An actuator system for extending and flexing a joint, including a multi-motor assembly for providing a rotational output, a rotary-to-linear mechanism for converting the rotational output from the multi-motor assembly into an extension and flexion of the joint, and a controller for operating the actuator system in several operational modes. The multi-motor assembly preferably combines power from two different sources, such that the multi-motor assembly can supply larger forces at slower speeds (“Low Gear”) and smaller forces at higher speeds (“High Gear”). The actuator has been specifically designed for extending and flexing a joint (such as an ankle, a knee, an elbow, or a shoulder) of a human. The actuator system may, however, be used to move any suitable object through any suitable movement (linear, rotational, or otherwise).