This paper presents the motion of recovering the balance for biped robot. Recently, it is expected that the humanoid robots can coexist to human beings. However, it may be possible for the robots to do harm the human beings. In other words, the relaxation of the collision with the human beings and the robots is necessary. Hence, it is necessary to become complication in order to absorb the crash each other. For the biomechanical studies, it is well known that the human beings are applied "three strategies", ankle strategy, hip strategy, and step strategy, to keep their balance for an external forces. Therefore the human beings can keep the balance by using ankle joint, hip joint. On the other hand, this strategy, which the ankle joint and/or hip joint is used, is applied to a certain impact. We propose the motion of knee joint to keep the balance for more large impact. The robots can keep their balance for more large impact to add the knee joint. The relation between the ankle and knee joint is based on Reaction Null-Space Method (RNS), which is used to control the postures of the free-flying space robots. The proposed method is confirmed by simulation results.