Muscles of the Arm

Muscles of the pectoral, back and shoulder regions | Main Anatomy Index | The cubital fossa

Muscles of the Arm

• There are four muscles in the arm: three flexors in the anterior fascial compartment supplied by the musculocutaneous nerve and one extensor supplied by the radial nerve.
• The anconeus muscle is located chiefly in the forearm, but it is described with the brachial muscles because it is morphologically and functionally related to the triceps brachii muscle.

The Biceps Brachii Muscle (pp. 539, 541)

• As its name "biceps" indicates, the proximal attachment of this long fusiform muscle has two heads (bi, two + cipital, from L. caput, head).
• The two bellies of the muscle unite just distal to the middle of the arm.
• The biceps is located in the anterior aspect of the arm in the anterior fascial compartment.

• Proximal attachments are: short head-tip of coracoid process of scapula; long head-supraglenoid tubercle of scapula.
• Distal attachments are: tuberosity of radius and fascia of forearm via bicipital aponeurosis.
• Innervation: musculocutaneous nerve (C5 and C6).

• When the elbow is extended, the biceps is a simple flexor of the forearm.
• It is also a powerful supinator when the forearm is flexed and when more power is needed against resistance.
• The biceps barely operates at all during flexion of the prone forearm.

• The tendon of the long head of biceps crosses the head of the humerus within the capsule of the shoulder joint and descends in the intertubercular groove of the humerus.

• The bicipital aponeurosis is a triangular membranous band that runs from the biceps tendon across the cubital fossa and merges with the deep fascia over the flexor muscles in the medial side of the forearm.
• The proximal part of the bicipital aponeurosis can be easily felt where it passes obliquely over the brachial artery and medial nerve.
• This aponeurosis affords protection for these and other structures in the cubital fossa. It also helps to lessen the pressure of the biceps tendon on the radial tuberosity during pronation and supination of the forearm.

Injuries involving the Biceps Brachii Muscle

• Sometimes the tendon of the long head biceps brachii is dislocated from the intertubercular groove in the humerus.
• This may occur during traumatic separation of the proximal epiphysis of the humerus.
• When this occurs, the arm is fixed in the abducted position and the head of the humerus can be felt in its normal position.

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The Brachialis Muscle (p. 541)

• This is a flattened, fusiform muscle that lies posterior to the biceps brachii.

• Proximal attachments are: distal half of anterior surface of humerus.
• Distal attachments are: coronoid process and tuberosity of ulna.
• Innervation: musculocutaneous nerve (C5 and C6)

• The strong brachialis is the main flexor of the forearm. It flexed the forearm in all positions and in slow or quick movements.
• When the forearm is being extended slowly, the brachialis steadies the movement by slowly relaxing.
• It always contracts during flexion of the elbow joint and is primarily responsible for maintaining flexion. Because of this, it is the workhorse among the flexor muscles if the elbow joint.

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The Coracobrachialis Muscle (p. 541)

• This is an elongated, narrow muscle in the superomedial part of the arm and is important mainly as a landmark (e.g., the musculocutaneous nerve pierces it).

• Proximal attachments are: tip of coracoid process of scapula.
• Distal attachments are: middle third of medial surface of humerus.
• Innervation: musculocutaneous nerve (C5, C6, and C7).

• The coracobrachialis helps to flex and adduct the arm at the shoulder joint. It also helps to stabilise this joint.

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The Triceps Brachii Muscle (p. 541)

• This is a large, fusiform muscle and is located in the posterior fascial compartment of the arm.
• It is associated with the small anconeus muscle at the elbow.
• As its name "triceps" indicates, it has three heads of proximal attachment: long, lateral, and medial.

• Proximal attachments are: long head-infraglenoid tubercle of scapula; lateral head-posterior surface of humerus, superior (and lateral) to the radial groove; medial head-posterior surface of humerus, inferior (and medial) to the radial groove.
• Distal attachments are: proximal end of olecranon and superior part of posterior surface of ulna.
• Innervation: radial nerve (C6, C7, and C8)

• The triceps is the main extensor or the forearm.
• Because the long head of the triceps crosses the shoulder joint, it also aids in extension and adduction of the arm.
• Its long head also steadies the head of the abducted humerus.

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The Anconeus Muscle (p. 547)

• This is a small triangular muscle and is on the lateral part of the posterior aspect of the elbow.
• It is usual blended with the triceps and should be considered part of the medial head of this muscle.

• Proximal attachments are: lateral epicondyle of the humerus.
• Distal attachments are: lateral surface of olecranon and superior part of posterior surface of ulna.
• Innervation: radial nerve (C7, C8, and T1)

• It helps the triceps to extend the forearm. It also abducts the ulna during pronation of forearm and contracts whenever this joint needs to be stabilised against flexion.

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