Shoulder Injuries, Part 1: Understanding the Shoulder Complex
Understanding the Shoulder Complex
The shoulder is one of the most mobile and adaptable regions of the human body. That remarkable range of motion depends on the coordinated interaction of multiple joints and functional articulations, numerous muscles, the rotator cuff, and the surrounding connective and neurological structures.
In this four-part series, we explore the shoulder from anatomy through rehabilitation.
Part 1 examines the joints and articulations that make up the shoulder complex.
Part 2 looks at the muscles that coordinate scapular and shoulder movement.
Part 3 focuses on the rotator cuff, clinical examination, imaging, and diagnosis.
Part 4 examines treatment, exercise, and progressive rehabilitation.
Rather than viewing shoulder pain as a problem with a single joint or structure, this series takes a broader approach. Understanding how the different components of the shoulder work together provides a foundation for assessing symptoms, restoring movement and strength, and developing an individualized rehabilitation plan.
Article Index:
Understanding the Shoulder Complex
The shoulder is more than the glenohumeral joint. Movement of the arm depends on coordinated motion between the glenohumeral, acromioclavicular and sternoclavicular joints, together with the functional movement of the scapula across the thorax. The subacromial region also provides an important interface through which the rotator cuff and surrounding tissues move during elevation of the arm.
The scapula plays a central role in this system. Seventeen muscles attach to the scapula, contributing to movement and control of the shoulder, neck, upper back, and arm. Together with the rotator cuff, capsule, ligaments, and other surrounding tissues, these structures allow the shoulder to produce an extraordinary range of movement while maintaining control under load.
More Than a Ball-and-Socket Joint
The glenohumeral joint provides much of the shoulder's remarkable mobility, but it does not work in isolation. Raising an arm, reaching overhead, throwing, lifting, or pushing requires coordinated movement throughout the shoulder complex.
This is why evaluating shoulder pain often requires looking beyond the location where symptoms are felt. Depending on the presentation, examination may consider glenohumeral movement, scapular motion, the acromioclavicular and sternoclavicular joints, muscle strength and endurance, neurological function, and the demands being placed on the shoulder.
Understanding these relationships does not mean that every variation in movement represents dysfunction or that a problem in one structure automatically causes problems elsewhere. Instead, it provides a framework for understanding how the shoulder functions as an integrated system.
In Part 1, we begin with the joints and functional articulations that make shoulder movement possible.
Anatomy and Biomechanics of the Shoulder Complex
Although we often refer to “the shoulder joint,” shoulder movement actually depends on the coordinated interaction of several joints and functional articulations.
For the purposes of understanding shoulder biomechanics, we can consider five important components of the shoulder complex:
Sternoclavicular Joint
Acromioclavicular Joint
Scapulothoracic Articulation
Glenohumeral Joint
Subacromial Interface
The first three are true anatomical joints. The scapulothoracic articulation is not a conventional joint but describes the movement of the scapula across the thoracic wall. Similarly, the subacromial region is better understood as a functional interface through which the rotator cuff tendons, bursa, and surrounding tissues move during shoulder motion.
The Shoulder Works as a System
These components do not function independently. Reaching, lifting, pushing, pulling, and raising the arm overhead require coordinated movement throughout the shoulder complex.
A limitation or change in one region may influence how movement is distributed elsewhere, but this does not automatically indicate dysfunction or injury. People can also demonstrate considerable variation in shoulder and scapular movement without experiencing pain.
For this reason, shoulder assessment should consider the individual as a whole. Depending on the presentation, this may include joint mobility, muscle strength and endurance, scapular movement, neurological function, activity demands, previous injury, and other regions that may be contributing to the person's symptoms or functional limitations.
Glenohumeral (GH) Joint
The glenohumeral joint is the primary ball-and-socket joint of the shoulder, formed by the head of the humerus and the glenoid of the scapula. Like other synovial joints, its articular surfaces are covered with cartilage and lubricated by synovial fluid, allowing relatively low-friction movement.
The glenohumeral joint contributes substantially to shoulder flexion, extension, abduction, adduction, and internal and external rotation. Its remarkable mobility is possible because the humeral head is considerably larger than the relatively shallow glenoid. Stability therefore depends not only on bone shape, but on the combined contributions of the labrum, joint capsule, glenohumeral ligaments, rotator cuff, surrounding muscles, and neuromuscular control.
Glenohumeral Instability
It is important to distinguish joint laxity from instability. Some degree of glenohumeral translation is normal, and some people naturally demonstrate considerably more joint laxity than others without experiencing pain or loss of function.
