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The Shoulder

Introduction

The shoulder is unique in the sense that it comprises four different joints and possesses unparalleled range of motion. This article aims to provide an overview of joint anatomy, deep and superficial musculature, common injuries, assessments, and programming implications for this joint. It is not intended to be exhaustive and is likely to be an evolving article as more detail gets added over time, but primarily seeks to provide actionable information for athletes and coaches alike, summarising complex research into bite sized chunks that can be easily understood and applied.

Joints

The major bones of shoulder include:

  • Humerus

  • Scapula (including acromion and coracoid process)

  • Clavicle

The four primary joints that comprise the shoulder are:

  • Glenohumeral joint

  • Acromioclavicular (AC) joint

  • Sternoclavicular joint

  • Scapulathoracic joint

The glenohumeral joint is made up of the ball (humeral head) and socket (glenoid), where the humeral head is larger than the glenoid it sits in. Due to this, only 25-30% of the humerus is in contact with the glenoid at any given time (aside from subluxations and dislocations, but more on that later!). The humeral head is also inclined relative to the humeral shaft at an angle of 130-150°. Due to these factors, maintaining a 1-2mm centre of rotation through a large range of motion at the shoulder is a result of an interplay between both static and dynamic stabilising forces. The greater and lesser tuberosity provide attachments for the rotator cuff musculature, which we will touch on in a moment.

The acromioclavicular (AC) joint is the junction between the acromion of the scapula and the clavicle, with an average size in adults of 9 x 19mm. Due to its anatomy it experiences a high axial load through a small surface area, which means that it can experience high levels of stress (stress = force ÷ cross sectional area). The stability of this joint is provided by static stabilisers including the acromioclavicular ligament.

The sternoclavicular joint is the junction of the clavicle and the sternum of the rib cage, and is the only articulation between the shoulder girdle and axial skeleton. This is evident when you witness a broken clavicle (collar bone), as the shoulder has lost its only connecting bone with the axial skeleton itself. There is a large difference between the end of the clavicle and the surface of the sternum, and therefore stability is provided by ligamentous structures.

The scapulothoracic joint is the junction between the scapula and the thoracic cage and is located on ribs 2-7. It is not a true ‘joint’ in the sense that it does not possess usual joint characteristics, but rather allows articulation of the scapula on the thorax. The convex shape of the posterior thoracic cage and the concave shape of the anterior scapula mean that there is a space between these two anatomical structures which we define as the scapulothoracic joint. For every 1° of scapulothoracic elevation there is approximately 2° of glenohumeral elevation, although this can vary. Finally, 17 muscles insert or originate at the scapula meaning that it plays a vital role in many different movements.

Muscles

Rotator cuff

The rotator cuff is made up of four muscles, each contributing to shoulder stability and movement. They co-contract with larger superficial muscles to stabilise the glenohumeral joint to prevent humeral translation and allow force couples to be established, therefore playing a hugely important role in shoulder health.

The four muscles of the rotator cuff are:

  • Supraspinatus

  • Infraspinatus

  • Teres minor

  • Subscapularis

The supraspinatus functions to elevate the arm and stabilises the glenohumeral joint. The infraspinatus acts to externally rotate the humerus, whilst also stabilising the glenohumeral joint against posterior subluxation. The teres minor also externally rotates the humerus and stabilise the glenohumeral joint. Finally the subscapularis internally rotates the humerus.

As the force capabilities of these smaller stabilising muscles is less than the larger superficial musculature of the shoulder, problems can arise if they are not trained directly, particularly as the internal rotators are much more powerful than the external rotators at the shoulder.

