Your arm contains three long bones that work as a coordinated system: the humerus in the upper arm, and the radius and ulna running side by side in the forearm. Together they form one of the most mechanically versatile structures in the human body, allowing everything from lifting heavy objects to threading a needle. What makes this trio especially interesting is how differently each bone behaves under load, and how the connections between them create movement patterns that no single bone could achieve alone.
The Humerus and Its Role as the Upper Arm’s Foundation
The humerus is the single bone of the upper arm, stretching from the shoulder to the elbow. At its top end, the humeral head is a smooth, rounded dome that sits in the shallow socket of the shoulder blade, forming a ball-and-socket joint with an extraordinary range of motion. At its lower end, the humerus flares into two bony bumps you can feel on either side of your elbow (the medial and lateral epicondyles) and a set of articular surfaces that connect with both forearm bones. The inner spool-shaped surface, called the trochlea, receives the ulna, while the rounded outer knob, called the capitulum, meets the radius.
Along its shaft, the humerus is roughly cylindrical near the shoulder and gradually becomes flatter and wider toward the elbow. A shallow groove spirals around the back of the shaft, carrying the radial nerve from the inner side to the outer side of the arm. This spiral groove gives the nerve a close relationship with the bone that becomes clinically relevant whenever the humerus breaks at mid-shaft.
Blood Supply to the Humerus
The humeral head depends on a network of small arteries that approach from the surrounding shoulder muscles. Two circumflex arteries loop around the surgical neck of the humerus to feed the head. Research quantifying these contributions found that the posterior circumflex artery provides about two-thirds of the blood supply to the humeral head, while the anterior circumflex artery supplies the remaining third.1PubMed. Quantitative assessment of the vascularity of the proximal part of the humerus An older anatomical study identified a key intraosseous vessel called the arcuate artery, which branches from the anterior circumflex artery and enters through the greater tuberosity, forming an arch inside the bone.2PubMed. The arteries of the humeral head and their relevance in fracture treatment This matters because fractures that disrupt these vessels can starve the humeral head of blood, leading to a condition where the bone tissue dies, a complication surgeons actively try to avoid when fixing shoulder fractures.
The shaft of the humerus gets its blood supply through a different route. A nutrient artery, typically branching from the brachial artery, enters through a small opening on the bone’s inner front surface. Anatomical study has shown that this artery forms a distinctive hairpin loop on the outer bone surface before plunging into the nutrient foramen and running downward inside the bone. Some people also have an accessory nutrient artery on the back of the shaft, near the radial nerve groove, fed by the radial collateral artery.3PubMed. Anatomic characterization of the humeral nutrient artery: Application to fracture and surgery of the humerus Surgeons take these vascular landmarks seriously when planning surgical approaches to the humeral shaft.
The Radius and Ulna as a Functional Pair
The forearm is unusual among limb segments because it has two bones rather than one, and their arrangement is the key to how you rotate your hand. The ulna is the larger of the two at the elbow, where it forms the bony point you lean on (the olecranon) and wraps around the trochlea of the humerus like a wrench gripping a bolt. The radius, by contrast, is slimmer at the elbow and broader at the wrist, where it forms the main connection with the hand.
At the elbow, the radius has a disc-shaped head that spins against the capitulum of the humerus and also rotates within a notch on the side of the ulna. Anatomical measurements show that the radial head is not always a perfect circle. Roughly six out of ten radial heads are slightly elliptical rather than circular, with a maximum diameter averaging around 22 mm and a minimum around 20 mm.4PubMed. Biometry of the radial head: biomechanical implications in pronation and supination That slight asymmetry has biomechanical consequences for how smoothly the forearm rotates and how replacement implants should be designed.
At the wrist, the roles reverse. The radius carries the broad articular surface that meets the carpal bones of the hand, while the ulna’s contribution is smaller and separated from the carpals by a disc of cartilage. This reversal means the radius is the main load-bearing bone at the wrist and the ulna is the main structural bone at the elbow, with the interosseous membrane stitching them together across the gap.
