
The most complex step in body modeling is the process to define segments. A segment has its mechanical identity (inertial properties). It can show both linear and angular motion, precisely speaking, the linear motion of the CM and the angular motion about its CM. In Kwon3D, a segment can have the following properties:
Three segment types are defined in Kwon3D: point, simple and complex. A point segment is defined by a point while a simple segment is defined by two points. A simple segment requires a distal point and a proximal point. The vector drawn from the proximal end to the distal end is used as the segment vector of the segment. A complex segment such as the trunk requires two distal points and two proximal points and the vector drawn from the midpoint of the proximal points to the midpoint of the distal points is used as the segment vector. It is logical that only the BSP ratios, such as mass ratios, CM ratios, and the ROG (radius of gyration) ratios, can be included in the body model since body model is commonly used for all trials in a study. The trial and subjectspecific BSPs are not allowed in the body model. Otherwise, one has to create several model files for a single study. Nevertheless, the trial and subjectspecific BSPs can still be used in the analysis. See the Handling Strategies page in the BSP Issues section for the details of the BSP types and strategies. One serious business in segment modeling is to define the segmental reference frame. See Segmental Reference Frame Setup for the details. Member points are those belong to the segment and to be used in the computation of the rotation matrix and the joint center using the procedures explained in the Computation of the Rotation Matrix and Computation of the Joint Center pages. At least 3 points must be observed in a segment for the computation of the rotation matrix and the joint center. Any points defined in the point modeling process except the secondary points can be used in specifying the member points. Segmental Reference Frame Setup The core of the segmental reference frame setup is to define the unit coordinate vectors of the frame. In Kwon3D, the unit coordinate vectors can be defined from points or vectors. The points and vectors that can be used in this stage are:
It is a bit unusual to see the velocity vector used in defining the unit coordinate vectors. Believe or not, there are some cases where one has to use the velocity vector of the whole body to define a reference frame. This type of reference frame is not really fixed to the body (segment), but can be used as the reference frame of a segment group. The fact that one can use the unit coordinate vectors of the previously defined reference frames raises an important issue. One has to consider how to define the reference frames before determining the order of the segments. A major improvement in Kwon3D 3.0 over 2.1 is the procedures for the reference frame setup. The procedure has been substantially simplified. In the core of the setup procedure are two axes: axes 1 and 2. Axis 1 is the one needs to be defined precisely while axis 2 doesn't have to be exact. As far as axes 1 and 2 correctly define a plane that is perpendicular to the third axis, it will be OK. This procedure can be summarized as
where n_{1}, n_{2} & n_{3} = the unit coordinate vectors, and a_{1} & a_{2} = the vectors defined for axes 1 and 2, respectively. In Kwon3D, there are 5 different ways to define a local reference frame:
The properties a reference frame can have are:
Both the primary points and the joint centers (secondary points) can be used in defining the unit coordinate vectors. Markers may have to be carefully placed to be used in defining the reference frame. The coordinates of the proximal joint center of the segment can be computed from those of the member points of the segment and its proximal segment using the procedures explained in the Computation of the Rotation Matrix and Computation of the Joint Center pages. 
© YoungHoo Kwon, 1998 