The Shoulder " Part Two "
Typically when the shoulder becomes painful and dysfunctional the movement tends to fall into a particular pattern of use by lifting the shoulder area, winging the scapula out from the thorax and leaning the neck towards the lifting shoulder. This movement pattern exposes the shoulder to abnormal forces and can provoke shoulder conditions such as impingement and tears of the rotator cuff muscles. The body makes preparations before an activity by pressing the scapula against the thoracic chest wall and engaging the core stability system.
For the rotator cuff muscles to function properly from their base the scapula must be held firmly as a stable point against the chest wall, otherwise they will suffer from a mechanical disadvantage and work abnormally. With normal scapular function the shoulder can work and avoid the unwanted extra movements of the neck and shoulder girdle. Accessory movements, as in all joints, play an important role in joint function. Accessory sliding and gliding movements occur naturally with the more normal movements of the joints .
A seal balancing a ball on the tip of its nose is how the interaction between the humeral ball and scapular socket has been described. The seal (the scapula) has to keep the ball (the humeral head) correctly and precisely centred over its nose ( the socket). The scapula has the job of successfully centring the large arm ball on the socket in all normal activities. If the ball is centred quickly and accurately this increases the precision and strength of arm movements. The shoulder has small muscles to control the arm and the arm is a long lever which generates a lot of force, with the added problem that the scapular muscles are working at poor angles.
The accuracy of placement of the head is vital and a millimetre or so can make a difference to the quality of movement involved, with the head able to slide or glide as it needs to. This kind of subtle, internal adjustment occurs in common movements such as raising the arm above the head, which is preceded by a small downward glide of the head on the socket. This allows the main shoulder movement muscles to exert the maximum force they need for strength. If accessory movements are lost this can force the muscles to work under strain, causing pain and stiffness in the joint.
All joints use accessory movements to accomplish normal motion for the joint in order to position themselves so their major moving muscles can work most efficiently. Without these very small inside movements the moving muscles strain to accomplish their goals, leading to pain and stiffness. The inability to position the scapula accurately and strongly is a common problem and may be the basis for many shoulder problems. The serratus anterior and lower trapezius, the main stabilising muscles of the scapula, may become under active and the upper trapezius, between the shoulder and the neck, overactive.
As the finer, more coordinated muscles of the upper arm become weaker and the cruder, less precise muscles become stronger, an abnormal rhythm of scapulohumeral movement develops. Typically we repeat arm actions again and again in a stereotypical fashion, often with the arm close to the body and the muscles working in the same short ranges. Many actions also involve pulling towards and inwards to the body as opposed to outwards and upwards, further reinforcing the tendency for the front muscles to become shortened and stronger and the posterior muscles to become longer and weaker.