This paper presents the development of an exoskeleton mounted on a mobile base to assist people in the Sit-to-Stand action and focused in the rehabilitation of the hip and knees joints. The prototype consists of linear actuators distributed in such a way that they assist the movements of extension and flexion, in the sagital plane, in the hip and knee joints; also, the mobile base allows the transport of a person of up to 90kg. The interaction between the user and the exoskeleton is carried out by processing EMG signals from the user via a Myo Armband and using a classifier to recognise five hand gestures that, in combination of them, generate the instructions to the exoskeleton. A PID control law was programmed in a microcontroller use as slave attached to an embedded computer for tracking the trajectories obtained from the Capture Motion System. The dynamical model of the system is presented in the paper as well as simulation and experimental results.