ANATOMY & PHYSIOLOGY OF THE SHOULDER - PART 1


The glenohumeral joint, or shoulder joint as it is commonly known, is a complex mechanical structure characterised by the greatest range of motion of any joint in the body. 
By Dirk Spits (B.Ed, MAAESS, AEP)

The glenohumeral joint, or shoulder joint as it is commonly known, is a complex mechanical structure characterised by the greatest range of motion of any joint in the body. However, the shoulder borders on a fine line between mobility and stability. In addressing shoulders injuries and complications it is important to learn in greater detail the anatomy and physiology of the shoulder.

Part 1 of this 3 part series illustrates the anatomy and physiology of the shoulder joint – it is important that you understand this to be able to train the shoulder appropriately. Part 2 of this series highlights common complications with the shoulder joint and part 3 common exercises you can prescribe for your clients in addressing these problems.

The shoulders’ function is predominantly mobility however we subject it to situations that require significant stability. It’s deficient in bony restraints subjecting it to a decreased ability to stabilise unless the muscles surrounding the shoulder are trained appropriately.

These characteristics lead to acute (short-term, sudden) and chronic (long-term overuse through repetition) shoulder problems that are common among people ranging from your average person attending a fitness centre to elite sportspeople.

Functional Anatomy of the Shoulder Joint

The shoulder joint is arguably labelled by many as a ball-and-socket joint. It is comprised of three bones – the scapula, clavicle and humerus. The surface area of the humeral head is on average three times larger than the glenoid cavity 1. Lippitt and Fredrick 2 state that the subchondral bone of the glenoid is relatively flat, however the cartilage surface is thicker peripherally than in the centre producing a curve that replicates the humeral head. This allows only one-third of the humeral head to be positioned in the glenoid cavity, producing a deficiency in bony restraint 2.

The absence of this bony restraint proposes significant intrinsic instability of the joint. The stability of the gleno-humeral joint is derived from the ligamentous and capsulating structures, articulating surfaces and synergistic activity of the rotator cuff and deltoid muscles. The ligaments of the glenohumeral joint prevent excessive displacement of the humeral head at the extremes of its range of motion. Nonetheless, they fail at times to prevent excessive movement of the glenohumeral head.

Common Complications of the Shoulder

Some of the most common complications to the shoulder joint include:

• Dislocations / Subluxations
• Scapular weakness and instability
• Shoulder tendonitis and bursitis
• Rotator Cuff or capsule tears and strains

Anatomical Components of the Shoulder Joint

The shoulder joint is comprised of many anatomical components.

Articular Capsule: The articular capsule is a thin loose sac that completely encircles the joint. It originates from the glenoid cavity to the anatomical neck of the humerus. The weakest part of this sac is the inferior region. The capsule aids in joint stability especially at the end ranges of motion. It also serves to lubricate the joint surfaces through its synovial membrane and is loaded with proprioceptive neurons (special nerve endings) called mechanoreceptors 3.

The shoulder also contains thousands of mechanoreceptors that communicate with the brain information of the position of the joint, speed of movement, force produced and pain in or around the joint.

Coracohumeral Ligament: This is a strong, broad ligament that assists in strengthening the superior part of the capsule. It extends from the coracoid process of the scapula to the greater tubercle of the humerus.

Glenohumeral Ligaments: These three strands of ligament extend from the glenoid cavity to the lesser tubercle and neck of the humerus. They provide minimal strength and stability.

Transverse Ligament: This is a narrow ligament that derives from the greater tubercle of the humerus and extends to the lesser tubercle of the humerus.

Glenoid Labrum: The glenoid labrum is narrow border of fibrocartilage surrounding the glenoid cavity. It enables the glenoid cavity to become minimally larger and deeper.

Bursaes: A bursa is a lubricated sac of tissue that helps prevent friction between two moving parts. There are four bursae associated with the shoulder joint. They include the subscapula bursa, subdeltoid bursa, subacromial bursa, and subcoracoid bursa.

Rotator Cuff

There are nine muscles that cross the shoulder joint. Two of these, the pectoralis major and latissimus dorsi are classified as axial muscles because they originate from the axial skeleton. The other seven muscles, consisting of the deltoid (anterior, middle and posterior), subscapularis, supraspinatus, infraspinatus, teres major, teres minor, and coracobrachialis are classified as scapular muscles due to their origin arising from the scapula.

