Lower body modeling with Plug-in Gait

This section describes lower body modeling with Plug-in Gait.

For details about the labeling skeleton templates to be used with Plug-in Gait lower body models, see Plug-in Gait labeling skeleton templates (VSTs) in Vicon Nexus.

Outputs from Plug-in Gait lower body model

Use a Plug-in Gait lower body model if you require the kinematic and kinetic calculation outputs listed in the following table. The output variables are prefixed by the appropriate context (L for left or R for right).

Joint angles, force, and moments are expressed in the three anatomical planes: sagittal, frontal, and coronal. Even if the joint powers are scalar, they can also be expressed in the anatomical planes in Vicon Polygon. Forces, moments and powers are normalized to the subject's height and body mass.

^a This output variable is calculated only if you use a Plug-in Gait model that contains the thorax.

Marker sets for Plug-in Gait lower body model

All Plug-in Gait marker sets are designed for the Newington-Helen Hayes model on which Plug-in Gait is based. The marker set for Plug-in Gait lower body modeling includes markers for the pelvis and the lower limbs.

There are two variations of the standard marker set for the lower body model:

• A single sacral (SACR) marker for the pelvis
• Two posterior superior iliac spine (PSIS) markers for the pelvis

These markers provide the same function; if you use two PSIS markers, Plug-in Gait calculates the midpoint between them and uses that to perform the calculations. If you use a single SACR marker, you identify that position to Plug-in Gait rather than having it calculated.

Using the two PSIS markers has the benefit of providing redundancy, so if one of the pelvis markers is missing, it is possible to reconstruct a virtual marker based on the remaining three markers. You can do this using a Rigid Body Fill in Nexus.

Marker placement for Plug-in Gait lower body model

To demonstrate where to attach the standard lower body model markers to your patient, the following images show front, back, and side views. Some markers are shown from two views to help you better determine their position on your patient.

The following image shows the front view. The left lower body markers are not labeled; place markers on the left side in a similar way to those on the right.

The following image shows the back view. The right lower body markers are not labeled; attach markers on that side in a similar way to those on the left (with some asymmetry, as described above).

The following image shows the right side view. The left side view is not shown; attach markers on that side in a similar way to the right markers (with some asymmetry, as described above).

Pelvis marker placement

The following markers are positioned on the patient's pelvis:

In some patients, especially obese individuals, the markers either can't be placed exactly on the ASIS, or are invisible in this position to cameras. In these cases, move each marker laterally by an equal amount along the ASIS-ASIS axis. The true inter-ASIS distance must then be manually measured and entered in the Properties pane at the bottom of the Subjects tab on the Resources pane. These markers, together with either the SACR marker or the LPSI and RPSI markers, define the pelvic coronal plane.

Lower limb marker placement

The following markers are positioned on each of the patient's lower limbs. For additional guidance, see the notes at the bottom of the table.

Notes

Knee markers To locate the precise point for placing the knee markers (LKNE, RKNE), passively flex and extend the knee a little while watching the skin surface on the lateral aspect of the knee joint. Identify where knee joint axis passes through the lateral side of the knee by finding the lateral skin surface that comes closest to remaining fixed in the thigh. This landmark should also be the point about which the lower leg appears to rotate. Mark this point with a pen. With an adult patient standing, this pen mark should be about 1.5 cm above the joint line, mid-way between the front and back of the joint. Attach the marker at this point.

If you are using a knee alignment device (KAD), see also KAD marker placement for Plug-in Gait lower body model.

Thigh markers The thigh markers (LTHI, RTHI) are used to calculate the knee flexion axis orientation. Place the LTHI marker over the lower lateral 1/3 surface and the RTHI marker over the upper lateral 1/3 surface of the thigh, just below the swing of the hand, although the height is not critical. The anterior-posterior placement of the marker is critical for correct alignment of the knee flexion axis. Try to keep the thigh marker off the belly of the muscle, but place the thigh marker at least two marker diameters proximal of the knee marker. Adjust the position of the marker so that it is lies in the plane that contains the hip and knee joint centers and the knee flexion/extension axis.

If you are using a KAD, the precise placement of the thigh markers is not crucial.

Tibia markers The tibia markers (LTIB, RTIB) are used to determine the alignment of the ankle flexion axis. Similarly to the thigh markers, place the LTIB marker over the lower 1/3 surface of the shank and the RTIB marker over the upper 1/3 surface of the shank. The tibial marker should lie in the plane that contains the knee and ankle joint centers and the ankle flexion/extension axis. In a normal patient, the ankle joint axis between the medial and lateral malleoli is externally rotated by around 20 degrees with respect to the knee flexion axis. The placements of the shank markers should reflect this. 

