Chapter 6: Elbow and Forearm

Introduction

The elbow and forearm are integral parts of the upper extremity, functioning as the bridge
between the shoulder and the hand. They are vital for performing a wide variety of tasks
that demand strength, mobility, and fine motor control, such as lifting, throwing, carrying,
and manipulating tools. The precise coordination of the bones, joints, ligaments, and
muscles in this region facilitates complex movements that allow the hand to be positioned
in space effectively. In this chapter, we dive into the anatomy and biomechanics of the
elbow and forearm complex, exploring their anatomical components and the role each
plays in functional movements.

Osteology (Bone Structure)

Humerus

The humerus, the long bone of the upper arm, plays a crucial role in forming the elbow
joint.

Distal Features:

Trochlea: The pulley-shaped structure on the medial side of the distal humerus that
articulates with the trochlear notch of the ulna. This joint enables the hinge-like motion of
elbow flexion and extension.
Capitulum: A rounded structure on the lateral side of the distal humerus, which articulates
with the radial head. This articulation facilitates both flexion/extension and rotational
movement.

Medial and Lateral Epicondyles: Bony prominences that serve as attachment points for
muscles of the forearm. The lateral epicondyle serves as the origin for the extensor
muscles, while the medial epicondyle is the origin for the flexor muscles.

Olecranon Fossa: A deep depression on the posterior side of the humerus that
accommodates the olecranon process of the ulna during elbow extension.

Coronoid and Radial Fossae: These anterior depressions receive the coronoid process of
the ulna and the radial head during elbow flexion, preventing hyperflexion.

Ulna

The ulna is a long bone located on the medial side of the forearm, playing a primary role in
elbow stability.

Proximal Features:

Olecranon Process: The prominent bony structure at the elbow’s posterior, which serves as
the lever arm for the triceps during extension.

Trochlear Notch: A C-shaped groove on the ulna that forms a tight articulation with the
trochlea of the humerus. This connection forms the hinge joint responsible for elbow
flexion and extension.
Coronoid Process: A smaller projection that fits into the coronoid fossa of the humerus
during flexion, providing stability to the joint.

Radial Notch: Located laterally on the proximal ulna, this feature articulates with the radial
head, allowing for the rotation of the radius.

Distal Features:

Ulnar Head: The rounded end of the ulna that articulates with the ulnar notch of the radius
at the wrist.

Styloid Process: A bony prominence at the distal end of the ulna that serves as a ligament
attachment site.

Radius

The radius is located laterally in the forearm and facilitates the rotational movements of the
forearm.

Proximal Features:

Radial Head: A disc-shaped structure that articulates with the capitulum of the humerus
and the radial notch of the ulna. This joint allows the radius to rotate around the ulna during
pronation and supination.
Neck: The narrow area directly below the radial head, where fractures are often seen in
cases of falls.

Radial Tuberosity: A bony prominence on the medial aspect of the radius where the biceps
brachii tendon inserts, crucial for forearm supination.

Distal Features:

Ulnar Notch: Located on the medial side of the distal radius, this feature articulates with
the ulnar head to form the distal radioulnar joint.

Styloid Process: A lateral projection at the distal radius, providing attachment for ligaments
that stabilize the wrist.

Arthrology (Joints and Articulations)

Elbow Joint Complex

Humeroulnar Joint:

Type: Hinge joint, allowing motion primarily in one plane—flexion and extension.
Kinematics: Flexion and extension occur in the sagittal plane around a mediolateral axis.
The range of motion is typically from 0° (full extension) to around 145° (flexion). As the ulna
moves along the humerus, the trochlear notch slides anteriorly during flexion and
posteriorly during extension.

Stability: The high congruence between the trochlea and trochlear notch contributes to the
joint’s stability, minimizing the risk of dislocation during normal movements.

Humeroradial Joint:

Type: Hinge joint, though it also permits slight rotation.

Kinematics: Flexion and extension occur, but the radial head also contributes to the
movement of the elbow through its articulation with the capitulum.

