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REHABILITATION SHOULDER IMPINGEMENT

highlighted as a main cause of dysfunction in the shoulder, are now commonly assessed in patients with impingement (9,10). Ellenbecker (11), Mottram (9) and Host (12) recognise the inability to control the movement of the scapula during upper limb movements and note this frequently accompanies the development of shoulder pain. The scapula has numerous functions contributing to both stability and mobility of the shoulder complex. It provides a base for muscle attachment and its appropriate orientation optimises the length tension relationships of these muscles (9). Kibler (8) commented that when the scapula loses its stability, the orientation of the glenoid fossa is compromised, preventing optimum function of both the glenohumeral joint and supporting musculature, namely the rotator cuff. Coordinated scapula movement is therefore essential and force couples exist to produce this smooth movement. It is when these force couples become dysfunctional that problems are thought to arise (13). The primary scapula force couple associated with shoulder elevation is between serratus anterior and trapezius; resulting in lateral rotation of the scapula (10). In the early range of elevation both the upper fibres of trapezius and serratus anterior are active. Later in range, when the axis of rotation moves towards the acromioclavicular joint, the lower fibres of trapezius and serratus anterior are dominant (1,14,15).

MOTOR CONTROL DYSFUNCTION

It is commonly accepted that many factors influence movement of the human body, including soft tissues, bone, joints and also the nervous system. It is the nervous system that controls the coordination of muscle activity that allows effective and efficient movement patterns to occur (16). A reduction in this control of motor activity can contribute to pain and dysfunction. Alternatively it is proposed that pain and dysfunction can cause an alteration in motor control.

Changes in sensory feedback, abnormal

reflex activation and reduced coordination have all been identified as contributing to deficits in the sensory control of movement (16). The basis of closed-loop motor control is accurate sensory feedback to the central nervous system (CNS). A reduced ability to detect movement and a reduced repositioning sense have occurred in populations with musculoskeletal dysfunction; for example subjects with low back pain (LBP) (17), post ankle sprain (18), subjects with shoulder instability (19) and subjects with osteoarthritis of the knee (20). Reduced sensory input to

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SERRATUS ANTERIOR CAN AFFECT THE SCAPULA FORCE COUPLE AND RESULT IN DYSFUNCTION AND PAIN

the CNS can lead to delayed reflex responses, impaired coordination of voluntary movements and faulty movement error detection and correction (16). In addition any change in muscle activity affects the sensation of movement. Therefore factors such as fatigue, antalgic movement patterns and pain-induced muscle inhibition could all result in altered motor control strategies that could potentially exacerbate the initial dysfunction. Hence a negative cycle is continually being reinforced (16).

Dysfunction of the scapula stabilisers has been highlighted clinically in subjects with impingement (9). This is consistent with studies in other anatomical regions that demonstrate stability muscle dysfunction associated with pain (16,21,22). Increased anterior glenohumeral laxity has been associated with poor scapula stability (4,23). Glenohumeral instability has also been associated with reduced firing of the rotator cuff (24), amplifying the joint instability further. The scapula stabilising muscles have been studied in both normal and painful shoulders to determine alterations in firing patterns with pain. An EMG study of 15 throwing athletes by Glousman et al (24) observed reduced serratus anterior activity in the group with unstable shoulders, although caution should be applied in extrapolating this data to

AN OVERALL IMBALANCE IN MUSCLE ACTIVITY BETWEEN TRAPEZIUS AND

impingement patients, despite instability being a potential cause of secondary impingement (5).

Ludewig and Cook (25) investigated the scapular muscle activity of 52 impingement diagnosed subjects. Their results support those of Glousman et al (24) by demonstrating statistically significant alterations in scapula muscle activation in impingement subjects. Ludewig and Cook (25) also measured the resulting effect on scapula positioning. The impingement subjects demonstrated a statistically significant reduction in serratus anterior activity with a resulting reduction in posterior tipping of the acromion and initial upward rotation of the scapula, accentuating the risk of subacromial impingement. Increased activity in both the upper and lower portions of trapezius also reached statistical significance from mid-range elevation upwards. However the reliability of using skin markers for kinematic analysis of true scapula positioning was not discussed in the above studies. Studies by Cools et al (26-28) have

also found altered scapular muscle activity in impingement populations. Cools et al (27) found overhead athletes with shoulder pathology to exhibit decreased force output of serratus anterior, resulting in an altered muscle balance between serratus anterior and trapezius. In the same study they

Figure 1: Right scapular winging 15

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