Superficial muscles

The primary muscles of the shoulder include:

  • Deltoid

  • Pectoralis major

  • Latissimus dorsi

  • Trapezius

  • Rhomboids

  • Serratus anterior

  • Pectoralis minor

  • Biceps

Injuries

Common injuries at the shoulder include:

  • Labrum tears – Bankart and SLAP lesions

  • Dislocation/subluxation

  • Rotator cuff tears

  • Impingement

  • Scapular dyskinesis

The labrum of the shoulder acts as a ‘suction cup’, enabling greater joint stability, whilst also deepening the joint itself. Damage to the labrum reduces resistance to humeral translation by 20%, which can therefore lead to joint laxity. Equally, following shoulder dislocation there is often damage to the labrum which further exacerbates the issue. Detachment of the labrum from the anterior-inferior glenoid rim is the "essential lesion" responsible for the high incidence of recurrent anterior shoulder dislocations. A bankart lesion is associated with an anterior dislocation of the humeral head, which compresses the labrum and leads to damage. A Superior Labrum Anterior and Posterior (SLAP) tear at the shoulder joint can be caused by repetitive use or acute injury, and is characterised by damage to the superior aspect of the labrum. Athletes involved in throwing sports such as baseball and cricket can be at higher risk of developing this particular injury, as well as swimmers and tennis players. 

Dislocations are a complete separation of the humeral head from glenoid, whereas subluxations are only a partial movement (25-50%) of the humeral head on the glenoid. A history of dislocation is a risk factor for future injury, as the labrum can become damaged as we’ve already outlined, leading to less joint stability. 

Scapular dyskinesis is where there are alterations in the normal position or motion of the scapula during coupled scapulohumeral movements. It is a non-specific response to a painful condition in the shoulder rather than a specific response to certain glenohumeral pathology. Altered scapular motion or position both reduce linear measures of the subacromial space, increase impingement symptoms, decrease rotator cuff strength, increase strain on the anterior GH ligaments and increase the risk of internal impingement.

 

Ranges of motion normative values


Normative values for range of motion allow us to compare and determine whether ranges are appropriate for an athlete, or whether limitations exist which may impact health and performance. 

Normative values for shoulder ranges of motion include:

  • Flexion 180 degrees

  • Extension 50-60 degrees

  • Abduction 150 degrees

  • Internal rotation 70-90 degrees

  • External rotation 90 degrees

  • Horizontal abduction 45 degrees

Implications for range of motion deficits may be an increased risk of injury, particularly when a sport or activity demands ranges that an individual is incapable of producing. Small limitations may not impact shoulder health in the short-term, but over a prolonged period may begin to lead to problems. 

Force production normative values

In line with the range of motion normative values that we’ve identified, force production normative values also exist which allow insight into whether an athlete has the force expression capabilities to meet the demands of their sport. 

For example, if an athlete is very limited in external rotation peak force, but has very high internal rotation peak force measures, then this may lead to shoulder health issues later down the line if not addressed. We already know that this is more likely given the morphology of the internal v external rotators, so preventative exercises are important if we are to reduce any predisposition to injury. 

Van Harlinger W. et al. (2005) identified shoulder force production normative values for a range of ages and in both male and female athletes. These are represented as peak force measures in kilograms (kg). 

As you can see, for a 20-24 year old male dominant side external rotation force norms are 10.0 ± 3.1kg. This is compared with 8.0 ± 3.1kg for the non-dominant side. 

Another key piece of information we can gather using peak force measures (typically using a hand held dynamometer as in this paper), is limb symmetry i.e. left v right shoulder force values. Although some sports may naturally lean towards asymmetry owing to their demands such as archery or golf, it may be helpful to understand if any major asymmetries exist which may impact the shoulder health of the athlete in the long term. 

Joint position sense

The ability of an athlete to understand their relative joint position in space is referred to as joint position sense.

Mechanoreceptors within joints are what provide proprioceptive feedback that enables an athlete to assess and adjust their relative joint position and are responsible for the detection of mechanical deformation at the joint. In the shoulder these are found in the shoulder capsule itself, as well as the labrum.

At the shoulder, these mechanoreceptors function to transduce mechanical deformation into neural signals that transmit proprioceptive information about joint position and motion. This results in an efferent signal for reflexive muscular contraction around the joint, which enables the protection of the joint when in compromised positions. Positioning is transmitted through the muscle spindle receptors measuring lengthening of the muscles and through the Golgi tendon organ measuring muscle tension.

