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Human Anatomy · Upper limb

Arm, Elbow and Forearm

The elbow does two entirely different jobs at once. It hinges, so the hand can be brought to the body, and it rotates, so the hand can be turned over without moving the shoulder. Two separate joints share one capsule to make that possible.

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10Questions

What you will be able to do

  • Name the compartments of the arm and forearm with their muscles, nerve and action.
  • Explain why brachialis rather than biceps is the main elbow flexor.
  • Name the boundaries and contents of the cubital fossa in order.
  • Describe the three articulations at the elbow and the ligaments checking them.
  • Explain the carrying angle and what alters it.
  • Describe the mechanism of pronation and supination, and test it correctly.
  • Explain how the common flexor and extensor origins produce golfer's and tennis elbow.
  • State what must be checked immediately in a supracondylar fracture, and why.

The arm: two compartments

Deep fascia divides the arm into two compartments, and the arrangement is beautifully simple: one compartment in front that flexes, one behind that extends, each with a single nerve.

Figure 1 · Compartments of the arm

Compartments of the arm Compartments with their muscles, nerve supply and shared action. COMPARTMENTS OF THE ARM Anterior flexes the elbow Biceps brachii Brachialis Coracobrachialis NERVE Musculocutaneous Posterior extends the elbow Triceps brachii Anconeus NERVE Radial One compartment, one nerve, one action. This pattern holds through the whole limb.
One compartment, one nerve, one action. This is the organising pattern of the whole limb, and it makes the muscles far easier to learn.

Three details are worth adding. Brachialis is the workhorse of elbow flexion, not biceps — it lies deep to biceps, crosses only the elbow, and works in every position of the forearm. Biceps is the more powerful supinator, and it flexes the elbow best when the forearm is already supinated. And triceps has three heads, one of which crosses the shoulder, so its length depends on shoulder position as well as elbow.

The cubital fossa

Figure 2 · The cubital fossa

The cubital fossa The three main contents of the cubital fossa in order from lateral to medial, with its boundaries. CUBITAL FOSSA CONTENTS, LATERAL TO MEDIAL T Tendon of biceps brachii A Artery brachial artery, dividing here into radial and ulnar N Nerve median nerve Remember it as T A N, running lateral to medial. BOUNDARIES Above a line between the two epicondyles Laterally brachioradialis Medially pronator teres Floor brachialis and supinator Roof deep fascia and the bicipital aponeurosis
Tendon, artery, nerve — lateral to medial. Small, and clinically loaded, because this is where needles go and where a supracondylar fracture does its damage.

A triangular hollow in front of the elbow. It matters clinically out of all proportion to its size, because the brachial artery and the median nerve pass through it, and because it is where blood is taken and cannulas are placed.

The bicipital aponeurosis deserves a mention. It is a sheet spreading medially from the biceps tendon across the fossa, and it lies directly over the brachial artery. It was once called the "grace of God" structure, because it protects the artery from a needle that has gone too deep.

The elbow joint

A synovial hinge joint with two articulations inside one capsule.

ArticulationBetweenAllows
Humeroulnar Trochlea of the humerus and the trochlear notch of the ulna Flexion and extension. This is the hinge, and it is very stable.
Humeroradial Capitulum of the humerus and the head of the radius Takes part in flexion, and permits the radius to rotate
Superior radioulnar Head of the radius within the annular ligament, against the ulna Pronation and supination. Shares the elbow capsule.
LigamentRunsChecks
Medial (ulnar) collateral Medial epicondyle to the ulna, in three bands Valgus force. The band injured in throwing athletes.
Lateral (radial) collateral Lateral epicondyle to the annular ligament Varus force
Annular A ring around the radial head, attached to the ulna at both ends Holds the radial head against the ulna while allowing it to spin

The carrying angle

With the elbow extended and the forearm supinated, the forearm does not continue in line with the arm. It angles slightly outwards — the carrying angle — which lets the arms swing past the hips when walking. It is normally a little greater in women, disappears on flexion, and is increased or reversed after a badly united supracondylar fracture. Comparing the two sides is a quick and useful examination.

Figure 4 · The elbow and radioulnar joints

Illustration to be added

Three panels. Panel one: the elbow opened from the front, showing the trochlea articulating with the trochlear notch of the ulna and the capitulum with the head of the radius, the medial and lateral collateral ligaments, and the annular ligament encircling the radial head. Panel two: a sagittal section through the humeroulnar joint showing the olecranon in its fossa in extension and the coronoid process in its fossa in flexion. Panel three: pronation and supination shown as two frames from the front, the radius parallel to the ulna in supination and crossed over it in pronation, with the interosseous membrane drawn between them and the superior and inferior radioulnar joints marked. Add a small inset showing the carrying angle with the elbow extended and supinated. Bone in warm ivory, cartilage pale blue, ligaments navy, gold leader lines.

