1Bain GI, et al. JISAKOS 2019;0:1–8. doi:10.1136/jisakos-2017-000187. Copyright © 2019 ISAKOS

Shoulder crane: a concept of suspension, stability, control and motionGregory Ian Bain,  1 Joideep Phadnis,2 Eiji Itoi,3 Giovanni Di Giacomo,4 Hiroyuki Sugaya,5 David H Sonnabend,6 James McLean7

State of the Art

To cite: Bain GI, Phadnis J, Itoi E, et al. JISAKOS Epub ahead of print: [please include Day Month Year]. doi:10.1136/jisakos-2017-000187

1Department of Orthopaedic Surgery and Trauma, Flinders University, Adelaide, South Australia, Australia2Shoulder & Elbow Surgery, Brighton & Sussex University Hospitals, Brighton, England3Department of Orthopaedic Surgery, Tohoku University School of Medicine, Sendai, Japan4Department of Orthopedic Surgery, Hospital for Special Surgery, Rome, Italy5Shoulder & Elbow Service, Funabashi Orthopaedic Sports Medicine Center, Funabashi, Japan6Department of Orthopaedic Surgery, University of Sydney, Sydney, New South Wales, Australia7Department of Orthopaedic Surgery and Trauma, Royal Adelaide Hospital, Adelaide, South Australia, Australia

Correspondence toProf Gregory Ian Bain, Department of Orthopaedic Surgery and Trauma, Flinders University, South Australia 5006, Australia; admin@ gregbain. com. au

Received 11 December 2017Revised 5 February 2019Accepted 12 February 2019

© International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine 2019. No commercial re-use. See rights and permissions. Published by BMJ.

AbSTrACTFramework and suspensory cascade This novel model uses the structure and workings of the industrial crane as a simile to explain the function of the human shoulder. As a crane consists of a base, axial tower, boom and suspensory cascade that move and position loads in space, the base consists of the pelvic platform, with outriggers (legs) that provide stability in human body. The axial tower consists of an articulated spinal column and thoracic platform, which are stabilised by the core muscles. The clavicular boom articulates with the anterior thoracic platform and is elevated by the trapezius from the posterior tower. The ’suspensory cascade’ extends from the skull and cervical spine to the trapezius and on to the clavicle, coracoclavicular ligaments, coracoid process, coracohumeral ligament and humeral head.Motion The rotator cuff muscles take origin from the scapula and coalesce with each other to form a multilayered rotator cuff tendon and cable, which cups to closely contain the humeral head. The four muscles insert into the common tendon and together share the load to stabilise and mobilise the arm in space. The coracoid is a pulley that allows the scapula to swivel on the coracoclavicular ligaments to enable adjustment of the angle of force transmission delivered by the rotator cuff to the humeral head.Stability and control The inferior glenoid and labrum are a fixed organ of compression, which coalesces with the hammock formed by the static inferior glenohumeral ligaments. The rotator cuff and deltoid compress the humeral head onto this static structure. The biceps tendon passes adjacent to the condensations of the coracohumeral ligament to insert into the mobile superior labrum and glenoid. Contraction of the biceps pulls the mobile superior labrum onto the humeral head and tightens the glenohumeral ligaments that wrap around the humeral head at the extremes of motion. The coracohumeral ligament is a sensory organ that interfaces with these structures and is well positioned to work as a servomechanism to redirect the rotator cuff in providing stability, control and motion.Level of evidence Level V.

InTroduCTIonShoulder evolution has been driven by the devel-opment of the orthograde posture with anatomic changes to accommodate the demands of a mobile, non-weight-bearing joint to create a brachiating, prehensile limb. There is an interplay of struc-tural osseous anatomy, static capsuloligamentous restraints and dynamic musculotendinous units.1 Static and dynamic stabilisers enable greater motion in the shoulder than in any other joint in the body.

Damage to one or more components will disrupt the balance between mobility and stability and place the shoulder at risk of injury.2–4

What are the new findings

► The shoulder crane consists of the primary parts: the base, axial tower, clavicular boom, suspensory cascade, pulley, and motor.

