Date Published: February 8, 2019
Publisher: Public Library of Science
Author(s): David H. Hawkes, Omid A. Khaiyat, Anthony J. Howard, Graham J. Kemp, Simon P. Frostick, Chunfeng Zhao.
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.
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]. Normal shoulder function, 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.
The shoulder is a complex system which relies on coordinated muscle activity to maintain stability [5–7]. Aberrant shoulder muscle coordination is considered to be an important aetiological component of shoulder impingement syndrome and shoulder instability [11, 12]. A recent randomised control trial has questioned the role of surgery in subacromial impingement syndrome and highlighted the important of rehabilitation . However, an intimate knowledge of normal shoulder inter-muscular relationships is required if the pathological movement patterns of patients are going to be treated effectively with physiotherapy.
The coordination between muscle groups is dynamic and changes during a task as the requirements for stability alter. In broad outline, the destabilising force of the deltoid, during the initial stage of shoulder elevation, is balanced by the coordinated activity of the rotator cuff, LD and TM. Stability requirements are lower during the mid-range of elevation and this is reflected in the reduced coordination between these muscle groups. At the end-range of movement the demand for muscular stability again increases and here this is provided by the rotator cuff. It is suggested that this fuller analysis of shoulder function in health will improve understanding of the alterations associated with pathology, and assist development of more targeted, evidenced-based, treatment strategies.