On expression of human body motion

In order to study a human body motion, a human body is sometimes replaced by a mechanical links system composed of human body elements. Each link is supposed to be connected by a spherical joint, if we neglect a small deformation or distortion at a human joint. Since it is assumed that a complex human body motion is performed by the superposition of the simple relative rotational motion of body elements on many human joints, it is necessary to resolve a body motion into the relative rotational motion for a study of a human body motion. Here, we discuss the method of resolving a relative rotational motion at a human joint and try to apply it to an actual swinging motion of arm.