► The core stabilises the tower, the periscapular muscles stabilise the scapular and the rotator cuff stabilises the glenohumeral joint.

► The suspensory cascade extends from the skull to the trapezius muscle, clavicle, coracoclavicular ligaments (CCLs), coracoid, coracohumeral ligament (CHL) and finally humeral head. The coracoid is a pulley that swivels below the clavicle to allow the rotator cuff to be realigned.

► The CHL is a sensory organ that interfaces with the rotator cuff and interval, biceps tendon, labrum and glenohumeral ligaments.

► The dynamic biceps tendon inserts into the mobile superior labrum, which is confluent with the static superior and middle glenohumeral ligamentous restraints that wrap around humeral head at the extremes of rotation.

► The shoulder crane is a biomechanical model, which explains shoulder suspension, stability, control and motion.

Preface

In 2015, the members of the Shoulder and Upper Limb Committee of ISAKOS published a text book titled ‘Normal and Pathological Anatomy of the Shoulder’. The editors were Gregory Bain, Eiji Itoi, Giovanni Di Giacomo and Hiro Sugaya.31 This book is a comprehensive state-of-the-art text on clinical anatomy of the shoulder and how it is affected by dysplasia, trauma, disease and degeneration. The concluding chapter titled ‘The Functional Shoulder’, brings together the many concepts presented by the authors throughout the book and presents a new model of normal shoulder anatomy.32

This article is based on the chapter ‘The Functional Shoulder’ and includes many of the concepts and images from this original chapter.32 The original publication has been modified for publication with the support of the editorial board of JISAKOS, on the basis that it offers an important academic and educational contribution to the readers and the literature.

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Knowledge of fundamentals of shoulder anatomy is integral to understanding pathomechanics and underpins treatment plans. Goss described the superior shoulder suspensory complex, which illustrates why acromioclavicular (AC) joint disruption unlinks the upper extremity from the axial skeleton.5 Digiovine et al defined the phases of pitching and how the chain of muscles function with precision and synchrony.6 However, it is chal-lenging to conceptualise how the axial skeleton suspends the upper limb and maintains glenohumeral stability and function while enabling the hand to be placed in space. This manuscript aims to provide an overview of recent advances in the under-standing of shoulder anatomy and present the ‘shoulder crane’ concept.

overview of the ‘shoulder crane’The mechanical workings of the shoulder girdle are analo-gous to a structural crane (figure 1). We propose the ‘shoulder crane’ model, which has a framework consisting of its primary part:

The base: pelvic platform with outriggers (legs), which provide stability and mobility.

Axial tower: articulated spinal column and thoracic platform, stabilised by the core muscles.

Clavicular boom: articulates with the anterior thoracic plat-form and is elevated by the trapezius from the posterior tower.

Suspensory cascade: extends from the skull and cervical spine to the trapezius, clavicle, coracoclavicular ligaments, coracoid process and coracohumeral ligament (CHL) to the humeral head.

Pulley: the scapula swivels on the coracoclavicular ligaments, allowing the scapula to change the direction of the glenoid face and the rotator cuff alignment to optimise shoulder function.

Motor: the rotator cuff tendons and capsule coalesce to form a multilayered structure, which stabilises the glenohumeral joint and powers the humeral head position.

Servomechanism as a balancing sensor: the CHL is a sensory organ positioned to assess soft tissue tension and provide feed-back to the rotator cuff.

Figure 1 The shoulder crane. The crane is constructed on top of the pelvic base and leg outriggers, which provide for stability and mobility. The articulated spinal tower and thoracic platform are stabilised by the core and periscapular muscles, respectively. The clavicular boom articulates with the anterior platform, at the sternoclavicular joint, and is elevated by the trapezius from the posterior tower. The scapula is suspended and swivels on the coracoclavicular ligaments, positioned by the powerful periscapular muscles, and traverses the ‘scapular track’. These factors are all designed to enable the rotator cuff to mobilise the shoulder while it keeps the glenoid aligned and stabilised with the humeral head throughout motion and loading (Copyright Dr Gregory Bain32).

