Arm

The arm refers to the region of the upper limb between the shoulder and the elbow. It is composed of a single long bone, the humerus, along with muscles, nerves, blood vessels, and connective tissues. The arm plays a critical role in connecting the forearm to the shoulder, facilitating movement and function.

Location

The arm is located in the upper limb, extending from the shoulder joint (glenohumeral joint) to the elbow joint. It lies between the shoulder girdle (proximal end) and the forearm (distal end). The arm is positioned laterally to the chest and anterior to the back, forming the upper portion of the appendicular skeleton.

Anatomy

The arm, also known as the brachium, is the region of the upper limb between the shoulder and the elbow. It is a cylindrical structure composed of a single long bone (humerus), multiple muscles, nerves, blood vessels, and soft tissues. The arm acts as a connection between the shoulder and the forearm, playing a vital role in upper limb structure and movement.

Below is a detailed description of the anatomy of the arm:

Bones of the Arm

The arm contains a single long bone:

Humerus

The humerus is the only bone of the arm and is the largest bone in the upper limb. It extends from the shoulder joint to the elbow joint and provides attachment points for muscles.

• Proximal End (Articulates with the Scapula at the Shoulder Joint):
  • Head of the Humerus: A rounded, ball-like structure that articulates with the glenoid cavity of the scapula to form the glenohumeral joint.
  • Anatomical Neck: The narrow area just below the head.
  • Greater Tubercle: A large lateral projection for the attachment of rotator cuff muscles.
  • Lesser Tubercle: A smaller anterior projection for muscle attachment.
  • Intertubercular Sulcus (Bicipital Groove): A groove between the tubercles that accommodates the tendon of the long head of the biceps brachii muscle.
• Shaft (Body):
  • The long cylindrical part of the humerus.
  • Contains the deltoid tuberosity on its lateral surface, where the deltoid muscle attaches.
  • The radial groove on the posterior surface accommodates the radial nerve and the deep brachial artery.
• Distal End (Articulates with the Radius and Ulna at the Elbow Joint):
  • Medial and Lateral Epicondyles: Bony prominences for muscle and ligament attachment.
  • Trochlea: A pulley-like surface that articulates with the ulna.
  • Capitulum: A rounded surface that articulates with the radius.
  • Olecranon Fossa: A depression on the posterior surface for the olecranon process of the ulna.

Muscles of the Arm

The muscles of the arm are divided into anterior (flexor) and posterior (extensor) compartments, separated by the intermuscular septa.

Anterior Compartment (Flexors)

The anterior compartment is responsible for flexion of the elbow and shoulder. It is innervated by the musculocutaneous nerve. The main muscles include:

• Biceps Brachii:
  • Origin:
    • Long Head: Supraglenoid tubercle of the scapula.
    • Short Head: Coracoid process of the scapula.
  • Insertion: Radial tuberosity and bicipital aponeurosis.
  • Features: A two-headed muscle on the front of the arm.
• Brachialis:
  • Origin: Anterior surface of the humerus.
  • Insertion: Coronoid process of the ulna.
  • Features: Lies deep to the biceps brachii and is a primary flexor of the elbow.
• Coracobrachialis:
  • Origin: Coracoid process of the scapula.
  • Insertion: Medial shaft of the humerus.
  • Features: Assists in flexing and adducting the arm.

Posterior Compartment (Extensors)

The posterior compartment is responsible for extension of the elbow. It is innervated by the radial nerve. The main muscle includes:

• Triceps Brachii:
  • Origin:
    • Long Head: Infraglenoid tubercle of the scapula.
    • Lateral Head: Posterior surface of the humerus above the radial groove.
    • Medial Head: Posterior surface of the humerus below the radial groove.
  • Insertion: Olecranon process of the ulna.
  • Features: A three-headed muscle that is the primary extensor of the elbow.
• Anconeus:
  • Origin: Lateral epicondyle of the humerus.
  • Insertion: Olecranon and proximal ulna.
  • Features: Assists the triceps in elbow extension.

