Abstract: An actuator includes a piston, a relief valve in fluid communication with the piston, and an input shaft for moving fluid in a chamber near the piston. The relief valve determines an amount of force needed to move the piston to compress a fluid. The actuator also includes at least one check valve which allows the input shaft to move back to an equilibrium position with a much lower force than a force needed to compress the fluid. The actuator is a passive device that can be used to prevent motion or isolate a base. The actuator is also incorporated in tuned mass damping systems.