The Deltoid, also know as the shoulder muscle, is a thick, triangular muscle that plays a major role in the function of The Shoulder Girdle . Given its wide-spanning, multi-planar origin, the Deltoid is typically divided into three segments (anterior, middle and posterior fibres) based on the plane in which they act; however, the Central Nervous System has been shown to independently control at least seven distinct sections of the Deltoid (SOURCE-18).
The triangular shape of the Deltoid is caused by its very broad origin and narrow base. It’s origin is typically divided into three parts, despite the muscle branching into seven distinct intramuscular tendons (SOURCE-2+18):
Clavicular (Anterior) Portion- originates from the superior surface and anterior border of the lateral 1/3rd of the Clavicle
Acromial (Middle or Lateral) portion- originates from the lateral margin and superior surface of the Acromion of the Scapula . The multipennate architecture of this portion branches into four intramuscular tendons
Scapular Spinal (Posterior) Portion- originates from the lateral 1/3rd of the Spine of the Scapula . This portion branches into two distinct intramuscular tendons
As the fibres descend the lateral Humerus they converge into a short but substantial tendon which attaches to theDeltoid Tuberosity, with anterior and posterior fibres adhering directly to this tendon (SOURCE-2+19). Fibres of this wide attachment on the lateral aspect of Humerus and the investing fascia are continuous with the Lateral Intermuscular Septum posteriorly and Brachialis and Brachial fascia anteriorly (SOURCE-19).
The Deltoid receives nervous innervation from the Axillary Nerve from C5, C6 Nerve Root s.
The Muscle Fibre Type composition of the Deltoid is described as relatively slow-twitch with an even proportion of Type I (~33%), Type IIa (~33%) and Type IIb (~33%) fibres (SOURCE-25). These proportions are highly variable, depending on factors such as exercise, age and gender (SOURCE-25). Over the long-term, endurance based training has been shown to induce a shift towards slow-twitch (Type-I) fibres in the Deltoid specifically (SOURCE-26). Conversely long-term strength / power based training induced hypertrophy of the Deltoid’s fast-twitch (Type II) fibres without a notable shift in the proportions of fibre types (SOURCE-26).
The Muscle Architecture of the Deltoid is generally described by its three parts, with the Anterior and Posterior Deltoid most similar. When compared to the Middle Deltoid, the Anterior and Posterior fibres are considerably longer and have no pennation angle while the middle fibres have a 31º angulation (SOURCE-12). Despite the muscle length of the individual segments being relatively similar, the Middle fibres have a greater mass which affords them a significantly greaterPhysiologic Cross-Sectional Area (PCSA), one of the largest of any muscle in the neighbouring area (SOURCE-12). The peculiar architecture of the Middle fibres suggests several key functions including (i) an efficient contraction as less motion or fibre excursion is required (ii) power/ stability, particularly in later GH Joint - Abduction given its substantial PCSA and (iii) fine motor control of The Shoulder Girdle as the Deltoid has multiple compartments with intricate fibre arrangements (SOURCE-12). While certain metrics are heavily contingent on the population evaluated, the following has been reported (anterior/middle/posterior fibres):5 cadavers, average age of ~73 years (SOURCE-12):
PCSA - 2.54 ± 0.59cm²/ 11.18 ± 1.57cm²/ 2.73 ± 0.12cm²
Mass - 40 ± 12g/ 67 ± 12g/ 51 ± 3g
Muscle Length - 153 ± 7mm
Muscle Fibre Length - 144 ± 8mm/ 55 ± 9mm/ 178 ± 6mm
Pennation Angle - 0º/31º/0º
MRI of 10 subjects (5 female, 5 male) 24-37 years, 158-188cm tall and a bodyweight of 50-86kg (SOURCE-13):
Collective PCSA - 25 ± 8.7 cm²
Muscle Length - 18.1 ± 1.8 cm²
As touched on above, the function of the Deltoid is heavily dependent on the direction of the fibres and the position of The Shoulder Girdle ; however they can all be described to move the Humerus in relation to the Scapula . The anterior fibres facilitate GH Joint - Flexion and Horizontal GH Joint - Adduction and assist with GH Joint - Internal Rotation (SOURCE-8+11). In contrast, the Posterior fibres help produce GH Joint - Extension and GH Joint - External Rotation . The Middle fibres are the primary muscle responsible for GH Joint - Abduction , which is the movement that occurs when all fibres of the Deltoid contract (SOURCE-8). The muscles large size and ability to be its own antagonist implicates it as also an important dynamic stabiliser of the Glenohumeral Joint , particularly against Inferior Glide on Humerus .
