Arm Muscles

The muscles of the arm occupy the region between the shoulder and elbow. Anatomically, the term arm refers specifically to this portion of the upper limb and does not include the forearm or hand.

The deep fascia of the arm and the medial and lateral intermuscular septa divide the musculature into two major compartments: an anterior compartment and a posterior compartment. The anterior compartment contains biceps brachii, brachialis, and coracobrachialis. The posterior compartment contains triceps brachii and anconeus.

The anterior muscles primarily produce flexion at the elbow, with biceps brachii also serving as a powerful supinator of the forearm. The posterior muscles primarily produce extension at the elbow. Several arm muscles also cross the shoulder joint and therefore contribute to movement and stabilization of the humerus.

Compartments of the Arm

Compartment	Muscles	Principal Nerve	Major Actions
Anterior	Biceps brachii, brachialis, coracobrachialis	Musculocutaneous nerve	Elbow flexion, forearm supination, shoulder flexion
Posterior	Triceps brachii, anconeus	Radial nerve	Elbow extension

Brachial Fascia

The muscles of the arm are surrounded by the brachial fascia, a continuation of the deep fascia of the shoulder and axillary regions. Distally, it continues with the antebrachial fascia of the forearm.

The brachial fascia sends strong medial and lateral intermuscular septa toward the humerus. These septa contribute to separation of the anterior and posterior muscular compartments and provide attachment for muscles.

Anterior Compartment of the Arm

The anterior compartment contains biceps brachii, brachialis, and coracobrachialis. These muscles are primarily associated with flexion movements and are supplied predominantly by the musculocutaneous nerve.

The brachial artery and its branches provide much of the arterial supply to this compartment.

Biceps Brachii

The biceps brachii is the prominent superficial muscle of the anterior arm. As its name indicates, it has two proximal heads: a long head and a short head. The two heads unite in the arm and form a common distal tendon.

Feature	Biceps Brachii
Long head origin	Supraglenoid tubercle of scapula and adjacent superior glenoid region
Short head origin	Tip of coracoid process of scapula
Insertion	Radial tuberosity and forearm fascia through bicipital aponeurosis
Actions	Supinates forearm and flexes elbow; assists shoulder flexion
Innervation	Musculocutaneous nerve

Long Head of Biceps Brachii

The tendon of the long head of biceps brachii originates within the shoulder joint region from the supraglenoid tubercle and adjacent superior glenoid labral area. It passes over the head of the humerus and exits the joint region into the intertubercular sulcus.

The tendon is intracapsular in its proximal course but remains extrasynovial, being enclosed by a synovial sheath as it passes through the joint region.

Short Head of Biceps Brachii

The short head arises from the coracoid process alongside coracobrachialis. It joins the long head to form the main muscular mass of biceps brachii.

Because both heads cross the shoulder joint, biceps can contribute to shoulder flexion and stabilization in addition to its major actions at the elbow and radioulnar joints.

Insertion of Biceps Brachii

The principal tendon of biceps brachii inserts onto the radial tuberosity. The orientation of this insertion gives the muscle considerable mechanical effectiveness for rotating the radius during supination.

A broad medial expansion from the tendon forms the bicipital aponeurosis.

Bicipital Aponeurosis

The bicipital aponeurosis is a broad fascial expansion extending medially from the distal biceps tendon into the deep fascia of the forearm.

It lies superficial to important structures in the cubital fossa, including portions of the brachial artery and median nerve, and therefore has important clinical and surgical relationships.

Actions of Biceps Brachii

Biceps brachii has two particularly important actions: supination of the forearm and flexion of the elbow. Its effectiveness depends on the position of the forearm and elbow.

Biceps is a particularly powerful supinator when the elbow is flexed. It also contributes strongly to elbow flexion when the forearm is supinated.

Biceps Brachii and Supination

During supination, the radius rotates around the ulna so that the palm turns anteriorly in the anatomical position. Because biceps inserts onto the radial tuberosity, contraction generates a rotational force on the radius.

Biceps acts with the supinator muscle to produce this movement.

Biceps Brachii and Elbow Flexion

Biceps contributes to elbow flexion, but its effectiveness varies with forearm position. Brachialis remains a major flexor regardless of whether the forearm is pronated or supinated.

Brachialis

The brachialis lies deep to biceps brachii and forms an important part of the floor of the cubital fossa. Because it inserts onto the ulna rather than the radius, its elbow-flexion action is relatively independent of forearm rotation.

Feature	Brachialis
Origin	Distal half of anterior humerus and adjacent intermuscular septa
Insertion	Coronoid process and tuberosity of ulna
Action	Flexes forearm at elbow
Innervation	Primarily musculocutaneous nerve; lateral portion may receive radial nerve contribution

Brachialis as an Elbow Flexor

Brachialis is a powerful and consistent flexor of the elbow. Its insertion on the ulna means that pronation and supination of the radius do not substantially alter its basic line of pull.

