The upper arm is the region between the shoulder and elbow joints. It contains five main muscles, divided into two compartments: the anterior (front) and the posterior (back) compartments. 

The muscles of the upper arm are: 

• Biceps brachii

• Brachialis

• Coracobrachialis

• Triceps brachii

• Anconeus.

Anterior muscles flex the arm at the shoulder and forearm at the elbow; the biceps also supinates the forearm.

Posterior muscles extend the forearm at the elbow. The triceps also assist with shoulder extension and adduction.

Biceps Brachii

The biceps brachii is the most visible muscle on the front of the arm. It has two heads:

• Long head

• Short head.

These two heads merge into one muscle belly.

The biceps brachii's main job is turning the palm upward (its most powerful action), bending the forearm, lifting the arm forward, and stabilizing the shoulder joint (long head).

• Origin: Long head from the supraglenoid tubercle of the scapula; short head from the coracoid process.

• Insertion: Both heads attach to the radial tuberosity and the forearm fascia via the bicipital aponeurosis.

• Innervation: Musculocutaneous nerve (C5–C6).

Coracobrachialis

The coracobrachialis lies deep to the biceps brachii. It helps move the arm forward at the shoulder, pulls the arm toward the body, and helps stabilize the shoulder when carrying weight.

• Origin: Coracoid process of the scapula.

• Insertion: Medial surface of the humerus, near the deltoid tubercle.

• Innervation: Musculocutaneous nerve (C5–C6).

Brachialis

The brachialis is located deep to the biceps and forms the floor of the cubital fossa (front of the elbow). It's the main flexor of the forearm at the elbow. 

This muscle is stronger than the biceps for pure flexion because it attaches to the ulna, which does not rotate.

• Origin: Front of the lower half of the humerus.

• Insertion: Ulnar tuberosity.

• Innervation: Musculocutaneous nerve (C5–C6) with help from the radial nerve (C7).

Triceps Brachii

The triceps brachii is the main extensor of the forearm at the elbow; it also helps extend and adduct the arm at the shoulder. It has three heads: long, lateral, and medial. 

• Origins:

  1. Long head – infraglenoid tubercle of the scapula.

  2. Lateral head – posterior humerus (above the radial groove).

  3. Medial head – posterior humerus (below the radial groove).

• Insertion: All heads join into one tendon that attaches to the olecranon of the ulna.

• Innervation: Radial nerve (C6–C8).

Anconeus

The anconeus is a small muscle on the back of the elbow. It assists triceps in elbow extension and helps stabilize the elbow joint.

• Origin: Lateral epicondyle of the humerus.

• Insertion: Lateral surface of the olecranon.

• Innervation: Radial nerve (C7–C8).

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