This informal CPD article ‘Biomechanical Analysis of the Shoulder — Part 2: Scapulohumeral Impingement, Subluxations and Therapeutic Implications’, 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 second part of a two-part contribution dedicated to the biomechanical analysis of the shoulder complex. In Part 1 [5], the vectorial dominances governing scapular and humeral positioning were analysed: the intrinsic asymmetry between scapular adductors and serratus anterior, the two patterns of the latissimus dorsi (Pattern A and Pattern B), and the dominance of humeral internal rotators over external rotators. This second part applies those principles to explain the mechanism of scapulohumeral impingement, examines subluxations of the shoulder complex, and discusses the therapeutic implications of this analysis.
The theoretical framework underpinning this analysis was established in the earlier contributions of this series: the physical model of muscle shortening and the concepts of Resistant Force and Work Force [1,9], vector analysis and the concepts of vectorial dominance and subdominance [2,6], and the analysis of the latissimus dorsi patterns in the frontal plane [3,4].
4. The Mechanism of Scapulohumeral Impingement
Scapulohumeral impingement is not a random event: it expresses the predictable mechanical consequence of opposing forces that converge according to measurable physical principles [2,7,8,16].
The mechanism is as follows. Simultaneously, the scapular adductors (trapezius, rhomboids, levator scapulae) approximate the scapula to the vertebral column, displacing it posteriorly [2,8,11]. At the same time, the humeral internal rotators (latissimus dorsi, pectoralis major, subscapularis, teres major) and the biceps brachii internally rotate and project the humeral head anteriorly within the glenoid cavity [2,7,8,16].
A simultaneous opposing movement is thus determined: the scapula displaces posteriorly whilst the humeral head displaces anteriorly [2,7,8]. This convergent movement accelerates the potential anterior impingement between scapula and humerus, reducing the subacromial space and determining compression of the interposed structures (supraspinatus tendon, subacromial bursa) [16,17,30,31].
Impingement is further amplified by the concomitant scapular adduction which, by approximating the scapula to the vertebral column and reducing physiological kyphosis at the T5 apex, modifies the orientation of the glenoid and further reduces the space available for the humeral head [2,8,16,17].
As described in the first article of the series [1], the progressive shortening of connective components of dominant muscles, as a function of the force × time product, produces an increase in Resistant Force at the expense of Work Force [1,9]. Applied to the shoulder, this principle explains why scapulohumeral impingement tends to become progressively established and chronic: the connective components of the internal rotator and scapular adductor muscles develop residual shortenings that maintain the impingement configuration even in the absence of active contraction [1,2,26].
4.1 Revision of the Concept of External Rotator Weakness
Vector analysis leads to a critical revision of the concept of external rotator "weakness" as a primary cause of scapulohumeral impingement [2,6,20].
In vectorial logic, in the absence of peripheral neurological pathologies, subdominant actions are not impeded by the "weakness" of agonist muscles but by excess tension in dominant antagonists [2,6]. In the case of the shoulder, it is not the humeral external rotators that are in contractile hypocapacity, but the internal rotators in excess tension (due to shortening of connective components) that impede external rotatory action [1,2,16].
It follows that strengthening of external rotators applied before vectorial rebalancing risks consolidating the compensatory pattern rather than correcting it [2,6]. The logical therapeutic sequence involves: first, reduction of Resistant Force in shortened dominant muscles (through prolonged stretching techniques that act on connective components), then strengthening to consolidate the correction obtained [1,2,6,28].
5. Subluxations of the Shoulder Complex
Further difficulties may be induced by the presence of subluxation of the sternoclavicular articulation and of the humeral head [7,8]. Subluxations are not merely an associated finding but may constitute a cause of shoulder symptomatology [7,8,21].
5.1 Subluxation of the Humeral Head
Subluxation of the humeral head is determined by selective shortening of the subscapularis and teres major [1,7,8]. Their shortening may also cause subluxation of the sternoclavicular articulation, through a cascade effect [7,8]. In this case, sternoclavicular subluxation is determined, as the final effector, by disruption of the horizontal fascia passing through the manubrium of the sternum, sustained by the subscapularis and teres major dysfunction [7,8].
5.2 Subluxation of the Sternoclavicular Articulation
Sternoclavicular subluxation may also be induced by shortening of the sternocleidomastoid [7,8,22], independently of the subscapularis and teres major dysfunction. In this case, differential evaluation involves rotation or inclination of the cranium to passively lengthen the sternocleidomastoid: if shortened, the subluxation worsens [7].
