Biomechanics of the Shoulder
Original Editor - Amanda Ager
The shoulder area is infamously known to be one of the most complex regions of the body to evaluate and rehabilitate. Because of the multiple joints involved during shoulder movement, it is prudent to refer to the area as the shoulder complex. To effectively rehabilitate a shoulder injury in clinical practice, it is important to have a functional knowledge of the underlying biomechanics of the shoulder complex.
Anatomy and Biomechanics
The shoulder complex involves 3 physiological joints and one floating joint:
- Glenohumeral (GH) joint,
- Acromioclavicular (AC) joint
- Sternoclavicular (SC) joint
- Scapulothoracic (ST) joint - known as a "functional joint".
The SC joint is the only bony attachment site of the upper extremity to the axial skeleton. The ST joint involves the gliding movement of the scapula along the rib cage during upper extremity movements and does not include a physical bone-to-bone attachment. The GH joint is of particular interest when understanding the mechanism of shoulder injuries because it is osteologically predisposed to instability.
The GH joint is comprised of a ball and socket synovial joint, where the head of the humerus (convex surface) articulates with the glenoid fossa (concave surface) of the scapula. Because of the relatively large surface area of the humeral head in relation to the fossa, the joint itself has limited bony congruency, and consequentially heavily depends on surrounds soft tissues for structural support. Moreover, it is estimated that only 25% of the humeral head articulates with the glenoid fossa at any one time during movement. The surrounding passive structures (the labrum, joint capsule, and ligaments) as well as the active structures (the muscles and associated tendons) act cooperatively in a healthy shoulder to maintain dynamic stability throughout movement.
An area most often involved in the cases of shoulder pain is the subacromial space, which includes the theoretical space between the coracoacromial arch and the head of the humerus. More specifically, the subacromial canal lies underneath the acromion, the coracoid process, the AC joint and the coracoacromial ligament. The space itself includes a bursa which provides lubrication for the rotator cuff (RC) tendons, the insertion for the long head of the biceps tendon, and the RC tendons themselves.
For more detailed anatomy visit Shoulder Anatomy
Biomechanics of Shoulder Movement
Glenohumeral (GH) Joint
- Flexion / extension
- Abduction / Adduction
- Medial / Lateral rotation
- Spin (pure flexion and extension)
- Inferior glide (Abudction)
- Superior glide (Adduction)
- Posterior glide (Medial rotation)
- Anterior glide (Lateral rotation)
Del Maso and colleagues have estimated that a maximum of 7.5 mm of upward translation of the humeral head may occur during range of motion movements, which is not an insignificant amount of migration for a large bony structure to experience within a compact space during a dynamic task. The success of a coordinated movement of the humeral head with normalized arthrokinematics, avoiding an impingement situation, requires the harmonious co-contraction of the RC tendons. Abnormal glenohumeral translations have been linked to pathological shoulders and it has been suggested to be a contributing factor for shoulder pain and discomfort, and may also lead to the damage of encompassing structures.
As illustrated by the force-vectors of their respected moment arms, the RC tendons collectively have been accredited with the compression of the humeral head within the glenoid fossa during movements. The individualized tendons of the RC complex are directly affiliated with limiting the translation of the humeral head in specific directions. The supraspinatous muscle contributes to preventing excessive superior translation, the infraspinatus and teres minor limit excessive superior and posterior translation, and the subscapularis controls excessive anterior and superior translation of the humeral head, respectively. An imbalance in the neural activation of any one of the RC muscles could easily cause a misalignment of the humeral head thus giving rise to an impingement of the subacromial structures during movement. Both the superior and anterior translation of the humeral head during movements are the leading biomechanical causes for an impingement syndrome.
Acromioclavicular (AC) Joint
The AC joint is a diarthrodial and synovial joint. It allows for axial rotations and antero-posterior glides. Because there are not direct attachements of muscles to the joint, all movements are passive and initiated by movements at other joints (such as the ST joint).
- Flexion / extension
- Abduction / adduction
- Medial / lateral rotation
- Posterior / anterior spin
- Inferior / superior spin
- Anterior / posterior glide
Scapulothoracic (ST) Joint
Scapulothoracic complex movements:
- Elevation and protraction = anterior elevation
- Elevation and retraction = posterior elevation
- Depression and protraction = anterior depression
- Depression and retraction = posterior depression
The movement of the scapula along the thoracic cage also directly influences the biomechanics of the shoulder complex as a whole, and can moreover predispose the development of an impingement syndrome. The healthy movement of the scapula along the thorax during arm elevation includes protraction, posterior tilting and lateral rotation, depending on the plane of movement (Figure 1).
