2D Motion Capture is Fake News

When it comes to true sports science, there is no doubt that 3D motion capturing is the gold standard. Everything else is just pretending.

3D Optical Motion Capture surrounds the environment with 3D cameras to create a capture volume.  Each camera emits an infrared light that bounces off reflective spherical ‘markers’ placed on the athlete while they are in the capture volume.  Those reflections are seen by the cameras to determine the location of each marker in 3D space.  This allows analysis of movement in all three axes and planes of movement, which is required for every sport. Marker position is recorded at a submillimeter accuracy – this is the gold standard of motion tracking.

2D Motion Capture uses one or more cameras typically set up in a line or in a few key positions around the capture volume.  Each camera records a video and/or marker data in one plane of motion.  This produces data of athlete movement in that particular plane, but is limited in accuracy and movement in the other two planes. For example: a 2D camera pointed towards the side of the athlete will record sagittal plane angles (flexion) and would produce reliable results as long as the athlete remains in the plane of motion (does not change direction). However, there is no reliable data on frontal or transverse plane angles or rotations.  This restricts the environment and the movement that the athlete can do in order to maintain an acceptable level of accuracy.

For those interested in only one plane of movement or axis of rotation, a 2D system is cost-effective and would produce reliable results.  However, if you require movement analysis of an athlete through multiple planes of movement (typical of just about every sport), 3D motion analysis is well known as the best tool for accurate and reliable information.

Schurr SA, Marshall AN, Resch JE, Saliba SA. Two-dimensional video analysis is comparable to 3D motion capture in lower extremity assessment. International Journal of Sports Physical Therapy. 2017;12(2):163-172.

Study Conclusion:
Moderate to strong relationships were observed between 2D video camera and 3D motion capture analyses at all joints in the sagittal plane, and the average mean difference was comparable to the standard error of measure with goniometry. The results suggest that despite the lack of precision and ability to capture rotations, 2D measurements may provide a pragmatic method of evaluating sagittal plane joint displacement for assessing gross movement displacement and therein risk of lower extremity injury.

– Kait

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