What is the must-having markers?

[Ruoli Wang]

Hi all, I have a question about marker set. In our lab, we used a Vicon Plug In whole-body marker set. I want to modified the gait2354_scale_markerset.xml to match our lab's. I realized the example marker set has more markers than ours. I suppose that more markers should give higher accuracy in scaling and IK etc. I want to decide if we need add some extra markers when collecting data in the future. Anyone has experiences which are the must having markers? (e.g. we did not have markers on medial knee, medial ankle and the ones on the metatarsal.)

Thank you in advance!!

[Chris Zirker]

It really depends on what you are studying. For walking, it is my experience that a plug-in gait marker set (which does not include any medially-placed markers on any of the extremities) does a good enough job. However, if you are studying running or some other high impact motion which could cause greater soft tissue artifact, you may need more "redundant" markers to reduce this noise. I have also had issues with the plug-in gait marker set giving me poor foot and ankle kinematics in which the foot rotates around the subtalar joint axis rather than plantar or dorsiflexing, so you may want to consider adding more foot markers. I only collect data on "normal" subjects, so I generally can fix the foot flat when scaling my models from static trials and I have found this to help reduce inaccurate subtalar joint movements during IK.

[Ruoli Wang]

Hi Chris,

Thanks for sharing.

I have another question about the input files for scaling. Did you use workstation to export .trc file? What about the .mot file? OpenSMAC can also convert .c3d to .trc and .mot. But it had a different marker set than PIG. Although .mot was optional, I guess it can increase the tracking accuracy.

[Ruoli Wang]

Hi, I have tried a static Vicon PIG marker set today without .mot, I also had very poor ankle tracking, very large eversion. How did you manage to fix the foot to be flat? And I found that 'no marker move' seems give better marker positions.