Kinematics of Scapular Motion

Abstract

Scapular motion has traditionally been described in a single planar two-dimensional model, with scapular upward rotation and acromial elevation as the end point [ 1 , 2 ]. Upper trapezius activation to pull the acromion up and serratus anterior activation to move the inferior border laterally were described as the key muscle force couples [ 1 , 3 , 4 ]. The magnitude of the upward rotation varied between studies but averaged 60°, establishing the 1:2 scapula /humerus motion ratio for total scapulohumeral rhythm (SHR).Recent studies show that scapular motion is actually multiplanar and three-dimensional. This work, using motion tracking systems and indwelling bone pins [ 5 -8 ] demonstrates that total scapular movement is a composite of motions (rotations around axes) and translations (sliding along a surface).The three observable rotary motions are upward/downward rotation around an axis perpendicular to the scapular body, internal/external rotation around a vertical axis along the medial border, and anterior/posterior tilt around a horizontal axis along the scapular spine. The absolute amount of each rotation varies within the studies. The indwelling bone pin study by Ludewig is probably the most accurate [ 7 ]. It shows that the resting position, in relation to the thorax, is 5.4° of upward rotation, 41.1° of internal rotation, and 13.5° of anterior tilt. As the arm moves to maximum elevation, the scapula moves in all three motions. It upwardly rotates 45°, posteriorly tilts 21°, and moves into internal then external rotation with a net change towards external rotation of 2° [ 9 ]. The largest part of these motions occurs in arm elevations above 80°.Two translations can occur in the presence of an intact clavicular strut and acromioclavicular (AC) joint. They are upward/downward sliding on the thorax due to clavicular upward/downward motion at the sternoclavicular (SC) joint and anterior/posterior sliding around the curvature of the thorax due to clavicular anterior/posterior motion at the SC joint.The clavicle and the SC and AC joints are major factors in creating the scapular positions, motions, and translations. The clavicle is the only bony connection of the scapula to the axial skeleton. This creates a stable strut, allowing controlled motion in several planes. To maximize scapular movement and scapulohumeral motion during maximal arm elevation, the clavicle retracts 16°, elevates 6°, and posteriorly rotates on its long axis 31° [ 9 ]. All these motions are