The Interosseous Membrane and Load Transfer
Between the radius and ulna stretches a tough sheet of fibrous tissue called the interosseous membrane. It is not just a partition between the two forearm compartments. It actively redirects force from one bone to the other, and without it, the forearm’s mechanical behavior changes dramatically.
Cadaver experiments have quantified what the membrane does. With the membrane intact and the forearm in a supinated position, the distal radius carried about 68% of an axial load applied at the wrist, while the distal ulna carried 32%. But at the elbow end, the split evened out to roughly 51% radius and 49% ulna, meaning force had been transferred from the radius across to the ulna on the way up. When researchers then cut the membrane, the proximal and distal loads in each bone equalized, proving the membrane was entirely responsible for that force transfer.5PubMed. The interosseous membrane affects load distribution in the forearm The membrane’s fibers run obliquely from the radius toward the ulna in a direction that pulls upward on the ulnar shaft, essentially redirecting wrist loads toward the elbow through the ulna.
A separate cadaver study added nuance by looking at what happens when the elbow’s alignment changes. When the radial head was firmly in contact with the humerus (a normal valgus alignment), the radius carried the vast majority of the load directly, and only a small fraction traveled through the membrane. In neutral forearm rotation, the distal ulna carried just under 3% of wrist load while the proximal ulna carried about 12%. But when that radial head contact was removed (simulating a varus alignment), the membrane had to work much harder, and the proximal ulna ended up carrying over 90% of the total load.6PubMed. Radioulnar load-sharing in the forearm. A study in cadavera This helps explain why radial head fractures can destabilize the entire forearm and why surgeons often choose to replace, rather than simply remove, a shattered radial head.
How the Forearm Rotates
Pronation (turning your palm face-down) and supination (turning it face-up) happen because the radius physically crosses over the ulna. The ulna stays largely fixed while the radius swivels around it, pivoting at both the elbow and the wrist. The axis of this rotation runs diagonally from the center of the radial head at the elbow to a small depression at the base of the ulnar styloid at the wrist.7Hand Clinics. Skeletal Anatomy of the Hand The total arc of forearm rotation typically ranges from 150 to 180 degrees, and up to about 30 degrees of apparent additional rotation can come from the wrist joints themselves.
The wrist end of this rotation depends on the distal radioulnar joint, where the ulnar head sits in a shallow groove on the radius called the sigmoid notch. Because the ulnar head is much smaller in circumference than the notch, the bony fit alone provides only about a fifth of the joint’s stability. The rest comes from ligaments and the interosseous membrane. In pronation, the dorsal radioulnar ligament is the more important stabilizer; in supination, the palmar radioulnar ligament takes over. Full dislocation of this joint typically requires damage to multiple structures at once.8PubMed. The stabilizing mechanism of the distal radioulnar joint during pronation and supination
The muscles that drive forearm rotation have their own quirks. The supinators (including the biceps) reach their peak mechanical advantage when the forearm is already somewhat pronated, around 40 to 50 degrees into pronation. The biceps is particularly dependent on elbow position, generating its greatest supination leverage at 90 degrees of elbow flexion, which is why you instinctively bend your elbow when trying to turn a stubborn screw. The pronators, by contrast, work best near neutral and are less affected by elbow angle. Brachioradialis acts as a centering muscle, pulling the forearm back toward neutral from either extreme.9PubMed. Moment arms of forearm rotators
Wrist Loading Patterns Change with Forearm Position
The forces passing through the distal radius and ulna are not constant. They shift depending on whether your forearm is pronated, supinated, or neutral. Cadaver testing found that the compressive forces through the distal ulna and the pull of the wrist extensors are both greatest when the forearm is pronated.10PubMed. Force variations in the distal radius and ulna: effect of ulnar variance and forearm motion Meanwhile, axial force through the ulna is broadly reciprocal to that in the radius, peaking in supination. Axial loading of the hand also creates bending forces in both bones, tending to bow the distal radius in one consistent direction while the ulna’s bending pattern varies between individuals.11PubMed. The load-bearing characteristics of the forearm: pattern of axial and bending force transmitted through ulna and radius
These shifting load patterns have practical consequences. Activities performed mostly in pronation, like typing, push-ups, or operating machinery, concentrate stress differently than activities in supination, like carrying a tray. People who develop ulnar-sided wrist pain often find that pronated gripping makes it worse, which tracks with the biomechanical data showing higher ulnar forces in that position.
