Abstract: An exoskeleton is configured to reduce muscle forces in a user's back during forward lumbar flexion. The exoskeleton includes a frame coupled to the user's trunk, a first link coupled to a first thigh and rotatably connected to the frame, and a second link coupled to a second thigh and rotatably connected to the frame. An actuator coupled to the frame includes a first element and a second element that rotate or translate relative to each other to generate a force or torque transmitted through lines connected to the thigh links. A lockout mechanism selectively couples components of the exoskeleton to restrict rotation of the first link relative to the frame in at least one direction, providing controlled support during flexion while permitting free movement during other activities.