Function: The humeroradial joint assists in distributing compressive forces and enabling a
wide range of functional movements of the forearm.

Forearm Joints

Proximal Radioulnar Joint:

Type: Pivot joint, allowing rotational movement for pronation and supination of the forearm.
Kinematics: The radial head rotates within the annular ligament, which forms a ring around
it. This joint facilitates rotational movement from 75° of pronation to 85° of supination.

Stability: The annular ligament and the quadrate ligament help stabilize the joint and
prevent dislocation during rotation.

Distal Radioulnar Joint:

Type: Pivot joint.

Kinematics: Like the proximal radioulnar joint, the distal radioulnar joint allows pronation
and supination. During pronation, the distal radius rotates around the ulna, and in
supination, the ulnar head rotates within the ulnar notch on the radius.

Stability: The triangular fibrocartilage complex (TFCC) provides significant stabilization to

Ligamentous Support

Medial (Ulnar) Collateral Ligament (MCL):

Posterior and Transverse Bundles: Provide additional support during elbow extension and
stabilize the ulna against external forces.

Lateral (Radial) Collateral Ligament (LCL) Complex:

Radial Collateral Ligament: Resists varus stress and protects against elbow dislocation,
particularly during axial loading.

Lateral Ulnar Collateral Ligament: Prevents posterolateral rotatory instability, which is

Annular Ligament: Encircles the radial head and allows for its smooth rotation during
forearm pronation and supination.

Interosseous Membrane: A fibrous sheet between the radius and ulna that transmits force
between the two bones during load-bearing activities. It also maintains the alignment of
the bones during forearm rotation.

Kinematics (Movement Mechanics)

Flexion and Extension:

The elbow undergoes movement in the sagittal plane around the mediolateral axis. The
extension range is from 0° (full extension) to around 145° of flexion. The elbow flexors,
particularly the brachialis, biceps brachii, and brachioradialis, generate torque during
flexion.

The extensor muscles, particularly the triceps brachii and anconeus, provide the opposite
motion to extend the arm. The biomechanics of this movement are influenced by muscle
attachment points and the positioning of the humerus and ulna.

Forearm Movements

Pronation and Supination:

Supination and pronation occur in the transverse plane, with the radius rotating around the
ulna at the proximal and distal radioulnar joints. Supination brings the palm upward
(anteriorly), while pronation turns the palm downward (posteriorly).

The forearm pronators (pronator teres and pronator quadratus) work to rotate the radius,
while the supinators (biceps brachii and supinator) reverse the action.
Musculature (Muscles Involved)

Elbow Flexors

Biceps Brachii: This muscle plays a major role in flexing the elbow and supinating the
forearm. It is most effective in a supinated forearm position. The biceps brachii has two
heads: the long head and the short head, which function together to produce significant
torque during elbow flexion.

Brachialis: A primary flexor of the elbow that operates independently of the forearm’s
position (pronated, neutral, or supinated). It acts as the workhorse of elbow flexion,
providing consistent force across various positions.

Brachioradialis: Flexes the elbow when the forearm is in a neutral position and is
particularly engaged during rapid, unresisted movements.

Elbow Extensors

Triceps Brachii: The triceps brachii is the most powerful extensor of the elbow. The long
head of the triceps also crosses the shoulder joint, assisting in shoulder extension. The
medial and lateral heads are responsible for extension during lower loads.

Anconeus: Assists with elbow extension and provides stability during pronation and
supination. Though not a primary elbow extensor, it plays a role in the final stages of
extension.
Forearm Supinators

Supinator: The supinator is responsible for slow and low-resistance supination, particularly
when the elbow is extended.

Biceps Brachii: In addition to its role as a flexor, the biceps is also a powerful supinator,
especially when the elbow is flexed to around 90°.

Forearm Pronators

Pronator Teres: Initiates pronation and assists in elbow flexion. It is most active in fast
pronation.

Pronator Quadratus: This muscle is the primary mover for slow and controlled pronation,
especially during unresisted tasks.

