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Using a joint center as tracking marker

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  • Using a joint center as tracking marker

    This query came to my mind while going through Visual3D (C-motion) guidelines (https://www.c-motion.com/v3dwiki/ind...e_note-Serge-1).

    The wiki reports about considering a joint center e.g., Femur Head Center to track the Thigh segment along with other skin-mounted markers. I am not sure if its a good practice to use a joint center as a tracking marker.

    Can anyone shed some lights on using the above idea and what may be the consequences on the resultant kinematics?

    Regards,
    Bhrigu K Lahkar, PhD


  • #2
    The joint center is a result of the model calculations which assume that the markers were placed accurately to match the model. And the model basically describes what we think the subject looked like but may not be 100% accurate for a subject - the model leg lengths are usually seen as identical but when I was working with clinical gait analysis I was always told that human leg lengths can vary and that there was an error if they were both entered into the calculations at the same length.
    I think that the C-motion link is very helpful but I agree that while joint centers may be accurate, they are estimated as a result of the calculations and marker placements, not measured so any marker placement errors can have an effect that might be hard to detect.
    Last edited by Edmund Cramp; January 17th, 2022, 11:33 AM. Reason: A minor grammar correction

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    • #3
      I use joint centers for tracking some times. One example would be a sparse marker set where the joint center is needed as a third tracking marker out of necessity. Another would be if you are using a marker cluster on the distal thigh to track the thigh segment, but you are getting quite a bit of translation at the hip (meaning the cluster is not capturing the bone motion well). In this case, adding the hip center (and maybe knee center or lateral knee marker in addition) can control the thigh motion better. The alternative is to constrain the joint using global optimization (but adding the joint center is much simpler). Overall, it depends on which markers are the most well behaved for your model.

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      • #4
        You've hit on the fundamental problem with biomechanics research over the past 30 years. Anybody who has ever put a marker on a joint centre/axis knows that it simply can't be done. The body is not a set of hinges or even gimbals. The centre of rotation changes as the joint moves. The early models (Helen Hayes/VCM/Vicon) were designed to approximate things and get away with as few markers as possible. More modern thinking is to track each bone individually in 3D. But this does have the problem that the bones can separate at the joints - obviously not true anatomically, but at least you know the data is bullsh*t - with a Vicon model it's all covered up.

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        • #5
          Tracking the joint center would function like a “soft” way to reduce joint dislocation. In the limiting case of having two real markers on the segment, and also the joint center of the parent joint, I think this would be nearly identical to modeling the joint as a ball-in-socket that cannot dislocate, since one marker is always pinned at the joint center.

          If you have three or more real markers on the segment, and you additionally track one its joint centers as a tracking marker, this would be akin to “pulling” the segment towards, but not completely to, its presumed anatomical connection at the joint, since Visual3D uses (un-weighted) least squares to determine 6DoF orientation if you have more than three tracking markers.

          Two disadvantages of this approach versus explicitly enforcing joint constraints via global optimization / inverse kinematics that I can think of:

          (1) You are assuming the joint center is defined correctly in the parent segment (e.g. pelvis), and the markers on the child segment (e.g. femur) cannot contribute any information to determining where the joint center is located. In IK, marker locations contribute information in both directions across a joint (thigh markers can “pull” the pelvis, and vice versa).

          (2) In cases with more than three markers (including the joint center) I suspect you are inducing something akin to L2 regularization in regression, where you are implicitly putting a maximum value on how much joint translation you are allowing to happen. But instead of having direct control over this value, like in IK, you are relying on the number of real markers to determine how strong of a “pull” towards the joint center you’re getting. The more real markers you have, the further from the joint center your segment can stray (or in other words, the less influence tracking the joint center will have on the kinematics).

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