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Abstract. [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.[Methods] 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.

ABSTRACT. Objective: Sit-to-stand motion (STS) is a dynamic motion utilized in fundamental activities of daily living and requires extensive joint movement in the lower extremities and the trunk and coordination of multiple body segments. The present study aimed to investigate whether aging affects the motor coordination of joint movements required to stabilize the horizontal and vertical movement of center of mass using the uncontrolled manifold (UCM) analysis. Method: We recruited 39 older adults with no musculoskeletal and/or neuromuscular conditions that affected STS, along with 21 healthy younger adults. All subjects performed five STS trials from a chair with the seat height adjusted to the length of their lower leg at a selfselected motion speed. Kinematic data were collected using a three-dimensional motion analysis system. We performed the UCM analysis to assess the effects of joint angle variance (elemental variable) to stabilize the horizontal and vertical movement of COM (performance variable) and calculated the joint angle variance that does not affect COM (VUCM), the variance that affects COM (VORT), and the synergy index (ΔV). Results: ΔV values in the horizontal direction were higher in the older adults than in the younger adults, but ΔV values in the vertical direction were lower in the older adults than in the younger adults. Conclusion: Older adults require increasing levels of stabilization of horizontal movement of COM after buttocks-off in the STS maneuver. As a result, variance in the joint angle of the lower extremities indicated no kinematic synergy for stabilizing the vertical movement of COM. Key words: Uncontrolled manifold analysis, Sit-to-stand, Aging (Phys Ther Res 20: 44-50, 2017)T he Sit-to-stand motion (STS) is a highly common motion that is performed as frequently as gait in daily life. It requires two tasks: transitioning the center of mass (COM) from a wide base of support (BOS) provided by the buttocks, thighs, and feet to a narrow BOS provided by the feet alone, and lifting the COM from the height of sitting to that of standing. STS also represents a motion for which the muscle strength of lower extremities is particularly important 1,2 ) . STS is associated with applied motions such as transfer and walking, and its acquisition is an essential factor to improve mobility capability. STS results from interactions characteristic of task-related, individual, and environmental constraints 3) . Alexander et al. 4) reported that aging affects STS acquisition, and that older adults with difficulty performing this maneuver require prolonged STS time. We reported that older subjects who experienced dif- The coordination of joint movements during STS motion 45 ficulty with this motion performed the STS using less energy and with more consideration for stability, relative to younger healthy subjects 5) . It is important to control the redundant degrees-offreedom (DOFs), which coordinate with each body segment to accomplish tasks. STS is a whole-body movement that requires simultaneous ...