Nerve Supply of the Arm

The nerves of the arm originate from the brachial plexus (C5–T1 spinal nerves):

• Musculocutaneous Nerve:
  • Supplies the anterior compartment muscles (biceps brachii, brachialis, coracobrachialis).
  • Provides sensory innervation to the lateral forearm.
• Radial Nerve:
  • Supplies the posterior compartment muscles (triceps brachii and anconeus).
  • Travels through the radial groove of the humerus.
• Median Nerve: Passes through the arm without innervating any muscles.
• Ulnar Nerve: Passes through the arm medially without innervating muscles, but provides sensation to the forearm and hand.
• Axillary Nerve: Supplies the deltoid and teres minor muscles near the shoulder.

Blood Supply of the Arm

The blood supply to the arm is primarily from the brachial artery, a continuation of the axillary artery:

• Brachial Artery: Runs along the anterior compartment of the arm, providing oxygenated blood to the muscles and soft tissues.
  • Branches:
    • Deep Brachial Artery (Profunda Brachii): Supplies the posterior compartment.
    • Radial Collateral Artery and Ulnar Collateral Artery: Contribute to the arterial anastomosis around the elbow.
• Venous Drainage:
  • Superficial veins: Cephalic vein (lateral side) and Basilic vein (medial side).
  • Deep veins: Brachial vein, accompanying the brachial artery.

Fascia and Compartments

The arm is enclosed by the brachial fascia, a tough connective tissue layer:

• Intermuscular Septa: Divide the arm into two compartments:
  • Anterior Compartment: Contains flexor muscles, the brachial artery, and musculocutaneous nerve.
  • Posterior Compartment: Contains extensor muscles and the radial nerve.

Surface Landmarks of the Arm

• Deltoid Muscle: Prominent muscle forming the rounded contour of the shoulder.
• Biceps Muscle: Visible during flexion of the arm.^[6]
• Medial Epicondyle: Bony prominence on the inner elbow.
• Lateral Epicondyle: Bony prominence on the outer elbow.
• Brachial Artery Pulse: Palpable in the medial part of the arm near the elbow.

Function

The arm, also known as the brachium, is the region between the shoulder and the elbow. It plays a fundamental role in facilitating a wide range of movements, providing stability, and enabling the execution of daily activities and specialized tasks. The arm’s structure, comprising the humerus, muscles, nerves, and blood vessels, works in coordination with the shoulder and forearm to perform critical motor and functional roles.

Here is a detailed explanation of the functions of the arm:

Facilitating Movement of the Forearm

The arm is essential for the movement of the forearm at the elbow joint. It enables both powerful and fine movements through the action of muscles located in the arm’s compartments:

Primary Movements:

Flexion:

• Bending the forearm toward the arm at the elbow joint.
• Muscles involved:
• Biceps Brachii (primary flexor of the elbow).
• Brachialis (strong flexor of the forearm).

Extension:

• Straightening the forearm away from the arm.
• Muscles involved:
• Triceps Brachii (primary extensor of the elbow).
• Anconeus assists in terminal extension.

These movements allow for functional activities like lifting objects, pulling, pushing, and interacting with the environment.

Positioning of the Upper Limb

The arm works as a lever that positions the hand and forearm to perform tasks effectively:

• Movements at the shoulder joint (flexion, extension, abduction, adduction) allow the arm to reposition and adjust the upper limb.^[1]
• By stabilizing the humerus, the arm ensures precise movements of the forearm and hand, such as reaching, grabbing, and manipulating objects.

Force Generation and Lifting

The arm plays a major role in generating strength for lifting, carrying, pushing, and pulling:

• The biceps brachii and brachialis generate force for powerful pulling and lifting motions.
• The triceps brachii provides strength for pushing and straightening the elbow under load.
• The humerus acts as a lever arm for these muscles, allowing efficient force transfer.

For example:

• Lifting a heavy object requires elbow flexion generated by the anterior arm muscles.
• Pushing or throwing involves elbow extension powered by the triceps brachii.

