# State of the Art: Bipedal Robots for Lower Limb Rehabilitation
## 3 Rehabilitation Application  
The lower limbs of the human body are mainly responsible for standing, maintaining balance, and walking. The requirements of these functions are the same in bipedal robots. Conversely, bipedal  
Figure 4: The model-based gait planning (**a**) The multi-link model; (**b**) The inverted pendulum model. (**(a)** was reproduced with permission from [63]; (**b**) was reproduced with permission from [64], Copyright Cambridge University Press, 2016.) robots are designed and controlled by the principle of realistically imitating people. As a result, bipedal robots and related technologies, such as stability control and motion planning, can be applied in human lower limb rehabilitation training and assisted walking. The application of bipedal robot technology to human lower limb rehabilitation training is limited. In 2000, with the support of the U.S. Defense Advanced Research Projects Agency, Berkeley's Human Engineering Laboratory (HEL), SARCOS Robotics, Oak Ridge National Laboratory (ORNL), and Millennium Jet began research on bipedal exoskeleton robots. Most bipedal exoskeleton robots are still in the initial stages of development. However, the technical achievements on bipedal robots have contributed to the development of lower limb rehabilitation robots, particularly for gait planning, stability control, and sensor applications. Of course, a re-examination of the theoretical approach helps to improve the robots. However, bipedal robots and rehabilitation robots cannot be equated. For rehabilitation robots, humans can act as a link in control, whereas the existing bipedal robot technology often does not consider this aspect. Therefore, developing and improving the control scheme from the unique perspective of the rehabilitation robot is necessary. Overall, the bipedal robot technology has a large volume of reference material for lower limb rehabilitation robots. Regardless of the type of lower limb rehabilitation robot, they all use bipedal robot control theory, especially for stability control and gait planning. Four common types of lower limb rehabilitation robots exist. * Wearable Exoskeleton Robot  
The wearable exoskeleton robot is one of the earliest proposed robots to assist with walking. Its biggest feature is that it allows the user's range of activities to be less restrictive. It can be used to assist patients with hemiplegia to walk, and to improve the body's normal function. This field has the highest number of researchers. The Berkeley lower extremity exoskeleton (BLEEX) (Figure 5a), developed by the University of California at Berkeley and the Ergonomics Laboratory, already has two versions [70]. The mechanical system of the bipedal exoskeleton robot was designed according to the biological characteristics of the normal adult man. A plurality of sensors and actuators mounted on the exoskeleton legs provide the necessary control information to the control center. The computer accepts this information while detecting the user's real-time status. By adjusting the drive torque to assist joints, people can walk long distances while carrying external load. Experiments show that when the load on BLEEX-1 is 34 kg, walking speed can reach 1.3 m/s. However, the human body feels only 2 kg of the external weight. When the load on BLEEX-2 is up to 45 kg, walking speed can be increased to 2 m/s. Cyberdyne HAL [71] (Figure 5b), Israel ReWalk (Figure 5c), and Rex Bionics' Rex (Figure 5d) are all wearable exoskeletons. Due to the complexity of the mechanical structure and the energy consumption of the equipment, the current wearable exoskeleton robot efficiency is low. Figure 5: The wearable exoskeleton robot. (**a**) BLEEX; (**b**) Cyberdyne HAL; (**c**); Israel ReWalk; (**d**) Rex Bionics’ Rex. ((**a,c**) were reproduced with permission from [72]; (**b,d**) were reproduced with permission from [73], Copyright Springer, 2013.) * Rehabilitation Equipment  
Rehabilitation equipment also uses bipedal robot technology.