This informal CPD article ‘Biomechanical Analysis of the Shoulder — Part 1: Vectorial Dominances in Scapular and Humeral Positioning’, was provided by Dr. Mauro Lastrico, Physiotherapist at AIFiMM Formazione, an organisation recognised by the Italian Ministry of Health as an authorised CME provider. They offer organised training courses in the Mézières Method, a rehabilitative and postural approach.
This article is the first part of a two-part contribution dedicated to the biomechanical analysis of the shoulder complex. It is part of a series of contributions applying the principles of physics to the musculoskeletal system. The first article, Clinical Assessment of Muscle Shortening [1], introduced the physical model of muscle shortening as viscoelastic deformation of connective components, distinguishing the mechanical behaviour of the contractile component from that of the connective component and defining the concepts of Resistant Force and Work Force [1,5].
The contribution Vector Analysis in Musculoskeletal Biomechanics [2] completed the theoretical framework by introducing vector analysis as a tool for identifying the muscular causes of alterations in physiological joint sequence, defining the concepts of vectorial dominance and subdominance and the dynamic equilibrium model [2,6].
The contributions dedicated to the analysis of the vertebral column in the frontal plane applied these principles to the interpretation of lateral deviations and vertebral rotations. The first part [3] introduced the interpretive principles of frontal plane analysis and described Pattern A of the latissimus dorsi, characterised by the predominance of the line of force from the iliac crest to the humerus, in which the principal mechanical resultant is the approximation between the ipsilateral scapula and hemipelvis, with the scapula adducted and descending and the clavicle descending [3]. The second part [4] described Pattern B of the latissimus dorsi, more frequently observed in clinical practice, characterised by the combined action of the upper fibres of trapezius, levator scapulae, rhomboids and upper portions of latissimus dorsi, with a resultant of scapular elevation and adduction, clavicular elevation and ipsilateral lateral thoracic convexity [4].
The present work applies these principles to the shoulder complex, demonstrating how muscular vectorial dominances determine predictable skeletal configurations and how scapulohumeral impingement, subluxations and painful shoulder syndromes can be interpreted as mechanical consequences of imbalances between asymmetrically distributed muscular forces [2,3,4,8]. This first part addresses the vectorial dominances that govern scapular and humeral positioning. The second part will examine the mechanism of scapulohumeral impingement, subluxations and their therapeutic implications.
1. The Shoulder Complex: Six Integrated Articular Relationships
The shoulder region represents one of the most complex areas of the entire musculoskeletal system [7,8,9]. This complexity derives from the multiplicity of joints and relationships that simultaneously contribute to the function and pathology of this district.
The shoulder complex involves six different articular relationships that operate in an integrated manner [7,8]: the scapulo-hyoid relationship, the scapulo-vertebral relationship, the scapulo-costal articulation (muscular in nature), the scapulohumeral articulation, the sternoclavicular articulation and the acromioclavicular articulation [7,8,9].
Any alteration of muscular equilibrium in one of these relationships may simultaneously involve the others, determining complex clinical pictures [2,7,8]. Analysis of this district therefore requires evaluation of which combination of articular relationships is involved in the specific clinical picture, identifying which muscular forces, through their selective shortenings, determine the skeletal configuration responsible for the symptom or impingement present [1,2].
Scapula rests upon the thoracic cage
2. Scapular Positioning: Vectorial Dominances and Adduction
2.1 Physiological Positioning
The scapula rests upon the thoracic cage and under physiological conditions is positioned at the side of the thorax, with the medial border aligned with the spinous process of T5 [7,10]. In the cranio-caudal direction, it is maintained by the antagonistic action between the muscles that elevate it and those that depress it [7,8,10].
2.2 Vector Analysis of Scapular Positioning
Analysis of the forces acting on the scapula reveals a clear asymmetry in muscular dominances [2,8]. The scapular elevator and adductor muscles comprise: the trapezius (upper fibres), with a line of force directed from the cervical column and occiput towards the acromion, which elevates and adduces the scapula; the levator scapulae, with a line of force directed from the cervical transverse processes towards the superomedial angle of the scapula, which elevates and adduces; the rhomboids, with oblique lines of force directed from the spinous processes of C7–T5 towards the medial border of the scapula, which adduct and elevate; the middle and lower fibres of trapezius, with oblique lines of force directed from the dorsal spinous processes towards the spine of the scapula and acromion, which adduct [2,8,11].
The only scapular abductor is the serratus anterior, which however possesses inferior vectorial potential relative to the sum of the adductor vectors [2,8,12]. As demonstrated through the parallelogram rule, the vector potentially expressible by the associated forces of the rhomboids and middle and lower fibres of trapezius is more than twice as long as the vector potentially expressible by serratus anterior [2]. This means that to equilibrate an adductor force on the scapula, serratus anterior must employ a traction force of more than double [2,12].
2.3 The Role of the Latissimus Dorsi in Scapular Positioning
The latissimus dorsi participates in the scapular positioning system with multiple lines of force [2,3,4,13]: from the iliac crest to the humerus, from the iliac crest to the lumbar vertebrae, from the lower thoracic vertebrae to the humerus, and from the last four ribs to the humerus. The predominance of one or more of these lines of force, in relation to selective shortening of the muscular components involved, enables the latissimus dorsi to produce different biomechanical effects both at the vertebral level and in the relationships between thorax, pelvis and upper limb [3,4,13].
Vector analysis of the latissimus dorsi has enabled identification of two principal patterns [3,4].
