It has a resting surface (2) with an opening (3) in correspondence with the area intended to support the user's trunk, in which there is a support element which has a contact surface (6, 60). It also has a height adjustment device (5, 50) of the contact surface (6, 60) and pressure sensors (7) located at different points on the contact surface (6, 60), intended to detect the pressure exerted by the support element on different points on the user's trunk, and a control unit (8) which receives signals from the pressure sensors (7) and, depending on them, acts on the height adjustment device (5, 50), modifying the height of the contact surface (6, 60) to the shape of the user's trunk.
