The Stable Shoulder

The primary source of stability of the shoulder is created dynamically by the shoulder musculature, rather than by the joint capsule or the static ligament structures. This is not to say that static structures provide no stability. The static ligamentous and capsular structures provide feed back about joint position and may act as the last line of defense against dislocation.

The shoulder has several important structures that allow  for movement and stability in the shoulder:


The ROTATOR CUFF is a cup like structure of muscles that drapes the humerus and provides support, joint, congruency, and a versatile array of motions.













BURSAS are small sacs of fluid that cushion and protect the tendons, allowing the tendons to glide without creating friction as they slide over nearby structures. The subacromial bursa protects the primary rotator cuff tendons from friction and from compression created by surrounding bony and soft tissue structures.














The LABRUM forms a cup for the ball-like head of the humerus to fit into. It functions like a gasket, creating a suction force to the humerus. When functioning normally, there is a negative pressure gradient that assists with shoulder joint congruence, decreasing work of the rotator cuff and increasing stability.














The complex arrangement of its musculature allows the shoulder to move freely, while simultaneously providing joint stability.

For more information see: Reinold et al on Current Concepts in the Scientific and Clinical Rationale behind exercises for glenohumeral and scapulothoracic musculature.

THE SHOULDER

Anatomy &  Biomechanics: Bones & Joints

The shoulder is one of the most complex joints in the body. Its dynamic structure allows for incredible mobility at the cost of stability.  The shoulder joint is formed by the HUMERUS (upper arm bone) and the SCAPULA (shoulder blade). These structures form a "ball and socket" joint, in which the humerus fits into the socket formed by the Glenoid (the cup-like surface of the scapula).                                                                   












The relationship of these structures explains the incredible motion available to the shoulder. The humerus is nearly 5x the size of that of its partner, the Glenoid, giving the humerus 100% freedom to rotate and glide over and around the Glenoid.  The foundations of the shoulder complex are its bony structures, which include: the acromion, the clavicle, the sternum, and the coracoid.


The ACROMION, a bony projection of the scapula, forms the "roof" of the shoulder joint and provides some mechanical protection to the shoulder against dislocation.


The CLAVICLE (collarbone) meets the acromion at the acromioclavicular joint and articulates with the sternum at the sternoclavicular joint. The clavicle acts like a strut to transfer stress from the arm into the torso, preventing injury and providing stability during dynamic movement.


The STERNUM  (the only attachment of the shoulder complex to the axial skeleton) and conducts forces  from the arm into the ribs and spine.













The CORACOID process is a hook-like bony projection from the scapula that provides an anchor for muscles of the arm and trunk. 


The CERVICAL & THORACIC spine and RIBS also play important roles in the mobility and stability of the shoulder joint. Proper movement and alignment of these structures is required to keep the shoulder in good working condition. Dysfunctions in ribs, thoracic spine, or cervical spine can create poor movement patterns that often lead to shoulder injury. Even the expansion of  the lungs during quiet breathing plays an important role in the mobility of the shoulder complex. With so much going on in the shoulder, it is important to be aware of its anatomy and biomechanics to maintain its dynamic structure.





THE SHOULDER: The Muscles  

The shoulder is a unique and mobile joint. In fact, it would be more correct to call it the SHOULDER COMPLEX. The muscles of the shoulder complex either connect the scapula and clavicle to the trunk, or connect the clavicle, scapula and body wall to the proximal (top) end of the humerus.

SUPERFICIAL MUSCLES: Pectoralis Major, trapezius (upper, middle and lower), latissimus dorsi, deltoid (anterior, middle & posterior).

DEEP MUSCLES: Pectoralis minor, Subclavius, levator scapulae, rhomboid (major & minor), teres major, serratus anterior, rotator cuff (supraspinatus, infraspinatus, teres minor and subscapularis.

MUSCLES OF THE SHOULDER & ARM: Biceps Brachii (long and short heads), coracobrachialis, & triceps brachii (lateral and medial heads).














The shoulder complex is able to move freely due to its intricate biomechanics. 

For more information see: Reinold et al on Current Concepts in the Scientific and Clinical Rationale behind exercises for glenohumeral and scapulothoracic musculature.

Primary Pain Generators in the Shoulder
    

66% of people will experience shoulder pain in their lifetime! The three major causes of shoulder pain are: rotator cuff injuries, subacromial bursitis, or biceps tendonitis.


The ROTATOR CUFF (“RC”) - Injury to the rotator cuff is the number one cause of diagnosed  shoulder pain by primary care physicians. Rotator cuff injuries are commonly associated with motions that require repeated overhead motions or forceful pulling motions, such as throwing or putting a heavy object on a high shelf. RC injuries result in decreased shoulder function, range of motion, and are often associated with pain.














SUBACROMIAL BURSITIS - The bursa is a fluid filled sac that protects the tendons, bones and muscles from friction created from interaction with one another. The subacromial bursa runs along the Supraspinatus tendon, protecting the tendon from friction and helping it to glide during dynamic motion. When the RC is weak, the humeral head moves upward and crushes the Supraspinatus tendon and its bursa into the acromion process. The result is an inflamed tendon and a bursa that become painful with movement and stiff, decreasing active range.
















BICEPS TENDONITIS – The tendon of the long head of the biceps also runs under the acromion. This tendon can be caught under the acromion in overhead arm movements, resulting in a thickened inflamed tendon. This injury commonly occurs when the arm decelerates from a throw, during which the tendon is stressed by high levels of tension.

SOLUTION – In order to avoid or treat these conditions, it is essential that the Scapulohumeral and Scapulothoracic rhythms, described by the timing of movements that allow for pain free full range of movement of the shoulder, are maintained. In addition, scapular stabilization and endurance are key!
















For more information see: Reinold et al on Current Concepts in the Scientific and Clinical Rationale behind exercises for glenohumeral and scapulothoracic musculature.

THE SHOULDER: What are the

Scapulohumeral and Scapulothoracic Rhythms?

The shoulder complex rhythms describe the timing of movements that allow full range of movement of the shoulder. Overall the rhythms occur at a 2:1 ratio of glenohumeral to scapulothoracic movement. That means that for every 2 deg. of shoulder flexion/abduction, the scapula must rotate upward roughly 1 deg. (vs. for abduction/extension). 180 deg. of shoulder abduction/flexion = 120 deg. of glenohumeral joint abduction + 60 deg. of scapulothoracic joint upward rotation.


















The Scapulohumeral rhythm serves at least two purposes:

1. It preserves the length-tension relationships of the glenohumeral muscles.

2. It prevents impingement between the humerus and the acromion.


Simultaneous movement of the humerus and scapula during shoulder elevation limits relative (arthrokinematic) movement between the two bones.

For more information see: Reinold et al on Current Concepts in the Scientific and Clinical Rationale behind exercises for glenohumeral and scapulothoracic musculature.

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