5.3 Clinical Assessment of Subluxations
Humeral subluxation is detected by bringing the upper limb into abduction so as to place subscapularis and teres major under tension [7]. Abduction simultaneously reveals the effect of shortening of these two muscles on three skeletal elements: subluxation of the humeral head, subluxation of the scapula and subluxation of the sternoclavicular joint [7,8].
A fundamental principle emerges from assessment: manually correcting the position of a single element worsens the subluxations of the other elements [7]. Containing two subluxations causes the third to worsen further. From this it is understood that correction must occur simultaneously on all three correlated elements [7,8].
Even when only one of the subluxations is present, treatment must include containment of all three potential subluxations, to prevent problems not yet manifest from becoming unmasked [7].
For assessment of sternoclavicular subluxation, the test is performed by exerting pressure on the articulation [7,8]. Physiological articular play should be of only a few millimetres. Excessive excursion indicates subluxation [7].
5.4 Systemic Implications of Subluxations
When present, subluxations activate the extrapyramidal system which intervenes for their protection, at the cost of destabilising skeletal regions even at a distance, where symptoms may manifest [7,8,18]. This mechanism explains why symptomatology apparently distant from the shoulder may originate from the presence of subluxations of the scapulohumeral and sternoclavicular complex [7,8].
Muscles that elevate and adduct the scapula
6. Synthesis of Acting Forces and Understanding of Shoulder Pathologies
Analysis of the acting forces clarifies the difficulties encountered in framing shoulder problems, particularly those unrelated to traumatic events [2,7,8,16].
The forces acting simultaneously on the shoulder complex may be synthesised as follows [2,7,8]. The muscles that elevate and adduct the scapula (upper trapezius, levator scapulae, rhomboids, middle and lower fibres of trapezius) displace the scapula posteromedially, approximating it to the vertebral column [2,8,11]. The pectoralis minor and omohyoid project the scapula anteriorly whilst maintaining adduction [7,8,15]. The latissimus dorsi, pectoralis major, subscapularis and teres major internally rotate and project the humeral head anteriorly within the glenoid [2,8,16]. The biceps brachii contributes to anterior and superior projection of the humeral head [7,8,19]. The specific shoulder configuration further depends on which of the two latissimus dorsi patterns (A or B) is prevalent [3,4].
The complexity of interactions between these forces explains why shoulder problems present with variable clinical pictures [7,8]. Each patient expresses a specific combination of vectorial dominances and selective shortenings that determines an individual skeletal configuration [1,2]. Therapeutic intervention must therefore begin with analysis of vectorial dominances in the shoulder complex, identifying which muscles, through shortening of their connective components, are primarily responsible for the impingement configuration [1,2,6].
Conclusions
Biomechanical analysis of the shoulder reveals a hierarchically organised system, where six integrated articular relationships are simultaneously influenced by asymmetric muscular dominances [2,7,8]. Scapular adduction represents the constant condition, determined by the vectorial superiority of adductor muscles over serratus anterior [2,8]. Anterior scapular projection, when present, is added to adduction without replacing it, through the action of pectoralis minor and omohyoid [7,8,15].
The specific configuration of scapular positioning depends on which of the two latissimus dorsi patterns is prevalent: in Pattern A [3], which is rare, the scapula is adducted and descending; in Pattern B [4], which is more frequent, the scapula is adducted and elevated, with an ascending clavicle and ipsilateral thoracic convexity. This distinction has direct implications for humeral head projection, which is predominantly superior in Pattern B and predominantly inferior in Pattern A [3,4,7].
The dominance of humeral internal rotators at the scapulohumeral articulation is an intrinsic anatomical asymmetry that, when not balanced by neural control or when altered by shortening of connective components, determines predictable patterns of internal rotation and anterior projection of the humeral head [1,2,8,16].
Scapulohumeral impingement expresses the mechanical consequence of the simultaneous opposing movement between scapula (in posterior adduction) and humeral head (in anterior projection), according to measurable physical principles [2,7,8,16,17]. Subluxations, when present, constitute further mechanical alterations that the system must compensate, with possible consequences at a distance [7,8,21].
The therapeutic approach is founded upon understanding these mechanisms [1,2,6]. As described in previous contributions, articular rebalancing requires a logical sequence: first, reduction of Resistant Force in shortened dominant muscles, then strengthening to consolidate the correction obtained [1,2,6]. Strengthening applied before vectorial rebalancing risks consolidating compensatory patterns rather than correcting them [2,6].
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References
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