Model Based Computations: Default Joint Names

Definition of a Joint

We use the word "joint" in two different ways in Visual3D.

First, a "joint" defines the point at which the joint reaction force is considered to be equal and opposite between two segments, and the net joint moment is considered to be equal and opposite between two segments.

Note that in a 6 DOF model, the joint isn't a specific point.

Note that the user cannot specify a joint. Visual3D determines that a joint exists if the proximal end of a kinetic segment is closer than the distal radius of a proximal segments distal endpoint.

Second, we have followed the literature in calling a landmark that is computed functionally as a functional joint. In Visual3D this is really a functional landmark.

If for example, a functional hip joint is computed and the landmark is not within the distal radius of the pelvis from the distal end of the pelvis, no joint will be created. This is not a problem for joint moments because the joint moment is really the proximal moment on the segment, so no joint is needed. This is, however, a problem for joint power because there will be no way to compute the joint angular velocity. If a joint has not been created where you expect it, check to see that he landmark is sufficiently close to the pelvis.

Default Joint Names

Visual3D creates joint names automatically for the default segments.

Visual3D contains a number of default segment names that are provided for the convenience of the user. These default segments contain geometry, the inertial properties, and the wire frame model. Users that create their own segment names need to add this information in the segment properties tab in Model Builder Mode

The calculation of Joint Moment, Joint Power, and Joint Force refer to the proximal end of the segment selected. For convenience we have mapped segment names to joint names automatically because we assumed that users would find it more intuitive.

Visual3D Segment Name	Anatomical Segment	Joint Name	Anatomical Joint
RFT	Right Foot	RANKLE	Right Ankle
RSK	Right Shank	RKNEE	Right Knee
RTH	Right Thigh	RHIP	Right Hip
LFT	Left Foot	LANKLE	Left Ankle
LSK	Left Shank	LKNEE	Left Knee
LTH	Left Thigh	LIHP	Left Hip
RPV	Pelvis	WAIST	Waist
RTK	Trunk and Pelvis	RWAIST	Waist
RTA	Thorax and Abdomen	RWAIST	Waist
RHE	Head	NECK	Neck
RAR	Right Upper Arm	RSHOULDER	Right Shoulder
RFA	Right Forearm	RELBOW	Right Elbow
RHA	Right Hand	RWRIST	Right Wrist
LAR	Left Upper Arm	LSHOULDER	Left Shoulder
LFA	Left Forearm	LELBOW	Left Elbow
LHA	Left Hand	LWRIST	Left Wrist

Custom Joint Names

The joint moment computed by Visual3D is expressed as the proximal moment on a segment.

For example, the moment between the shank and thigh is commonly referred to as the Knee Moment, but is actually computed as the proximal moment on the shank.

If the link model is defined with a shank and a thigh segment, the "joint" name listed in the model based combo boxes is assigned automatically to be the "Knee" (as shown in the table above).

If the link model is defined with a shank segment and no thigh segment, the joint moment at the proximal end can be computed, but the joint is given the name of the shank segment (e.g. 3 letter acronym).

Custom Segment Joint Names

If the user has created a custom segment name, Visual3D uses that segment name.

Given a segment

NEW_SEG

The Joint will appear as

NEW_SEG