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Rathi S.1,2, Taylor N.3, Green R.1
1La Trobe University, College of Science, Health and Engineering, Melbourne, Australia, 2St John of God Healthcare, Physiotherapy, Bendigo, Australia, 3La Trobe University, Department of Physiotherapy, Melbourne, Australia
Background: The rotator cuff muscles are considered to have an important role as dynamic stabilizers of the shoulder joint. The proposed stabilizing mechanism of rotator cuff muscles is to limit excessive humeral head translation. To understand why shoulder function is often compromised following rotator cuff injury, it is important to investigate the role of rotator cuff muscles in limiting glenohumeral joint translation in living subjects. However, an accurate measurement of glenohumeral joint translation in vivo has been challenging.
Purpose: This study aimed to measure the effect of rotator cuff muscle contraction on glenohumeral joint translation in living subjects using real time ultrasound (RTUS) and electromyography.We hypothesized that in a healthy shoulder, contraction of the rotator cuff muscles will limit the humeral head translation in a direction-specific manner, effectively stabilizing the glenohumeral joint against anterior and posterior directed translation forces.
Methods: Twenty healthy adults with no history of shoulder pathology were recruited. Six intramuscular electrodes were inserted in the rotator cuff muscles (supraspinatus, upper and lower infraspinatus, teres minor, upper and lower subscapularis). Anterior and posterior glenohumeral translations were measured in two positions (shoulder neutral, abduction), two views (anterior, posterior) and six testing conditions (rest, isometric internal rotation (IR), isometric external rotation (ER), translation force, translation force with IR and translation force with ER). To assess whether there were significant differences in the humeral head position with different conditions, analysis was completed using four separate 2-way ANOVAs for each position (abduction and neutral) and for each view (anterior and posterior). The EMG activity of all muscles in each condition were analysed by using four separate 2-way ANOVAs for each position.
Results: There was reduced glenohumeral translation with rotator cuff muscle contraction in the neutral anterior (F2, 38 = 17.8, p 0.01), neutral posterior (F1.6, 31.0 = 44.3, p 0.01) and abducted posterior (F1.5, 28.8 = 5.2, p 0.02) positions. There were also differences between the amount of translation limited by anterior and posterior rotator cuff muscles in response to anterior and posterior translation forces (p 0.05), indicating that their activity was, to a certain extent, direction specific. For example in both neutral and abducted positions, contraction of the posterior rotator cuff muscles, infraspinatus and teres minor, appeared to tether anterior translation of the humeral head. This direction-specificity was also reinforced by the observation that no rotator cuff muscle was activated greater than 20% of maximum voluntary contraction when not acting as the prime mover for that condition.
Conclusion(s): Our results confirm that the rotator cuff functions as a stabilizer of the glenohumeral joint by limiting humeral head translation and this is likely to be in a direction-specific manner.
Implications: The RTUS technique described can be used for investigating the effect of rotator cuff dysfunction and its effect on the mechanics of the shoulder joint. Similar to other conditions like low back pain and patellofemoral pain syndrome, it is possible that a reappraisal of the stabilizing role of the rotator cuff may lead to significant changes in the assessment and treatment of shoulder disorders.
Funding acknowledgements: This project received funding from the La Trobe University Sport, Exercise and Rehabilitation Research Focus Area.
Topic: Musculoskeletal: peripheral
Ethics approval: This study was approved by the La Trobe University Human Ethics Committee (14-079).
All authors, affiliations and abstracts have been published as submitted.