It therefore contributes to elbow flexion in pronated, neutral, and supinated forearm positions.

Coracobrachialis

The coracobrachialis is a slender muscle in the proximal medial arm. It extends from the coracoid process to the medial humeral shaft.

Feature	Coracobrachialis
Origin	Tip of coracoid process
Insertion	Middle third of medial humerus
Actions	Flexes and adducts arm at shoulder; contributes to glenohumeral stabilization
Innervation	Musculocutaneous nerve

Musculocutaneous Nerve and Coracobrachialis

The musculocutaneous nerve characteristically pierces coracobrachialis after arising from the lateral cord of the brachial plexus. This relationship is an important anatomical feature of the proximal arm.

The nerve then descends between biceps brachii and brachialis, supplying the muscles of the anterior compartment.

Musculocutaneous Nerve

The musculocutaneous nerve is the principal motor nerve of the anterior arm. It supplies coracobrachialis, biceps brachii, and brachialis.

After providing its muscular branches, it continues distally as the lateral cutaneous nerve of the forearm, providing sensory innervation to the lateral forearm.

Blood Supply of the Anterior Compartment

The anterior arm receives blood primarily from muscular branches of the brachial artery. Additional contributions arise through arterial networks associated with the shoulder and elbow.

Posterior Compartment of the Arm

The posterior compartment is dominated by triceps brachii, the principal extensor of the elbow. The small anconeus muscle is functionally associated with the posterior extensor group.

The posterior compartment is supplied by the radial nerve and principally by branches of the profunda brachii artery.

Triceps Brachii

The triceps brachii is the large muscle of the posterior arm. It has three heads: long, lateral, and medial. These converge into a common tendon that inserts onto the olecranon of the ulna.

Head	Origin
Long head	Infraglenoid tubercle of scapula
Lateral head	Posterior humerus superior to radial groove
Medial head	Posterior humerus inferior to radial groove

Feature	Triceps Brachii
Insertion	Olecranon of ulna and adjacent forearm fascia
Primary action	Extends forearm at elbow
Additional action	Long head extends and adducts arm at shoulder
Innervation	Radial nerve

Long Head of Triceps Brachii

The long head originates from the scapula and therefore crosses both the shoulder and elbow joints. In addition to extending the elbow, it can contribute to extension and adduction of the arm at the shoulder.

Its position also allows it to contribute to stabilization of the humeral head during upper limb movement.

Lateral Head of Triceps Brachii

The lateral head arises from the posterior humerus superior to the radial groove. It forms much of the visible superolateral muscular contour of the posterior arm.

It contributes strongly to elbow extension, particularly when substantial force is required.

Medial Head of Triceps Brachii

The medial head arises primarily from the posterior humerus inferior to the radial groove and lies deep to the long and lateral heads through much of its course.

It contributes to elbow extension across a wide range of loading conditions.

Triceps Tendon

The three heads of triceps converge distally into the triceps tendon, which inserts principally onto the olecranon of the ulna.

Because the ulna forms the principal hinge relationship with the humerus at the elbow, triceps contraction directly produces extension of the forearm.

Elbow Extension

Triceps brachii is the principal extensor of the elbow. Extension increases the angle between the arm and forearm and is important during pushing, throwing, climbing, rising with the assistance of the upper limbs, and many athletic activities.

Anconeus

The anconeus is a small triangular muscle located at the posterolateral elbow. It is closely associated functionally with triceps brachii.

Feature	Anconeus
Origin	Lateral epicondyle of humerus
Insertion	Lateral olecranon and proximal posterior ulna
Actions	Assists elbow extension and contributes to stabilization of elbow
Innervation	Radial nerve

Function of Anconeus

Anconeus assists triceps during elbow extension and helps stabilize the elbow joint. It may also contribute to small movements of the ulna associated with pronation.

Radial Nerve in the Arm

The radial nerve is the principal nerve of the posterior arm. It enters the posterior compartment and travels with the profunda brachii artery along the posterior humerus in the region of the radial groove.

It supplies triceps brachii and anconeus before continuing toward the lateral elbow and forearm.

Radial Groove

The radial groove is an oblique groove on the posterior surface of the humeral shaft. The radial nerve and profunda brachii vessels travel in close relationship to this region.

This anatomical relationship is clinically important because fractures of the humeral shaft can injure the radial nerve.

Profunda Brachii Artery

The profunda brachii artery, also called the deep artery of the arm, is a major branch of the brachial artery. It accompanies the radial nerve into the posterior compartment.

It supplies triceps and contributes to arterial anastomoses around the elbow.