Pronation and supination

This is the elbow's second job, and it is worth understanding as a mechanism rather than memorising as a fact.

The ulna stays still. The radius rotates around it, crossing over it in pronation and lying parallel in supination. The rotation happens at two joints working together — the superior radioulnar joint at the elbow and the inferior radioulnar joint at the wrist — connected by the interosseous membrane.

MovementMusclesNerveNote
Supination Biceps brachii, supinatorMusculocutaneous, radial Stronger than pronation. This is why a right-handed screw tightens clockwise.
Pronation Pronator teres, pronator quadratusMedian Weaker. Pronator teres is also an elbow flexor.

Test it properly

Pronation and supination must be tested with the elbow flexed to a right angle and held against the side. Otherwise the patient simply rotates at the shoulder and appears to have full range when they have none. This is one of the commonest errors in a student examination.

The forearm: two compartments again

Figure 3 · Compartments of the forearm

Compartments of the forearm Compartments with their muscles, nerve supply and shared action. COMPARTMENTS OF THE FOREARM Anterior flexes wrist and fingers, pronates Superficial: pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis Deep: flexor digitorum profundus, flexor pollicis longus, pronator quadratus NERVE Median, except flexor carpi ulnaris and half of flexor digitorum profundus, which are ulnar Posterior extends wrist and fingers, supinates Superficial: brachioradialis, the wrist extensors, extensor digitorum, extensor carpi ulnaris Deep: supinator, the three thumb muscles, extensor indicis NERVE Radial, mostly through its posterior interosseous branch Almost everything in front is median; almost everything behind is radial. Learn the exceptions.
Front is median, back is radial — then learn the two exceptions. Flexor carpi ulnaris and half of flexor digitorum profundus are ulnar.

The same principle as the arm, with more muscles. The important simplification is that most anterior muscles arise from a common flexor origin on the medial epicondyle, and most posterior muscles from a common extensor origin on the lateral epicondyle.

That single fact explains the two commonest elbow complaints in the clinic. Overloading the wrist extensors irritates their shared origin at the lateral epicondyle — tennis elbow. Overloading the flexors irritates the medial one — golfer's elbow. Neither is really a problem of the elbow joint at all.

What goes wrong here

ProblemAnatomy behind itWhat to check
Supracondylar fracture
a childhood emergency		 The lower humerus is thin from front to back here. The brachial artery and median nerve lie directly in front. Radial pulse, hand colour and warmth, and median nerve function — immediately, and repeatedly.
Cubital tunnel syndrome The ulnar nerve passes behind the medial epicondyle in a shallow groove, close to the skin. Numbness in the little and half the ring finger, weak grip, worse with the elbow bent.
Tennis elbow Common extensor origin at the lateral epicondyle Pain on resisted wrist extension; tenderness just distal to the epicondyle.
Golfer's elbow Common flexor origin at the medial epicondyle Pain on resisted wrist flexion and pronation.
Pulled elbow
in a young child		 The radial head is not yet fully formed, so a sharp pull slips it out from under the annular ligament. A child who will not use the arm, holding it slightly flexed and pronated, after being pulled up by the hand.
Olecranon bursitis A superficial bursa between skin and the olecranon A swelling at the point of the elbow with surprisingly little loss of movement.

Where students get this wrong

Calling biceps the main elbow flexor

Brachialis is. It crosses only the elbow and works in every forearm position. Biceps is the stronger supinator.

Testing rotation with the elbow straight

The patient rotates at the shoulder instead. Elbow flexed to a right angle, tucked into the side.

Thinking the radius and ulna both rotate

The ulna is the fixed post. The radius crosses over it.

Missing the vascular check in a supracondylar fracture

The brachial artery lies directly in front of the fracture site. Pulse, colour and warmth are the first things to record, and they must be rechecked.

Treating tennis elbow as a joint problem

It is a tendon origin problem at the lateral epicondyle. The joint itself is usually normal.

Forgetting where the ulnar nerve runs

Behind the medial epicondyle, superficial and exposed. It explains the tingling when you knock your elbow, and it explains cubital tunnel syndrome.

Check yourself

Ten questions on this chapter. Tap one to see the answer and the reasoning.

Q1. The main flexor of the elbow, working in every position of the forearm, is:
  1. (A) Biceps brachii
  2. (B) Brachialis
  3. (C) Brachioradialis
  4. (D) Pronator teres

Answer: (B) Brachialis. It lies deep to biceps, crosses only the elbow, and its line of pull is unaffected by forearm rotation. Biceps is the more powerful supinator.

Q2. The contents of the cubital fossa, from lateral to medial, are:
  1. (A) Nerve, artery, tendon
  2. (B) Tendon, artery, nerve
  3. (C) Artery, nerve, tendon
  4. (D) Tendon, nerve, artery

Answer: (B) Tendon of biceps, brachial artery, median nerve. Remember it as T A N running lateral to medial.

