The supraspinatus is a relatively small muscle of the upper back that runs from the supraspinatous
fossa superior of the scapula 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 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.
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. 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.
Innervation The supraspinatus muscle is supplied by the
suprascapular nerve, 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 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 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 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. 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; 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. Sharkey et
al. 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. Additional images
References
External links Origin, insertion and nerve supply of the
muscle at Loyola University Chicago Stritch School of Medicine
supraspinatus+muscle at eMedicine Dictionary GoogleBody - Supraspinatus muscle