Summary of Arm Muscle Innervation

Muscle	Principal Nerve
Biceps brachii	Musculocutaneous nerve
Brachialis	Musculocutaneous nerve, with possible radial contribution laterally
Coracobrachialis	Musculocutaneous nerve
Triceps brachii	Radial nerve
Anconeus	Radial nerve

Summary of Arm Muscle Attachments and Actions

Muscle	Major Distal Attachment	Major Action
Biceps brachii	Radial tuberosity	Supination and elbow flexion
Brachialis	Coronoid process and ulnar tuberosity	Elbow flexion
Coracobrachialis	Medial humerus	Shoulder flexion and adduction
Triceps brachii	Olecranon	Elbow extension
Anconeus	Olecranon and proximal ulna	Assists elbow extension

Elbow Flexors

Three major muscles contribute substantially to elbow flexion: brachialis, biceps brachii, and brachioradialis. Brachialis and biceps are muscles of the arm, while brachioradialis is anatomically located in the forearm.

Their relative contributions vary with forearm position and the mechanical demands of the movement.

Brachialis and Forearm Position

Because brachialis inserts onto the ulna, it remains mechanically effective during elbow flexion regardless of whether the forearm is pronated or supinated.

Biceps and Forearm Position

Biceps brachii is particularly effective as an elbow flexor when the forearm is supinated. Its mechanical contribution changes as the radius rotates during pronation.

Forearm Supination

Supination is produced principally by biceps brachii and supinator. Biceps is particularly powerful when rapid or resisted supination is performed with the elbow flexed.

Relationship to the Shoulder Joint

Biceps brachii, coracobrachialis, and the long head of triceps cross the shoulder joint. Consequently, the muscles of the arm are not limited functionally to movements of the elbow.

Muscle	Shoulder Action
Biceps brachii	Assists shoulder flexion and contributes to stabilization
Coracobrachialis	Flexes and adducts arm
Triceps long head	Extends and adducts arm

Cubital Fossa

The cubital fossa is a triangular depression on the anterior aspect of the elbow. Several arm muscles form its boundaries or floor.

Brachialis forms much of the floor, while the lateral border is formed by brachioradialis and the medial border by pronator teres. The distal biceps tendon passes through the fossa toward the radial tuberosity.

Biceps Tendon in the Cubital Fossa

The distal biceps tendon can be palpated within the cubital fossa during resisted elbow flexion. It lies lateral to the brachial artery in the typical arrangement of major structures within the fossa.

Brachial Artery

The brachial artery is the principal artery of the arm. It begins as a continuation of the axillary artery near the inferior border of teres major and descends through the anterior arm.

Near the elbow it enters the cubital fossa and usually divides into the radial and ulnar arteries.

Relationship of the Median Nerve

The median nerve descends through the arm in close relationship to the brachial artery. It typically does not provide motor innervation to the major muscles of the arm, but its course is important when considering the neurovascular anatomy of the anterior compartment.

Arm Muscles During Pulling

Pulling movements commonly require coordinated elbow flexion by biceps brachii and brachialis. Biceps becomes particularly important when the movement also requires forearm supination.

Arm Muscles During Pushing

Pushing movements require extension of the elbow by triceps brachii. The amount of triceps activity depends on resistance, shoulder position, and the direction of the applied force.

Arm Muscles During Lifting

Lifting an object toward the body often requires coordinated elbow flexion and stabilization of the shoulder. Biceps brachii and brachialis generate flexion force, while shoulder and forearm muscles position the upper limb appropriately.

Arm Muscles During Throwing

Throwing requires rapid coordination between muscles of the shoulder, arm, forearm, and trunk. Triceps contributes to rapid elbow extension during acceleration, while the elbow flexors participate in control and deceleration during other phases of the movement.

Arm Muscles and Joint Stability

Although ligaments and articular geometry provide substantial passive stability, muscles crossing the shoulder and elbow provide important dynamic stabilization. Their contractions help control joint position while external forces act on the upper limb.

Length-Tension Relationships

Muscles crossing more than one joint change length as either joint moves. Biceps brachii and the long head of triceps are examples of two-joint muscles whose mechanical performance is influenced by both shoulder and elbow position.

Biceps Reflex

The biceps reflex is elicited by tapping the biceps tendon, producing contraction of biceps brachii and elbow flexion under normal conditions.

The response is used clinically to assess the integrity of the musculocutaneous nerve and associated cervical spinal segments, particularly C5 and C6.

Triceps Reflex

The triceps reflex is elicited by tapping the triceps tendon proximal to the olecranon. A normal response produces contraction of triceps and extension at the elbow.

The reflex assesses the radial nerve and associated cervical spinal segments, particularly C7 and C8, with C7 providing a major contribution.