Although posterior tilting is generally understood as primarily an acromioclavicular joint motion, the tilting that occurs at the scapula during arm elevation is crucial in order to minimize the encroachment of soft tissues passing under the acromial arch. The normal contribution of the ST joint is generally expressed as the ratio of ST movement with regards to that occurring simultaneously at the GH articulation. The scapulohumeral rhythm is quantified by dividing the total amount of shoulder elevation (humeralthoracic) by the scapular upward rotation (scapulothoracic). Within the scientific literature, the scapulohumeral rhythm is generally accepted to be 2:1, which represents 2° of humeral elevation for every degree of scapular upward rotation.
The stability of the ST joint relies on the coordinated activity of the 18 muscles that directly attach to the scapula. The scapular muscles must dynamically control the positioning of the glenoid so that the humeral head remains centered and permits arm movement to occur. When a weakness or neuromuscular dysfunction of the scapular musculature is present, normal scapular arthrokinematics become altered, and ultimately predisposes an individual to an injury of the GH joint.
- Increased medial rotation
- Decreased superior rotation, and
- Decreased posterior tilting
These movement alterations are believed to increase the proximity of the rotator cuff tendons to the coracoacromial arch or glenoid rim, however, there are still points of contention as to how the movement pattern deviations directly contribute to the reduction of the subacromial space.
For the sake of clarification, the current literature differentiates between an internal impingement and an external impingement. An impingement that involves a decreased space towards the coracoacromial arch is said to be an external impingement, whereas an internal impingement involves the glenoid rim, and can be associated with a GH instability. Regardless of the classification, the dysfunctional shoulder mechanisms can further the progression of rotator cuff disease and must therefore be understood as a neuromuscular impairment.
The neuromuscular control of the scapula relies on the balanced team-work between the global movers and the fine-tuning stabilizing muscles of the shoulder complex. Again, because of the floating nature of the scapula along the thorax, it too, must rely on the kinship between the cortical direction provided by the nervous system and the resulting action of the MSK system. We can therefore affirm, that the shoulder complex is among the most kinematically complex regions of the human body, and requires a high level of neuromuscular stability throughout movement. The neuromuscular control of the shoulder also requires a well-developed sense of motor control and proprioception.
Thoracic Spine Movement
During right arm flexion: The upper thoracic vertebrae right side flexes, right rotates and extends. The first and second ribs descend, while the 4-6th ascend and the 3rd acts as an axis.
Limitation of motion in any of these structures will adversely affect the biomechanics of the shoulder girdle and may produce or predispose the shoulder girdle to pathological changes.
Static Structures and Mechanoreceptors
The static structures of the shoulder complex, which includes the labrum (a fibrocartilaginous ring), the capsule, cartilage, ligaments, and fascia collectively act as the physical restraints to the osseous matter and provides a deepening effect to the shallow glenoid fossa.
Further to their passive stabilization role, they also provide additional protection via the various mechanoreceptors embedded within their fibers. Mechanoreceptors can be understood as the neural sensors that provide afferent input to the central nervous system for motor processing and descending motor commands for the execution of movements.
Mechanoreceptors are characterized by their specialized nerve endings that are sensitive to the mechanical deformations of tissues, and therefore contribute to the modulation of motor responses of the adjacent muscles. Mechanotendinous receptors (muscle spindles and golgi tendon organs), capsuloligamentous receptors (ruffini and pacinian corpuscles) as well as cutaneous receptors (meissner, merkel and free nerve endings) are responsible for our sense of touch, vibration, proprioceptive positioning, as well as provide the feedback regarding muscle length, tension, orientation, further to the speed and strength of the contractions of the muscle fibers.
It is clear that the passive structures of the shoulder provide a neurological protection mechanism through feed forward and feed back input, that directly mediates reflex musculature stabilization about the glenohumeral joint.
Further to the intricate network of passive ligatures that conjoin adjacent bones, the importance of the surrounding musculature cannot be overstated. Active muscle contractions are essential for maintaining the stability of the shoulder complex.
