Purpose] This study aimed to clarify the motion strategy used by young healthy adults during sit-tostand motion (STS) and to investigate the relationship between forward tilt movement by the trunk and shanks, which is considered to contribute to the forward shift of COG, and the load on the lower extremity joints accept, from the standpoint of mechanical energy flow. [Subjects] The subjects were 10 young healthy males without disease and/or present history of illnesses that would affect STS.

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... thods] Three kinds of task conditions were adopted: normal STS (condition N), STS with the trunk tilted as forward as possible (condition TM), and STS with the trunk as vertical as possible (condition TV). The kinematic data during STS were collected using a three-dimensional motion analysis system, and the kinetic data were collected by force plates. [Results] The average negative power in the proximal portions of the shank under condition TV was significantly higher than that under conditions N and TM. [Conclusion] Shank forward tilt appears to be primarily a reactive movement facilitating knee extension rather than contributing to moving COG forward. Thus, less trunk forward tilt movement results in the requirement of higher ability of shank forward tilt movement.