Service robots must comply with very demanding safety requirements in order to guarantee that a human can be assisted without any risk of injury. This paper presents a detailed multibody model of the interaction between a single link manipulator and a human head–neck to study the different and more significant parameters involved in the design of the manipulator. The multibody model is first validated through comparison with experimental results obtained in a testbed, which has been built for this purpose. The testbed consists of a flexible pendulum with an inertial wheel attached to the pendulum shaft and a head–neck dummy of 1 degree of freedom (DOF). A phenomenological model of the robot-arm foam soft cover has been developed by fitting experimental results obtained in a compressive test performed on the foam. Once the multibody model is qualitatively validated, several simulations are carried out. The aim of the simulations is to study the effect of different design parameters in the head injury. In particular, the effects of the link flexibility, of the joint compliance, and of the soft cover are detailed.