Shoulder pain is one of the most typical musculoskeletal ailments that cause people to seek physiotherapy. Of those shoulder pain complaints, disorders of rotator cuff tendons are the most common, with rotator cuff tendinopathy accounting for 35% to 50% of diagnoses taken.
For optimal glenohumeral movement and stability there is a need for balanced rotator cuff function. The rotator cuff centres the humeral head against the glenoid labrum, controlling humeral head translations in addition to producing and controlling rotations and elevations in glenohumeral joint. In throwing actions there is the need for efficient and optimal movement control and stability in many areas of the kinetic chain, including the scapulo-thoracic joint, thoracic and lumbopelvic area, as well as the hip and lower leg. In tennis-players experiencing upper limb overuse injuries, such as shoulder injury, there is poor quality of energy transfer from the trunk to the upper limb, reducing ball velocity and an elevating levels of energy absorbed by the shoulder and upper extremity compared with non-injured players (Martin et al 2014). The researches also stated that altered energy transfer presents before the onset of clinical symptoms, identifying the breakdown in the kinetic components prior to pathology.
Functional role of infraspinatus muscle
The infraspinatus muscle produces glenohumeral external rotation in various elevation angles and controls humeral head translation in superior direction. Forming a force-couple with subscapularis muscle, infraspinatus also provides translation control of the humeral head in antero-posterior directions .
Infraspinatus also has:
- forward flexion, abduction and internal rotation actions (Heuberer et al 2015).
- feedforward activation pattern (Day et al 2012).
- myofascial connections to supraspinatus, rhomboids and triceps brachii muscles (Stecco 2015, Myers 2001) .
Infraspinatus dysfunction in pain and pathology
It has been demonstrated that induced subacromial pain and during subacromial impingement, infraspinatus activation level decreases (Reddy et al 2000, Sole et al 2014, Diederichsen et al 2009). Professional beach volleyball players are also found to have high frequency of infraspinatus atrophy and significantly reduced infraspinatus strength of the hitting shoulder (Lajtai G et al 2012). Additionally, infraspinatus can also present as a restriction to normal joint range, limiting shoulder internal rotation especially in upper arm elevation, a scenario producing aberrant movement pattern in both the glenohumeral and scapulothoracic joint.
Because the shoulder pain is so common many management strategies in physiotherapy are utilized in rotator cuff tendinopathy to include manual therapy techniques, posture education, various strengthening and stretching exercises to name only a few. One third of patients complaining of shoulder pain still do have ongoing symptoms and dysfunction regardless of treatment. So, what have we been missing ?
In musculoskeletal disorders there are multiple areas where the things may go wrong. In the periphery there is an alteration of muscle function in pain and pathology, including changes in the timing and duration of activation, decreased or increased activation and altered pattern of muscle recruitment order. But it is not just in the periphery – hardware or ‘branch post offices’ – where things get messy. There are also changes in central nervous system, in our software or ‘central post office’. Alteration in central nervous system have been documented in many musculoskeletal disorders for example in low back pain, neck pain and whiplash associated disorders. It has been recently hypothesized that part of the deficits of rotator cuff tendinopathy is related to reorganization of somatosensory and motor cortices such as in low back pain, phantom limb pain and complex regional pain syndrome (Myers et al., 2006; Roy et al., 2009; van Vliet and Heneghan, 2006, Tsao et al 2011, Tsao et al 2008).
In healthy people it has been shown that cortical representation of infraspinatus muscles are symmetrical between sides regarding both excitability and location (Ngomo et al 2013). Age or gender does not influence infraspinatus cortical representation yet, in rotator cuff tendinopathy the cortical representation can be altered.
Ngomo et al (2015) did an interesting study using transcranial magnetic stimulation (TMS) regarding possible central motor representation changes of infraspinatus muscle in thirty-nine patients (18 women, 21 men) with unilateral rotator cuff tendinopathy. The group sought to clarify whether such changes might be related to pain intensity, pain duration and physical disability.
Researchers found that there was a significant inter-hemispheric asymmetry of infraspinatus active motor threshold. On the affected side, active motor threshold was higher compared to unaffected side indicating decreased corticospinal excitability. Duration of pain (>12 months), but not its intensity, appeared to be a factor related to the lower excitability of the infraspinatus representation. It is interesting to note for example that in phantom limb pain and in other neuropathic pain syndromes, cortical reorganization is related to magnitude of pain – not the length of pain (Moseley and Flor 2012).
In the discussion the researchers stated the results were consistent with the previous reports of alterations in corticospinal excitability for shoulder muscles in patients with other musculoskeletal shoulder disorders. For example, it was found increased active motor threshold (decreased corticospinal excitability) in subjects with non-traumatic shoulder instability (Alexander 2009). There are also other TMS studies in other types of musculoskeletal disorders that have been shown variable alterations in corticospinal excitability whether increased or decreased activation (On et al 2004; Strutton et al 2003; Strutton et al 2003).
Are there then some implications for rehabilitation in light of these findings? For people in pain we always need to consider the need to offer pain education so as to reduce fear of pain and fear of movement. This fear itself might be a contributing and maintaining factor in pain, associated movement control impairments and disability. Furthermore, in order to change, modify and improve cortical representation so as to decrease ongoing pain and dysfunction we could argue that at least in part we have to focus our rehabilitation program on voluntary, cognitive low-threshold motor control retraining. This relates not only to the glenohumeral joint but through the whole kinetic chain. With skilled motor control retraining we can find and build a new, versatile and healthy way to move, restoring pain free function and redrawing the cortical map.
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