Infraspinatus muscle
Encyclopedia
In human anatomy, the infraspinatus muscle is a thick triangular muscle
, which occupies the chief part of the infraspinatous fossa
. As one of the four muscles of the rotator cuff
, the main function of the infraspinatus is to externally rotate the arm and stabilize the shoulder joint.
and laterally to the middle facet of the greater tubercle
of the humerus
.
The muscle arises by fleshy fibers from the medial two-thirds of the infraspinatous fossa, and by tendinous fibers from the ridges on its surface; it also arises from the infraspinatous fascia
which covers it, and separates it from the teres major and teres minor.
The fibers converge to a tendon, which glides over the lateral border of the spine of the scapula, and, passing across the posterior part of the capsule of the shoulder-joint, is inserted into the middle impression on the greater tubercle
of the humerus
.
The trapezoidal insertion of the infraspinatus onto the humerus is much larger than the equivalent insertion of the supraspinatus, the reason why the infraspinatus is involved in rotarcuff tears about as frequently as the supraspinatus.
, which may communicate with the joint cavity.
The infraspinatus and teres minor rotate the head of the humerus outward (external, or lateral, rotation); they also assist in carrying the arm backward (extension of the glenohumeral joint).
Additionally, the infraspinatus reinforces the capsule of the shoulder joint.
and pectoralis minor
— evolved from a primitive muscle sheet that connected the coracoid to the humerus
. In late reptilians and early mammals, this muscle structure was displaced dorsally; while most of its components evolved into the pectoralis major, some fibers eventually attached to the scapula and evolved into the supraspinatus, the infraspinatus, and parts of the subscapularis.
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
, which occupies the chief part of the infraspinatous fossa
Infraspinatous fossa
The infraspinatous fossa of the scapula is much larger than the supraspinatous fossa; toward its vertebral margin a shallow concavity is seen at its upper part; its center presents a prominent convexity, while near the axillary border is a deep groove which runs from the upper toward the lower...
. As one of the four muscles of the rotator cuff
Rotator cuff
In anatomy, the rotator cuff is the group of muscles and their tendons that act to stabilize the shoulder. The four muscles of the rotator cuff, along with the teres major muscle, the coracobrachialis muscle and the deltoid, make up the seven scapulohumeral muscles of the human body.-Function:The...
, the main function of the infraspinatus is to externally rotate the arm and stabilize the shoulder joint.
Origin and insertion
It attaches medially to the infraspinous fossa of the scapulaScapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
and laterally to the middle facet of the greater tubercle
Greater tubercle
The greater tubercle of the humerus is situated lateral to the head of the humerus and posteriolateral to the lesser tubercle.Its upper surface is rounded and marked by three flat impressions....
of the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
.
The muscle arises by fleshy fibers from the medial two-thirds of the infraspinatous fossa, and by tendinous fibers from the ridges on its surface; it also arises from the infraspinatous fascia
Infraspinatous fascia
The infraspinatous fascia is a dense fibrous membrane, covering the Infraspinatous muscle and fixed to the circumference of the infraspinatous fossa; it affords attachment, by its deep surface, to some fibers of that muscle....
which covers it, and separates it from the teres major and teres minor.
The fibers converge to a tendon, which glides over the lateral border of the spine of the scapula, and, passing across the posterior part of the capsule of the shoulder-joint, is inserted into the middle impression on the greater tubercle
Greater tubercle
The greater tubercle of the humerus is situated lateral to the head of the humerus and posteriolateral to the lesser tubercle.Its upper surface is rounded and marked by three flat impressions....
of the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
.
The trapezoidal insertion of the infraspinatus onto the humerus is much larger than the equivalent insertion of the supraspinatus, the reason why the infraspinatus is involved in rotarcuff tears about as frequently as the supraspinatus.
Relations
The tendon of this muscle is sometimes separated from the capsule of the shoulder-joint by a bursaBursa (anatomy)
A bursa is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of slimy fluid . It provides a cushion between bones and tendons and/or muscles around a joint. This helps to reduce friction between the bones and allows free movement...
, which may communicate with the joint cavity.
Innervation
The suprascapular nerve innervates the supraspinatous and infraspinatous muscles. These muscles function to abduct and laterally rotate the arm, respectively.Action
The primary function of the infraspinatus is extension, horizontal (transverse) extension and lateral rotation of humerus at the shoulder joint. It is the major external rotator of the shoulder. When the arm is fixed, it abducts the inferior angle of the scapula. Its synergists are teres minor, subscapularis, and the deltoid.The infraspinatus and teres minor rotate the head of the humerus outward (external, or lateral, rotation); they also assist in carrying the arm backward (extension of the glenohumeral joint).
Additionally, the infraspinatus reinforces the capsule of the shoulder joint.
Evolutionary variation
The pectoral muscles — the pectoralis majorPectoralis major muscle
The pectoralis major is a thick, fan-shaped muscle, situated at the chest of the body. It makes up the bulk of the chest muscles in the male and lies under the breast in the female...
and pectoralis minor
Pectoralis minor muscle
The pectoralis minor is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major.-Origin and insertion:...
— evolved from a primitive muscle sheet that connected the coracoid to the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
. In late reptilians and early mammals, this muscle structure was displaced dorsally; while most of its components evolved into the pectoralis major, some fibers eventually attached to the scapula and evolved into the supraspinatus, the infraspinatus, and parts of the subscapularis.