The kinematics of the shoulder complex were studied by non-invasive motion analysis and by developing a computer program which calculates the three-dimensional rotations of the shoulder . Because of the large range of motion at the shoulder, it was necessary to investigate and address several singularities. The mathematics used in the computer program created local coordinate systems on body segments and used Euler angles in the form of a joint coordinate system as introduced by Grood and Suntay (12).

Six young men, who had no previous shoulder injury, performed a series of arm movements while 60 Hertz positional data were collected using a motion analysis system. The data were then read into the program and three angles were calculated:​ the internal/external rotation of the humerus, the rotation of the humerus in a transverse plane relative to the thorax and the elevation angle.