Stabilization of the Elbow Joint

The arm ensures stability and control at the elbow joint during movement:

• The triceps brachii and anconeus stabilize the elbow during extension, preventing hyperextension.^[8]
• The brachialis and biceps brachii stabilize the elbow during flexion, ensuring smooth, controlled movement.
• The ligaments around the elbow joint, such as the collateral ligaments, work with the arm muscles to maintain joint alignment.

Stabilization is essential during weight-bearing activities (e.g., pushing against a surface) or when resisting external forces.

Enabling Precision and Fine Motor Control

The arm provides the necessary stability and mobility to position the hand for fine motor activities:

• Actions like writing, typing, or buttoning a shirt require the arm to hold the forearm and hand steady while allowing precise adjustments.
• The coordinated activity of the biceps, triceps, and forearm muscles enables subtle control for tasks involving tools, utensils, or devices.

The arm acts as a bridge, transmitting forces to the forearm and hand to execute delicate or precise actions.

Supporting Weight-Bearing and Load Transmission

The arm supports the transmission of forces during weight-bearing activities, such as:

• Pushing oneself off the ground (e.g., rising from a chair).[^7]
• Supporting the body during activities like push-ups or plank exercises.

In these cases:

• The humerus acts as a weight-bearing structure.
• The triceps brachii and associated ligaments stabilize the elbow and prevent collapse under pressure.

Shock Absorption

The arm helps absorb and dissipate forces to protect the bones, joints, and muscles from injury:

• The biceps brachii and brachialis muscles cushion the impact during activities like catching or pulling.
• The humerus, with its strong structure, distributes forces generated during falls, impacts, or high-intensity movements to minimize damage.

Protection of Neurovascular Structures

The arm protects important nerves and blood vessels that supply the upper limb:

• The radial nerve, median nerve, and ulnar nerve pass through the arm, supplying motor and sensory innervation to the forearm and hand.^[5]
• The brachial artery runs along the anterior compartment, providing blood supply to the arm and forearm.
• These structures are safeguarded by the surrounding muscles, fascia, and the humerus.

This protection ensures that vital nerve and blood supply reach the forearm and hand.

Contribution to Posture and Stability

The arm, particularly the humerus and its associated muscles, contributes to postural stability and body alignment:

• Muscles such as the biceps brachii and triceps brachii help stabilize the shoulder joint and humerus during weight-bearing and dynamic activities.^[3]
• The arm works with the shoulder girdle to provide support for upper body posture.

This stabilization is especially critical during tasks that involve balancing, lifting, or holding objects overhead.

Functional Activities of Daily Living

The arm enables a variety of daily functional activities by working in coordination with the shoulder and forearm:

• Lifting and Carrying: Picking up and holding objects.
• Pushing and Pulling: Opening doors, moving objects, or pushing a cart.
• Reaching: Extending the arm to access objects at varying distances.
• Throwing and Swinging: Performing tasks in sports or physical work.
• Holding and Stabilizing: Keeping objects steady for precision work.

These functions are essential for performing tasks at home, work, and recreational activities.

Clinical Significance

The arm is a critical region for movement, strength, and function, making it prone to various injuries and conditions that can affect daily activities.^[2] Its role in connecting the shoulder to the forearm and housing vital structures makes it clinically significant.

Fractures

Humeral Shaft Fractures are common due to falls, direct trauma, or accidents. These may injure the radial nerve, leading to conditions like wrist drop.

Nerve Injuries

• Damage to the radial nerve (e.g., in mid-shaft humerus fractures) or musculocutaneous nerve can impair muscle function and sensation.
• Ulnar nerve compression at the elbow can cause weakness in hand muscles (cubital tunnel syndrome).

Muscle Strains and Tears

Overuse or sudden forceful movements can lead to tears in the biceps brachii or triceps brachii, causing pain and loss of strength.

Vascular Conditions

Injury to the brachial artery (e.g., in fractures) can compromise blood flow to the forearm and hand, requiring urgent intervention.

Tendon Ruptures

Rupture of the biceps tendon, often due to lifting heavy objects, causes weakness in elbow flexion.