This page is the FAQs regarding modeling.

Question:When I define a segment using lateral and medial markers at both proximal and distal ends the segment shows as calibrated in the ‘segments’ tab. However, when I use one central marker to define the joint at the proximal end and two markers to define the medial and lateral points at the distal end, then the segments show as not calibrated. Is it necessary to define both proximal and distal ends of a segment using medial and lateral markers?

Answer (click to expand)

It isn't necessary to specify medial and lateral markers. If, however, you specify only one marker, you must also specify a meaningful non-zero radius for the segment. If you specify the proximal joint, the radius isn't terribly important because it is only used for computing the moment of inertia. If you specify only the lateral or medial side, the radius is very important because it represents the distance to the proximal joint. Either three or four targets/landmarks are required to define the frontal plane and the segments ends. A segment end can be defined with:

• lateral target and a radius
• medial target and a radius
• “joint” target and a radius
• lateral and a medial target

When Visual3D requires a radius, you can enter an absolute value (e.g. 0.05 = 5cm) however this value is constant for all participants' data to which you apply the model template, rather than “scaling” to each participant. We therefore suggest that radii are defined by use of an expression. For instance the radius at the knee can be defined as half the distance between the lateral and medial knee markers. This can be entered into the “radius” field as 0.5*distance(lateral_knee marker_name,medial_knee_marker_name). See Segmental definitions

Question:How do I model the fingers?

Answer (click to expand)

It can be challenging to model fingers, but a digitizing pointer can be used to construct landmarks on the fingers. Refer to Modeling Fingers page for recommendations on using a digitizing pointer with the fingers.

Question: I am interested in calculating shoulder angles and moments during a throwing motion, but I am not 100% sure about the information to input into Visual3D. In order to obtain the joint moment for the shoulder, what information would I have to put into “Resolution Coordinate System” or “Cardan Sequence”? Does this depend on the Cardan Sequence used to calculate joint angles?

Answer (click to expand)

A joint angle is a relative angle between two segments, one of which is termed the “reference segment” and is assumed to be stationary for the calculation. Calculation of a joint angle is typically done by taking rotations around the reference segment's X axis, a floating Y axis, and the non-reference segment's Z axis (X-Y-Z) in turn (hence a joint angle is NOT a vector). The final orientation of any segment relative to a reference segment depends on this rotation sequence. Modelling upper extremity movements can be problematic due to the large ranges of motion, in all three planes, at the shoulder. This means there is no single definition that is anatomically meaningful for the shoulder’s full range of motion. For a joint moment you can use either a “Resolution Coordinate System” or “Cardan Sequence” but not both. In the upper extremity it probably makes most sense to use a “Resolution Coordinate System”. This is because selecting the proper “Cardan Sequence” and interpreting the data at the shoulder can be tricky. For example in an X-Y-X rotation sequence, “gimbal lock” can occur when the Y rotation (shoulder ab-duction) approaches 90 degrees. This results in any interpretation of the data using this sequence being somewhat meaningless. Also, be aware that the joint moment is a vector (i.e. it has “magnitude” and “direction”), but when you resolve it into a “Cardan Sequence” it loses its properties as a vector. Finally, we suggest that IK (global optimisation) is used to “pin” the shoulder joint together by constraining translations.

Question: We are currently conducting a study which involves our subjects going from a standing to a sitting position. We are using a CODA pelvis but invariably during the trial one or both of the ASIS markers are becoming occluded and the pelvis segment is being ‘lost’. Is there another way we can define the pelvis using, say, markers on the iliac crests or greater trochanters?

Answer (click to expand)

When you define segments in Visual3D, you select ‘anatomical’ targets (markers) which are used to define the segmental local coordinate system (orientation of the segment), and you also select ‘tracking’ targets. For the CODA pelvis the ‘anatomical’ targets are the ASIS and PSIS targets. Most people also use these to track the segment during movement trials, but that is not essential. For any segment, including the pelvis, the tracking targets do not need to be the same as the anatomical targets, i.e. in addition to the anatomical targets you can place tracking targets on any locations on the segment where there is minimal soft tissue artefact, and the targets are not occluded during the motion. This means that you can add any targets you wish to the list of tracking targets for the pelvis. If you open a static trial (Model→Create→Hybrid Model) and apply the model template, you will see a list of the segments in the model. If you double click on the Pelvis, a pop up dialog will appear. In the bottom of the dialog, there is a list of tracking targets. If you select the iliac crest targets, they will be used to track the pelvis. Once you have added these targets to the list of tracking targets, you will need to save the updated model template (Model→Save Model Template).

The iliac crest targets are acceptable tracking targets for the pelvis, but the greater trochanter is actually an anatomical landmark on the femur, so we don’t recommend using greater trochanter targets as tracking targets for the pelvis. Also, there is typically also a lot of soft tissue between the greater trochanter and the greater trochanter targets, so they can be unreliable. See Segmental definitions Marker set guidelines

Question: In our protocol we collect data while participants walk across our lab in the positive Y direction and also in the negative Y direction. This messes up our pelvis angles. What can we do?

Answer (click to expand)

There are two simple ways to deal with this. The first is always have your participants walk in one direction only! Obviously this may not always be possible, and certainly not if you have already collected your data and are dealing with the issue retrospectively. The second is to create a 'virtual lab' segment. A virtual lab will be aligned with the direction of travel, and therefore should avoid the problems you are having. See Example virtual lab Virtual lab tutorial

Question: Our model uses the same marker to track two segments – the knee marker tracks the thigh and the shank. Why are there translations along and rotations about all three axes at the knee, when the model is not a 6DoF model?

Answer (click to expand)

This can be quite confusing, and there is some debate as to what things should be called, even amongst ourselves. When a segment is tracked, and its pose estimated, using markers that do not track any other segment then the segment is free to translate and rotate independently of any other segment. This independent pose estimation is what is meant by a 6DoF model. When two adjoining segments, such as the thigh and shank, share a tracking marker then the pose estimation of each segment is calculated using information from each segment’s tracking markers. While three rotations and three translations are still possible at the joint between the segments, the pose of each is not independent to the other, as some information (the shared marker) applies to both segments, hence we do not refer to that kind of model as a 6DoF model.