Motion Capture Markers

[Dan Robbins]

Hi All,

We are looking at purchasing some new markers for our 3D motion capture system as our current batch is looking worse for wear.

Reflecting on the data collection we have completed so far, I suspect the markers we currently use which have a the neck/stalk between the marker and rubber base which is adhered to the participant being analysed might not be the best style for our uses.

I have noticed particular issues around the pelvis during cycling when the stalk seems to increase the movement of the marker substantially.

Before I write off this type of marker completely, I am interested to hear peoples thoughts on the following points:

1. Is this a common issue when using this marker type?
2. Is there a benefit for this marker type?
3. Is there a marker size/diameter you would recommend for a lab which typically records:

a. Gait
b. Cycling
c. Exercise performance
d. Balance assessments/perturbations
e. Movement screens

The lab is approximately 3 m x 3 m x 16 m with an effective capture volume of approx 2 m x 2 m x 4 m.

The motion capture system is a Motion Analysis Corporation system with a mixture of Osprey and Kestrel cameras.

All help and advice will be greatly appreciated.
Regards
Dan

[Robert D. Catena]

Re: Motion Capture Markers

1. Is this a common issue when using this marker type?

• I have found markers with a stem to proportionally amplify the errors of skin motion artifact and vibrations in our gait studies, so I try to avoid them whenever possible.

2. Is there a benefit for this marker type?

• On a straight line, a marker with a stem combined with non-stem markers will allow you to have three markers not inline to track 3d motion. The Helen Hayes markers set uses stem markers on the thigh and shank for this reason. But instead of the shank stem marker, I have transitioned to a medial shank marker a few centimeters above the medial malleolus. It allows for the 3 non linear markers and still avoids obstructing walking and the error mentioned above. I would also suggest to think of alternative locations for nonstem markers for your applications before going with a stem marker.
• a stem marker also may be needed in areas where surrounding soft tissue could obstruct the marker view.

3. Is there a marker size/diameter you would recommend for a lab which typically records:

a. Gait
b. Cycling
c. Exercise performance
d. Balance assessments/perturbations
e. Movement screens

• The smallest you can get away with should be best. Your camera company should be able to help with this. I have also found that some marker suppliers are willing to give you a sample of a few different sizes for you to test out before you make a large purchase

[Ross Miller]

Re: Motion Capture Markers

Other than what Robert already mentioned, a substantial stalk could also be useful if trying to measure a very subtle motion with limited camera resolution. For gait I think most of the subtle motions (e.g. knee internal rotation) are not measured very accurately anyway, but I could see stalks being helpful for something like quiet standing.

Ross

[Edmund Cramp]

Re: Motion Capture Markers

It's important to understand the model that you are using when you consider changing marker sizes or locations - for example, the Helen Hayes model placed markers on short wand on the thigh and shank to define two planes (ankle-shank-knee and knee-thigh-ASIS) that allowed the HHH software model to measure limb rotations on each leg. Replacing the shank and thigh markers with markers on the skin surface without changing the model has significant effects on the limb rotation measurement accuracy.

In a more subtle way, changing the size of the markers placed directly on the skin surface will move the measured location (the marker centroid) - switching from 24mm markers to 5mm markers moves the measured location by 10mm. This may affect model calculations.

As far as measurement artifacts with markers on stalks - these can be reduced by careful mounting of the markers to minimize the motion - the original Helen Hayes wand markers used a long base, aligned with the limb, to provide better attachment and minimize vertical artifact motion. The software then processed the markers based on their location to reduce the motion artifact components.

[Dan Robbins]

Re: Motion Capture Markers

Thanks to all for the responses so far.

If I am interpreting the information correctly, some makers with stalks are handy - but not every marker requires a stalk.

All info provided has been very helpful and will certainly assist in the final decision.

Thanks again.
Dan