The authors, using cinematic methods have determined the positions of Instant Rotation Center (IRC) in upper and lower radio-ulnaris joints, in the movement of prosupination : -- At the upper radio ulnaris joint, the IRC divide themselves into 2 zones (I and II), corresponding to the centres of 2 circles of different diameters, each one respectively corresponding to large and small anatomical curves of the head of the radius. -- At the lower radio ulnaris joint, the IRC divide themselves into a third zone (III), situated opposite the head of ulna. The prosupination movement is effectuated around 2 axes delta I - II, and delta II - III : supination around delta I - III, and pronation around delta II - III in a first phase, ten around delta I - II in a second phase. The movement is not impaired if the ligamentum annulare radii is cut, but it is considerably impaired if the ligamentum quadratum is cut.