A large vertebrate skeletal muscle divided into an ascending, descending, and transverse portion, attaching the neck and central spine to the outer extremity of the scapula. It functions in scapular elevation, adduction, and depression.
The deltoid muscle, a triangular muscle on the human shoulder.
The shoulder or glenohumeral joint is a ball and socket joint formed between
the humerus and scapula. Due to the shallowness of the socket and relatively
loose connections, the shoulder joint allows for a wide range of motion;
however, this wide range makes the joint unstable and thus more
prone to dislocation and injury than other joints.
Two other joints make up the shoulder; the
acromioclavicular joint of the clavicle and scapula, which allows the arm to be
raised above the head, and the sternoclavicular joint of the clavicle and
sternum, which plays an important role in facilitating movement of the upper
arm and connecting it to the rest of the
Muscles that act on the shoulder can be
classified as extrinsic, intrinsic, pectoral, or upper arm. Upper arm
muscles will be discussed in a later section since they primarily promote forearm movement.
Extrinsic Shoulder Muscles
Extrinsic muscles of the shoulder originate
from the trunk and attach to the bones of the shoulder. They can be further
subdivided into superficial and deep layers.
As suggested by the name, superficial
muscles lie on the surface. There are two superficial extrinsic muscles.
Trapezius – The trapezius is the most superficial muscle of the back and forms a
broad flat triangle.
Attachments – The trapezius
originates from the skull and spine of the upper back and neck. It attaches to
the clavicle and scapula.
Actions – The superior region
supports the arm and elevates and rotates the scapula, the intermediate region
retracts the scapula, and the inferior region rotates and depresses the scapula.
Dorsi – The latissimus dorsi originates from the
lower back and covers a wide area.
Attachments – The latissimus
dorsi originates from the lower spine and ribs and the upper pelvis and fascia
of the deep trunk muscles. The muscle converges into a tendon attaching to the
Actions – Extends, adducts, and
medially rotates the upper arm.
Three deep muscles lie below the superficial
muscles of the shoulder.
Scapulae – A small, strap-like muscle that joins the
neck to the scapula.
Attachments – Originates from
the side of the spine in the neck and attaches to the scapula.
Actions – Elevates the scapula.
Major – Sits inferiorly to the levator scapulae.
Attachments – Originates from
the spine in the upper back and attaches to the scapula in an inferior position
to the levator scapulae attachment.
Actions – Retracts and rotates
Minor – Sits between the levator scapulae and rhomboid major, with which
it is paired in action and function. It retracts and rotates the scapula.
Intrinsic muscles originate from the
scapula or clavicle and attach to the humerus. There are six intrinsic muscles,
four of which form the rotator cuff.
– The deltoid muscle is a triangular muscle which
covers the shoulder. The action of the muscle is complex, with the components
acting in opposing and separate ways during the course of a contraction.
Attachments – The deltoid
muscle originates from the scapula and clavicle and attaches to the lateral surface
of the humerus.
Actions – The anterior region
assists the pectoralis major during transverse flexion of the shoulder and
acts weakly in strict transverse flexion. The lateral region assists in
shoulder flexion when the shoulder is rotating, although it also assists the
transverse abduction of the shoulder. The posterior region is the hyperextensor of the shoulder, contributing to transverse
Major – The teres major is a thick flattened muscle
connecting the lower scapula with the humerus.
Attachments – Originates from
the posterior of the scapula and attaches to the humerus.
Actions – Adducts the shoulder
and assists in rotation of the arm.
The rotator cuff is a group of four muscles
that pull the ball of the humerus into the shallow
socket of the scapula, adding required stability. The rotator cuff complex is composed
of the supraspinatus, infraspinatus, subscapularis, and teres minor all of which originate
from the scapula and connect to the humerus. The supraspinatus is involved in
abduction of the arm in association with the deltoid, while the other muscles facilitate
rotation of the arm.
Pectoral muscles lie in the chest and exert
force through the shoulder to move the upper arm. Three pectoral
muscles interact with the shoulder.
Major – The pectoralis major is a large, fan-shaped
muscle covering the chest. It is comprised of clavicular and sternocostal
Attachments – The clavicular
region originates from the clavicle and the sternocostal region originates from
the sternum and the fascia of the oblique muscles of the abdomen. Both attach
to the humerus.
Actions – Adducts and rotates
the upper arm.
Minor – The pectoralis minor muscle is smaller and
lies beneath the pectoralis major.
Attachments – The pectoralis
minor originates from the third to fifth ribs and attaches to the scapula.
Actions – Supports and
depresses the scapula.
Anterior – The serratus anterior is located in the
lateral wall of the chest.
Attachments – The muscle is
formed of several strips originating from the second to eight ribs, each
of which attaches to the scapula.
Actions – Supports the scapula allowing
for elevation of the upper arm.
Extension (upper limb backwards behind back)
Produced by the posterior deltoid,
latissimus dorsi, and teres major.
Flexion (upper limb forwards past chest)
Produced by the biceps brachii (both
heads), pectoralis major, anterior deltoid, and corocobrachialis.
Abduction (upper limb away from trunk,
spreading arms wide)
Produced by the supraspinatus and deltoid.
Past 90 degrees, the scapula needs to be rotated by the trapezius and serratus anterior to achieve abduction.
Adduction (upper limb towards trunk,
bringing arms down to side)
Produced by contraction of pectoralis
major, latissimus dorsi, and teres major.
Medial Rotation (rotation of arm inwards to
Produced by contraction of subscapularis,
pectoralis major, latissimus dorsi, teres major, and anterior deltoid.
Lateral Rotation (rotation of arm outwards
away from the abdomen)
Produced by contraction of the
infraspinatus and teres minor.