Understanding The Shoulder-Part 2

                                                                                         

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.

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 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.

A millimetre or so can make a big difference in the accuracy of placing the humeral head and so to the efficiency of movement which results, provided the head can slide and glide. These small and unseen corrections happen in frequently used actions such as lifting the arm above the head. A small glide of the head downwards initially allows the main moving muscles to get into the best position to be at their strongest. Loss of accessory movements leads to abnormal movement, increased muscle strain and develops into joint stiffness and 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.

About the author:
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, Physiotherapists in Bournemouth, back pain, orthopaedic conditions, neck pain and injury management. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.

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