In recent years, there has been intense interest in collaborative robots, both for industry and household applications. While significant progress has been made, physical human-robot interaction is still presenting a challenging problem that has not been satisfactorily solved. When a human is interacting with another human, the forces they exchange represent a communication channel and a continuous stream of information flows between them. When a human is interacting with a robot, the forces applied by the robot are interpreted by the human that in turn reacts to them; obviously, people are expecting the robot to also react to the forces they are applying.