System of Rigid Bodies
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In was mentioned earlier that the human body segments are considered as rigid bodies for analytical convenience. Since the human body is composed of several segments (14 - 15, depending on the model used), it can be considered as a system of rigid bodies. In spite of some similarities, a system of rigid bodies is quite different from a system of particles in the sense that the elements of the system are rigid bodies that have not only the mass but also the volume (moment of inertia). Therefore we need to develop a new set of equations dedicated to a system of rigid bodies.

Some of the mechanical quantities of a system of rigid bodies can be directly computed by summing those of the segments, such as the momentum and the kinetic energy. But some other quantities deserve more attention. The pages included in this section are

Whole Body Center of Mass
Whole Body MOI
Whole Body Angular Momentum

The human body is in fact more than a simple collection of rigid bodies. The segments are inter-linked at the joints and, for this reason, the human body is often called as the linked-segment system. The fact that the segments interact each other adds additional complexity to the system and the related issues are covered in a separate section in Volume II: Theories and Practices in Motion Analysis.

Joint Kinetics
Joint Torque
Data Interpretation
Joint Energetics


Young-Hoo Kwon, 1998-