A propulsion system for a manually-operated mobility device, the propulsion system comprising: a hand rim configured to receive a propulsion force from a user of the manually-operated mobility device; a wheel hub fixedly attached to a wheel of the manually-operated mobility device; and a planetary gear system rotatably coupling the hand rim to the wheel hub, the planetary gear system comprising a sun gear, a ring gear, and one or more planetary gears disposed between the sun gear and the ring gear, wherein the one or more planetary gears are supported by a planet carrier, wherein one of the sun gear and the ring gear is fixedly coupled to the hand rim and the other of the sun gear and the ring gear is fixedly coupled to the wheel hub; wherein the manually-operated mobility device is shiftable between: a pushing propulsion mode in which the hand rim and the wheel hub rotate in a same rotational direction, and a rowing propulsion mode in which the hand rim and the wheel hub rotate in opposite rotational directions; wherein the planet carrier causes the manually-operated mobility device to transition between the pushing propulsion mode and the rowing propulsion mode by releasably attaching to a frame of the manually-operated mobility device; characterized in that the planet carrier rotates relative to a frame of the manually-operated mobility device in the pushing propulsion mode and is rotatably fixed relative to the frame in the rowing propulsion mode. a hand rim configured to receive a propulsion force from a user of the manually-operated mobility device; a wheel hub fixedly attached to a wheel of the manually-operated mobility device; and a planetary gear system rotatably coupling the hand rim to the wheel hub, the planetary gear system comprising a sun gear, a ring gear, and one or more planetary gears disposed between the sun gear and the ring gear, wherein the one or more planetary gears are supported by a planet carrier, wherein one of the sun gear and the ring gear is fixedly coupled to the hand rim and the other of the sun gear and the ring gear is fixedly coupled to the wheel hub; wherein the manually-operated mobility device is shiftable between: a pushing propulsion mode in which the hand rim and the wheel hub rotate in a same rotational direction, and a rowing propulsion mode in which the hand rim and the wheel hub rotate in opposite rotational directions; wherein the planet carrier causes the manually-operated mobility device to transition between the pushing propulsion mode and the rowing propulsion mode by releasably attaching to a frame of the manually-operated mobility device; characterized in that the planet carrier rotates relative to a frame of the manually-operated mobility device in the pushing propulsion mode and is rotatably fixed relative to the frame in the rowing propulsion mode. a pushing propulsion mode in which the hand rim and the wheel hub rotate in a same rotational direction, and a rowing propulsion mode in which the hand rim and the wheel hub rotate in opposite rotational directions; wherein the planet carrier causes the manually-operated mobility device to transition between the pushing propulsion mode and the rowing propulsion mode by releasably attaching to a frame of the manually-operated mobility device; characterized in that the planet carrier rotates relative to a frame of the manually-operated mobility device in the pushing propulsion mode and is rotatably fixed relative to the frame in the rowing propulsion mode.