Abstract: An apparatus for maintaining a support platform and a payload mountable thereon in near neutral equilibrium relative to a base structure. The support platform is spaced from the base structure by a plurality of actuators which provide the support platform with six degrees of freedom relative to the base structure about longitudinal, lateral and vertical axes. A substantially upright strut is mountable between the base structure and support platform for supporting the weight of the support platform and payload. First and second balance assemblies counteract forces imparted on the strut when the payload is moved or pivoted longitudinally and/or laterally. The forces counteracted by the first balance assembly and the second balance assembly reduce the load imparted on the actuators by the payload.