Attach Plug-in Gait markers to a patient

After adding the patient's measurements to the subject node (see Take subject measurements for Plug-in Gait), you attach the markers to the patient.

Vicon cameras capture the movement of the retroreflective markers rather than the body to which they are attached. To enable Nexus to determine the movement of the subject's underlying skeleton, ensure that the subject's clothing will not occlude markers or by cause the markers to move excessively. If possible, attach the markers directly to the patient's skin. During capture, ensure that there is nothing else reflective in the capture volume.

To attach markers to a patient:

1. Select the appropriate size and quantity of Vicon markers to be used. Typically, 14mm markers are used for gait analysis studies.

2. Securely attach the Vicon markers to the patient in accordance with the marker arrangement defined in your chosen template. A front view of the most commonly used set (for Plug-in Gait lower body modeling) is shown in the following image. The left lower body markers are not labeled; place markers on the left side in a similar way to those on the right.

   Important: Note that, as shown in the following image, some asymmetry is desirable as it helps the auto labeling routine distinguish right from left. In a lower body marker set, you can place the THI and/or TIB markers asymmetrically. For a full body set, you can place the THI, TIB, UPA and FRM markers asymmetrically. Similarly, avoid symmetrical placement of marker clusters or groups of markers and also ensure markers are asymmetrical within each cluster/group.

   
   
   
   

   Tip

   • The THI and TIB markers anterior-posterior position is critical for identifying the orientation of the knee and ankle flexion axis.
   • If you are using a single sacral marker (SACR) in a Plug-in Gait lower body or full body marker set, attach the SACR marker instead of the two PSIS markers.
   • Because you need at least three markers to obtain six degrees of freedom, best practice is to place four markers on the pelvis (LASI/RASI/LPSI/RPSI), so that even if one of the four markers is occluded during motion capture, the required three markers will still be visible to the cameras.

   
   

   For detailed descriptions and images of marker positions, see Models and templates in the Plug-in Gait Reference Guide

You can now capture a calibration trial and reconstruct the markers (see Calibrate a labeling skeleton).