A shoulder joint is provided. The shoulder joint includes a main support; a fold-out arm including an upper arm and a lower arm, the upper arm operable to pivot about a pin and the lower arm operable to pivot on a spindle on one end of the upper arm; and a first wheel attached to the pin, and a second wheel attached to the spindle. The upper arm and the lower arm are mechanically linked via the two wheels in such a way that rotation of the upper arm about the pin cause rotation of the second wheel and the lower arm relative to the upper arm, resulting in a bending or straightening of the arm.