Arm Bone diagram: Radius, Proximal, Function, Humerus

Edited By Team Careers360 | Updated on Jul 02, 2025 05:11 PM IST

The humerus, ulna, and radius are the three bones that make up an arm.The arm of the human body is a critical portion that permits movement along the shoulder, elbow,helpful wrist and fingers for day-to-day activities.

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Humerus

The humerus, which resembles the femur bone found in the legs, is the longest bone in the human arm.
From the shoulder to the elbow, it is extended.
The body, the lower extremities, and the upper extremity make up its three main components.
The upper extremity features a slender neck, a hemispherical head, and two processes that extend from it.It moves across the shoulder by articulating with the scapula (shoulder bone).
Overall cylindrical in shape, the body's bottom has a prism-like appearance.
The elbow joint is made up of two depressions or fossae, two smooth articular surfaces (trochlea and capitulum), and two projections (epicondyles) on the lower extremity.
The radius and ulna, respectively, and the capitulum and trochlea articulate.
The ulna bone projects into the two depressions of the fossae.
The forearm and finger muscles join to the bones at the epicondyles, which are found on either side of the bones.

This Story also Contains

Humerus
Function
Radius
Ulna

Proximal Landmarks

The head, anatomical neck, surgical neck, greater and lesser tuberosity, and intertubercular sulcus are the distinguishing features of the proximal humerus.

The head is located at the top of the humerus. The anatomical neck divides this from the bigger and lesser tuberosities, and it faces medially, upward, and backward.

The larger tuberosity has an anterior and a posterior surface and is situated laterally on the humerus. Three of the rotator cuff muscles, the supraspinatus, infraspinatus, and teres minor, attach to the superior, middle, and inferior aspects of the greater tuberosity, respectively.

The lesser tuberosity is situated further medially on the bone and is significantly smaller. Only the anterior surface is present. Here is where the subscapularis, the last rotator cuff muscle, attaches.

The intertubercular sulcus, a deep groove, divides the two tuberosities. In this groove, the tendon of the long head of the biceps brachii emerges from the shoulder joint.

Lips are the designation for the intertubercular sulcus' margins. The lips of the intertubercular sulcus are inserted by the pectoralis major, teres major, and latissimus dorsi muscles. The phrase "a woman between two majors" can help you recall this since the latissimus dorsi attaches between the pectoralis major laterally and the teres major on the medial lip.

The surgical neck runs from the tuberosities immediately distal to the humeral shaft. Here, the circumflex humeral arteries and the axillary nerve rest against the bone.

Shaft

Several muscles attach to the humerus' shaft at this location. It is seen to be round proximally and flattened distally in cross section views.

On the lateral side of the humeral shaft is a roughened area where the deltoid muscle attaches. The deltoid tuberosity is the name given to this.

The radial (or spiral) groove is a small depression that extends parallel to the deltoid tuberosity down the posterior surface of the humerus. In this groove are the profunda brachii artery and radial nerve. Along the humerus' shaft, the following muscles are connected to it:

Coracobrachialis, deltoid, brachialis, and brachioradialis are located anteriorly.

Lateral and medial heads of the triceps are located in the back (the spiral groove demarcates their respective origins).

Distal Region

The lateral and medial margins of the distal humerus form medial and lateral supraepicondylar ridges. The lateral supraepicondylar ridge is more roughened, indicating the region of common origin of the forearm extensor muscles.

The lateral and medial epicondyles are extracapsular bone projections that are immediately distal to the supraepicondylar ridges. At the elbow, both can be felt. The medial is the largest and farther-reaching of the two. It is possible to feel the ulnar nerve as it passes through a groove on the medial epicondyle's backside.

The trochlea is situated medially and reaches onto the back of the bone distally. The capitulum, which articulates with the radius, is located lateral to the trochlea.

Three depressions called the coronoid, radial, and olecranon fossae are also seen on the distal part of the humerus. When the elbow is flexed or extended, they allow room for the forearm bones.

Articulations

The glenohumeral joint is created when the proximal area of the humerus articulates with the glenoid fossa of the scapula (shoulder joint).

Distally, at the elbow joint, the trochlea of the humerus articulates with the trochlear notch of the ulna and the capitulum of the humerus with the head of the radius.

