General classification of upper extremity muscles
The entire muscular system of the human upper limb is generally divided into two major topographical groups:
- Muscles of the pectoral girdle — their main task is to connect the girdle bones, such as the clavicle and scapula, to the trunk, as well as to control the shoulder joint.
- Muscles of the free upper limb — this group is responsible for direct movements of the arm, forearm, and hand in space.
Directly belonging to the pectoral girdle (mm. cinguli membri superioris) are 6 muscles, whose coordinated work allows the shoulder joint to perform complex biomechanical tasks. These include:
- Deltoid muscle (m. deltoideus)
- Supraspinatus muscle (m. supraspinatus)
- Infraspinatus muscle (m. infraspinatus)
- Teres minor muscle (m. teres minor)
- Teres major muscle (m. teres major)
- Subscapularis muscle (m. subscapularis)
Deltoid muscle (m. deltoideus)
This is a large, superficially located muscle that forms the rounded contour of the shoulder.
Anatomical landmarks:
- Points of origin (O): muscle bundles start from the lateral third of the clavicle, the acromion, and the spine of the scapula.
- Insertion site (I): the fibers converge and firmly attach to the deltoid tuberosity of the humerus (tuberositas deltoidea humeri).
Functional features (A): The muscle has a complex structure, and its bundles can work both independently and together. With a general contraction, it abducts the arm (up to 60°). The anterior bundles are responsible for flexion and internal rotation (pronation) of the arm, while the posterior bundles perform the opposite task—extending the arm and rotating it outward (supination).
Blood supply and innervation: The tissue is supplied by the posterior circumflex humeral artery (a. circumflexa humeri posterior) and the thoracoacromial artery (a. thoracoacromialis). Control is provided by the axillary nerve — n. axillaris (segments C₅–C₆).
Clinical feature: To reduce friction between the inner surface of the muscle and the greater tubercle of the humerus, there is a synovial subdeltoid bursa (bursa subdeltoidea).
Supraspinatus muscle (m. supraspinatus)
An important element of the scapular muscular apparatus, hidden beneath the trapezius muscle.
- Origin (O): fills the supraspinous fossa of the scapula (fossa supraspinata scapulae).
- Insertion (I): the tendon attaches to the superior facet of the greater tubercle of the humerus (tuberculum majus humeri).
- Biomechanics (A): acts as a synergist to the deltoid muscle, being the initiator of arm abduction (responsible for the first 15–20° of movement). In addition, it lies tightly against the joint, stabilizing it.
- Innervation (N): suprascapular nerve, n. suprascapularis (C₅–C₆).
- Blood supply (V): suprascapular artery (a. suprascapularis) and circumflex scapular artery (a. circumflexa scapulae).
Clinically, this muscle is extremely important as it is part of the rotator cuff — a structure most frequently subject to injuries and degenerative changes in sports medicine and traumatology.
Infraspinatus muscle (m. infraspinatus)
A wide, flat muscle occupying most of the posterior surface of the scapula below its spine.
- Origin (O): starts from the walls of the infraspinous fossa of the scapula (fossa infraspinata scapulae).
- Insertion (I): attaches to the middle facet of the greater tubercle of the humerus (tuberculum majus humeri).
- Function (A): is a powerful external rotator (rotates the arm outward). A special role of the muscle is that upon contraction, it pulls back the capsule of the shoulder joint, effectively preventing it from being pinched between the articular surfaces during active movements.
- Innervation (N): like the supraspinatus muscle, it is innervated by the suprascapular nerve — n. suprascapularis (C₅–C₆).
- Blood supply (V): receives arterial blood from a. circumflexa scapulae and a. suprascapularis.
Briefly: answers to topical questions
Where does the teres minor muscle originate and insert?
The teres minor muscle (m. teres minor) originates from the upper part of the lateral border of the scapula. The muscle is located in the infraspinous bed, covered by the infraspinous fascia, and its upper edge adjoins the lower edge of the infraspinatus muscle. It attaches to the greater tubercle of the humerus, specifically weaving into its lower facet, lying below the tendon of the infraspinatus muscle.
What functions does the subscapularis muscle perform?
The subscapularis muscle (m. subscapularis) performs the functions of inward rotation of the arm, lowering the raised arm, adduction of the arm, and anterior-posterior movement of the arm. In addition, as part of the rotator cuff, it provides dynamic stabilization of the shoulder joint and rotation of the humeral head. The deep tendinous bundle of the muscle forms the floor of the tendon sheath of the biceps, contributing to its stability, while the superficial fibers form the roof of the intertubercular sulcus.
Which muscles form the rotator cuff of the shoulder?
The rotator cuff of the shoulder is formed by a complex of muscles whose tendons weave into the joint capsule of the shoulder and attach to the tubercles of the humerus. It includes:
- Subscapularis muscle (m. subscapularis)
- Supraspinatus muscle (m. supraspinatus)
- Infraspinatus muscle (m. infraspinatus)
- Teres minor muscle (m. teres minor)
These muscles provide dynamic stabilization of the joint and rotation of the humeral head.
What is the blood supply and innervation of the teres major muscle?
The teres major muscle (m. teres major) is innervated by the thoracodorsal nerve (segments C5–C8), which is formed from the posterior cord of the brachial plexus. Information on the blood vessels supplying this muscle is absent in sources. Together with the latissimus dorsi and subscapularis muscles, it participates in internal rotation of the arm, adduction of the arm, and lowering the raised arm.
Two 'spinatus' muscles (supraspinatus and infraspinatus), two 'teres' muscles (major and minor), the deltoid, and the subscapularis — this is how easily you can remember all 6 muscles of the shoulder girdle.
Class and exam questions
Which muscles initiate shoulder abduction?
The process of abduction is initiated by the supraspinatus muscle (it provides the first 15–20 degrees), after which the deltoid muscle actively joins in, bringing abduction up to 60 degrees.
Which structure protects the joint capsule from impingement?
This function is performed by the infraspinatus muscle. During movements, it pulls back the capsule of the shoulder joint, preventing it from getting pinched between the bones.
From which spinal cord segments do the discussed muscles receive innervation?
The deltoid, supraspinatus, and infraspinatus muscles are innervated by branches of the brachial plexus, whose fibers originate from segments C₅–C₆.
What else to study on this topic
- Anatomy of the teres minor muscle (m. teres minor)
- Anatomy of the teres major muscle (m. teres major)
- Topography and functions of the subscapularis muscle (m. subscapularis)
- Structure and biomechanics of the rotator cuff
- Blood supply to the shoulder joint and anastomoses of the scapular circle
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