It is often reported that in the healthy shoulder the Supraspinatus facilities the initial 15º of Abduction, after which it assists the Deltoid until roughly 90º of Abduction (SOURCE-6). One supporting factor for this is that the Moment Arms of the Anterior and Middle fibres of the Deltoid are smaller than those of the Supraspinatus , Infraspinatus and Subscapularis at low Abduction angles (0-40°). However by 60° these Moment Arms have more than doubled which is reflected in a peak activity of the Anterior + Middle fibres occurring between 60º-90º (SOURCE-5). The Moment Arm of the Anterior Fibres is further influenced by the extent of Humeral Rotation where added GH Joint - External Rotation can produce a favourable mechanical advantage even at 0º Abduction (SOURCE-5). Conversely, Abduction performed with GH Joint - Internal Rotation diminishes this advantage (SOURCE-5).The notion that the Supraspinatus initiates Abduction has been contended by recent literature which recorded near simultaneous pre-activity of both the Middle Deltoid and Supraspinatus prior to shoulder elevation, irrespective of load or plane (SOURCE-7). In either case, the contribution of these muscles are in close proximity and when one is impaired, the other may compensate (SOURCE-6+5). In terms of the total Torque contribution to Abduction, the Middle fibres contribute ~ 35-65%, the Anterior fibres ~ 2% (although this varies with rotation) and the remaining percentage is accounted for by the Rotator Cuff (SOURCE-5)
Activity of the Anterior fibres also increases with verticality of the upper body, making Vertical Push exercises more favourable than Horizontal Push exercises (SOURCE-5). During more vertical exercises a wider hand grip will also better recruit the Anterior fibres while during more horizontal exercises this same effect is induced with a closer grip (SOURCE-5).
The Posterior Deltoid fibres display a seemingly opposing role where they do not contribute to Abduction in the Scapular plane between 0-90º (SOURCE-5). Instead, these fibres have an GH Joint - Adduction Moment Arm in the Scapula plane which diminishes with increased Abduction (SOURCE-5). At higher Abduction ranges (~ 110º) the orientation of these fibres no longer contribute to GH Joint - Adduction and become more favourable to Abduction (SOURCE-5). Higher range Scaption with added GH Joint - Internal Rotation markedly increases activity of the Posterior fibres (SOURCE-5). Conversely, during Horizontal Pull movements Humeral Rotation does not appear to have a significant influence on activity of these Posterior fibres (SOURCE-5).
The predominate pathomechanic associated with the Deltoid relates to Subacromial Impingement . In the healthy shoulder, muscles such as the Latissimus Dorsi and Rotator Cuff (specifically Infraspinatus , Teres Minor and Subscapularis ) afford a inferior translatory bias on the Head of Humerus which offsets the superior bias of the Deltoid in order to maintain adequate space within the Suprahumeral Joint (subacromial space) (SOURCE-4+5). Failure of these inferior pulling muscles predisposes an overbearing pull from the Deltoid, migration of Humeral Head superiorly and consequent impingement against the undersurface of the Acromion during movements such as arm elevation (SOURCE-5+7+15+20). The Deltoids superior pull is most notable at the initiation of elevation (SOURCE-15+20). Left unchecked, continued impingement may compromise neighbouring soft-tissues such as the Rotator Cuff , Glenoid Labrum or Long Head of Biceps which then predisposes Glenohumeral Instability .
Given its large encapsulating structure around The Shoulder Girdle , the Deltoid is a major stabilising structure. During certain shoulder movements, the Deltoid serves as its own antagonist. GH Joint - Internal Rotation is partly facilitated by the concentric contraction of the Anterior Deltoid, while simultaneously the Posterior Deltoid is eccentrically loaded (SOURCE-8). For GH Joint - External Rotation , these roles are reversed (SOURCE-8). The coordinated nature of these eccentric and concentric contractions facilitate smooth, quality motion (SOURCE-8). Insufficient Deltoid action can therefore limit the shoulder’s capacity and quality of rotation.
