In this paper we present a motorized hip orthosis to assist elderly people while walking, stair climbing and during the sit-to-stand transitions. The design is based on biomechanics considerations. Ranges of motion, velocities and torques of the hip joint rotations were studied from the literature to define precise design specifications. In order to fit with these requirements, an amplification mechanism inspired by excavators was developed and implemented. Our prototype’s workspace is therefore sufficient for most common activities. The paper also presents a theoretical model of the motorized orthosis which illustrates the device capabilities. Additional tests with the prototype showed that excellent transparency can be achieved with the device, in particular when using an extra sensor to detect small position variations. The assistance capabilities are limited at higher frequencies but can typically go up to 30% for a 70 kg subject during walking at a cadence of 100 steps/min.