Glenohumeral instability occurs when excessive or poorly controlled movement of the humeral head becomes symptomatic, producing problems such as pain, apprehension, recurrent subluxation or dislocation, or difficulty performing particular activities. Instability may develop following trauma, repetitive loading, or in association with generalized joint laxity, and its presentation varies considerably between individuals.
The joint capsule and glenohumeral ligaments contribute to passive stability. The inferior glenohumeral ligament complex (IGHL) is particularly important in limiting excessive translation when the arm is elevated and rotated. The rotator cuff and other shoulder muscles provide dynamic control by helping coordinate the position of the humeral head during movement.
Changes in posterior shoulder mobility, muscle function, strength, coordination, or activity demands may also influence how the glenohumeral joint moves and tolerates load. These findings should be interpreted within the individual's overall clinical presentation rather than assuming that one restriction or structural finding is responsible for shoulder pain.
Fascial and Soft-Tissue Relationships
The tissues surrounding the glenohumeral joint are continuous with muscular and fascial structures extending across the shoulder girdle, scapula, chest, and upper back. These anatomical relationships provide pathways for transmitting force and coordinating movement throughout the shoulder complex.
However, the presence of fascial continuity does not necessarily mean that tension or restriction in one region is causing dysfunction elsewhere. When clinically relevant, these surrounding tissues can be considered alongside joint mobility, muscle strength, neurological function, movement tolerance, and the demands being placed on the shoulder.
Acromioclavicular (AC) Joint
The acromioclavicular (AC) joint is formed between the acromion of the scapula and the lateral end of the clavicle. It is a small synovial plane joint that allows subtle but important movement between the scapula and clavicle.
Although movement at the AC joint is relatively small, it plays an important role in overall shoulder motion. As the arm is elevated, the scapula must rotate, tilt, and adjust its position relative to the clavicle. Movement at the AC joint, together with motion at the sternoclavicular joint, allows the scapula to reposition across the thorax and contributes to the shoulder's large functional range of motion.
The AC joint is also a common site of injury and age-related change. Trauma, particularly a fall directly onto the shoulder, can injure the AC joint and its supporting ligaments, producing an AC joint separation. Degenerative changes are also frequently seen, particularly with increasing age, but imaging findings do not necessarily indicate that the AC joint is the source of shoulder pain.
Sternoclavicular (SC) Joint
The sternoclavicular (SC) joint is formed between the medial end of the clavicle, the manubrium of the sternum, and the first costal cartilage. It is a saddle-type synovial joint containing an articular disc that contributes to joint stability and helps distribute forces.
The SC joint is particularly important because it provides the only direct bony connection between the upper limb and the axial skeleton.
Movement occurs in three dimensions. The clavicle can elevate and depress, move forward and backward through protraction and retraction, and rotate around its long axis. These relatively small movements contribute significantly to the ability of the scapula to reposition as the arm moves overhead.
Although SC joint problems are less common than many other causes of shoulder pain, the joint can be affected by trauma, instability, and degenerative or inflammatory conditions. Dislocations are uncommon, but posterior SC joint dislocation is particularly important because of the major blood vessels, airway, and other structures located immediately behind the joint and requires urgent medical assessment.
Scapulothoracic (Scapulocostal) Articulation
The scapulothoracic articulation describes the movement of the scapula across the posterior thoracic wall.
Unlike the glenohumeral, acromioclavicular, and sternoclavicular joints, it is not a true anatomical joint. Instead, movement occurs through layers of muscle and connective tissue between the scapula and rib cage.
Muscles including the serratus anterior, trapezius, rhomboids, levator scapulae, and pectoralis minor contribute to positioning and controlling the scapula. Their coordinated activity allows the scapula to move through elevation, depression, protraction, retraction, upward and downward rotation, as well as anterior and posterior tilting and internal and external rotation.
Scapulothoracic movement depends on coordinated motion at the sternoclavicular and acromioclavicular joints. As the arm is elevated, the scapula progressively changes position on the thorax, contributing to the overall range of shoulder motion and providing a changing base from which the humerus can move.
Scapular Movement and Coordination
Variations in scapular position and movement are sometimes described as scapular dyskinesis. These changes may be associated with shoulder pain or altered shoulder function in some individuals, but they are not necessarily abnormal or the cause of symptoms.
Scapular movement varies considerably between individuals and also changes with task, load, speed, fatigue, and athletic demands. For this reason, scapular motion should be interpreted within the context of the person's symptoms, strength, function, and activity rather than judged against a single “ideal” movement pattern.
