The 3-dimensional movement of the shoulder is a complex process of different highly coordinated muscles. The wide range of motion (ROM) of the shoulder joint is strongly connected to the historical developments when the humans started walking on two instead of all four extremities. When the human changed his way of locomotion the arms became increasingly useful for manipulation. The ROM increased by different explicit developments as follows:

Especially the reduction of the glenoid surface in relation to the circumference of the humeral head in the evolution enabled this expanded ROM on the account of form fitted stability.

This loss of stability needs to be compensated by the muscles of the rotator cuff. External rotation is limited by a slight retrotorsion of the humeral head in relation to the axis of the humerus [1, 2]. Comparing the gleno-humeral joint to the hip joint owning three degrees of freedom in motion, the gleno-humeral joint gains its flexibility in motion from additional anterior to posterior and cranial to caudal translation possibilities of the humeral head onto the shallow surface of the glenoid caused by the flexible glenoidal labrum. Thus, the shoulder joint reaches five degrees of freedom in motion and can be characterised as a “force stabilized” joint.

Even during normal shoulder motion the contact area between the humeral head and the glenoid varies and a roll- sliding mechanism supports the wide range of motion. Elevation of the arm de-centres the humeral head to a more posterior region of the glenoid, from a specific point the force vectors change their force approach and a sliding of the humeral head results [3, 4]. Nevertheless nearly the entire shoulder motion is a combination of motion of the shoulder girdle and the scapula-thoracic junction. During motion, only at the end of external rotation the scapula is approximated to the thoracic wall and removed from there at the end of internal rotation. In the sagittal plane anterior flexion is possible up to 170° and posterior flexion up to 50°. The rise of the arm sideways is divided into two phases, the abduction phase until 100° and the rise above 100° so-called elevation up to the end of approximately 170°. In the first phase the motion arises almost only from the gleno-humeral joint, all motion above this level is provided by a combination of the scapula, the gleno-humeral joint and the scapula-thoracic sliding.

The muscle insertion at the scapula is a direct connector to the thoracic wall with its own basic tension. Already small changes of this sensibly balanced muscular construct affect the position of the scapula in relation to the posterior thoracic wall. This situation might be recognized when the Serratus anterior muscle is either destroyed or paralyzed known as Scapula alata syndrome. In this situation the scapula reacts with a protrusion of the patient’s back in an abnormal position presenting a rare appearance with the potential to limit functional activity such as the ability to lift, pull, and push weight.

The synergistic work of the shoulder girdle allows for the broad range of motion since the glenoidal cavity is always put in a perfect position to allow increased motion. The trapezoid muscle and its minor supporters such as the levator scapulae muscle and sternocleidomastoideus muscle are stabilizing factors regarding cranial shoulder movement. The central forces, which are restraining anterior movement are distributed from the acromio-clavicular joint and the clavicle to the sterno-clavicular joint. The sliding suspension of the scapula-thoracic area is combined with the supporting sling of the superficial back muscles and the serratus muscle the only attachment to the thorax. Increasing abduction also increases the lever and in this combination also the forces applied to the scapula, thorax and clavicle. The multidirectional suspension of the scapula with its broad sliding freedom towards the thorax is nevertheless an isolated fixation concept and can thus be seen from a static point of view. It is unique that the shoulder joint is free of ligaments to provide an adequate fixation to the trunk.

The force transmission of the shoulder is provided by the surrounding muscles of the shoulder girdle. In the 0° -position in terms of a hanging arm, the forces to centre the humeral head on the glenoidal cavity are equivalent to the weight of the arm and the force vector aiming to the centre. The basic tension of the rotator cuff allows for a centering of the humeral head onto the glenoid. If the arm is abducted in the sagittal plane, this motion is majorly beard by the supraspinatus and deltoid muscles. In case of performing weight bearing the rotator cuff has to respond to this weight by developing immense forces to balance the humeral head in the right anatomical position. With increasing abduction the lever and its resulting forces are changing and the humeral head is ascending to the fornix humeri on the relatively flat glenoidal cavity. To hold against this ascending movement of the humeral head the adductor muscles have to re-center the humeral head.

Many statements exist regarding the disproportion of the humeral head’s circumference and the much smaller glenoidal cavity being an aberrated development. But at a closer look, this development seems to be a perfect construction. The arrangement of the periarticular muscles shows that these muscles are able to centre the humeral head onto the glenoidal cavity according to the motion required with a fine adjustment of these mechanisms. Since the scapula is not rigidly fixed to the thorax, in the evolution process there was no need for increased bone or ligament fixation providing more stability. Instead the suspension of the shoulder is very flexible and allows for a combined sliding on the thoracic and rolling manoeuvres on the glenoidal cavity always being neuromuscularly stabilised by the rotator cuff. Only in cases when neurological or traumatic reasons are affecting the composition the risks of instability arises. Accordingly a misfunctional muscular balance leads to a rise of the humeral head against the fornix humeri or the roof of the shoulder. In this situation the fornix becomes an abutment to the humeral head with the clinically apparent typical impingement symptoms when in most cases the supraspinatus muscle tendon is compromised. In a normal physiologically working shoulder the greater tuberosity dives under the lateral edge of the acromion without pincing the supraspinatus tendon. Repeated squeezing of the supraspinatus tendon may harm it with, in the worst case, triggering a tendon tear.

To understand the stabilizing structures of the humeral head the differentiation into static and dynamic mechanisms is very useful. The dynamic or active stabilizers are the surrounding muscles of the shoulder girdle and corresponding tension whereas the passive stabilizers are the anatomical construct including the osseous and capsular structures.