Body Features

The humerus's body has three surfaces and three edges.

Borders

Anterior
Lateral
Medial

Surfaces

Antero-lateral
Antero-medial
Posterior

Function

The upper limb's function depends on 13 muscles that are attached to the humerus and which help move the hand, elbow, and other upper limb-related joints.

Radius

The radius is the name of the lateral forearm bone.It is a long bone with a proximal end, shaft, and distal end as its main structural components. The distal end articulates with the carpal bones at the wrist and the head of the ulna, while the proximal end's head articulates with both the distal humerus and the proximal ulna. The interosseous membrane, a thick connective tissue, holds the shaft (body) securely to the ulna.

Proximal radius

The head, neck, and radial tuberosity are carried by the proximal end of the radius. The complex elbow joint is made up of the capitulum of the humerus and the concave superior surface on the disc-shaped head of the radius. The proximal radioulnar joint is created by positioning the radial head's periphery, also known as the articular circumference of the head of the radius, inside the radial notch of the ulna and wrapping it in the annular ligament.

The neck is formed by the thin section of the radius that is distal to the head. The biceps brachii muscle attaches onto the medially directed, oval-shaped radial (bicipital) tuberosity that is located just below the neck

Radial shaft

The shaft of the radius is a lengthy segment of bone that continues distally from the neck and radial tuberosity. It is thin proximally but enlarges towards the wrist, when it broadens to create the distal end of radius. The radial shaft has a modest lateral curve and is triangular in cross-section for much of its length.Anterior, posterior, and interosseous boundaries make up its three borders.

On the medial side of the bone, the anterior boundary is located. It begins immediately distal to the radial tuberosity and passes diagonally to the side of the shaft. The middle of the shaft is where the posterior boundary is most noticeable as it is located on the posterior face of the radius. The acute interosseous boundary faces the ulna medially. The middle radioulnar joint is created when this border connects to the ulna's interosseous border via the fibrous interosseous membrane.

An anterior, posterior, and lateral surface are also present on the radial shaft. While its rear surface is located between the interosseous and posterior borders, its anterior surface is located between the anterior and interosseous borders. The anterior and posterior borders are separated by the lateral surface, which is slightly convex.

Distal radius

The radius's shaft enlarges to create a wide, rectangular distal end with a four-sided cross-section that extends beyond the ulna's distal end.

The distal radius's front surface is smooth, concave, and anteriorly angled. The distal radioulnar joint is formed by the ulnar notch, a concavity on the medial surface that receives the head of the ulna. On the other hand, the lateral surface of the distal radius is rough and extends inferiorly as the radial styloid process.

Two facets on the inferior surface, also known as the carpal articular surface, are in contact with the scaphoid and lunate carpal bones. The dorsal tubercle (also known as Lister's tubercle), a large bony projection on the distal radius's posterior surface, is located between the grooves that transmit the tendons of the forearm muscles.

Ulna

The longer of the two parallel forearm bones, the ulna is the medial bone of the forearm. Similar to the radius, the ulna is composed of a proximal end, a shaft, and a distal end. The distal humerus and the head of the radius are articulated with the proximal end. On the other hand, the distal end has a head that joins with the distal radius. The shaft (body) of the ulna and the shaft of the radius are connected by the interosseous membrane.

Proximal ulna

The distal humerus and the head of the radius interact with the proximal ulna, a huge hook-shaped bone. It has the ulnar tuberosity, sublime tubercle, radial notch, coronoid process, olecranon, and trochlear notch.

The triceps brachii muscle inserts at the olecranon, a conspicuous, proximal projection from the posterior portion of the proximal ulna. When the elbow joint extends, its uppermost section bends anteriorly like a beak and fits into the olecranon fossa of the distal humerus to prevent hyperextension. The c-shaped trochlear notch is formed by the concave anterior surface of the olecranon. Similar to the jaws of a wrench, this notch is covered in articular cartilage and articulates with the trochlea of the distal humerus to form a hinge that allows flexion and extension movements at the elbow.

The coronoid process, which is accommodated by the coronoid fossa on the anterior face of the distal humerus during flexion, projects anteriorly as the lower lip of the trochlear notch. The sublime tubercle, a little tubercle on the proximal part of the medial border of the coronoid process, is where the anterior band of the ulnar collateral ligament attaches. The proximal ulna carries the ulnar tuberosity, where the brachialis muscle inserts, just distal to the coronoid process.