Q3. The annular ligament:
  1. (A) Connects the two epicondyles
  2. (B) Encircles the head of the radius and attaches to the ulna
  3. (C) Checks valgus force at the elbow
  4. (D) Attaches the biceps tendon to the ulna

Answer: (B) It forms a ring holding the radial head against the ulna while allowing it to spin during pronation and supination.

Q4. During pronation:
  1. (A) The ulna rotates around the radius
  2. (B) The radius crosses over the ulna
  3. (C) Both bones rotate equally
  4. (D) Rotation occurs at the humeroulnar joint

Answer: (B) The ulna is the fixed post. The radius rotates around it, crossing over it in pronation, using the superior and inferior radioulnar joints together.

Q5. Pronation and supination must be tested with the elbow flexed to a right angle because:
  1. (A) It is more comfortable for the patient
  2. (B) Otherwise the patient substitutes shoulder rotation
  3. (C) The radioulnar joints only work in flexion
  4. (D) It isolates pronator quadratus

Answer: (B) With the elbow straight, shoulder rotation mimics forearm rotation convincingly. Flexing the elbow and tucking it in removes the substitution.

Q6. Tennis elbow involves which structure?
  1. (A) The medial collateral ligament
  2. (B) The common flexor origin
  3. (C) The common extensor origin at the lateral epicondyle
  4. (D) The annular ligament

Answer: (C) The shared origin of the wrist extensors at the lateral epicondyle. It is a tendon problem, not a joint problem, and pain is reproduced by resisted wrist extension.

Q7. Which nerve lies behind the medial epicondyle?
  1. (A) Median
  2. (B) Radial
  3. (C) Ulnar
  4. (D) Musculocutaneous

Answer: (C) The ulnar nerve, superficial and exposed in its groove. It explains the tingling on knocking the elbow, and it is the site of cubital tunnel syndrome.

Q8. In a supracondylar fracture of the humerus in a child, the first priority is to check:
  1. (A) Range of elbow movement
  2. (B) The carrying angle
  3. (C) The radial pulse and median nerve function
  4. (D) Shoulder rotation

Answer: (C) The brachial artery and median nerve lie directly in front of the fracture site. Circulation and nerve function must be checked immediately and rechecked.

Q9. Most muscles of the anterior forearm are supplied by the median nerve. The exceptions are:
  1. (A) Pronator teres and palmaris longus
  2. (B) Flexor carpi ulnaris and the medial half of flexor digitorum profundus
  3. (C) Flexor pollicis longus and pronator quadratus
  4. (D) Flexor digitorum superficialis only

Answer: (B) Those two are supplied by the ulnar nerve. Everything else in the anterior compartment is median.

Q10. A young child will not use an arm after being pulled up by the hand, holding it flexed and pronated. The likely problem is:
  1. (A) Supracondylar fracture
  2. (B) Pulled elbow, with the radial head slipping from the annular ligament
  3. (C) Cubital tunnel syndrome
  4. (D) Olecranon bursitis

Answer: (B) The radial head is not yet fully formed, so a sharp longitudinal pull lets it slip from under the annular ligament. The history is characteristic.

Quick review

Everything on this page, in one screen

  • Arm: anterior flexes, musculocutaneous; posterior extends, radial.
  • Brachialis is the main elbow flexor. Biceps is the stronger supinator.
  • Cubital fossa contents, lateral to medial: Tendon, Artery, Nerve.
  • Elbow = humeroulnar hinge + humeroradial + superior radioulnar, all in one capsule.
  • Annular ligament holds the radial head while letting it spin.
  • Carrying angle: forearm angles outwards in extension and supination. Altered after a badly united supracondylar fracture.
  • In rotation the ulna stays still and the radius crosses over it. Test with the elbow flexed and tucked in.
  • Forearm: front mostly median, back mostly radial. Exceptions are flexor carpi ulnaris and half of flexor digitorum profundus, which are ulnar.
  • Lateral epicondyle = common extensor origin = tennis elbow. Medial epicondyle = common flexor origin = golfer's elbow.
  • Supracondylar fracture: check pulse, colour, warmth and median function immediately, and again.
  • The ulnar nerve runs behind the medial epicondyle, superficial and exposed.

Further reading

BookWhat it adds here
B D Human Anatomy, Volume 1: Upper Limb and Thorax
Chaurasia		 Compartment-by-compartment detail at examination level.
Anatomy and Human Movement
Palastanga, Field and Soames		 The mechanics of pronation and supination, and the carrying angle.
Clinical Anatomy by Regions
Snell		 The cubital fossa and the clinical consequences of injury here.
The Concise Book of Muscles
Jarmey		 A quick muscle-by-muscle reference for the forearm.

Reviewed by the Physiotherapist India Team. · Human Anatomy contents