The present invention relates a mechanical differential actuator for interacting with a mechanical load. The mechanical differential actuator comprises first and second semi-active sub-actuators, a velocity source and first and second mechanical differentials having three interaction ports each. The first mechanical differential includes a first interaction port coupled to the velocity source, a second interaction port and a third interaction port coupled to the first semi-active sub-actuator. The second mechanical differential includes a first interaction port coupled to the velocity source, a second interaction port and a third interaction port coupled to the second semi-active sub-actuator. Finally, the second interaction ports of the first and second mechanical differentials are coupled together to form an output which is configured so as to be coupled to the load.