If you are using a KAD, the ankle dorsi-plantar flexion axis is assumed to be parallel to the knee flexion axis unless:

• The tibial torsion is manually measured and entered in the Properties pane at the bottom of the Subjects Resources pane.

or

• One marker on each medial malleolus is attached (RMED, LMED). During the static trial, Plug-in Gait automatically calculates the tibial torsion as the angle between the knee flexion and the ankle dorsi-plantar flexion axes.

Toe and heel markers If the toe markers (LTOE, RTOE) cannot be placed level with the heel markers (LHEE, RHEE), you must configure Plug-in Gait to compensate for this. For details of the relevant settings, see Plug-in Gait Static pipeline in the Vicon Nexus User Guide. 

KAD marker sets for Plug-in Gait lower body model

In addition to the standard lower body marker sets, an alternative marker set enables you to use a knee alignment device (KAD).

The KAD is a light-weight, spring-loaded G-clamp, whose adjustable jaws bridge the knee and whose stem is aligned with the knee flexion axis.

One standard-sized marker is fixed to the tip of the stem and two markers are mounted on the ends of two additional rods fixed to the device. The three markers are equidistant from the point where the stem meets the jaws of the clamp, enabling the 3D position of this point, known as the 'virtual knee marker', to be calculated.

Instead of the THI and TIB markers in the standard model, the KAD markers (left and right KAX, KD1, and KD2) are used to calculate the orientation of the medio-lateral axes of knee and ankle respectively.

The KAD is applied to the patient during a static trial to enable Plug-in Gait to calculate:

• The offset angle between the knee flexion axis orientation as calculated by using the KAD markers and knee flexion axis orientation as calculated by using the THI markers (Thigh Rotation Offset). When a KAD is present, the ankle dorsi-plantar flexion axis is assumed to be aligned with the knee flexion axis unless the tibial torsion measurement is entered.
• The angle between the ankle dorsi-plantar flexion axis orientation as calculated by using the KAD and the tibial torsion measurement and the ankle dorsi-plantar flexion axis orientation as calculated using the TIB markers (Shank Rotation Offset).

These calculations eliminate the reliance on the anterior posterior position of the THI and TIB markers.

A variation of the KAD marker set defines the additional LMED and RMED markers for the medial malleoli.

When you use a KAD, the tibial torsion measurement is critical for Plug-in Gait to identify the correct orientation of the ankle dorsi-plantar flexion axis. In fact, if the tibial torsion is left at 0 (zero), the ankle flex axis is assumed to be aligned with the knee flex axis. To enable Plug-in Gait to automatically calculate the tibial torsion measurement, attach the LMED and RMED markers on the medial malleoli of your patient.

When the static trial has been processed, you can remove the KAD and the MED markers, and for dynamic trials, place the KNE marker exactly where the KAD pad used to be on the femural epicondyle.

KAD marker placement for Plug-in Gait lower body model

The following images show front, back, and side views to demonstrate where to attach the lower body model markers to your patient. You do this before capturing a static trial as described in the Vicon Nexus User Guide. Some markers are shown from two views to help you better determine their position on your patient.

The following image shows the front view. It includes the knee alignment device (KAD) marker variations, which are inset and shown with dashed lines to indicate they are optional.

The left lower body markers are not labeled; attach markers on that side in a similar way to those on the right (with some asymmetry as described in Marker placement for Plug-in Gait lower body model).

The following image shows the back view. It includes the knee alignment device (KAD) marker variations, which are inset and shown with dashed lines to indicate they are optional.

The right lower body markers are not labeled in this figure; attach markers on that side in a similar way to those on the left (with some asymmetry as described in Marker placement for Plug-in Gait lower body model).

The following image shows the right side view. It includes the knee alignment device (KAD) marker variations, which are inset and shown with dashed lines to indicate they are optional.

The left side view is not shown; attach markers on that side in a similar way to the right markers (with some asymmetry as described in Marker placement for Plug-in Gait lower body model).

If a knee alignment device (KAD) is used, it is attached instead of the LKNE and RKNE markers for the static trial only. Before dynamic capture and modeling, it must be removed and LKNE and RKNE markers attached instead.

The KAD markers are constructed in such a way as to enable the model to calculate a position for a virtual knee marker, which corresponds to the external pad. So, the external pad should be positioned as described for the knee marker.

The following table shows the KAD marker labels included for static trials.

For dynamic trials, the KAD must be removed, and the left or right KNE marker positioned at the same point as the external KAD pad.