In this paper, preliminary studies are presented to model and design a device that will be part of an exoskeleton actuated by Pneumatic Artificial Muscles (PAM's). The aim of the device is to assist the user in lifting weights, avoiding backbone lesions. The device increases the strength of the operator. It uses sensors to detect the force to be applied and to detect the position of the user's trunk. The complete dynamical model of the system consists of the model of a planar robot with 3 rotation joints and the PAM's model, that is taken from some references. At this stage only the mechanical part of the prototype is adjustable.