Shoulder muscles are categorized as intrinsic and extrinsic.
This is based on their attachment points.
The scapula, clavicle, and humerus are the bones of the shoulder.
The intrinsic muscles originate from the scapula and/or clavicle and attach to the humerus.
Together with the coracobrachialis, which is an upper arm muscle, the intrinsic muscles
compose the scapulohumoral group.
Meanwhile, the extrinsic ones originate from the torso, and attach to the clavicle, scapula,
or humerus.
Let’s discuss the intrinsic muscles first.
There are six – the deltoid, teres major, and four rotator cuff muscles, which are the
teres minor, subscapularis, supraspinatus, and infraspinatus.
The deltoid has three heads – anterior, intermediate or middle, and posterior, which
converge at their attachment point to the humerus at the deltoid tuberosity.
These three heads have different origins and different actions.
The anterior fibers arise from the anterior border and upper surface of the lateral third
of the clavicle.
The intermediate fibers originate from the lateral border of the acromion.
Finally, the posterior fibers originate from the lower lip of the posterior border of the
spine of the scapula.
The anterior fibers assist the pectoralis major, causing flexion and medial rotation.
The intermediate fibers take over arm abduction after the supraspinatus abducts the first
15 degrees.
Finally, the posterior fibers cause extension and lateral rotation.
When all three heads of the deltoid contract simultaneously, the deltoid is the prime mover
of arm abduction along the frontal plane.
Teres major is on the inferior border of the quadrangular space, which is found between
teres minor and major, the long head of triceps brachii, the humerus, and the subscapularis.
It originates from the posterior surface of the inferior angle of the scapula and attaches
to the medial lip of the intertubular groove of the humerus.
The teres major adducts and extends the humerus, and medially rotates the humerus.
It also helps stabilise the humeral head in the glenoid cavity or fossa.
Now we come to the four rotator cuff muscles, which all originate from the scapula and attach
to the humerus.
In addition to their individual actions, these muscles together pull the humeral head into
the glenoid fossa, which is in the scapula.
This provides stability to the glenohumeral joint.
Teres minor originates from the middle half of the lateral border of the scapula and attaches
to the inferior facet of the greater tubercle of the humerus.
It laterally rotates the arm.
Subscapularis originates from the subscapular fossa on the costal surface of the scapula
and attaches to the lesser tubercle of the humerus.
It medially rotates the arm.
Supraspinatus originates from the supraspinous fossa of the scapula and attaches to the superior
facet of the greater tubercle of the humerus.
It abducts the arm up to 15 degrees and then helps the deltoid in abduction from 15-90
degrees.
Infraspinatus originates from the infraspinous fossa of the scapula and attaches to the middle
facet of the greater tubercle of the humerus.
It is the main external rotator of the shoulder.
It also adducts the inferior angle of the scapula.
Note that this muscle works with the deltoid and teres minor.
Now we come to the final topic of this video – the extrinsic shoulder muscles.
Again, these muscles originate from the torso and attach to the scapula, clavicle, or humerus.
The extrinsic shoulder muscles can be organized into two layers – deep and superficial.
The deep muscles include the rhomboid major and minor, as well as the levator scapulae.
The rhomboid minor is above the rhomboid major.
Rhomboid minor originates from the spinous processes of C7-T1.
Rhomboid major originates from the spinous processes of T2-T5 vertebrae.
Both rhomboids attach to the medial border of the scapula.
Both rhomboids serve to retract and rotate the scapula.
The levator scapulae originate from the transverse processes of the C1-C4 vertebrae and attach
to the medial border of the scapula.
They elevate the scapulae.
The superficial muscles include the trapezius and latissimus dorsi.
The trapezius lies overtop of the rhomboids and the levator scapulae.
The trapezius originates from the skull, the nuchal ligament, and the spinous processes
of C7-T12.
The fibers attach to the clavicle, acromion, and the scapula spine.
The trapezius has three heads – upper, middle, and lower.
The upper fibers of the trapezius elevate the scapula.
The middle fibers retract the scapula, and the lower fibers depress the scapula.
Finally, we come to the latissimus dorsi.
This muscle originates from the spinous processes of T7-T12, the iliac crest, the thoracolumbar
fascia, and the inferior three ribs.
The latissimus dorsi’s fibers converge into a tendon that attaches to the intertubular
sulcus of the humerus.
The latissimus dorsi causes extension of the humerus, medial rotation of the humerus, and
it causes adduction of the humerus, together with teres major and pectoralis major.