The musculature of the shoulder region can be subdivided into the global movers of the shoulder and the fine-tuning stabilizers of the individual articulations. The larger muscles such as the trapezius, the levator scapula, the pectorali, the deltoids, the serratus anterior, the latissimus dorsi, the rhomboids, the teres major, the biceps, the coracobrachialis, and triceps muscles are responsible for various synergistic activities during shoulder movements. Conjointly as agonist and antagonist couplings, they allow for the gross motor movements of the upper quadrant. More specifically to the GH joint, the fine-tuning stabilizers are just as important to the shoulder complex as the global movers for coordinated and smooth shoulder movements.
The stabilizing muscles of the GH articulation, the supraspinatus, subscapularis, infraspinatus, and teres minor,are often summarized as the rotator cuff (RC) complex, and attach to the humeral head within the glenoid fossa. Collectively, they act as the dynamic stabilizers of the GH joint by maintaining a centralized positioning of the humeral head within the glenoid fossa, in both static and dynamic conditions. It has been suggested that the tendons of the rotator cuff muscles blend with the ligaments and the glenoid labrum at their respected sites of attachments, so that the muscle contractions can provide additional stability by tightening the static structures during movement.
The synchronized contractions of the RC muscles must maintain the centralized positioning of the humeral head during movements in order to avoid the physical encroachment of tissues, predominantly anteriorly or superiorly to the GH joint, which has been linked to injury and pain amongst the shoulder region. As previously noted, due to the anatomical passage of the common RC tendon within the subacromial space, the RC tendons are particularly vulnerable to compression, abnormal friction, and ultimately an impingement (pinching) during active tasks. Proper alignment of the glenohumeral head is important for the healthy engagement of the shoulder joint in activities of daily living.
- Suprak DN, Osternig, L.R., van Donkelaar, P., & Karduna, A.R. Shoulder joint position sense improves with elevation angle in a novel, unconstrained task. Journal of Orthopedic Research. 2006;24(3):559-569.
- Bushnell BD, Creighton, R.A., & Herring, M.M. Bony instability of the shoulder. J Arthroscopy and Related Surgery. 2008;24(9):1061-1073.
- Anderson BC. Office orthopedics for primary care: Diagnosis.Philadelphia; 2005.
- Neer CS. 2nd impingement lesions. Clin Orthop Relat Res. 1983(173):70-77.
- Neer CS. Anterior acromioplasty for the chronic impingement syndrome in the shoulder: a preliminary report. J Bone Jt Surg. 1972;54:41-50.
- Põldoja E, Rahu, M., Kask, M., Weyers, I., & Kolts, I. Blood supply of the subacromial bursa and rotator cuff tendons on the bursal side. Knee Surg Sports Traumatol Arthrosc. 2016.
- Tillmann B, & Gehrke, T. Funktionelle anatomie des subakromialen raums. Arthroskopie. 1995;8:209-217.
- Craig J, et al. Chapter 12: Biomechanics of the shoulder.Philadelphia: Lippincott Williams & Wilkins.; 2001.
- Dal Maso F, Raison, M., Lundberg, A, Arndt, A., Allard, P., Begon, M. Glenohumeral translation during range of motion movements, activities of daily living, and sports activities in healthy participants. Clin Biomech (Bristol Avon). 2015;30(9):1002-1007.
- Milgrom C, Schaffer, M., Gilbert, S., & van Holsbeeck, M. Rotator cuff changes in asymptomatic adults. The effect of age, hand dominance and gender. J Bone Jt Surg. 1995;77:296-298.
- Jam B. New paradigms in rotator cuff retraining. APTEI Report. 2004.
- David G, Jones, M., & Magarey, M. Rotator cuff muscle performances during gleno-humeral joint rotations: An isokinetic, electromyographic and ultrasonographic study. Paper presented at: Manipulative Physiotherapists Association of Australia Conference Proceedings., 1997; Melborne, Australia.
- Sahrmann SA. Diagnosis and treatment of movement impairement syndromes.St. Louis.; 2002.
- Struyf F, Nijs, J., Baeyens, J.P., Mottram, S., Meeusen, R. Scapular positioning and movement in unimpaired shoulders, shoulder impingement syndrome, and glenohumeral instability. Scandinavian Journal of Medicine and Science in Sports. 2011;20(3):352-358.
