A power transmission device (100) comprising a first transmission (110) configured to receive a power from an input device (200)； a second transmission (130) configured to deliver the power to an output device (500)； and connecting unit (120) configured to connect the first power transmission unit and the second power transmission unit, and to deliver a pressure of a transfer fluid occurring due to the first power transmission unit to the second power transmission unit.A motion assistance device (10) comprising a driver (210) configured to mount to a user at a position corresponding to a proximal part of the user, and to generate a power；a joint assembly (400, 600) corresponding to a distal part of the user, the joint assembly configured to provide an assistance force to the user based on the power； anda power transmission device (100).
