81 and being arranged vertically in the assembled frame configuration as can be seen in fig2 and also in fig1 which shows the details the elastic coupling elements between the backrest 21 and the shoulder supports 22 and 23 comprises for example a leaf spring such a spring comprises stacked metal plates for example of stainless steel and preferably of the same dimensions so that there is only one reference a leaf spring has a high damping coefficient for travels i e very low amplitudes of deformation in addition the essentially flat shape thereof is compatible with its mode of attachment as above and with the covering over the frame the frame is assembled from the bent front plate 26 the twice folded rear plate 25 and the angle bracket formed by folding the plate in fig8 as follows first the angle bracket is positioned inside the angle formed by the 90 u00b0 folding of the horizontal part 71 of the rear plate 25 four holes 4 millimeters in diameter are pierced in said rear plate using the holes 28 of the upper part 91 of the angle bracket as a piercing guide so that there is no misalignment between the holes 28 on the angle bracket and the corresponding ones on the plate 25 then the horizontal upper part 91 of the angle bracket 27 and the horizontal part 71 of the rear plate 25 are attached to each other for example with four rivets through the aforementioned holes second and in the same manner five holes are pierced in the vertical lower part 90 of the angle bracket 4 millimeters in diameter using the five holes 74 of the vertical lower part of the plate 25 as a guide and then.