As the shoulder movement becomes less well balanced a typical pattern develops of raising the whole shoulder area, tipping the neck slightly over to the same side and allowing the scapula to wing away from the chest at the back. This biomechanical pattern forces abnormal stresses onto the shoulder structures, allowing impingement or the development or rotator cuff tears. In normal movement and use of the arm the body’s neural preparation system ensures the core is engaged to stabilise the spine and the scapula is stabilised by the muscles connecting it to the thorax.
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 .
The shoulder socket can be looked at as if it were a seal balancing a ball on its nose. The nose is the socket and the seal has to position itself just right to keep the large ball balanced successfully. The shoulder blade does a similar job and keeps the large ball centred on the socket whatever the arm is trying to do. If the control is kept within certain optimal limits the power and accuracy of arm placing and function is significantly enhanced. The arm is a long lever and the muscles of the shoulder region are small and without bulk like some other bodily muscles. The angles the rotator cuff muscles work at are also disadvantageous.
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.
Accessory movements are necessary for all joints to allow them to perform normal movement, positioning themselves for the most efficient action of the major joint muscles. If the small adjusting movements are not present, strain can develop in the power muscles as they strive to make the motion required, resulting in pain and loss of movement. The most common presenting complaint may be the inability to achieve good stability of the scapula, with the stabilising muscles, the lower trapezius and the serratus anterior, becoming under active. This often goes with the upper trapezius muscle at the side of the neck becoming overactive.
The rotator cuff muscles are functionally different in action from those muscles which give the scapula stability against the chest wall. The scapulothoracic muscles are designed to do low level holding of a posture for a time. The cuff muscles, however, typically act in a different way by acting quickly to perform a particular movement and then go into relaxation while they wait for the next required action. If weakness develops in the muscles of the rotator cuff then the scapular muscles can be recruited to complete the movement. Gradual domination of the scapular activity over the cuff activity can occur by inhibition.
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.
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