Various embodiments provide, an apparatus for emulating an impulse force exerted by a skeletal muscle, the apparatus including a mass and an actuator, the actuator being adapted to exert a unidirectional force, a unidirectional torque, a unidirectional moment or a unidirectional couple, on the mass, the apparatus being configured in use to selectively couple together the mass and the actuator so that the unidirectional force, a unidirectional torque, a unidirectional moment or a unidirectional couple exerted on the mass by the actuator emulates the impulse force.