Biomechanics and Torque Production

Elbow Flexors: The flexors generate peak torque at about 90° of elbow flexion. Torque is
influenced by muscle length, moment arm, and muscle contraction velocity. The force
generated by these muscles is crucial for lifting, carrying, and fine manipulation tasks.
Elbow Extensors: The extensor muscles also peak in torque production at 90° of flexion,
where the moment arm is the longest. The triceps brachii, being the largest muscle in the
region, is key for both functional extension and stability.

Pronation and Supination: The torque produced by the pronators and supinators varies
depending on forearm position. Supination torque is often greater than pronation torque,
particularly when the elbow is flexed.

Clinical Considerations

Carrying Angle:

Cubitus Varus and Valgus: Deviation in the carrying angle can affect arm alignment and
function. Cubitus valgus is associated with a greater than normal carrying angle and is
often seen in post-fracture cases. Cubitus varus is a condition of the elbow often referred
to as “gunstock deformity.”

Ligament Injuries:

The medial collateral ligament (MCL) is particularly vulnerable to repetitive valgus stress,
such as in baseball pitchers, leading to ligament tears or sprains. The lateral collateral
ligament (LCL) is responsible for stabilizing against varus forces, but injury can lead to
instability.
Fractures and Dislocations:

Radial Head Fractures: Common in high-impact falls, often leading to pain and limited
mobility at the elbow.

Conclusion

1. Functional Complexity: The elbow and forearm are integral to the upper extremity's
function, contributing significantly to a variety of complex tasks such as lifting, throwing,
and precision grip.
2. Anatomical Importance: The bones of the elbow and forearm (humerus, ulna, and
radius) provide a stable and mobile structure essential for forearm and elbow motions.
3. Elbow Joint Mechanics: The elbow joint's hinge-like structure, formed by the
humeroulnar and humeroradial articulations, facilitates flexion and extension,
contributing to activities like pushing and lifting.
4. Forearm Rotation: The distal and proximal radioulnar joints enable pronation and
supination, allowing the forearm to rotate and the hand to assume different functional
positions.
5. Torque and Force: Flexor and extensor muscles, especially the biceps brachii and
triceps brachii, generate significant torque at various elbow flexion angles, crucial for
stability and movement control.
6. Ligamentous Support: Ligaments such as the MCL and LCL play critical roles in
stabilizing the elbow, preventing dislocations, and supporting the arm during high-load
activities.
7. Dynamic Stability: The stability of the elbow and forearm relies on a combination of
ligamentous structures, muscular support, and joint congruency, enabling functional
range of motion without compromising stability.
8. Pronation and Supination: Muscles like the pronator teres and supinator are essential
for controlled forearm rotation, contributing to functions such as tool use and driving.
9. Impact of Muscle Length: The force-generating capacity of the elbow flexors and
extensors is influenced by muscle length and joint angle, with peak torque occurring at
approximately 90° of flexion.
10. Elbow Injury Mechanisms: The elbow is prone to both acute and chronic injuries,
including ligament sprains, fractures, and dislocations, which can significantly impair
mobility and function.
11. Clinical Relevance: Understanding the biomechanics of the elbow and forearm is
essential for diagnosing and treating conditions like tennis elbow, golfer’s elbow, and
elbow arthritis.
12. Overuse Injuries: Repetitive activities, especially those involving high velocity or force,
can lead to overuse injuries such as medial collateral ligament tears, common in athletes
who perform repetitive overhead motions.
13. Role of Rehabilitation: Effective rehabilitation of elbow and forearm injuries relies on
strengthening key muscles (flexors, extensors, pronators, and supinators), restoring joint
mobility, and improving proprioception.
14. Postural Implications: The carrying angle of the elbow plays a significant role in overall
upper limb function and affects posture, with deviations potentially leading to
biomechanical inefficiencies or discomfort.
15. Prevention and Performance: Maintaining flexibility, strengthening the elbow and
forearm muscles, and using proper technique can reduce the risk of injury and optimize
the performance of both everyday activities and athletic tasks.