Supraspinatus muscle

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Supraspinatus muscle
Supraspinatus muscle back4.png
Position of the supraspinatus muscle (red) seen from the back.
Supraspinatus.PNG
Posterior view of muscles connecting the upper extremity to the vertebral column. Supraspinatus muscle is labeled in red at right, while it is covered by other muscles at left.
Latin musculus supraspinatus
Gray's p.440
Origin supraspinous fossa of scapula
Insertion superior facet of greater tubercle of humerus
Artery suprascapular artery
Nerve suprascapular nerve
Actions abduction of arm and stabilizes humerus see part on controversy of action.
TA A04.6.02.006
FMA FMA:9629
Anatomical terms of muscle

The supraspinatus (plural supraspinati, from Latin supraspinatus) is a relatively small muscle of the upper back that runs from the supraspinatous fossa superior of the scapula (shoulder blade) to the greater tubercle of the humerus. It is one of the four rotator cuff muscles and also abducts the arm at the shoulder. The spine of the scapula separates the supraspinatus muscle from the infraspinatus muscle, which originates below the spine.

Structure[edit]

The supraspinatus muscle arises from the supraspinous fossa, a shallow depression in the body of the scapular above its spine. The supraspinatus muscle tendon passes laterally beneath the cover of the acromion. Research in 1996 showed that the postero-lateral origin was more lateral than classically described.[1][2]

The supraspinatus tendon is inserted into the superior facel of the greater tubercle of the humerus.

  • The distal attachments of the three rotator cuff muscles that insert into the greater tubercle of the humerus can be abbreviated as SIT when viewed from superior to inferior (supraspinatus, infraspinatus, and teres minor).

The acronym SITS regarding the rotator cuff muscles is completed by including the subscapularis muscle, which unlike the other rotator cuff muscles attaches to the lesser tubercle of the humerus.[3]

Innervation[edit]

The supraspinatus muscle is supplied by the suprascapular nerve (C5 and C6), which arises from the superior trunk of the brachial plexus and passes laterally through the posterior triangle of the neck and through the scapular notch on the superior border of the scapula. After supplying fibers to the supraspinatus muscle, it supplies articular branches to the capsule of the shoulder joint.

This nerve can be damaged along its course in fractures of the overlying clavicle, which can reduce the person’s ability to initiate the abduction.

Function[edit]

Contraction of the supraspinatus muscle leads to abduction of the arm at the shoulder joint. It is the main agonist muscle for this movement during the first 10-15 degrees of its arc. Beyond 30 degrees the deltoid muscle becomes increasingly more effective at abducting the arm and becomes the main propagator of this action.

The supraspinatus muscle is one of the musculotendinous support structures called the rotator cuff that surround and enclose the shoulder. It helps to resist the inferior gravitational forces placed across the shoulder joint due to the downward pull from the weight of the upper limb.

The supraspinatus also helps to stabilize the shoulder joint by keeping the head of the humerus firmly pressed medially against the glenoid fossa[disambiguation needed] of the scapula.

Without a functioning supraspinatus, the physician must start abducting the patient's arm and eventually the patient will be able to finish abduction if the deltoid is functional, which is common because the supraspinatus is innervated by the suprascapular nerve from the superior/upper trunk of the brachial plexus. The deltoid is innervated more distally by the axillary nerve, which arises from the posterior cord of the brachial plexus.

Controversy about Action[edit]

A 1963 study, "Function of the Supraspinatus Muscle and its Relation to the Supraspinatus syndrome - An experimental in Man" by B Van Linge and J D Mulder in Leiden, Holland showed that the supraspinatus does not cause the abduction of the first 30 degrees, as widely accepted, but rather is a synergist muscle. This means that it assists the deltoid, but is not, by itself the abducting muscle. In this study, the supraspinatus was anaesthetised, and the deltoid muscle was still able to complete a full range of abduction, however was not able to sustain an isometric contraction for more than one minute. From this study, it is suggested that the "true" action of the supraspinatus is to hold the capsule in position to allow greater functional strength and stamina of the deltoid muscle.[4]

In support of this study, one should also consider the basic mechanics of the forces involved in abduction of the shoulder. Supraspinatus is a smaller and weaker muscle compared to deltoid on three counts: deltoid has three large components - anterior, middle and posterior fibre groups; secondly the middle fibres have a multipennate arrangement believed to pack more muscle power into a relatively compact space;[5] thirdly it attaches to the deltoid tuberosity half way down the humerus adding to the mechanical advantage to abduct the arm. Thus the bulk, arrangement and insertion of the deltoid fibres are designed for the power needed to overcome the load of the weight of the arm plus any load in the hand. By contrast, supraspinatus is a much smaller muscle with convergent fibres leading to a tendon which attaches on the highest facet on the greater tubersosity of the humerus, thereby affording it minimal traction on the arm. The arm is a very long lever with the added weight of muscles and other soft tissues.If supraspinatus were capable of initiating abduction it would not only need the power to move a heavy lever but would need to overcome the intertia offered by the stationary limb. Sharkey et al (1994)[6] identified that the whole of the rotator cuff group contributes to abduction of the arm, reducing the work of deltoid by 41%.It strongly suggests that the rotator cuff act synergically in concert with deltoid to stabilise the head of the humerus whilst deltoid provides the turning moment at the gleno-humeral joint to abduct the arm. Indeed if deltoid is palpated when abduction is initiated, active contraction of the muscle can be detected suggesting co-contraction of deltoid with the rotator cuff rather than after initiation by any of the rotator cuff muscles.

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