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.
Once the scapula is stable against the chest wall by the action of the serratus anterior muscles this gives a stable base for the rotator cuff muscles to work from, allowing them to act without being disadvantaged. This also allows the correct actions in the neck and surrounding muscles and prevents the abnormal movement pattern developing. As in all joints the accessory muscles play a major role in the normal functioning of the shoulder joint, where these movements are particularly important. Accessory movements are small gliding and sliding motions which occur in concert with larger, more obvious movements.
The way the ball and the socket interact has been compared to the act of a seal balancing a ball on its nose. The socket represents the nose and the seal’s job is to keep that ball balanced correctly. The function of the scapula is to do just that, to maintain the centring of the large humeral head on the the glenoid socket in normal arm use. The accuracy and strength of subsequent shoulder function is increased by the optimal control of this ability. The muscles controlling the shoulder are relatively small compared to the length of the lever of the arm and are not as massive as the hip muscles, with the added disadvantage that they have to work at unhelpful angles.
Placing the humeral head in exactly the right position within a millimetre or so can greatly increase the movement efficiency of the glenohumeral joint as long as sliding and gliding can occur. The internal anatomical adjustments automatically happen in usual movements such as lifting the arm above the head. The first stage is a small glide downwards of the humeral head on the socket, to allow the major muscles to exert their greatest strength. Abnormal movement can occur if the little accessory movements are lost, straining the muscles and causing pain.
Joints require accessory movements to occur within them so that they can move normally and attain the optimum position for the main joint power muscles to act. If there is an absence of the small internal joint adjustments then the primary moving muscles can suffer strain and result in loss of movements and pain as they struggle to achieve the required movement. The underlying problem for a variety of shoulder problems may be an interruption in the ability to keep the scapula stable. Under activity in the stability muscles of the scapula, the serratus anterior and the lower trapezius, is accompanied by overactivity in the upper trapezius.
The muscles of the scapula differ from those of the rotator cuff in the way that they work. The muscles between the thorax and the scapula are postural in nature and used to contracting at a low level for long periods. The rotator cuff muscles of the shoulder blade become active only when required and then relax once that activity has been completed. If the rotator cuff muscles weaken the scapular muscles try and compensate by becoming more active so that the movement can be completed. As the more crude scapular muscles dominate, the smaller and subtler cuff muscles are inhibited.
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.