The shallow, rounded depression known as the radial notch is located directly distal to the trochlear notch on the lateral surface of the proximal ulna. The proximal radioulnar joint is created when the radial notch articulates with the radial head's circumference.

Ulnar shaft

The ulna's shaft is wider near its proximal end and narrows distally as it approaches its head. The ulnar shaft has three boundaries and, like the radius, has a triangle cross-section for the majority of its length (anterior, posterior and interosseous).

The front border is smooth and rounded. It starts immediately medially to the ulna's tuberosity and descends along the anteromedial shaft. The anterior border terminates distally on the posterior side of the shaft, not far from the styloid process' base. The entire length of the forearm can be palpated subcutaneously to feel the posterior boundary, which extends from the styloid process to the posterior aspect of the olecranon. The medially protruding interosseous border of the ulna, which joins the ulna to the radius, is pointed, similar to the radial shaft, and acts as a place of attachment for the fibrous interosseous membrane.

The anterior, posterior, and medial surfaces of the ulnar shaft are all present. With the exception of slight roughening at its distal end, which indicates where the pronator quadratus muscle is attached, the anterior surface, which is located between the interosseous and anterior borders, is essentially smooth. The posterior surface of the ulnar shaft is the area between the interosseous and posterior borders. This surface has linear marks that show the many places where muscles attach. Between the anterior and posterior boundaries of the ulnar shaft, the medial surface is smooth and transversely convex.

Distal ulna

A tiny, rounded head and an ulnar styloid process make up the distal ulna. The distal radioulnar joint is formed by the lateral convex articular surface of the head of the ulna and the ulnar notch of the distal radius. The triangular fibrocartilage (TFC), an articular disc that separates the head of the ulna from the carpal bones, articulates with the inferior surface of the ulna. As a result, the head of the ulna has no direct role in the development of the wrist joint. The dorsomedial portion of the wrist can feel the short, rounded ulnar styloid process, which extends distally from the posteromedial aspect of the distal ulna.

Frequently Asked Questions (FAQs)

1. What are the bones called in the arm?

Your arm is made up of three bones: the upper arm bone (humerus) and two forearm bones (the ulna and the radius).

2. What causes humerus bone pain?

The rotator cuff tendinitis is the most common cause. Tendon swelling characterizes this disorder. Impingement syndrome, in which the rotator cuff becomes trapped between the acromion (the region of the scapula that covers the ball) and humeral head, is another typical reason for shoulder pain (the ball portion of the humerus).

3. What causes the pain in the center of my arm?

Muscle strain, bruising, and post-exercise DOMS are common reasons for pain in the center of the bicep. Most minor injuries heal on their own, but more serious ones could need medical attention and physical therapy. Pain in the left arm may occasionally be a sign of a heart attack.

4. What is the term for above the elbow?

The term "lateral epicondyle" refers to the external (lateral) hump located just above the elbow. The medial epicondyle, or the lump on the inside of the arm immediately above the elbow, is where the majority of the muscles that straighten the fingers and wrist come together and attach.

5. The arm has how many joints?

The six main joints covered here (from proximal to distal) are the sternoclavicular, acromioclavicular, shoulder, elbow, radioulnar, and wrist joints. The upper limb has a wide range of precise movements linked with it to enable us to interact with our surroundings successfully.

6. How does the humerus contribute to elbow movement?

The humerus contributes to elbow movement through its distal end, which forms part of the elbow joint. The capitellum of the humerus articulates with the radial head, allowing for flexion and extension of the elbow, as well as rotation of the forearm. The trochlea of the humerus articulates with the ulna, further facilitating these movements.

7. What is the role of the medullary cavity in long bones like the humerus and radius?

The medullary cavity is the hollow center of long bones, including the humerus and radius. It serves several important functions: it contains bone marrow, which produces blood cells; it reduces the overall weight of the bone while maintaining strength; and it allows for some flexibility in the bone, which helps absorb shock and prevent fractures.

