In budo and sports, most coaching words are abstract and intuitive, and their true meanings and scientific foundations are ambiguous. To understand complex movements scientifically, it is important to reduce them into simple models and to analyze them on the basis of mechanical laws. In this paper, as the first step in the theoretical study of movements, we consider the mechanics of bar models, which are applicable to various movements in budo and sports. In particular, we focus on the effects of inertial force on human movements, which have so far been overlooked. We also discuss the restorative force of muscles and gravity, which act in cooperation with inertial force. As a result, we found the mechanisms of some typical movements. For example, we interpret “kinetic link” as the combination of inertial force acting on a rigid body and restoring the force of muscles, and clarify the scientific meaning of “weight shift” in terms of the equivalence of gravity and inertial force.
