[Arm rotation during abduction of the shoulder in the scapular plane]

The arm rotation during abduction of the shoulder in the scapular plane was investigated in 11 volunteers. The degree and direction of the arm rotation were measured using five potentiometers, and were analysed by a computer system. In eight volunteers, the arm always rotated externally (1.5 degrees-4.9 degrees of rotation for each 10 degrees of abduction); in one volunteer it rotated internally (0.9 degree of rotation for each 10 degrees of abduction); and in the two remaining volunteers it rotated without showing a constant tendency during the abduction. The degree of arm rotation was correlated to the tilting angle of the acromial articular surface which was measured radiographically. A tendency of external rotation of the arm is suggested to occur during abduction in the scapular plane along with the decrease of the tilting angle of the acromial articular surface.