Disclosed is a motorized assistance system with a tibial orthosis including: a tibial fastening that covers the user's shin; an ankle fastening articulated with a plantar fastening at the axis of the ankle; and a processor. The tibial fastening can include a support of a motorized winch, in which a strap is hooked, whose free end is fixed to the foot, so that the winch takes up the strap when a foot position sensor indicates that it is not resting on the ground. Alternatively, the system can have a femur orthosis with two femur actuators that take up or release two strap in parallel, one attached to a harness or belt and the other to the tibial fastening. The system may include additionally or alternatively a hip orthosis.