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These mechanisms elevated and rotated the arm, to place the hand in space, and perform functional activities.

The spinal towerThe base consists of outriggers (legs) that provide leverage for the entire construct. The pelvis base is a platform for the axial tower.

The axial tower consists of an articulated spinal column on which is constructed the thoracic platform. Man-made cranes have a straight steel tower, which are strong but rigid. The human spinal tower has multiple vertebral segments in a sinu-soidal shape of the lumbar, thoracic and cervical spine. This confers greater flexibility but requires a complex array of core muscles to maintain control and stability.

The sternoclavicular joint is strategically positioned at the anterior aspect of the elevated platform. The clavicle is the boom (or jib) of the crane, which elevates and lateralises the point of suspension. The main ligament attachment sites are inset from each end of the bone, with the shock absorber (articular disc) at each end of the clavicle.

The coracoacromial arch is a gantry composed of the lateral clavicle, coracoacromial ligament, acromion and scapular spine (figure 2) (Dr Peter Hales from Perth, Australia coined the term). A ‘gantry’ is a bridge-like framework or supportive structure (modified from www. collinsdictionary. com). The ‘coracoacro-mial gantry’ is unique to bipedal animals, most notably the brachiating animals, and is one of the evolutionary modifications that have allowed the arm to be elevated from the body. The gantry creates a ‘pseudo articulation’ between the coracoacro-mial arch and the rotator cuff. The gantry has various parts, each with its own function.

The pillars of the gantry are the coracoid process and the spine of the scapula, which provide cantilever support for the cora-coacromial arch. The trapezius muscle inserts into its superior surface, so that it can elevate the entire gantry and with it the arm, providing an important contribution to abduction strength.

The acromion projects laterally over the humeral head, increasing the moment arm of the deltoid, and the coracoacro-mial ligament transfers tension from the acromion to the cora-coid process. The deltoid is a strong multipennate muscle that attaches to the lateral acromion and works with the trapezius and supraspinatus to abduct the shoulder.

The inferior surface of the coracoacromial arch consists of the thin and somewhat flexible osseous acromion and the cora-coacromial ligament. During abduction, the deltoid contracts to narrow the subacromial space, and the flexible arch then moulds itself to become a ‘soft pivot’ for the rotator cuff. The evolu-tion of the coracoacromial ligament has allowed the acromion to better absorb the forces of the deltoid, and therefore enabled the deltoid/acromial complex to be a more effective shoulder abductor.

Cascade of suspensionThe clavicular boom is elevated by the trapezius muscle (‘boom guy line’), which originates from the cranium and the cervical spine. The apex of suspension is above the thoracic platform, at the top of the spinal tower. From the apex of the tower (cranium) to the humerus, there is a cascade of osseous and intervening suspensory structures. For each articulation, there is a set of ‘boom guy line’ muscles, which provide dynamic control of the articulation. The coracoid is suspended from the lateral aspect of the clavicular boom by the coracoclavicular ligaments, and in

Figure 2 Shoulder gantry. The periscapular muscles mobilises the scapula, while the rotator cuff mobilises and stabilises the humeral head (copyright Dr Gregory Bain).32 Overhanging the glenohumeral joint is the shoulder gantry (left), which is composed of the clavicle, coracoacromial ligament (CAL), acromion and scapular spine. The trapezius muscle (posterior) elevates the gantry, hinging on the sternoclavicular joint (anterior-medial). The AC joint fibrocartilaginous disc buffers the compressive forces, while the coracoclavicular ligaments resist the tensile forces. The shoulder triangle has three sides (right): medial base: thoracic platform; anterior: clavicular boom; posterior: scapular body/periscapular muscles, which control the scapula. There are three angles: anterior hinge: sternoclavicular joint; posterior: axial skeleton; lateral: coracoclavicular ligaments, on which the scapula is suspended and swivels. The clavicle and shoulder triangle lateralise the glenohumeral joint. Lateral to the lateral angle of the triangle is the glenoid, then the centre of rotation of the humeral head and finally the rotator cuff insertions. The rotator cuff, scapula and humeral head are a functional unit, with the rotator cuff providing stability and motion.

