Marker Cluster Placement

[James R Borrelli]

Hi,
I'm a long time reader, first time poster.

Does anyone know why I would not want to place clusters of markers on the anterior aspects of the thigh and shank as opposed to the lateral aspect?

I am working with Motion Analysis' motion capture system, Cortex. I am working with a full body model. The model includes the shank, thigh, pelvis, trunk, upper arm, lower arm and head. There are also markers on the hands. I am tracking the shank, thigh, pelvis and trunk with clusters. For my experiments, the subjects stand still and are caused to exhibit some type of, mostly, upper body motion and/or arm motions.

When the hands are at the side (hanging straight down), I get some occlusion of the thigh clusters. I was wondering (and did a fair amount of searching) if anyone has used clusters on the anterior part of the thigh as opposed to the lateral part of the thigh.

I did a little experimenting myself. I put markers on myself with thigh clusters on the lateral aspects of the thigh and shank (Side_Clusters.jpg) and with the clusters on the anterior aspects (Front_Clusters.jpg).

During my experimenting, I stood still and did "the hokey pokey", the "muscle-man", "jazz-hands", etc, in an attempt to produce all the arm and body motions a subject may exhibit.

If you are familiar with Cortex, the time line feature shows a colored line for all the samples when a marker has been successfully identified and white space when it has not. I did two trials for each setup. I have attached a snapshot of the time-line for both trials with the clusters on the front (Front_Time_Line_1.jpg and Front_Time_Line_2.jpg) and one time-line for the trial with clusters on the side (Side_Time_Line_2.jpg).

It appears that Cortex was more successful in identifying the markers when I placed the clusters on the anterior aspect of the thigh and shank.

[Robert D. Catena]

Re: Marker Cluster Placement

Markers (and clusters especially) are better on the side of the body because they typically stay in the sagittal spatial plane. If on the front of the extremities, they would move between the coronal and transverse spatial planes with extremity flexions during most human movements (especially gait type movements). When that happens your cameras are less likely to see the cluster perpendicular to its plane, and more likely to see the cluster markers blocking each other. Or even possibly lose some of the markers on the cluster completely like in instances of being placed on the anterior tibia during a knee flexion with an extended hip (as during the preswing phase of gait). A lateral shank marker cluster would not have the same issues during the preswing phase of gait.

If your research just involves stance (as your experiments did) then marker cluster position should not matter as much.

[James R Borrelli]

Re: Marker Cluster Placement

Thanks for the response Dr. Catena.

That is a good point about markers becoming occluded when the knee is flexed and the hip is extended (<-not particularly a problem for my work).

With regard to your comment:

"If on the front of the extremities, they would move between the coronal and transverse spatial planes with extremity flexions during most human movements (especially gait type movements). When that happens your cameras are less likely to see the cluster perpendicular to its plane, and more likely to see the cluster markers blocking each other."

Would this still be an issue if I have cameras viewing the sagittal, transverse and coronal planes? I failed to mention before, I am using twelve (12) cameras. They are mounted about eight (8) feet above the ground, looking (angled) down. There are two (2) in the front, back, left and right for a total of eight (8) and one (1) more camera in each corner.

I expected placing marker clusters on the front of the thigh would cause the clusters to be "busier" as a result of muscle activity compared to lateral placement, can anyone speak to this concern?

[Robert D. Catena]

Re: Marker Cluster Placement

It might help to know what your experimental task is. These are more general comments:

With camera placement surrounding the capture volume, how far away these cameras are from your capture volume will determine the amount of shoulder/hip extension or knee flexion (motions that cause anterior marker clusters to face downward) that would result in a cluster marker occlusion, with closer cameras being more susceptible to marker occlusion with smaller joint movements.

There are two additional considerations.
One you brought up is the muscle movement under the skin causing cluster movement. Since the muscles for typically the greatest motion (flexion/extension) are mostly located on the front and back of segments, contraction of these muscles would cause your clusters to have additional motion artifact.

The second additional thought is the task. If the person has to interact with some object as part of the task then the object location with respect to camera and limb locations should be considered.

[James R Borrelli]

Re: Marker Cluster Placement

Thanks again Dr. Catena.

I am abandoning this idea due to the potential for occlusion and motion artifacts from muscle contraction.