Interactive generation of reactive motions for virtual humans as they are hit, pushed and pulled are very important to many applications, such as computer games. In this paper, we propose a new method to simulate reactive motions during arbitrary bipedal activities, such as standing, walking or running. It is based on momentum based inverse kinematics and motion blending. When generating the animation, the user first imports the primary motion to which the perturbation is to be applied to. According to the condition of the impact, the system selects a reactive motion from the database of pre-captured stepping and reactive motions. It then blends the selected motion into the primary motion using momentum-based inverse kinematics. Since the reactive motions can be edited in real-time, the criteria for motion search can be much relaxed than previous methods, and therefore, the computational cost for motion search can be reduced. Using our method, it is possible to generate reactive motions by applying external perturbations to the characters at arbitrary moment while they are performing some actions.