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turn, the coracohumeral and glenohumeral ligaments suspend the humerus from the coracoid.

The scapula swivels below the coracoclavicular ligaments, while the acromioclavicular joint (ACJ) capsule restrains anterior and posterior translation of the scapula. They form part of the superior suspensory ring of the shoulder, and disruption of this ring produces acromioclavicular joint instability.7 It is interesting to note that the conoid and trapezoid ligaments are in continuity and, when seen from the anterior aspect, appear like an open book.8 The conoid passes over the medial prominence of the coracoid process and wrapping around the prominent tubercle at the posterior angle of the clavicle to be the primary suspen-sory restraint. In addition, with clavicle rotation, the ligament shortens and lengthens by wrapping around the clavicle, analo-gous to the distal biceps tendon wrapping around the proximal radius. The trapezoid wraps around the medial side of the cora-coid process and passes laterally to the inferior surface of the clavicle to be the primary restraint against medialisation of the scapula. The primary restraint to anteroposterior translation is the acromioclavicular joint capsule, especially the superior and posterior aspects. These three structures each provides a primary restraint in one direction.8

A pulley is a wheel that supports movement and changes force direction along its circumference. The scapula is a pulley stra-tegically positioned in the middle of the ‘suspensory cascade’ between the clavicle and humerus. The scapula changes direc-tion by swivelling on the coracoclavicular ligaments, allowing the scapula to change the direction of the glenoid face and the rotator cuff alignment to optimise shoulder function.

The scapular body is a large, thin, triangular bone, which overlies the thoracic platform. Its extensive surface area serves as an attachment for the many muscles of the shoulder girdle.

The multiple, powerful periscapular muscles span from the spinal column and the thoracic cage. These muscles control scapular rotation and translation across the thoracic cage. The scapula is almost a sesamoid bone positioned between the peri-scapular muscles that control the scapula and the rotator cuff that controls the humeral head. These two major muscle groups work together to position the humerus in space. The scapula traverses the thoracic cage along the ‘scapular track’. The rotator cuff muscles control the humeral head across the ‘glenoid track’.9

The scapula is stabilised and mobilised by an important func-tional triangle (figure 2). The sides and angles of the triangle consist of:

Medial side: the fixed thoracic platform.Anterior angle: the sternoclavicular joint.Anterior side: the clavicular boom that lateralises and elevates

the scapular pulley.Lateral angle: the coracoclavicular ligaments that suspend and

swivel the scapula.Posterior side: the scapula and its periscapular muscles, which

power and dynamically stabilise the scapula.Posterior angle: the periscapular muscles that attach to the

articulating tower.Note: the three sides and corners of all have different func-

tions. The position of the scapula is defined by the:1. Angle of elevation of the sternoclavicular joint.2. Length of the clavicular boom.3. Tension in the periscapular muscles.

The scapular pulley is mobilised on the fixed thoracic cage (‘scapular track’), directed by the static anterior restraints, as determined by the periscapular muscles. The function of this triangle is to stabilise and mobilise the scapular pulley, so that it can align the rotator cuff to stabilise and mobilise the

glenohumeral joint. The scapula is oriented so that the glenoid faces anterolaterally, defining the functional plane of the shoulder.

The CHL passes from the base of the coracoid process through the rotator cuff interval, deep to the biceps tendon, into the supe-rior glenohumeral ligament (SGHL) and cable, enveloping and inserting into the supraspinatus and subscapularis tendons, just before their insertion into the greater and lesser tuberosities.3 4 10 The CHL at first glance appears to be a significant ligamentous structure that acts like a ‘string on a ball’, to suspend the humeral head in the glenohumeral joint (figure 3A,B). However, its histology is similar to that of capsular tissue but with numerous sensory nerves (figure 3C). As a sensory organ, it is perfectly positioned to interpret the interplay of these important dynamic and mobile stabilising structures (figure 3D). In retracted rotator cuff tears, the CHL will need to be released to enable the cuff to be mobilised. Contracture of the CHL will restrict external rotation, as seen in frozen shoulder.11

Glenohumeral jointThe glenohumeral joint is the primary articulation of the shoulder girdle.