8. How does the capitellum of the humerus facilitate arm movement?

The capitellum is a rounded projection on the lateral part of the distal humerus. It articulates with the concave surface of the radial head, forming a ball-and-socket type joint. This arrangement allows for smooth rotation of the radial head against the capitellum during forearm pronation and supination, as well as flexion and extension of the elbow.

9. How does the capitellum-radial head articulation contribute to the stability of the elbow joint?

The capitellum-radial head articulation forms a ball-and-socket type joint that allows for smooth rotation of the forearm. This arrangement distributes forces across a wider surface area, enhancing joint stability. Additionally, the shape of the radial head and its snug fit against the capitellum helps resist lateral forces on the elbow, further contributing to joint stability.

10. What is the function of the coronoid process of the ulna?

The coronoid process is a triangular projection on the anterior part of the proximal ulna. It fits into the coronoid fossa of the humerus when the elbow is flexed, providing stability to the elbow joint. It also serves as an attachment site for the brachialis muscle, which is a powerful flexor of the elbow.

11. How does the structure of the humerus contribute to its strength?

The humerus is structured for both strength and mobility. Its shaft is cylindrical and hollow, providing strength while minimizing weight. The bone's ends are expanded and reinforced with trabecular bone, allowing for better distribution of forces. The shaft also has ridges and tuberosities for muscle attachments, which not only provide leverage for movements but also contribute to the bone's overall strength.

12. How does the anatomical neck of the humerus differ from its surgical neck?

The anatomical neck of the humerus is a narrow groove just below the humeral head, marking the attachment of the joint capsule. The surgical neck, located slightly lower, is the area just below the tubercles of the humerus. The distinction is important because fractures of the surgical neck are more common and can affect the blood supply to the humeral head, while the anatomical neck is less prone to injury.

13. What is the function of the bicipital groove on the humerus?

The bicipital groove, also known as the intertubercular groove, is a longitudinal depression on the proximal humerus. Its primary function is to provide a passage for the long head tendon of the biceps brachii muscle. This groove helps guide and stabilize the tendon during arm movements, particularly during rotation of the humerus.

14. What is the role of the lateral epicondyle of the humerus?

The lateral epicondyle is a bony prominence on the outer side of the distal humerus. It serves as the common origin for the extensor muscles of the forearm, including those that extend the wrist and fingers. This arrangement provides leverage for these muscles, allowing for powerful and precise movements of the hand and fingers.

15. How does the structure of the trochlea contribute to elbow stability?

The trochlea is a spool-shaped structure at the distal end of the humerus that articulates with the ulna. Its unique shape, with a groove running between two ridges, allows it to fit snugly into the trochlear notch of the ulna. This interlocking arrangement provides significant stability to the elbow joint, particularly during flexion and extension movements.

16. What is the primary function of the radius in the arm?

The radius is one of the two bones in the forearm that primarily functions to allow rotation of the forearm and wrist. It works in conjunction with the ulna to enable pronation (palm down) and supination (palm up) movements, which are crucial for tasks like turning doorknobs or pouring liquid from a container.

17. How does the proximal end of the radius differ from its distal end?

The proximal end of the radius, located near the elbow, is smaller and rounded, forming a disc-like shape called the radial head. This end articulates with the capitellum of the humerus. The distal end, near the wrist, is broader and flatter, forming part of the wrist joint and articulating with the carpal bones.

18. What is the significance of the radial tuberosity?

The radial tuberosity is a bony prominence on the medial side of the proximal radius. Its primary significance is that it serves as the attachment site for the biceps brachii muscle tendon. This attachment allows the biceps to effectively flex the elbow and supinate the forearm.

19. How does the structure of the radial head contribute to its function?

The radial head has a disc-like shape with a concave superior surface. This shape allows it to articulate smoothly with the capitellum of the humerus, enabling rotational movements of the forearm. The radial head also acts as a pivot point for forearm rotation and helps distribute forces from the hand and wrist up through the arm.

20. How does the radius contribute to wrist stability and movement?

The distal end of the radius forms a major part of the wrist joint. It has a concave articular surface that interfaces with the scaphoid and lunate bones of the wrist. This articulation allows for flexion, extension, and side-to-side movements of the wrist. The styloid process of the radius also provides attachment for ligaments that stabilize the wrist joint.

