Methods are provided for controlling a powered lower limb device. A stance phase control method is disclosed in which the required joint torque is determined based on the difference between two joint angles, such as the knee joint and the ankle joint. A swing control method is also disclosed that employs feedback-based minimum jerk trajectory control. A joint assembly for use in a modular lower limb device is also provided. The joint assembly includes a reconfigurable slider-crank mechanism that is configurable to provide a plurality of different ranges of rotational travel, rotational speeds, and torques, for customization according to different anatomical joints. The joint assembly may include a compact coupling device for coupling a ball screw of the slider-crank mechanism to an output shaft of a motor. When employed to form a modular orthosis, the joint assembly may be adapted for self-alignment as its length adjustment method during setup.