The glenoid projects from the lateral scapula and forms the major articulation of the shoulder girdle. The glenoid is posi-tioned perpendicular to the body of the scapula, which is intrin-sically stable, especially with the support of the rotator cuff. A retroverted glenoid is known to be detrimental, associated with posterior glenohumeral instability, abnormal kinematics and osteoarthritis.

The capsule–labral complex is of primary importance in shoulder stability.12 The inferior labrum (5–9 o’clock) has a rounded convex surface, which increases the glenoid depth up to 50% and provides a bumper effect.13–15 It has a stable inter-face with the articular cartilage and sits on a rigid bony founda-tion. The inferior labrum and the inferior glenohumeral joint (figure 4A,B) together form a ‘fixed organ of compression’.13

In contrast, the superior labrum has a loose, mobile inter-face with no bony foundation and attaches off the rim, away from the glenoid articular margin. It is concave in cross-sec-tion, meniscal in nature and follows the contour of the glenoid surface.13 The superior labrum is a ‘mobile organ of tension’. The superior labrum is continuous with the SGHL and middle glenohumeral ligament (MGHL) and the CHL. These structures together wrap around the humeral head and are static restraints, which enhance joint stability throughout motion, especially rotation.13–16 As the long head of the biceps tendon inserts into the mobile superior labrum, it transmits a dynamic compo-nent to the static glenohumeral restraints. Biceps is known to be a head depressor, but it also tensions the mobile superior labrum onto the humeral head, and thereby tightens the static glenohumeral ligaments, throughout humeral rotation. In the athlete, this tension band effect, in conjunction with the sensory feedback of the CHL, is likely to be important in optimising performance.

When fixing the torn labrum, it is important to perform an anatomical repair. The inferior labrum should be repaired onto the glenoid face and the superior labrum well off the face. If the inferior labrum is fixed off the face, the shoulder will remain unstable. If the normally mobile anterior superior labrum is fixed to the glenoid face or anterior rim, it will create a painful stiff shoulder.

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Shoulder spacesThe ‘shoulder crane and gantry construct’ has various working parts, which require spaces or ‘pseudo-articulations’ between them to allow the functional units to work effectively.

The subacromial space is the potential space between the acromion and rotator cuff. With shoulder abduction from 0° to 90°, the supraspinatus mobilises the glenohumeral joint, and the tendon traverses the subacromial space. From 135° to 180°, abduction is predominantly scapulothoracic motion, with minimal translation of the rotator cuff, which is positioned on the under-surface of the coracoacromial arch.

The size and shape of the acromion are important factors in the development of rotator cuff tears.17 18 The ageing coracoacro-mial ligament loses its resilience, becomes stiffer and places greater force on the rotator cuff, predisposing it to impingement, degeneration and tearing.

The scapulothoracic space is a potential space. The multiple periscapular muscles coordinate how the scapular moves across the chest wall along the ‘scapular track’. Winging is dysfunc-tional scapular motion, where the scapula posture results in prominence of the medial scapula border. The four basic causes of winging are osseous (ie, clavicle fractures), articular (ie,

acromioclavicular joint instability), muscular (ie, fatigue) and neurological (ie, long thoracic nerve palsy).

Mobilisation of the shoulder craneThe ‘core’ muscles are counterbalance stabilisers for the plat-form and articulated tower (figure 1). This model highlights the importance of the lower limb and truncal core muscles and why they are critical to good shoulder function and rehabilitation.