In Pattern A [3], the configuration of the vertebral column is dominated by the principal line of force of the latissimus dorsi from the iliac crest to the humerus. The predominant mechanical resultant is represented by approximation between the ipsilateral scapula and hemipelvis, with consequent ipsilateral vertebral concavity. In this configuration the scapula presents as adducted and descending, and the clavicle, rather than following a horizontal course, also presents as descending, with the acromioclavicular joint below the sternoclavicular joint [3]. This is a rare pattern in clinical practice [3,4].
In Pattern B [4], significantly more frequent, the configuration is characterised by the combined action of the upper fibres of trapezius, levator scapulae, rhomboids and upper portions of the latissimus dorsi. The overall resultant determines: elevation and adduction of the scapula, elevation of the clavicle, ipsilateral lateral thoracic convexity and elevation of the hemipelvis [4]. This pattern involves multiple lines of force distributed across different skeletal segments, rendering it biomechanically more stable and clinically more frequent than Pattern A [3,4].
The overall resultant of muscular forces acting on the scapula is therefore, in the vast majority of cases, in adduction and elevation (Pattern B) [4,8,11]. The element that is always present, outside specific pathologies related to congenital or acquired skeletal malformations, is scapular adduction, with consequent reduction of physiological kyphosis at the T5 apex [2,8].
2.4 Anterior Projection of the Scapula
In this context the term "anterior projection" is used instead of the traditional "shoulder anteposition" [2,7]. This terminological choice is not arbitrary but derives from a precise need for biomechanical clarity. In common usage, a misconception has become established that associates shoulder anteposition with scapular abduction. Vector analysis demonstrates instead that the scapula moves anteriorly in association with adduction [2]. Using "anterior projection" clarifies that we are describing an anterior displacement of the scapula that occurs whilst maintaining or increasing adduction.
In parallel with adduction, the scapula may be anteriorised through the action of two specific muscles [7,8].
The pectoralis minor, with its insertion between the coracoid process of the scapula and the ribs, determines anterior projection of the shoulder through approximation of the coracoid to the ribs [7,8,14]. As a consequence, in upright stance, the inferior angle of the scapula becomes prominent. Anterior projection determined by pectoralis minor occurs in association with scapular adduction, not as a substitute for it [7,8].
The omohyoid presents a particular structure, with an interposition of connective tissue between the muscle fibres [7,15]. The portion of omohyoid between the scapula and connective tissue projects the scapula forwards. Anterior projection from the omohyoid also associates with scapular adduction, determining a configuration in which the scapula is both approximated to the vertebral column and displaced anteriorly [7,15].
Humeral internal rotator muscles
3. Scapulohumeral Relationship: Rotatory Dominances and Humeral Head Projection
3.1 Muscular Control of Humeral Positioning
The position of the humerus within the glenoid cavity and its movements are under the control of the scapulohumeral muscles, brachio-scapular muscles, costo-brachial muscles and the latissimus dorsi [7,8,9]. These muscles are asymmetrically distributed and possess different vectorial potential, giving rise to predictable dominances [2,8].
3.2 Dominance of Humeral Internal Rotators
Analysis of rotatory forces on the humerus reveals a clear asymmetry [2,8,16].
The humeral internal rotator muscles comprise: the latissimus dorsi, pectoralis major, subscapularis and teres major. These are numerically superior muscles, with larger muscle masses and longer, more oblique vectors [2,8,16,24]. The external rotator muscles comprise: the supraspinatus (with limited rotatory component), infraspinatus and teres minor. These are numerically inferior muscles, with smaller masses and shorter vectors [8,16,17].
Vectorial dominance is entirely in favour of the internal rotators [2,8,16]. This intrinsic asymmetry explains why humeral internal rotation with anteriorisation of the humeral head is one of the most frequent patterns in subacromial impingement syndromes [16,17]. As described in the contribution on vector analysis [2], anatomical vectorial dominances become particularly evident in neurological conditions such as spastic paresis, where the loss of supraspinal inhibitory control allows intrinsic dominances to manifest fully [2,18].
3.3 Anterior Projection of the Humeral Head
If, in addition to the humeral internal rotators, the biceps brachii is taken into consideration, an overall dominance in internal rotation and anterior projection of the humeral head within the glenoid results [2,7,8]. The biceps, through its proximal insertion on the supraglenoid tuberosity and the coracoid process (long head and short head), contributes to anterior and superior displacement of the humeral head [7,8,19].
3.4 Superior and Inferior Projection of the Humeral Head
In addition to anterior displacement and internal rotation, the humeral head may move towards superior or inferior impingement relative to the glenoid [7,8,16].
The muscles that determine superior projection of the humeral head are: the biceps brachii, the horizontal fibres of the latissimus dorsi in Pattern B [4] and the deltoid [2,7,8]. The muscles that determine inferior projection are: the long head of triceps and the inferior components of the rotator cuff [7,8]. Clinical experience and radiographic findings confirm that superior projection of the humeral head is significantly more frequent [7,8,16], consistent with the overall vectorial dominance in elevation and with the greater clinical frequency of Pattern B compared with Pattern A [3,4].
In the rare Pattern A [3], where the line of force of the latissimus dorsi from the iliac crest to the humerus predominates, the resultant is in depression and internal rotation of the humeral head, with a descending scapula. In this case inferior projection of the humeral head prevails [3,7].
In the distal portion of the humerus, the overall resultants of the acting muscles produce: internal rotation, posterior flexion and adduction [2,7,8]. This explains the typical clinical configurations observable in upright stance, where the humerus presents internally rotated, with the elbow slightly flexed and adducted to the trunk [7,8,20].
The second part of this contribution will demonstrate how these vectorial dominances converge to produce the mechanism of scapulohumeral impingement, and will examine the role of subluxations and the therapeutic implications of this analysis.
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References
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