# State of the Art: Bipedal Robots for Lower Limb Rehabilitation
## 1 Introduction
Japan has many other scientific research institutions engaged in bipedal robot development and theoretical research work, such as Matsushita Electric Works, Fujitsu, Farrah, and Hitachi. They have performed significant research on bipedal robots and have achieved some success. In parallel with the Japanese developments, [PERSON] put forward the \"Passive Dynamic Waking\" theory in 1989 [41; 42]. This theory improved the system of bipedal robots. The next year, [PERSON] et al. proposed the use of neural networks to achieve stable dynamic walking and achieved their goal with the SD series bipedal robots [43; 44]. This was the first time intelligent algorithms were integrated with gait planning. In 1997, [PERSON] and [PERSON] et al. of the Massachusetts Institute of Technology proposed a virtual model control (VMC) strategy in the research of the bipedal robot Spring Flamingo [45]. The use of VMC effectively avoided the cumbersome robot dynamics and inverse kinematics calculations. This was helpful for the research of bipedal robots. In 2005, the University of Florida developed a bipedal walking robot, Rabbit, to show the world its running ability [46]. The walking ability of bipedal robots was again improved. At present, the research and development of bipedal robots are mainly concentrated in Japan, the U.S., China, South Korea, and France. However, the focus of each country's research is different. Japanese bipedal robots are biased toward simulating human movements and living characteristics. European countries developed bipedal robots biased toward medical services, and the American research bipedal robot has focused on military applications. Over the past decade, new bipedal robot products were being constantly developed. In 2010, Japan's Advanced Industrial Science and Technology (AIST) released an entertaining bipedal robot HRP-4C [20]. Although HRP-4C is an entertainment robot meant for singing and dancing, it is technologically advanced. PETMAN robots were developed by Boston Dynamics in 2013 [47], a lifelike humanoid robot mainly used to test protective clothing. Surena III was the latest generation of bipedal robots launched by Tehran University in 2015 [24]. Its body can move backward while standing on one foot. In 2016, Boston Dynamics showcased Atlas, developed for the U.S. military [25]. Atlas can not only walk like a human body with its legs, but can also adapt to a variety of outdoor terrain. These bipedal robots, as shown in Figure 2, represent the top level of bipedal robot development. The theory of bipedal locomotion has basically matured, although room for improvement exists in stability. The researchers should allow the bipedal robot to identify the external environment information, so that it knows how and where to go. The implementing agency, cooperating with the bipedal locomotion, needs to be improved and perfected. Once this is attained, bipedal robots will be able to complete more tasks. Conversely, the movement of bipedal robots is similar to that of humans, and many bipedal robot technologies can be applied to rehabilitation, such as structural design, stability control, and gait planning. So, the development of bipedal robots will promote the development of rehabilitation, and rehabilitation will become one of the important applications of bipedal robots.