The trapezius is a major muscle with an extensive origin (occiput to lumbar vertebrae) and an extensive insertion into the shoulder ‘gantry’ (lateral clavicle, acromion and scapular spine), which enables it to spin the scapula like a wing nut. The supe-rior part elevates the gantry, middle part retracts the scapula and inferior part rotates the scapula.

The powerful deltoid has unipennate anterior and posterior portions and a multipenate lateral portion (figure 5).19 20 Each portion will act in a different way depending on the position of the arm.21

Each rotator cuff muscle takes origin from the wide scapular body and coalesce into a common tendon cuff, which inserts into

Figure 3 The coracohumeral ligament (CHL). (A) The CHL is a significant structure with a wide coracoid attachment. Laterally it attaches to the subscapularis and supraspinatus and contributes to the rotator cuff cable and biceps pulleys. The two components of the CHL are a four-bar linkage, which restricts the extremes of the motion of the humeral head throughout circumduction (permission from Di Giacomo et al, Atlas of functional shoulder anatomy. Milan: Springer-Verlag; 2008.33 (B) The CHL suspends the humeral head. Mechanically, the CHL appears to tethers the humeral head, like a ‘ball on a string’ (copyright Dr Gregory Bain32). (C Sensory nerves within the CHL, identified with protein gene product(PGP) 9.5 sensory neuronal marker. (D) CHL attachments. The CHL envelops the subscapularis and supraspinatus tendons, spans the rotator cuff interval and is closely related to the biceps tendon. The cable links the various components of the rotator cuff. (Permission sort from Di Giacomo et al, Atlas of functional shoulder anatomy. Milan: Springer-Verlag; 2008.33

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the rotator cable and the tuberosities of the proximal humerus (figure 6).22

Each rotator cuff muscle has a different intramuscular tendi-nous configuration.

Supraspinatus: bipennate muscle with single tendon.Subscapularis: multipennate with thick upper tendon.Infraspinatus: multipennate with oblique and transverse

heads.23

The rotator cuff is a network of interlacing tendons and ligamentous structures including CHL, glenohumeral liga-ments and the ‘rotator cuff cable’ (semicircular ligament of the humerus).24 25 At arthroscopy, the ‘cable’ is visualised as a

1 cm wide ligamentous ‘suspension bridge’, spanning and rein-forcing the deep surface of the cuff from subscapularis to teres minor.25–27 The ‘cable’ closely contains the head for stability and allows the individual cuff muscles to create different effects depending on the position of the humerus.28

Muscle synchronisation and coordination is important for shoulder function. This includes the lower limb, core, periscap-ular and rotator cuff muscles. EMG studies have demonstrated that with shoulder abduction, the supraspinatus activates first. However, prior to arm movement, the scapula is stabilised by the trapezius and deltoid.29 With abduction beyond 135°, the inferior trapezius rotates the scapula, boosting abduction,

Figure 4 Inferior and superior and labrum composite histology image of the glenoid labrum at 5 and 12 o’clock. (A) The inferior labrum is a fixed organ of compression. The convex bumper of the inferior labrum is mounted onto of the osseous glenoid. There is no defect between the glenoid, labrum and articular surface. (B) Superior labrum is a mobile organ of tension, attached off the glenoid face, with a synovial fined cleft between the labrum and the glenoid. It is a mobile organ of tension. The superior labrum is continuous with the static restraints (SGHL and MGHL), which become taut at the extremes of rotation. Biceps contraction pulls the mobile superior labrum onto the humeral head to increase containment and secondarily tightens the associated static restraints (SGHL and MGHL). The coracohumeral ligament (CHL) is closely associated with the biceps, SGHL and rotator cuff attachments to provide sensory feedback. (Copyright Dr Gregory Bain32). (C) The CHL drapes the rotator cuff, biceps tendon, superior labrum, SGHL and MGHL. This sensory organ is perfectly positioned to receive the feedback on the tension in the rotator cuff and the ligament, the position of the humeral head and if it is subluxating. This servomechanism can fine tune rotator cuff function for function and elite performance. MGHL, middle glenohumeral ligament; SGHL, superior glenohumeral ligament.