We will explore scapular movement, muscular coordination, and scapular dyskinesis in greater detail in Part 2: Muscles, Scapular Movement and Coordination.
Subacromial Interface
The subacromial region lies between the humeral head and the coracoacromial arch, which is formed by the acromion, coracoid process, and coracoacromial ligament. Important structures within this region include the supraspinatus tendon, other portions of the rotator cuff, the long head of the biceps tendon, and the subacromial-subdeltoid bursa.
As the arm moves, these tissues must accommodate changing loads and positions within the shoulder complex. The relationship between the humerus, scapula, rotator cuff, and surrounding tissues changes continuously throughout shoulder movement.
From “Impingement” to Rotator Cuff-Related Shoulder Pain
Pain over the front or side of the shoulder, particularly during arm elevation, was traditionally described as Subacromial Impingement Syndrome (SIS). The theory proposed that the rotator cuff tendons or bursa were mechanically compressed beneath the acromion during movement.
We now know that this explanation is often too simplistic. Similar symptoms can occur without evidence of a structure being mechanically “pinched,” and structural findings on imaging do not always correspond with pain.
The term rotator cuff-related shoulder pain (RCRSP) is increasingly used to describe a broad group of shoulder presentations involving pain associated with the rotator cuff and surrounding structures. Assessment considers the patient's symptoms, strength, movement, activity demands, load tolerance, and functional limitations rather than assuming that narrowing of the subacromial space is the primary cause.
This distinction becomes particularly important when considering treatment. The goal is not necessarily to “open the subacromial space” or correct a mechanical impingement, but to identify relevant factors and progressively restore the shoulder's ability to tolerate movement and load.
Anatomy of the Shoulder
Want to explore the anatomy of the shoulder in greater detail?
Our Shoulder Anatomy video reviews the bones and major anatomical structures that form the shoulder complex and provides a useful visual foundation for understanding how these regions work together.
Conclusion
The shoulder is far more than a single ball-and-socket joint. Its remarkable range of motion depends on the coordinated interaction of the glenohumeral, acromioclavicular and sternoclavicular joints, the scapulothoracic articulation, and the subacromial interface.
Understanding how these components work together provides an important foundation for evaluating shoulder pain and injury. Changes in mobility, strength, coordination, or load tolerance in one region may influence movement elsewhere, but these findings must be interpreted within the context of the individual's symptoms, function, activity demands, and clinical examination.
Just as importantly, anatomical or movement variations do not automatically represent dysfunction. Effective assessment involves determining which findings are actually relevant to the person experiencing shoulder pain rather than attempting to correct every variation from an assumed ideal.
In Part 2: Muscles, Scapular Movement and Coordination, we turn our attention to the 17 muscles that attach to the scapula and examine how they contribute to shoulder movement, strength, and control. We will also take a closer look at scapular dyskinesis, what it means, and when changes in scapular movement may be clinically relevant.
References - Part 1
Kibler WB, Sciascia A. Current concepts: Scapular dyskinesis. Br J Sports Med. 2010;44(5):300–305.
Lewis J. Rotator cuff related shoulder pain: Assessment, management and uncertainties. Man Ther. 2016;23:57–68.
Lo CN, Leung BPL, Sanders G, Li MWM, Ngai SPC. The major pain source of rotator cuff-related shoulder pain: A narrative review on current evidence. Musculoskelet Care. 2023;21(2):285–293.
Ludewig PM, Reynolds JF. The association of scapular kinematics and glenohumeral joint pathologies. J Orthop Sports Phys Ther. 2009;39(2):90–104.
Requejo-Salinas N, Lewis J, Michener LA, et al. International physical therapists consensus on clinical descriptors for diagnosing rotator cuff related shoulder pain: A Delphi study. Braz J Phys Ther. 2022;26(2):100395.
Sciascia A, Kibler WB. Current views of scapular dyskinesis and its possible clinical relevance. Int J Sports Phys Ther. 2022;17(2):117–130.
Wilk KE, Macrina LC, Reinold MM. Non-operative rehabilitation for traumatic and atraumatic glenohumeral instability. N Am J Sports Phys Ther. 2006;1(1):16–31.
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DR. BRIAN ABELSON DC. - The Author
Dr. Brian Abelson is a chiropractor, author, and educator with more than 30 years of clinical experience in musculoskeletal care, rehabilitation, and sports performance. His educational work focuses on musculoskeletal conditions, human movement, sports biomechanics, and evidence-informed approaches to treatment and rehabilitation.
He is the developer of Motion Specific Release (MSR) and Clinical Director of Kinetic Health in Calgary, Alberta, Canada.
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