Abstract
The shoulder relies heavily on coordinated muscle activity for normal function owing to its limited osseous constraint. However, previous studies have failed to examine the sophisticated interrelationship between all muscles. It is essential for these normal relationships to be defined as a basis for understanding pathology. Therefore, the primary aim of the study was to investigate shoulder inter-muscular coordination during different planes of shoulder elevation. Twenty healthy subjects were included. Electromyography was recorded from 14 shoulder girdle muscles as subjects performed shoulder flexion, scapula plane elevation, abduction and extension. Cross-correlation was used to examine the coordination between different muscles and muscle groups. Significantly higher coordination existed between the rotator cuff and deltoid muscle groups during the initial (Pearson Correlation Coefficient (PCC) = 0.79) and final (PCC = 0.74) stages of shoulder elevation compared to the mid-range (PCC = 0.34) (p = 0.020–0.035). Coordination between the deltoid and a functional adducting group comprising the latissimus dorsi and teres major was particularly high (PCC = 0.89) during early shoulder elevation. The destabilising force of the deltoid, during the initial stage of shoulder elevation, is balanced by the coordinated activity of the rotator cuff, latissimus dorsi and teres major. Stability requirements are lower during the mid-range of elevation. At the end-range of movement the demand for muscular stability again increases and higher coordination is seen between the deltoid and rotator cuff muscle groups. It is proposed that by appreciating the sophistication of normal shoulder function targeted evidence-based rehabilitation strategies for conditions such as subacromial impingement syndrome or shoulder instability can be developed.
Highlights
Shoulder pathology, such as subacromial impingement syndrome or shoulder instability, is prevalent and has a substantial impact on patient’s quality of life [1, 2]
Significant differences in signal amplitude were seen across all movements for middle deltoid (MD), posterior deltoid (PD) and middle trapezius (MT)
There were no significant differences in muscle activation between the movements for anterior deltoid (AD), lower trapezius (LT), serratus anterior (SA), teres major (TM), latissimus dorsi (LD), SSP, ISP, SUBS, rhomboid major (RM)
Summary
Shoulder pathology, such as subacromial impingement syndrome or shoulder instability, is prevalent and has a substantial impact on patient’s quality of life [1, 2]. Essential for many activities of daily living, requires the integration of strength, range of motion and muscular endurance. Understanding this is the basis for understanding pathology. It has long been recognised that the rotator cuff muscles are crucial in limiting this translation and maintaining GH joint stability [5,6,7]: their contraction ‘stiffens’ the joint, establishing a stable fulcrum for arm movement [8]. More recently coordinated activity of all the major shoulder muscle groups during a functional shoulder elevation task has been shown, implying a wider muscular contribution to stability [9]
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