Shoulder, elbow, and epicondylitis: vector analysis of the upper limb

Shoulder pain, scapulohumeral impingement, and epicondylitis can derive from muscular imbalances identifiable through vector analysis. The shoulder is a system of six interconnected articular relationships in which the scapula is constantly adducted, the humerus is dominated by the internal rotators, and joint conflict is the predictable mechanical consequence of converging opposite forces. Complexity progressively decreases from the shoulder toward the hand.

The attached PDF document, available for free download, develops the complete vector analysis with images and bibliographic references.

Six articular relationships, one integrated system

The shoulder complex involves six articular relationships operating in an integrated way: scapulo-hyoid, scapulo-vertebral, scapulo-costal (muscular articulation), scapulo-humeral, sterno-clavicular, and scapulo-clavicular. Any alteration in muscular equilibrium in one of these relationships may simultaneously involve the others, giving rise to complex clinical patterns.

Scapular adduction as a constant condition

The scapular adductors — upper trapezius fibres, levator scapulae, rhomboids, middle and lower trapezius fibres, vertebro-humeral portions of the latissimus dorsi — dominate over the serratus anterior, the only scapular abductor with the arm alongside the body, which has a totally subdominant force line. Outside specific pathologies, the scapulae are always adducted, with consequent reduction of the physiological kyphosis at T5.

Scapular adduction is almost always associated with elevation — latissimus dorsi pattern B. Only in the rare pure pattern A does the scapula appear adducted and depressed, with a descending clavicle.

Anterior projection of the scapula

Parallel to adduction, the scapula can be projected anteriorly by the pectoralis minor — which approximates the coracoid process toward the ribs — and by the omohyoid. The term "anterior projection" clarifies that what is described is an anterior displacement of the scapula occurring while maintaining or increasing adduction, not replacing it. The common association between forward shoulder and scapular abduction is biomechanically inaccurate.

Dominance of the humeral internal rotators

Latissimus dorsi, pectoralis major, subscapularis, and teres major dominate over supraspinatus, infraspinatus, and teres minor. The result is internal rotation and anterior projection of the humeral head within the glenoid. If the brachial biceps is also considered, the overall dominance is in internal rotation, anterior projection, and superior projection of the humeral head.

The scapulohumeral conflict mechanism

Scapulohumeral conflict is the predictable mechanical consequence of simultaneous opposite movements: the scapula in posterior adduction and the humeral head in anterior projection. Superior projection of the humeral head — determined by the biceps, the horizontal fascicles of latissimus dorsi pattern B, and the deltoid — is the most frequent pattern. Inferior projection — from the iliac crest-to-humerus force line of latissimus dorsi pattern A — is rare.

Subluxations

Subluxation of the humeral head is determined by selective shortening of the subscapularis and teres major. Their shortening may also cause sternoclavicular joint subluxation with a cascade effect. Sternoclavicular subluxation may also be induced by shortening of the sternocleidomastoid.

Correction must occur simultaneously on all correlated elements. Even when only one subluxation is clinically present, treatment must include containment of all three potential subluxations to avoid de-latentisation of problems not yet manifested.

Elbow: flexor dominance and action reversal

The elbow flexors totally dominate over the triceps brachii. The flexors behave as flexors only when the hand is the mobile point. If the hand is the fixed point — as in a floor exercise — the flexors reverse their action and become extensors. The triceps participates as a secondary motor.

Elbow hyperextension, if not linked to ligamentous laxity, is due to the chain relationship between scapular adductors and anterior upper-limb muscles. The verification is simple: in the supine position, passive elevation of the arm produces mechanical scapular abduction; if the elbow flexes, this signals that lengthening of the scapular adductors translates into shortening of the flexors.

Valgus deviation is determined by shortening of the brachioradial muscles, forearm supinators. Varus deviation is rare. The most frequent picture is the tendency toward valgus with the forearm in semi-pronation.

Wrist and hand

The vector dominance at the wrist is in ulnar deviation, produced by shortening of the dorsal and palmar flexors. In the hand, dominance is in flexion. Complexity progressively decreases from the centre toward the periphery.

Physical foundations of the model.
This article applies the AIFIMM biomechanical model.
Its physical foundations are developed in three sequential articles, best read in order:
1. How muscle shortening generates joint conflict — why muscles shorten and the Resistant Force / Working Force model
2. Do antigravity muscles really oppose gravity? — how segmental malalignment raises Resistant Force
3. Why joint conflict develops: vector analysis of muscular forces — how the responsible forces are identified and predicted

This topic is part of the online course Systemic and Segmental MSK Biomechanics.