While in the context of overdominance the superior translatory bias of the Deltoid on the Head of Humerus is considered pathological, in healthy individuals this same function forms the chief dynamic resistance to excessive inferior translation (SOURCE-4+15). Inferior stability is facilitated predominately by the Deltoid’s Middle and Posterior fibres, while the Anterior fibres serve as a posterior stabiliser (SOURCE-15). The Deltoid’s role as an inferior stabiliser is most notable at low shoulder elevation ranges (SOURCE-4+15). Insufficiency of the Deltoid may compromise stability of Glenohumeral Joint , particularly in an inferior direction.
While Axillary Nerve lesions are rare, the are one of the most commonly injured nerves in The Shoulder Girdle (SOURCE-2+8+21). Given the Axillary Nerve affords the Deltoid both motor and sensory innervation, nerve dysfunction may manifest as Deltoid weakness, altered sensation or atrophy in chronic instances (SOURCE-8). While GH Joint - Abduction is most likely affected, full range of motion may still be possible through compensation of the Supraspinatus (SOURCE-2). Axillary Nerve injury is most often attributed to over-stretching during Anterior Glenohumeral Dislocations and frequently occur concomitantly with fractures of the Surgical Neck of Humerus (SOURCe-8+21). The precise site at which the Axillary Nerve is compromised may be indicated by the presentation of symptoms.
Quadrilateral Space Syndrome (QSS)- the narrow Quadrilateral Space forms a passage for the Axillary Nerve and Posterior Humeral Circumflex Artery which may be subject to compression when this space is reduced (SOURCE-22+23). Both neurovascular structures bifurcate near this space into anterior and posterior branches, for the Axillary Nerve this division happens within the space in 88% of specimens (SOURCE-23). As multiple structures are therefore potentially implicated, this syndrome has distinct clinical manifestations based on the neurovascular structure(s) that has been compromised:
Neurologic QSS - compression of the Axillary Nerve or one of its major branches results in generalised pain within The Shoulder Girdle that may be felt more posteriorly and altered sensation of its cutanous branches which supply the skin over the distal two-thirds of the posterior deltoid (SOURCE-22+23). Compression of the posterior branch may compromise motor function of the Teres Minor and Posterior Deltoid , while compression of the anterior branch may compromise the Anterior and Middle Deltoid (SOURCE-24)
Vascular QSS - compression of the Posterior Humeral Circumflex Artery is likely to result in signs of acute ischemia including Pain , pallor (pale skin) or absent pulses, thrombosis or embolism (appearing bluish/ purple due to a insufficient oxygen) of The Hand and Fingers (SOURCE-22+23+24)
Anything that encroaches on the Quadrilateral Space has the capacity to compress the neurovascular structures, most commonly this is attributed to thick fibrous bands that extend from the Long Head of Triceps Fascia to the Teres Minor (SOURCE-23). These bands are pulled taut during movements such as GH Joint - Abduction , GH Joint - External Rotation and GH Joint - Internal Rotation , making these motions provocative (SOURCE-23). Other known causes include Muscle hypertrophy with repetitive overhead movements or space occupying lesions such as paralabral cysts, bony Fracture fragments and benign tumours (SOURCE-23).
In terms of Fascia l connections, the Trapezius has been included in the following:
Fascial Lines - The Deltoid to Lateral Intermuscular Septum connection defines the Superficial Back Arm Line . There are however, fibres that are extended from the muscle to the local Brachialis which creates a crossover between the SBAL and the Deep Front Arm Line .
Myofascial Chains - the lateral fibres of the Deltoid are included in the Lateral Arm Chain (SOURCE-1)
A restriction at any point along the line/ chain could result in Pain or dysfunction of the Deltoid.
The accumulation of myofascial trigger points or Inflammation with the Deltoid may result in Referred Pain . While this Pain is typically most pronounced local to the muscle, it may refer down the lateral upper arm (SOURCE-14).
Deltoid dysfunction may be a predisposing factor to the following pathologies or vice versa:
Glenohumeral Instability - the Deltoid is a major dynamic stabiliser of the Glenohumeral Joint , affording the most significant resistance to inferior translation of the Head of Humerus (SOURCE-4+15).
