Muscles abducting the arm at the shoulder joint

Supraspinatus

Deltoid

Supraspinatus

Supraspinatus arises from the medial two-thirds of the supraspinous fossa and the deep surface of the dense fascia which covers the muscle. The muscle and the tendon which forms within it pass laterally below trapezius, the acromion process and the coracoacromial ligament to cross over the top of the shoulder joint. The tendon of supraspinatus blends on its deep surface with the capsule of the shoulder joint prior to inserting into the upper of the three facets on the greater tubercle of the humerus.

Nerve supply

Supraspinatus is supplied by the suprascapular nerve, root value C5, 6, a branch from the upper trunk of the brachial plexus. The skin over the muscle is supplied from roots C4 and T2.

Action

Supraspinatus initiates the process of abduction at the shoulder joint, being more important during the early part of the movement than later when deltoid takes over. The role of supraspinatus is probably twofold during this movement. It braces the head of the humerus firmly against the glenoid fossa to prevent an upward shearing of the humeral head( this has been likened to a “foot on the ladder” where a small force applied at one end will produce a rotatory rather than a shearing movement) while at the same time producing abduction. After the initial 20° of abduction, when the stronger deltoid takes over, supraspinatus acts to hold the humeral head against the glenoid fossa.

Functional activity

Supraspinatus is one of the four muscles which form a musculotendinous cuff(or rotator cuff) around the head of the humerus. They function to keep the head of the humerus in the glenoid fossa during movements of the shoulder joint.

Palpation

Contraction of supraspinatus can be felt through trapezius if the examiner’s fingers are pressed into the medial part of the supraspinous fossa when the subject initiates abduction at the shoulder joint. In the anatomical position, the tendon of supraspinatus is covered by the acromion process but it can be palpated if the subject medially rotates the shoulder with his or her hand resting passively in the small of the back. During this manoeuvre, the greater tubercle moves anteriorly so that the tendon can now be rolled against the bone by a medial to lateral pressure of the examiner’s finger against the tubercle. The tendon of supraspinatus is the most frequently damaged soft tissue in the shoulder region and techniques, such as transverse frictions, injection and ultrasound are often applied to this exact location. In severe cases the tendon may be  sufficiently eroded to cause its rupture, which then affects the ease with which abduction can occur. In such cases, or when supraspinatus is paralysed, the patient can still initiate abduction by leaning slightly to the side so using gravity. Alternatively, the patient may use the opposite arm to push the affected limb away from the side, or jerk the hips to “kick” the elbow out. By each of these actions a small yet sufficient degree of abduction occurs to enable the powerful deltoid to take over.

Deltoid

Deltoid is a coarse, thick, triangular muscle which gives the shoulder its rounded contour. Functionally it can be divided into three parts, anterior, posterior and middle, of which only the middle portion is multipennate. It has an extensive attachment to the pectoral girdle. In front, the fibres attach to the anterior border of the lateral third of the clavicle, whilst behind, they come from lower lip of the crest of the spine of the scapula. The most anterior and posterior fibres both run obliquely, in an uninterrupted manner, to the deltoid tuberosity on the lateral surface of the shaft of the humerus.

The middle muscle fibres are more complex because of their multipennate arrangement. These shorter oblique fibres run from four tendinous slips which are attached to the lateral margin of the acromion process to join three intersecting tendinous slips which ultimately run to the deltoid tuberosity of the humerus. Consequently, these shorter, more numerous middle fibres of deltoid, working under considerable mechanical disadvantage when active, give this part of the muscle great strength.

Deltoid is separated from the coracoacromial arch and the upper and lateral aspects of the shoulder joint(and the tendons lying on it) by the subacromial bursa.

Nerve supply

Deltoid is supplied by the axillary nerve, root value C5, 6. The skin covering deltoid is supplied by roots C4 and 5.

Action

Deltoid is the principal abductor of the arm at the shoulder joint, the movement being produced by its middle, multipennate fibres. However, deltoid can only produce this movement efficiently after it has been initiated by supraspinatus.

The true plane of abduction is in line with the blade of scapula, that is in slight flexion, and for this the anterior and posterior fibres are active in order to maintain the plane of abduction by acting as “guy ropes”. The tendency for deltoid to produce an upward shearing of the head of the humerus is resisted by the muscles of the rotator cuff, that is by subscapularis anteriorly, teres minor and infraspinatus posteriorly, and supraspinatus from above.

The anterior part of deltoid is a strong flexor and medial rotator of the humerus, while the posterior part is a strong extensor and lateral rotator, and can help in the transfer of the strain of heavy weights carried in the hand to the pectoral girdle. The posterior part of deltoid is also active during adduction of the arm, to counteract the medial rotation produced by pectoralis major and latissimus dorsi.

Functional activity

Deltoid is active in abduction when the middle fibres contract concentrically, but the massive development and multipennate nature of the muscle are probably due to the fact that many activities of the upper limb require that it be maintained or “held” in this position for long periods of time. Consequently, the middle fibres contract statically when performing activities with the arms in front of the trunk; they than lower the arm back to the side by operating in an eccentric mode.

Palpation

If the seated subject is asked to raise the arm to 60° of abduction in the plane of the scapula, the triangular bulk of deltoid can be felt and seen. Palpating the upper surface of the acromion process and moving the fingers laterally from its edge, the depressions in the muscle caused by the tendinous intersections can be felt if anteroposterior pressure of the fingers is applied.

The anterior and posterior fibres can be made to stand out more clearly if, in the same position as above, the subject is asked to maintain the position against resistance first anteriorly and then posteriorly.

Paralysis of deltoid severely affects the functioning of the shoulder joint and therefore of the upper limb.