The Biceps Connection and the Lacertus Fibrosus
The biceps does not simply attach to the radius and pull. It also sends a flat sheet of tissue called the lacertus fibrosus (or bicipital aponeurosis) that fans out across the top of the forearm muscles and blends into the fascia covering the ulna. This structure is easy to overlook, but it meaningfully improves the biceps’ mechanical leverage. When the lacertus fibrosus was experimentally released in cadavers, the biceps lost about 28% of its effective lever arm for elbow flexion and a full 50% of its lever arm for supination.12PubMed. The biomechanical role of the lacertus fibrosus of the biceps brachii Muscle In other words, this thin tissue sheet is responsible for a substantial chunk of the biceps’ real-world pulling power, and tearing it (which sometimes happens along with a distal biceps tendon rupture) can make the muscle feel weaker than the tendon injury alone would predict.
Nerves That Hug the Bones
Three major nerves cross the elbow on their way to the hand, and each one has a stretch where it is pressed directly against bone. Those contact zones make the nerves vulnerable to injury from fractures, compression, or repetitive stress.
The radial nerve wraps around the back of the humeral shaft through the spiral groove, then exits to the front of the arm by piercing the lateral intermuscular septum. Because the nerve is tethered at this exit point, it has limited ability to slide out of the way when the bone moves or breaks.13Scientific Reports. Location of the radial nerve along the humeral shaft between the prone and lateral decubitus positions at different elbow positions A meta-analysis found that radial nerve palsy occurs in roughly 12% of humeral shaft fractures, making it the most commonly injured nerve in upper-arm breaks.14PubMed Central. Localisation of the radial nerve at the spiral groove: A new technique Wrist drop, the inability to extend the wrist and fingers, is the hallmark sign.
The ulnar nerve takes a different risky path, passing behind the medial epicondyle of the humerus through a shallow channel you know as the “funny bone” groove. During elbow flexion, the nerve is stretched and compressed against the bone. This mechanical deformation is measurable: the nerve elongates by roughly 4 to 8 mm at 90 degrees of elbow flexion and up to 10 mm at 135 degrees, with increasing strain on the nerve’s coverings and reduced blood flow through its tiny feeding vessels.15PubMed Central. Geometric and biomechanical perspectives on ulnar nerve compression in cubital tunnel syndrome: pathomechanisms and surgical strategies Chronic compression at this site produces cubital tunnel syndrome, the second most common nerve entrapment in the upper limb, which typically causes numbness in the ring and little fingers and weakness of the hand’s intrinsic muscles.
The median nerve crosses the front of the elbow, lying just in front of the inner quarter of the humeral condyle, separated from bone by only about 2 mm on average.16PubMed. The course of the median and radial nerve across the elbow: an anatomic study This tight proximity means that elbow dislocations and certain fracture patterns can injure the median nerve, though it is less commonly affected than the radial or ulnar nerve at this level. The median nerve is more famously compressed further downstream, at the wrist, where it produces carpal tunnel syndrome.
