In the field of physical therapy, observational motion analysis is important to predict the impairment and/or to judge the effect of physical therapy. Many kinds of knowledge are needed to observe a human movement and to integrate its phenomena and impairment. It is very important to have a knowledge of mechanics, because human motion is governed by the laws of mechanics. In this paper, especially, I would like to mention both mechanical stress and various forces in a knowledge of mechanics.

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... hanical stresses include the following: load, tension, compression, bending, shear and torsion. Various forces are displayed in this article, including gravity, inertial force and reaction force. I think that to understand these stresses and forces related to human movement may be useful for the prediction of impairment.