Subacromial Impingement - as detailed underPathomechanics, a lack of inferior translation on the Head of Humerus via insufficiency of muscles such as the Latissimus Dorsi , Subscapularis , Teres Minor or Infraspinatus results in an overpowering superior translation from the Deltoid. This directly reduces the subacromial space, leaving the joint vulnerable to impingement.
Scapular Dyskinesis - restriction in the posterior shoulder (which includes posterior fibres of the Deltoid) is associated with forward positioning of the Scapula (excess Scapulothoracic Joint - Protraction and Anterior Scapular Tilting ) (SOURCE-3). Through arm elevation, this may limit the necessary Scapulothoracic Joint - Retraction and Posterior Tilting. Additionally, posterior shoulder tightness may restrict Horizontal GH Joint - Adduction (SOURCE-3).
Axillary Nerve Compression - as described underPathomechanics.
Flattened Deltoid - a loss of muscles typical rounded appearance may indicate paralysis or Anterior Glenohumeral Dislocation (SOURCE-8)
Swallow Tail Sign- a loss or weakness during GH Joint - Extension compared to the asymptomatic side indicates perturbed functionality of the Posterior Deltoid, either due to weakness or Axillary Nerve palsy (SOURCE-8)
Manual Muscle Testing of the Deltoid is performed differently for each segment.
Anterior Deltoid - the Anterior portion of the Deltoid can be assessed for relative strength seated against gravity or side-lying without gravity. Compensation from the Coracobrachialis , Middle Deltoid, Clavicular fibres of the Pectoralis Major , Serratus Anterior , Biceps Brachii and Upper and Lower Trapezius are common:
Seated - arm in 90º GH Joint - Flexion and slight GH Joint - Adduction and GH Joint - Internal Rotation . The patient then attempts to raise their arm above their head against the practitioners resistance
Side-Lying - patient lies on the contralateral side with ipsilateral arm in slight GH Joint - Abduction and supported by the therapist who passively adds 90º GH Joint - Flexion .The patient then attempts to flex their shoulder further against the resistance of the therapist
Middle Deltoid - middle fibres of the Deltoid can be assessed for relative strength seated against gravity or lying supine without gravity. Compensation from the Upper Trapezius , Long Head of Biceps or lateral flexion of the Trunk is common:
Seated - with arm straightened and in >90º GH Joint - Abduction . The patient attempts to Abduct their arm further against the resistance of the therapist. This test can also be repeated in the Scapula plane
Supine - patient lies supine with arm neutral and straightened. The patient Abducts their arm as high as 90º against the resistance of the practitioner
Posterior Deltoid - posterior fibres of the Deltoid can be assessed for relative strength lying prone against gravity or seated without gravity. Compensation from the Infraspinatus and Teres Minor is common:
Prone - patient lies prone with arm in ~75º GH Joint - Abduction and 90º Elbow - Flexion so that the forearm hangs freely over the edge of the table. The patient attempts to further Horizontally Abduct their arm against the resistance of the practitioner by raising their Elbow towards the ceiling
Seated - patient is seated with arm in ~75º GH Joint - Abduction . The patient attempts to Horizontally Abduct their arm by directing their Elbow posteriorly. Compensation from the Rhomboids and Middle Trapezius are common
As the most superficial muscle over The Shoulder Girdle , the Deltoid is easily palpable. From muscles origins off the lateral Clavicle , Spine of Scapula and Acromion, fibres may be palpated along their entire length as they converge inferiorly towards theDeltoid Tuberosity, roughly half way down the lateral shaft of Humerus .
The following details treatment techniques relevant to the distinct portions of the Deltoid.
