Segment Group Modeling
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A segment group is a group of segments such as the whole body or the right leg. A body model generally defines the scope of the mechanical system in question but it can be divided into several segment groups. For example, one may be interested in quantifying the angular momentum of the body parts (the trunk & head, right arm, left arm, and the legs) as well as the whole body. Or the segment groups may be overlapped depending on the analyst's interest. The whole body is in fact a segment group composed of all the body segments. In summary, a body model may need more than one segment groups defined.

Some confusion arises when the subject uses foreign objects such as racquet, bat or walker. The question is whether the foreign object should be included in the body model as a part of it or not. In other words, does one need to study the motion of the human-foreign object system or the motion of the human body alone. In reality, the relationship between the human body and the foreign object is no different from that among the segments and it depends on the situation. Although the body and the foreign object maintains contact, there may not be any constraining force acting on the foreign object. Likewise, although the segments are held together at the joints, the actual joint force can be zero at a given joint at a given instant. Excluding a foreign object that maintains contact with the body completely from the mechanical system is one problem, but including a foreign object that is apart from the body is another problem.

The solution for both cases presented above is to allow to define multiple segment groups. This way, one can define body parts such as right & left legs as well as the whole body and/or one can define multiple segment groups with and without the foreign objects in a model. To include every segment/body in the model and to define multiple segment groups (body parts) is the ultimate solution. Precisely speaking, a body model does not confine the mechanical system in question. Instead, the segment groups defined within a model do. A model can have multiple mechanical systems.

Along this line, Kwon3D allows to define multiple segment groups (body parts) in a model. Each segment group has its own CM. A reference frame can be defined and assigned to a segment group if necessary. A segment group is simply a collection of segments and different groups need not be mutually exclusive. In fact, they may overlap each other. A segment group only requires the list of the member segments that were defined earlier in the segment modeling stage.

Young-Hoo Kwon, 1998-