The paper discusses a robotic system (RS) for the rehabilitation of the lower limbs of patients. The system consists of an active parallel 3-PRRR robot and a passive part of the RRRR orthoses. A 3D model of a passive RRRR orthosis’s joint structure was developed, taking into account patients’ fit with different anthropological data. The requirements for the body mass index of patients are formulated to optimize the design parameters. This made it possible to determine the rehabilitation system’s workspace, taking into account the minimum and maximum regulation of the passive orthosis joints based on optimization methods. The simulation results are presented. © 2022, The Author(s), under exclusive license to Springer Nature Switzerland AG.