21. What is the olecranon process, and how does it relate to arm movement?

The olecranon process is a bony prominence at the proximal end of the ulna. It forms the point of the elbow and serves as the attachment site for the triceps brachii muscle. When the triceps contract, they pull on the olecranon, causing elbow extension. This process is crucial for pushing movements and straightening the arm.

22. How do the radius and ulna work together in forearm rotation?

The radius and ulna work together in forearm rotation through a unique arrangement. The radius can rotate around its long axis at both its proximal and distal ends, while the ulna remains relatively stationary. During pronation, the radius crosses over the ulna, and during supination, it uncrosses. This mechanism allows for the palm-down and palm-up movements of the hand.

23. What is the interosseous membrane, and what is its function in arm movement?

The interosseous membrane is a fibrous sheet that connects the radius and ulna along their lengths. It serves several functions: it provides stability to the forearm, acts as an attachment site for some forearm muscles, and helps transmit forces from the hand through the forearm to the humerus. It also plays a role in coordinating the movements of the radius and ulna during forearm rotation.

24. What is the significance of the radial notch on the ulna?

The radial notch is a small, concave surface on the lateral side of the proximal ulna. It articulates with the circumference of the radial head, forming the proximal radioulnar joint. This articulation is crucial for the rotational movements of the forearm (pronation and supination) as it allows the radial head to pivot within the radial notch.

25. What is the carrying angle of the arm, and how does it relate to the humerus and radius?

The carrying angle is the outward angle formed between the extended forearm and the upper arm when viewed from the front. It's typically greater in females than in males. This angle is created by the slight outward tilt of the trochlea of the humerus and the corresponding shape of the proximal ulna. The radius follows this angle, which allows the forearms to clear the hips when walking and carrying objects.

26. What is the significance of the radial styloid process?

The radial styloid process is a bony projection at the lateral and distal end of the radius. It serves as an attachment point for ligaments that stabilize the wrist joint. The styloid process also acts as a bony landmark for clinicians when assessing wrist injuries or performing procedures. Its prominence can be felt on the thumb side of the wrist.

27. How does the structure of the proximal radioulnar joint allow for forearm rotation?

The proximal radioulnar joint is formed by the articulation between the head of the radius and the radial notch of the ulna. The radial head is shaped like a disc with a concave superior surface that fits against the capitellum of the humerus. This arrangement allows the radius to rotate around its long axis within the radial notch of the ulna, enabling pronation and supination of the forearm.

28. What is the significance of the nutrient foramen in long bones like the radius and humerus?

The nutrient foramen is a small opening in the shaft of long bones that allows blood vessels and nerves to enter the bone. It's crucial for supplying nutrients to the bone tissue and bone marrow. In the radius and humerus, the direction of the nutrient foramen (pointing towards the elbow) is used to determine the growth end of the bone during development.

29. How does the structure of the radial tuberosity enhance the function of the biceps brachii?

The radial tuberosity is a rough, oval-shaped prominence on the medial side of the proximal radius. It serves as the attachment site for the distal tendon of the biceps brachii muscle. This positioning provides an optimal lever arm for the biceps, enhancing its ability to flex the elbow and supinate the forearm. The roughened surface of the tuberosity also ensures a strong attachment for the tendon.

30. What is the role of the coronoid fossa of the humerus?

The coronoid fossa is a small depression on the anterior surface of the distal humerus, just above the trochlea. Its primary role is to accommodate the coronoid process of the ulna when the elbow is fully flexed. This allows for a greater range of motion in elbow flexion by preventing bone-to-bone contact at the extremes of movement.

31. How does the shape of the radial head contribute to load transmission in the elbow?

The radial head has a disc-like shape with a concave superior surface that articulates with the capitellum of the humerus. This shape allows it to distribute forces effectively across the elbow joint. When a load is applied to the hand, about 80% of the force is transmitted through the radiohumeral joint (between the radius and humerus), with the radial head playing a crucial role in this load transmission.

32. What is the function of the deltoid tuberosity on the humerus?

The deltoid tuberosity is a rough, V-shaped area on the lateral surface of the humeral shaft. It serves as the insertion point for the deltoid muscle. This positioning allows the deltoid to effectively abduct the arm (raise it to the side). The roughened surface of the tuberosity provides a strong attachment point, enabling the powerful movements of the deltoid muscle.

