A new method to quantify intra-articular relationships between articular surfaces of the glenohumeral joint during discrete poses representing the late preparatory phase of throwing is presented. This method is based on 3D bone reconstructions from medical imaging data processed into finite helical axis parameters. With the shoulder moving in the anatomical planes from 90 degrees abduction and 90 degrees external rotation into the apprehension test pose, the centre of the humeral head posteriorly translated on the glenoid and rotated about a finite helical axis, which was positioned at the joint contact. The data are contrasted with Kaltenborn's convex-concave rule explaining intra-articular kinematics of the glenohumeral joint as a ball-and-socket joint. The data show at all conditions that the glenohumeral joint does not act as a ball-and-socket joint. Consequently, the mobilization techniques used in manual therapy, which are based on this convex concave rule, should be adapted.