The region between the shoulder joint and the elbow joint is called the upper arm or arm. That between elbow and wrist is called forearm. Brachium and antebrachium are anatomical terms for arm and forearm respectively but are not used that commonly.
Muscles of arm can be classified as that of a flexor compartment or anterior compartment [act to flex the elbow joint] and that of extensor or posterior compartment. The anterior compartment is anterior to the humerus and the posterior compartment is posterior to it.
The compartments are formed by lateral and the medial intermuscular septa.
Muscles of the flexor compartment of the arm are the biceps brachii, the brachialis, and the coracobrachialis.
The extensor compartment consists of a single large muscle, the triceps brachii.
Muscles of Arm- Anterior Compartment
There are three muscles located in the anterior compartment of the upper arm – biceps brachii, coracobrachialis, and brachialis.
All the anterior compartment muscles of arm are all supplied by the musculocutaneous nerve, a branch of brachial plexus. Arterial supply is by the muscular branches of the brachial artery.
Biceps Brachii
The biceps brachii is named so because its origin is by two heads. One of the unique features of Biceps is that although the majority of the muscle mass lies in front of the humerus, it has no attachment to the humerus bone itself. Another unique feature is that it crosses two joints shoulder and elbow.
Origin
Biceps has two heads of origin-
- The short head takes origin at the coracoid process
- The long head originates from supraglenoid tubercle of the scapula, respectively
The long head after origin remains tendinous as it passes the shoulder joint and the intertubercular groove of the humerus
The tendon of the short head is adjacent to the tendon of the coracobrachialis, another muscle of the arm.
Both heads of the biceps join to form a single muscle belly, but though together, do not have confluent fibers and remain distinct.
As an anatomical variation, additional heads usually one or two have been reported to arise from humerus in about 10% of the people.
Insertion
The insertion of biceps brachii is on the radial tuberosity. The two heads rotate 90 degrees externally before inserting onto the radial tuberosity. The short head inserts distally and long head proximally on the tuberosity.
Close to the musculotendinous junction of this muscle, a thick fascial band called the bicipital aponeurosis or the lacertus fibrosus originates and goes medially as a band to insert on the ulnar part of forearm fascia.
A bursa [bicipitoradial bursa] is present between the biceps tendon and radial tuberosity to ensure smooth motion during pronation and supination of the forearm.
Function
The supination of the forearm and flexion of the elbow are the most important functions of the biceps muscle.
Supination occurs at the proximal radioulnar joint whereas flexion occurs at ulnohumeral joint. It is a movement in the flexed elbow where the palm is turned upward. [The reverse motion that turns the palm downwards is called pronation]
AS a flexor, the biceps muscle has maximum contribution when the forearm is supinated.
Brachialis, brachioradialis, and supinator muscles are the main flexors of the elbow when the forearm is pronated.
Biceps also weak flexor and abductor of the shoulder joint.
The short head also acts as a shoulder joint stabilizer when a heavyweight is carried in the arm.
Coracobrachialis
The coracobrachialis muscle lies deep to the biceps brachii in the arm.
It is the smallest of all three muscles that attach to the coracoid process (the other two muscles are the short head of the biceps brachii muscle and pectoralis minor).
The coracobrachialis muscle is found at the upper and medial part of the arm.
Origin
The muscle, as the name suggests originates from the coracoid process of the scapula. Its tendon here partly blends with the short head of biceps.
Insertion
The muscle inserts at the anteromedial surface of the humeral shaft between the brachialis muscle and the medial head of the triceps
Function
Coracobrachialis is an adductor of the shoulder joint and to a small extent causes internal rotation.
Brachialis
The brachialis muscle lies deep to the biceps brachii and is found more distally than the other muscles of the arm. It forms the floor of the cubital fossa.
Origin
Brachialis originates from the anterior surface of the humerus near the insertion of the deltoid muscle,
Insertion
The fibers converge to a thick tendon, which is inserted into the coronoid process and the tuberosity of the ulna.
Nerve Supply
The brachialis is also supplied by the radial nerve in addition to the musculocutaneous nerve [C5, C6] which supplies all the flexor muscles of arm.
Arterial Supply
Muscular branches of the brachial artery and the recurrent radial artery.
Function
The brachialis flexes the arm at the elbow joint. In fact, it is the prime mover of elbow flexion. Unlike the biceps, the brachialis does not insert on the radius and does not participate in pronation and supination of the forearm.
Attachments: Originates from the medial and lateral surfaces of the humeral shaft and inserts into the ulna tuberosity, just distal to the elbow joint.
Muscles of Arm-Posterior Compartment
Triceps Brachii
Triceps brachii muscle is a three-headed muscle which is the muscle of arm in the posterior compartment.
Origin
The triceps has three heads of origin [see the image of the humerus for dedtails]
- Long head
- From the infraglenoid tubercle of the scapula.
- Lateral head
- Originates from the posterior surface of the humerus, lateral and superior to the radial groove.
- Extends from the greater tubercle down to the region of the lateral intermuscular septum.
- Medial head
- Arsies inferior to the groove of the radial nerve from the posterior surface of the humerus
- From the medial intermuscular septum
- Distally from the lateral intermuscular septum
The medial head is covered in most parts by the lateral and long heads and can be seen is only visible distally on the humerus.
Insertion
The heads converge onto one tendon and insert into the olecranon of the ulna.
Function
Triceps is a powerful extensor of the elbow joint. Insertion also occurs on to the posterior wall of the capsule of the elbow joint and the fascia of the forearm.
It is an antagonist of the biceps and brachialis muscles. It also stabilizes the elbow joint while hand does fine movements such as writing.
Nerve Supply
The triceps muscle is supplied by the radial nerve. In some individuals, the long head is supplied by the axillary nerve.
Clinical significance
Biceps Tendon Rupture
The long head of biceps can rupture either following degeneration or excessive force.
This leads to Popeye sign, a characteristic bulge where the muscle belly is. The name is given after the famous cartoon character Popeye.
Stretch Reflexes
Biceps and triceps reflexes are important parts of the nervous system examination. Biceps reflex tests C5 spinal segment and nerve mainly and triceps reflex tests predominantly C7.
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