ASIS, ankles etc which are placed in bony parts and less affected by soft tissues. But why is the knee marker not tracked and why do the Acromium markers have less weight compared to ASIS?

I have collected data using OptiTrak's Helen Hayes lower body model which does not have three markers per segment. So:

1) How do I decide what weights to set? Should the weights be the same for scaling (single IK) vs Inverse kinematic?
2) Which markers do I select for IK and which do I not track? What are the consequences of not tracking markers such as knee etc on the results? Would not tracking markers in an entire segment affect the results?

I am analysing soft tissue artefacts

Thanks,
Vignesh

[Radhakrishnan Vignesh]

Any luck with this question?

[Jacob J. Banks]

1) Aside from the advice provided in the online tutorial, all I can add is that in my experience Marker Weighting is often "more art than science" and that it depends on what your research question is. Use an educated guess to set your weights and then try some modifications out to see how that impacts things.

And once you decide on a weighting, still continue to preview your IK results for each sim (don't assume things will look correct).

2) I'm guessing that in the case of the tutorial (without looking), that the knee marker isn't tracked because it was removed during dynamic trials. It was only used for scaling purposes and then rigid clusters tracked the thigh and shank.

Bonus) Weighting aside, if your interested in 'soft tissue artifact', you may be better off looking directly at your marker coordinates and not trying to fit them to a model.

Best,
Jake Banks

[Radhakrishnan Vignesh]

Hello Jacob,

Thanks for your reply. Sorry but I have some follow up questions:

1) Regarding the weights, I am assuming weights which are less influenced by soft tissues should be greater than ones which would be more affected by the same. If I did not have a rigid cluster for thighs as the markertset I used just requires one marker on the thigh, by that logic would I set higher weights for markers placed on bone parts as they have less soft tissues?

2) Is there a particular reason knee markers and ankle markers are removed during dynamic trials despite them being placed on bony segments?

3) Sorry, but could you explain the bonus point? My goal is to see if the inverse kinematic process, which was and is designed to reduce marker errors due to soft tissues, has a greater error when tested on people with greater body fat percentage on thigh and shin.
How would I be able to compare the errors without a model?

Sorry for the inconvenience

Thanks

[Radhakrishnan Vignesh]

Hello Jake,

Happy new year. Sorry but could you further elucidate how to compare soft tissue artefacts without feeding it into a model?

Thanks,
Vignesh

[Jacob J. Banks]

Sorry for the delayed response... sort of forgot I replied to a post!
1 & bonus) I don't really have much to add, but I would challenge you to rethink your research question &/or design. From the information provided, your results would be very dependent upon marker weighting, which as I as stated prior can be a bit arbitrary. Otherwise, if your determined to use a model/IK tool, you could always track as you see fit and then extract the IK resulting model marker positions via an API. Then compare those resulting marker global positions to your 'raw' marker positions (sort of like your tracing error would report, but can look at specific markers a bit better with your own API). I'd also suggest looking at a variety of weighting combinations.

2) Dynamic tasks (particularly gait) tend to knock off medial markers on the lower limbs.

Sorry I can't be of more assistance and for the delayed response.
Jake Banks

[Radhakrishnan Vignesh]

Hello Jake,

Thanks for your reply. Yes, I played around with different marker weights ensuring markers which are less affected by soft tissues(heel,hip etc) are weighted more than thigh or shin, and calculated marker errors for each of them. With all the different configurations, I got that error of thigh marker was higher in people with greater body fat percentage than with people with lesser body fat percentage, some significantly so. Not sure about the analysis so going through with different settings.

But thanks for your assistance. It helped a lot

Regards
Vignesh