6-Degree of Freedom Approach to Motion Capture

General Principles:

At the NIH, a 6-Degree of Freedom approach to motion capture is used. The motion of each body segment is tracked independently.

Calibration Targets are reflective markers placed on the surface of the subject's skin over specific skeletal landmarks. Tracking Targets are "clusters" of markers mounted to lightweight rigid "shells" and attached to each limb segment.

Each shell provides a relatively rigid surface for positioning targets. Ideally, the Neoprene wraps do not affect limb movement, yet decrease the amount of movement between the shell and the underlying skeleton.

A minimum of 3 non-colinear targets per shell are required for data collection; however, 4 are traditionally used. This provides redundancy and helps compensate in the event a target is momentarily obscured.

The subject is instructed to stand still in an anatomically neutral position and motion capture data is collected for 1 to 2 seconds. Analysis software uses the Calibration Targets to determine the orientation of the Tracking Targets relative to the underlying skeleton.

Prior to data collection of the motion of interest, the Calibration Targets are removed leaving only the Tracking Targets in place.

Example Target Application (lower body, only left side shown):

A rigid shell made from heat-shapeable plastic is placed on the top of the subject's foot with the anterior-most target positioned near the head of the 5th metatarsal (figure 1). Velcro patches on the shell provide mounting points for the Tracking Targets. Elastic tape is snugly wrapped around the foot and across the shell to hold it in place (figures 2, 3). Reflective spheres on Velcro pedestals are then placed on the Velcro patches of the shell. An adhesive Velcro patch is placed on the subject's lateral malleolus and another reflective sphere, on a shorter pedestal, is attached (figure 4). This last target serves as a Calibration Target. Similarly, in a rather unique situation, the target located on the shell over the 5th metatarsal head is frequently assigned a dual role as both a Tracking Target and a Calibration Target.

 
Velcro-sensitive Neoprene is wrapped snugly around the calf (figure 5). A shell with Velcro backing is applied to the lateral portion of the wrap (figure 6). Reflective targets are attached to the shell with Velcroed pedestals (figure 7). The femoral lateral epicondyle is located and another target attached as a Calibration Target (figures 8, 9). Again, please note the non-colinear arrangement of the targets on the shell.

 
The shell of the femur is positioned and attached in a similar method using the Velcro wrap method (figure 10). A Calibration Target is applied to the superior aspect of the greater trochanter (figure 11).
 

Finally, a rather wide Velcro wrap is secured around the pelvis of the subject, and a Calibration Target is applied to the iliac crest. Two last Tracking Targets are then applied, one slightly anterior and one posterior to the existing targets, as shown (figures 12, 13).
 

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