Quantitative evaluation of human rising movements from chair in terms of physical strain imposed on joints

We have already formulated a dynamics-switching optimal control model that reproduces human rising movements from a chair, and clarified its effectiveness and the existence of the optimal relation between the model´s two factors (input weight and switching time) indispensable for sit-to-stand movements. In order to quantitatively evaluate human sit-to-stand movements in terms of physical strain imposed on the joints, this research quantifies the influence of the two factors on the joint torques during movement. The following results are derived: (1) as the upper body´s forward bending increases, i.e., the input weight becomes smaller and the switching time becomes larger, the relation among the three joint toques changes from a tendency for their magnitude to decrease in the order of knee, ankle, and hip joints to a tendency to increase in the same order; (2) in an infinite number of optimal combinations of the two factors, there exist three optimal conditions for the three maximum absolute joint torques to take their respective minima, and the sit-to-stand movements reproduced under the three optimal conditions are very close to measured movements. These results suggest that the human sit-to-stand mechanism usually produces movements to nearly minimize the physical strain imposed on the joints, though it can produce various kinds of sit-to-stand movements depending on circumstances.