Damage to these receptors can lead to proprioceptive deficits that can lead to joint instability, for example following labrum tears where the receptors are located. The exact implication for this is unclear, however we might assume that this may lead to impaired joint position sense.

There are two different types of mechanoreceptors which have slightly different roles to play in shoulder joint position sense:

Type 1 receptors (Ruffini endings) are slow adapting, responding to prolonged and constant stimuli such as stretch, compression, and rotation. 

Type 2 receptors (Pacinian/Krause), are rapid-adapting receptors that respond to the beginning and end of stimuli.

The implications for this are that instability of a joint is associated with reduced joint position sense, which may in turn lead to poorer understanding of relative joint positioning in space. Research has demonstrated a link between fatigue and reduced joint position sense in swimmers and rugby players, which may lead to an athlete getting into compromised positions more frequently when in a fatigued state. The implications for this for shoulder health and performance are certainly intriguing!

Finally, assessing joint position sense may be achieved through the laser-pointer assisted angle reproduction test (LP-ART) which assesses an athlete’s ability to replicate joint positions relative to the contralateral limb.

Other considerations

There are other factors to consider when addressing shoulder health and performance. Not only are localised mobility and force production capabilities essential, but so are force production qualities throughout the kinetic chain. For example, if we do not possess sufficient lower limb force capabilities, then we are likely to be more reliant on producing force at the shoulder during throwing actions, for example. If we can ensure good levels of lower limb strength, trunk strength and the ability to transfer forces through the kinetic chain effectively, we will be in a much better position to improve shoulder function and health. 

The take home therefore is to consider shoulder training as part of a wider holistic programme, recognising that the shoulder is only one part of the kinetic chain that facilitates movement during overhead actions. 

Summary

The shoulder is a complex joint which requires specific interventions to enhance athlete health and performance. By understanding not only the capabilities of the athlete from a range, force and capacity perspective but also the demands of the sport, we can bring the athlete closer to where they need to be. In doing so we can minimise risk of future injury, maintain shoulder health and contribute to enhanced performance. The shoulder should be trained directly, but also within the context of an holistic view of performance taking into consideration the whole kinetic chain.

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References

Balke et al. (2011) The laser-pointer assisted angle reproduction test for evaluation of proprioceptive shoulder function in patients with instability. Arch Orthop Trauma Surg 131:1077–1084

Bankart ASB. (1923) The pathology and treatment of recurrent dislocation of the shoulder joint. Br Med J. 2:1132-1133.

Horsley, I. (2012). Proprioception and the Rugby Shoulder. 10.5772/27977.

Kibler WB, Ludewig PM, McClure PW, et al. (2013) Clinical implications of scapular dyskinesis in shoulder injury: the 2013 consensus statement from the 'Scapular Summit' Br J Sports Med 47:877–885.

Lippitt SB, Vanderhooft JE, Harris SL, Sidles JA, Harryman DT II, Matsen FA HI. (1993) Glenohumeral stability from concavity-compression: a quantitative analysis. J Shoulder Elbow Surg. 2:27-35.

Mathewson, M. A., Kwan, A., Eng, C. M., Lieber, R. L., & Ward, S. R. (2014). Comparison of rotator cuff muscle architecture between humans and other selected vertebrate species. The Journal of experimental biology, 217 (Pt 2), 261–273.

Terry GC, Chopp TM. (2000) Functional Anatomy of the Shoulder. J Athlet Train. 35(3): 248-55.

Van Harlinger W. et al. (2005) Upper Limb Strength: Study Providing Normative Data for a Clinical Handheld Dynamometer. PMR 7:2, 135-140.

Witherspoon, J. W., Smirnova, I. V., & McIff, T. E. (2014). Neuroanatomical distribution of mechanoreceptors in the human cadaveric shoulder capsule and labrum. Journal of anatomy, 225(3), 337–345.

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