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. 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.