The following Stretching techniques can be used to restore length in the following portions of the Deltoid:
Anterior Fibres
Door Frame Shoulder Stretch - self-guided anterior shoulder stretch with large Horizontal GH Joint - Abduction range
Shoulder Dislocates - mobility exercise that emphasises the greatest circumduction range possible
Split Stance Biceps Stretch ( Thumb up variation) - self-guided anterior shoulder stretch with large GH Joint - Extension range
Dowel External Rotation Stretch - self-guided GH Joint - External Rotation stretch with overpressure
Middle Fibres
GH Joint - Adduction across body
Posterior Fibres
Genie Stretch - a position of maximal Horizontal GH Joint - Adduction at 90º GH Joint - Flexion was shown to maximally elongate these fibres (SOURCE-9)
Bent Over Lat Stretch (horizontal adduction variation)
As a prime mover of The Shoulder Girdle with anterior, lateral and posterior fibres, the Deltoid is involved in many Push and Pull exercises. Significantly greater activity of the Anterior and Middle Deltoid fibres occurs when performing free weight Horizontal Push exercises when compared to their machine equivalent (SOURCE-5).Early Phase:
Rotator Cuff Pendulums - oscillatory motion that afford a distracting glide to the Humeral Head
GH Joint - Flexion >120º with GH Joint - External Rotation - great recruitment of the Anterior (69±24%) and Middle (73±16%) fibres in terms of Maximum Voluntary Isometric Contraction and moderate recruitment of the Posterior fibres
GH Joint - Abduction >120º with GH Joint - External Rotation - great recruitment of the Anterior (62±28%) and Middle (64±13%) fibres in terms of Maximum Voluntary Isometric Contraction and moderate recruitment of the Posterior fibres
Scaption >120º with GH Joint - Internal Rotation - great recruitment of the Anterior (72±23%) and Middle (83±13%) fibres in terms of Maximum Voluntary Isometric Contraction and moderate recruitment of the Posterior fibres
Prone Horizontal Abduction - variation at 100º with GH Joint - External Rotation displays very high activity of the Middle (82±32% MVIC) and Posterior (88±33% MVIC) Deltoid
GH Joint - External Rotation at 90º GH Joint - Abduction - both standing (59±33% MVIC) and prone (79±31% MVIC) versions display good recruitment of the Posterior fibres (SOURCE-5)
Prone Shoulder External Rotations - adds gravity or light load to Apprehension Test position
Banded Horizontal Adduction - low load isotonic horizontal adduction exercise
Scapular Punches - isotonic exercise that emphasises Scapulothoracic Joint - Protraction
Mid-Phase:
DB Front Raises - emphasise the Anterior Fibres, with greatest activity recorded with a Elbow - Pronation grip (SOURCE-10)
DB Side Raises - places greatest emphasis on the Middle Fibres (SOURCE-10)
Inverted Rows - rudimentary isotonic horizontal pull exercise that utilises bodyweight
Overhead Press - great recruitment of the Anterior (62±26%) and Middle (72±24%) fibres in terms of Maximum Voluntary Isometric Contraction and moderate recruitment of the Posterior fibres (SOURCE-5)
DB Shoulder Press - unilaterally loaded overhead press variation
Seated Row - rudimentary weighted isotonic horizontal pull movement
Push-Up - bodyweight isotonic horizontal push exercise
One Arm Row - unilateral DB version of Seated Row
Face Pulls - bilateral isotonic horizontal pull exercise that emphasises GH Joint - External Rotation
Upright Row - weighted isotonic vertical pull exercise that emphasises Upper Trapezius and Deltoid
Bench Press - isotonic horizontal push exercise with capacity for high loads
Incline DB Bench Press - unilaterally loaded Bench variation on variable incline
Bent Over Row - weighted isotonic horizontal pull exercise that emphasises entire posterior chain
Late Phase:
Push Press - wholebody, explosive variation of the Overhead Press
DB Hang Clean - unilateral clean progression, often performed explosively
Bottoms-Up Kettlebell Walk - typically isometric exercise for entire arm musculature with perturbation from walking
Lu Raises - large GH Joint - Abduction range with no Humerus rotation to promote Scapulothoracic Joint - Upward Rotation
Kneeling Landmine Press - wholebody, explosive vertical pressing exercise with large overhead range
Split Stance Landmine Press - crossbody, standing variation of the Kneeling Landmine Press
DB Snatch - unilateral isotonic, explosive shoulder exercise with large overhead range
DB Snatch to Step-Up - wholebody DB Snatch variation that emphasises diagonal functional patterns
Bird-Dog Row - One Arm Row variation that emphasises Posterior Sling
The following lists key considerations when Dry Needling the Deltoid (SOURCE-17):Starting Position:
Patient is best seated upright to access all portions of the Deltoid
Procedure:
Typically 30-50mm needles are used
Needle is inserted perpendicular to skin using a flat palpation
Precautions:
Cephalic Vein - (anterior fibres)
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