- Dayanidhi S, Orlin, M., Kozin, S., Duff, S., Karduna, A. Scapular kinematics during humeral elevation in adults and children. Clin Biomech (Bristol Avon). 2005;20(6):600-606.
- Wu G, van der Helm, F.C., Veeger, H.E. et al. ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion - Part II: shoulder, elbow, wrist and hand. J Biomech. 2005;38:981-992.
- Ludewig PM, & Braman, J.P. Shoulder impingement: biomechanical considerations in rehabilitation. Man Ther. 2011;16(1):33-39.
- Paine RM, & Voight, M.L. The role of the scapula. J Orthop Sports Phys Ther. 1993;18:386-391.
- Voight ML, & Thomson, B.C. The role of the scapula in the rehabilitation of shoulder injuries. J Athl Train. 2000;35(5):364-372.
- Paine R, & Voight, M.L. The role of the scapula. Int J Sports Phys Ther. 2013;8(5):618-629.
- Zhao KD, Van Straaten, M.G., Cloud, B.A., Morrow, M.M., An, K-N., & Ludewig, P.M. Scapulothoracic and glenohumeral kinematics during daily tasks in users of manual wheelchairs. Frontiers in Bioengineering and Biotechnology. 2015;3:183-193.
- Ludewig P. M. CTM. Alterations in shoulder kinematics and associated muscle activity in people with symptoms of shoulder impingement. Phys Ther. 2000;80:276-291.
- Lukasiewicz A. C. MP, Michener L., Pratt N., & Sennett B. . Comparison of 3-dimensional scapular position and orientation between subjects with and without shoulder impingement. J Orthop Sports Phys Ther. 1999;29:574-583.
- Ludewig PM, & Reynolds, J.F. . The association of scapular kinematics and glenohumeral joint pathologies. J Orthop Sports Phys Ther. 2009;39(2):90-104.
- Gombera MM, & Sekiya, J.K. Rotator cuff tear and glenohumeral instability: a systematic review. Clin Orthop Relat Res. 2014;472(8):2448-2456.
- Soslowsky LJ, Thomopoulos, S., Esmail, A. et al. . Rotator cuff tendinosis in an animal model: Role of extrinsic and overuse factors. . Annals of Biomedical Engineering. 2002;30(8):1057–1063.
- Morgan R, & Herrington, L. The effect of tackling on shoulder joint positioning sense in semi-professional rugby players. Physical Therapy Sport. 2014;15:176-180.
- Lephart SM, Pincivero, D.M., Giraldo, J.L., & Fu, F.H. The role of proprioception in the management and rehabilitation of athletic injuries. Am J Sports Med. 1997;25(1):130-137.
- Kennedy JC, Alexander, I.J., & Hayes, K.C. . Nerve supply of the human knee and its functional importance. Am J Sports Med. 1982.;10:329-335.
- Edouard P, Gasq, D., Calmels, P., Ducrot, S., Degache, F. Shoulder sensorimotor control assessment by force platform: feasibility and reliability. Clin Physio Funct Imaging. 2012;32(5):409-413.
- Witherspoon JW, Smirnova, I.V., & McIff, T.E. Neuroanatomical distribution of mechanoreceptors in the human cadaveric shoulder capsule and labrum. J Anat. 2014;225:337-345.
- Kandel E, Schwartz, J., & Jessell, T. Principles of Neuroscience.New York.: McGraw-Hill.; 2000.
- Wamer JJ, Lephart, S., & Fu, F.H. Role of proprioception in pathoetiology of shoulder instability. Clin Orthop Relat Res. 1996;330:35.39.
- Janwantanakul P, Magarey, M.E., Jones, M.A., & Dansie, B.R. Variation in shoulder position sense at mid and extreme range of motion. Arch Phys Med Rehabil. 2001;82:840-845.
- Brukner P, & Khan, K. et al. Chapter 17: Shoudler Pain. In: Pike C, ed. Clinical Sports Medicine. 3rd Ed. ed. Autralia: McGraw-Hill Autralia Pty Ltd.; 2010: 244-259.
- Jobe C. Evaluation of impingement syndromes in the overhead throwing athlete. J Athl Train. 2000;35(3):293.
- Vafadar AK, Côté, J.N., & Archambault, P.S. . Inter-rater and Intra-rater reliability and validity of three measurement methods for shoulder position sense. . Journal of Sport Rehabilitation Technical Report. 2015(19):2014-0309.