Four of these scapula muscles, referred to as the, ‘Rotator Cuff’ function as an integral part of the shoulder. The rotator cuff is comprised of the supraspinatus, infraspinatus, subscapularis and teres minor. The rotator cuff acts to stabilize, strengthen and depress the humeral head (during overhead movements). However, as stated by Turkel et al 4, no single structure acts to stabilise the glenohumeral joint.

Subscapularis (sub – below, scapularis - scapula)
The subscapularis has a main role of internally rotating the humerus. The subscapularis acts in synergy with a number of other muscles such as the teres major, pectoralis major and latissimus dorsi 5. It should be noted that the subscapularis can also produce flexion, extension, adduction or abduction of the shoulder joint depending on the positioning of the arm.

Supraspinatus (supra – above, spinatus – spine of scapula)
The supraspinatus is a rounded muscle that lies deep to the trapezius. It main role is to assist the deltoid in abducting the humerus

Infraspinatus (infra – below, spinatus – spine of scapula)
Its main role is to laterally rotate and adduct the humerus.

Teres Minor

The teres minor is a cylindrical, elongated muscle that is often inseparable from the infraspinatus 6. It laterally rotates, extends and adducts the humerus.

For effective functioning of glenohumeral articulation, essentially a coordinated action of stabilisers (biceps, deltoids and rotator cuff muscles) and compliant curtailment of articular surfaces and ligaments needs to occur (louis)

 

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Now that we have a basic understanding of the anatomy and physiology of the shoulder joint we can now begin to explore the methodologies and exercises behind training the shoulder appropriately.

References:

1. Anderson JAD: (1984) Shoulder pain and tension neck and their relation to work Scand J Work Environ Health 10:435-442.

2. Ballantyne, B. S. O’Hare, J. Paschall, M. Pavia-Smith, A. Pitz, J. Gillon, and G. (1993) Soderberg Electromyographic activity of selected shoulder muscles in commonly used therapeutic exercises. Phys. Ther, 73:668-677.

3. Barber, D. and Cieminski, C. (1995) Shoulder and Elbow Training for Baseball National Strength and Conditioning Association October.

4. Basmajian, J.V., and Deluca, C.J. (1985) Muscles Alive: Their Functions Revealed by Electromyography. Baltimore, Williams and Williams, pp. 265-289

5. Bejelle A, Hagberg M, Michaelsson, G. (1979) Clinical and ergonomic factors in prolonged shoulder pain among industrial workers. Scand J Environ Work Health 5:205-210.

6. Bigiani, L.U., Kelkar, R., Flatrow, E.L., Pollock, R.G. and Mow, V.C. (1996) Glenohumeral Stability Clinical Orthopaedics and Related Research 330: 13-28

7. Blasier, R.B., Soslowsky, L.J., Malicky, D.M., Palmer, M.L. and Michigan, A.A. (1997) Posterior glenohumeral subluxation active and passive stabilization in the biomechanical model. The Journal of Bone and Joint Surgery 79-A(3): 433-440.

8. Brostrom, L.A., Kronberg, M., Nemth, G. and Oxelback, U. (1991) The effect of shoulder muscle training in patients with recurrent shoulder dislocations. Scandinavian Journal of Rehabilitation Medicine 24(1): 11-15.

9. Carson, W.G. (1989) Rehabilitation of the Throwing Shoulder Clinical in Sports Medicine 8:657-689

10. Chek, P. (2000) Pattern Overload Articles, Chek Institute, USA.

11. Ciccotti, M.G. (1988) Rotator Cuff Injury and Surgery Rothman Institute

12. Cummings G.S., Crutchfield, C.A., Barnes, M.R. (1985) Soft Tissue Changes in Contractures. Orthopedic Physical Therapy Series. Stokesville Publishing Co. Volume 1.

13. Dvorak, J., Dvorak, V. (1990) Manual Medicine Diagnostics 2nd Edition Thieme Medical Publications.

14. Spits, D & Nunn, C (2000) An Introduction to the Anatomy and Physiology of the Shoulder and it’s Rehabilitation

 

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