This paper proposes a novel leg orthosis for lower limb rehabilitation robots of the sitting/lying
type. It consists of three joint mechanisms: hip, knee and ankle, and two sets of links: thigh and
crus. Each driving motor is located close to the associated joint and the rotational axis of each joint mechanism is unique and stable. These features make it outperform the similar mechanisms in stability and dynamic performance. Different forms of eccentric slider-crank mechanisms are applied in the three joint mechanisms, respectively, such that they can be optimized independently. The optimization problems for the hip and knee joint mechanisms, characterized as strongly nonlinear, are developed respectively. Then, a particle swarm optimization algorithm is used to obtain the optimal solutions, which are subsequently validated by comprehensive comparisons. Moreover, the kinematics necessary for motion control and trajectory tracking are investigated, which denote the relationships between the displacements and velocities of the joint mechanisms, lead screws and the end effector. Finally, this paper illustrates the feasibility of the application of the leg orthosis to actual rehabilitation exercises by a simulation example.