Time-constrained Optimization of Sit-to-stand Movements in Contact with the Environment

This study aims to evaluate effects of different lower limb joint torque constraints on the motion trajectory of an sit-to-stand movement (STS) grasping an assistive device by simulating the STS with an assistive device using a human model with different lower limb joint torque constraints at various second lengths. This is an initial study for estimating lower limb muscle strength from the motion trajectory of the STS grasping an assistive device. In this study, instead of measuring many human subjects, optimal motion trajectories are calculated and simulated. Equations of the STS using the assistive device are first established using a rigid link model with both hands grasping the assistive device. Then, equations of motion and physical constraints such as joint range of motion and torque limitations are used as constraint conditions, and the sum of squares of joint torque, which calculates the action with the minimum muscle load on the body, is used as an evaluation function to optimize the STS. During optimization, calculations are performed with several types of joint torque conditions and end-time, i.e., the time required to complete the operation. By comparing several types of joint torque condition movements, the differences in the effects of different joint torque constraints on the motion trajectory of the STS grasping the assistive device can be determined.