According to a first aspect of the invention there is provided a motion generator comprising an effector for applying forces, moments and movements to an effector pay load relative to a surface connected to one or more elongate rigid struts, each strut being connected at one end thereof by a first joint to the effector and being connected at its other end by a second joint to an associated rocker, the rocker comprising a rocker arm and having a pivot axis which is generally parallel with the surface, such that the movement of the rocker arm through a generally vertical are about the pivot axis leads to movement of the effector, and forces applied to an associated rocker lead to forces being applied to the effector, in which the movement of a rocker and forces applied by the rocker are controlled by an actuator, the actuator being in the form of, or comprising an elongate belt, cable, or rope drive which is connected by either end thereof to an associated rocker to apply a force to a point on the associated rocker away from the pivot axis of the rocker, the motion generator further comprising means for applying tension to the belt, cable or rope drive during movement of an associated rocker.