Open chain movement: refers to the movement that the proximal end of the limb (the part close to the trunk) is fixed and the distal joint (the part far away from the body) is movable, such as the flexion and extension of sitting legs. Closed chain movement: refers to the movement of distal limb fixation and proximal joint movement, such as squatting. Closed-chain exercise can play a protective role without increasing joint shear force, which is closer to functional rehabilitation training.