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while deltoid and supraspinatus stabilise the glenohumeral joint.

Composite activities (eg, throwing) involve a series of articula-tions from ground to hand. This coordinated sequence is a series of ‘levers on levers’, enabling the amazing velocity of the hand at the time of ball release.

How does the shoulder crane differ from a man-made crane?The man-made crane has wide outriggers, straight steel tower, with a mobile boom and a counter balance weight. It lifts objects, rotates the boom and lowers the object. The construct is stiff, rigid and cannot mobilise without deactivating the crane.

The shoulder crane is mobile, with a flexible ‘S’ shaped artic-ulated spinal tower stabilised by the core muscles. The clavicular boom, lifts and orientates the upper limb and hand to grasp and bring objects to the mouth.

A man-made servomechanism is an electronic device that operates via negative feedback, where the expected position is compared with the actual position of the mechanical system as measured by a transducer at the output. Any difference between the actual and desired values (an ‘error signal’) is amplified (and converted) and used to drive the system in the direction necessary to reduce or eliminate the error. The sensory CHL is well positioned to be the servomechanism to fine tune rotator cuff function, which is essential for glenohumeral stability and optimal performance (figure 4c). It seems no coincidence that sensory organ disorders include: dysfunctional coordination of multidirectional instability, rotator interval capsulitis30 and painful biceps pathology.

Figure 5 The deltoid muscle can be divided into seven segments. The anterior segments (A1, A2 and A3) converge and attach to the anterior insertion. The middle segment (M1) attaches to the middle insertion. The posterior segments (P1, P2 and P3) converge and attach to the posterior insertion (used with permission from Yoshimasa21; modified from Rispoli et al20).

Figure 6 Rotator cuff muscles and their intramuscular tendons. The supraspinatus tendon has a single tendon within a bipennate muscle. The subscapularis has four tendons that span the insertion. The infraspinatus has an oblique head that is an effective head depressor, and a transverse head that is an effective external rotator (images courtesy of Dr Afsana Hasan, Adelaide32).

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The viscoelastic soft tissues are flexible, supple and absorb forces. As the forces are concentrated to a few areas, it is not surprising that the athlete with bad technique, which overtrains to the point of fatigue, gets soft tissue injuries.

dISCuSSIonThe ‘shoulder crane’ framework consists of a base, axial skel-eton, clavicular boom, suspensory cascade, pulley, motor and arm. It highlights that the lower limbs and core muscles stabilise the articulated spinal tower, the periscapular muscles stabilise the scapular and the rotator cuff stabilises the humeral head. The shoulder crane positions the arm in space and then gracefully brings it back to the body. Then the elbow adjusts the reach, the forearm provides rotation and the wrist is a universal joint that allows the hand to have strength of grasp and precision of pinch in almost any orientation of motion.

This concept does not replace previous published concepts of shoulder function5 6 but provides a framework on which they can be applied. Athletes, trainers, therapists and clinicians may better understand the normal and pathological shoulder and assist in design of training programmes to optimise technique and perfor-mance. These concepts may have an even more important role in defining the effect of any deviation in technique, training and fatigue that will have a negative impact on performance and increase the risk of overuse injuries. It refocuses rehabilitation to the base, platform, core, periscapular and rotator cuff muscles.

The relationship of the dynamic biceps tendon, mobile supe-rior labrum/SGHL, cable and the sensory CHL is particularly fascinating. Future research into sensory feedback and motor coordination is required to better understand the crane model and to advance athletic performance, recovery and reduce overuse injuries.

Contributors GIB: co-editor in the initial book, development of concepts in this manuscript, writing, figure preparation, rewritting of manuscript. JP: assisted with development of concepts, writing of intimal chapter and manuscript, figure preparation, rewritting of manuscript. EI, HS and GDG: co-editor in the initial book, support and editing of text. JM: assistance in editing of text. DHS: assistance with concepts, editing and rewriting.

Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

Competing interests None declared.

Patient consent for publication Not required.

Provenance and peer review Commissioned; externally peer reviewed.

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