Musculocutaneous Nerve Injury

Injury to the musculocutaneous nerve can weaken elbow flexion and forearm supination because of impaired biceps and brachialis function.

Elbow flexion is not necessarily lost completely because brachioradialis and other muscles supplied by different nerves can still contribute.

Radial Nerve Injury in the Arm

The functional consequences of radial nerve injury depend strongly on the level of the lesion. A sufficiently proximal lesion can weaken elbow extension by affecting branches to triceps, while lesions farther distally may preserve substantial triceps function.

Because the radial nerve lies close to the humeral shaft, it is particularly relevant in fractures involving the shaft of the humerus.

Humeral Shaft Fracture

A fracture involving the humeral shaft may injure the radial nerve as it travels through the radial groove. The profunda brachii vessels are also closely related to this region.

Biceps Tendinopathy

The tendon of the long head of biceps can develop degenerative or inflammatory changes, particularly where it passes through the shoulder and intertubercular region.

Because of its close relationship to the glenohumeral joint and rotator cuff, symptoms in this region may coexist with other shoulder disorders.

Proximal Biceps Tendon Rupture

Rupture of the long head of biceps tendon can allow the muscle belly to retract distally, producing a characteristic alteration in the contour of the anterior arm.

Elbow flexion may remain relatively well preserved because the short head of biceps and other flexors remain intact, although supination strength can be affected.

Distal Biceps Tendon Rupture

Rupture of the distal biceps tendon disrupts the connection between biceps brachii and the radial tuberosity. This can substantially reduce the muscle's ability to generate forceful supination and can also weaken elbow flexion.

Triceps Tendon Injury

The distal triceps tendon can undergo partial or complete rupture near its insertion on the olecranon. Complete disruption can substantially impair active elbow extension.

Biceps Muscle Strain

Biceps brachii can be injured during sudden or excessive loading, particularly during resisted elbow flexion or supination. Injury can involve the muscle belly, musculotendinous junction, or tendons.

Triceps Muscle Strain

Triceps injuries may occur during forceful pushing, throwing, weight lifting, or sudden eccentric loading. The severity depends on the extent and location of muscular or tendinous disruption.

Muscle Testing

The arm muscles can be assessed by testing their characteristic actions against resistance while observing and palpating muscular contraction.

Movement	Major Arm Muscle
Elbow flexion with forearm supinated	Biceps brachii and brachialis
Elbow flexion regardless of forearm rotation	Brachialis
Forearm supination	Biceps brachii
Shoulder flexion and adduction	Coracobrachialis
Elbow extension	Triceps brachii
Assisted elbow extension	Anconeus

Testing Biceps Brachii

Biceps can be tested by asking the individual to flex the elbow against resistance with the forearm supinated. The muscle belly and distal tendon can be palpated during contraction.

Resisted supination can further emphasize biceps function.

Testing Brachialis

Brachialis can be emphasized during resisted elbow flexion with the forearm pronated, a position that reduces the mechanical contribution of biceps brachii.

Testing Triceps Brachii

Triceps is tested by asking the individual to extend the elbow against resistance. The muscle can be observed and palpated along the posterior arm and its tendon near the olecranon.

Key Anatomical Features

Feature	Key Point
Anatomical extent	Shoulder to elbow
Main compartments	Anterior and posterior
Anterior compartment muscles	Biceps brachii, brachialis and coracobrachialis
Posterior compartment muscles	Triceps brachii and anconeus
Anterior compartment nerve	Musculocutaneous nerve
Posterior compartment nerve	Radial nerve
Powerful elbow flexor	Brachialis
Powerful supinator	Biceps brachii
Principal elbow extensor	Triceps brachii
Biceps insertion	Radial tuberosity
Triceps insertion	Olecranon
Major anterior arterial supply	Brachial artery and muscular branches
Major posterior arterial supply	Profunda brachii artery

Anatomical and Functional Importance

The muscles of the arm form the major muscular systems controlling flexion and extension at the elbow while also contributing to forearm rotation and shoulder movement. Their organization into anterior and posterior compartments closely reflects their innervation and principal functions.

The anterior compartment combines the powerful elbow-flexion function of brachialis with the flexion and supination functions of biceps brachii. Coracobrachialis acts primarily at the shoulder and provides an important anatomical relationship with the musculocutaneous nerve.

The posterior compartment is dominated by triceps brachii, whose three heads converge on the olecranon to form the principal extensor mechanism of the elbow. The long head also crosses the shoulder, allowing it to participate in movement and stabilization of the arm at the glenohumeral joint.

The close relationships between these muscles and the musculocutaneous nerve, radial nerve, brachial artery, profunda brachii artery, humerus, cubital fossa, and shoulder make the arm an important region for understanding both functional and clinical anatomy.