Reliability retro-reflective markers

[Barbara Schupak]

I used 19 mm passive retro-reflective markers during 3-D analysis of obstacle crossing at 60 Hz. Markers positioned on the navicular and heel frequently disappeared after crossing the obstacle. Was this a function of marker size or velocity/acceleration of movement? Thank you for your kind comments.
Barbara Schupak

[Ton van den Bogert]

Re: Reliability retro-reflective markers

If there are other markers nearby, the markers may not be separated in some the camera views. The camera will then merge them and the centroid will be in the wrong place to match other camera views. You could try smaller markers to reduce that problem.

Another common cause of marker dropout is that they are obscured. In an obstacle crossing study, the obstacle might come between the marker and one or more cameras. I would check this first.

I have never seen marker tracking fail due to velocity or acceleration, even in fast running.

Ton van den Bogert

[Dave Gutekunst]

Re: Reliability retro-reflective markers

To add a small point to what Dr. van den Bogert wrote, tracking the navicular can be problematic even without an obstacle, as it is essentially blocked from camera views from the anterior, lateral, and posterior sides by the foot itself. If the navicular is dropping out when the swing foot passes the stance shank, you may need some creative camera placement to successfully track it (i.e., perhaps some oblique views from the anterior-medial & posterior-medial).

Do you find that the navicular and heel markers are dropping out solely near the obstacle?

[Barbara Schupak]

Re: Reliability retro-reflective markers

To add a small point to what Dr. van den Bogert wrote, tracking the navicular can be problematic even without an obstacle, as it is essentially blocked from camera views from the anterior, lateral, and posterior sides by the foot itself. If the navicular is dropping out when the swing foot passes the stance shank, you may need some creative camera placement to successfully track it (i.e., perhaps some oblique views from the anterior-medial & posterior-medial).

Do you find that the navicular and heel markers are dropping out solely near the obstacle?

Thank you for your kind comments. To answer your question, the markers drop out after crossing the obstacle, pop in and out until heel strike. Barbara Schupak

[Barbara Schupak]

Re: Reliability retro-reflective markers

Thank you for your kind comments. They are most helpful. I will try smaller markers and check the cameras again. Barbara Schupak

[Barbara Schupak]

Re: Reliability retro-reflective markers

Thank you for your kind comments. To answer your question, the markers drop out after crossing the obstacle, pop in and out until heel strike. Barbara Schupak