33. How does the interosseous border of the radius contribute to forearm function?

The interosseous border of the radius is a sharp ridge running along the medial aspect of the bone. It serves as an attachment site for the interosseous membrane, which connects the radius to the ulna. This arrangement helps coordinate the movements of the two bones during forearm rotation, provides stability to the forearm, and serves as an attachment site for some of the deep muscles of the forearm.

34. What is the significance of the olecranon fossa of the humerus?

The olecranon fossa is a deep depression on the posterior surface of the distal humerus. Its primary function is to accommodate the olecranon process of the ulna when the elbow is fully extended. This allows for a greater range of motion in elbow extension by preventing bone-to-bone contact. The depth of the olecranon fossa contributes to the stability of the extended elbow.

35. How does the structure of the proximal radius contribute to its ability to rotate?

The proximal end of the radius has a disc-shaped head with a concave superior surface and a cylindrical circumference. The superior surface articulates with the capitellum of the humerus, while the circumference fits into the radial notch of the ulna. This arrangement allows the radius to both pivot on its long axis (for pronation and supination) and rotate against the capitellum (for flexion and extension), enabling the complex movements of the forearm.

36. What is the role of the medial epicondyle of the humerus?

The medial epicondyle is a bony prominence on the inner side of the distal humerus. It serves as the common origin for the flexor muscles of the forearm, including those that flex the wrist and fingers. This arrangement provides leverage for these muscles, allowing for powerful gripping and grasping movements. The medial epicondyle is also a key landmark for locating the ulnar nerve.

37. How does the structure of the radial neck contribute to forearm function?

The radial neck is the slightly narrowed portion of the radius just below the radial head. Its structure allows for the attachment of the annular ligament, which wraps around the radial neck and holds the radius in place against the ulna. This arrangement permits the radius to rotate freely within the ligament during pronation and supination while maintaining stability at the proximal radioulnar joint.

38. What is the significance of the greater and lesser tubercles of the humerus?

The greater and lesser tubercles are bony prominences on the proximal humerus. The greater tubercle serves as an attachment site for the supraspinatus, infraspinatus, and teres minor muscles (rotator cuff muscles), while the lesser tubercle is the attachment site for the subscapularis muscle. These tubercles provide leverage for the rotator cuff muscles, enabling them to rotate and stabilize the shoulder joint effectively.

39. How does the structure of the ulnar notch of the radius contribute to forearm rotation?

The ulnar notch is a concave articular surface on the medial side of the distal radius. It articulates with the head of the ulna, forming the distal radioulnar joint. This structure allows the distal end of the radius to rotate around the relatively stationary ulna during pronation and supination. The shape of the ulnar notch facilitates smooth rotation while maintaining joint stability.

40. What is the function of the anatomical neck of the humerus?

The anatomical neck of the humerus is a narrow groove just below the humeral head. It marks the boundary between the articular surface of the humeral head and the site of attachment for the joint capsule of the shoulder. This structure helps define the extent of the articular surface and plays a role in the biomechanics of the shoulder joint by influencing the range of motion and stability.

41. How does the structure of the radial shaft contribute to its strength and function?

The radial shaft has a triangular cross-section, which provides strength while minimizing weight. It has three borders (anterior, posterior, and interosseous) and three surfaces, which provide attachment sites for various muscles. The slight curvature of the shaft allows it to bow away from the ulna, creating space for the interosseous membrane and muscles, and facilitating forearm rotation.

42. What is the role of the capitellum in elbow and forearm movements?

The capitellum is a rounded projection on the lateral part of the distal humerus that articulates with the radial head. This ball-and-socket-like arrangement allows for smooth rotation of the radial head during forearm pronation and supination. It also contributes to flexion and extension of the elbow. The capitellum's shape and position enable it to distribute forces effectively across the elbow joint during various arm movements.

43. How does the structure of the proximal ulna contribute to elbow stability?

The proximal ulna has several key structures that contribute to elbow stability. The trochlear notch forms a tight articulation with the trochlea of the humerus, providing stability during flexion and extension. The olecranon process extends behind the elbow, limiting hyperextension. The coronoid process fits into the coronoid fossa of the humerus during flexion, adding further stability. Together, these structures create a highly stable hinge joint.