Growth Plates Around the Elbow
Children’s elbows contain six separate growth centers (secondary ossification centers) that gradually appear on X-rays during childhood and fuse to the main bone during adolescence. A radiological study found that all six centers fuse between the ages of 15 and 19, are consistently still open at 14 or younger, and are uniformly fused above 19. Fusion happens earlier in girls than boys across all six centers. The order of fusion starts with the trochlea, then the capitulum, followed by the lateral epicondyle, medial epicondyle, olecranon, and finally the radial head.17MOJ Orthopedics & Rheumatology. Radiological Assessment of Closure Time of Around Elbow Secondary Ossification Centers, In Khartoum Hospital, Khartoum North Hospital, Omdurman Hospital, Police Hospital, and Umbadda Hospital, In Khartoum, Sudan (December 2009-2010)
This sequence matters clinically because unfused growth plates in children can be mistaken for fractures on X-rays, and certain fracture patterns (like avulsions of the medial epicondyle) specifically target these still-growing centers. Knowing which centers should be visible and which should be fused at a given age helps radiologists distinguish normal development from injury.
How These Bones Change with Age
Like other skeletal sites prone to osteoporotic fractures, the proximal humerus undergoes measurable changes in bone architecture as people age. Research in healthy men found progressive cortical thinning combined with overall bone expansion, a pattern that closely mirrors what happens at the distal radius.18PubMed Central. Age-related changes in proximal humerus bone health in healthy, white males The bone compensates for getting thinner walls by making the overall diameter slightly larger, which partially preserves bending strength. But eventually the cortex becomes thin enough that a fall onto an outstretched hand or the side of the shoulder can fracture the proximal humerus. These fractures are among the most common in older adults, trailing only hip and wrist fractures in frequency among osteoporosis-related breaks.
The distal radius follows its own aging trajectory and is the classic “fragility fracture” site: the fracture you get when you trip and catch yourself with an outstretched palm. In younger adults, the same fall mechanism tends to break the scaphoid (a wrist bone) or dislocate the elbow instead, because the denser bone redirects the failure point elsewhere. As bone density declines, the weakest link shifts to the distal radius.
What Arm Bones Reveal About Ancient Lives
Bioarchaeologists study the muscle attachment sites on ancient arm bones to reconstruct how past populations used their bodies. When muscles are repeatedly stressed, the ridges and bumps where they attach to bone can become enlarged and roughened, forming what are called musculoskeletal stress markers or entheseal changes. A study of ancient Thule Eskimo groups from northwest Hudson Bay found distinct differences in upper-limb stress markers between males and females, suggesting gender-specific activity patterns that were not otherwise visible from the archaeological record alone.19International Journal of Osteoarchaeology. Activity‐induced musculoskeletal stress markers (MSM) and subsistence strategy changes among ancient Hudson Bay Eskimos
Interpreting these markers is not straightforward, though. Body size and age both affect how prominent muscle attachment sites become, independent of activity level. A study of a central California Amerind population tested whether meaningful activity patterns could still be detected after statistically accounting for those confounding factors. The answer was partially yes: after controlling for size and age, the remaining differences between males and females were consistent with male-specific activities like throwing during hunting.20PubMed. Muscle markers revisited: activity pattern reconstruction with controls in a central California Amerind population The humerus is a particularly rich source of these markers because so many muscles with different functions attach along its length, from the deltoid tuberosity to the epicondylar ridges.
The Evolutionary Backstory of the Arm Skeleton
The three-bone arrangement of the arm is not a uniquely human design. It traces back hundreds of millions of years to the transition from fins to limbs in our aquatic ancestors. Musculoskeletal modeling of early tetrapod fossils has revealed how the functional demands on the humerus, radius, and ulna shifted during this period. Early tetrapods like Acanthostega and Pederpes had restricted ability to rotate the humerus along its long axis compared to their fish-like predecessors, but they gained increased muscular leverage for pulling the limb backward (retraction) and greater mobility in elbow flexion and extension.21PubMed Central. Evolution of forelimb musculoskeletal function across the fish-to-tetrapod transition
In practical terms, the earliest land-dwelling vertebrates traded the sweeping rotational freedom of a fin for a limb that could plant itself on the ground and push the body forward. The elbow became a true hinge, and the forearm developed the ability to adjust hand angle independent of the upper arm. Over subsequent evolutionary time, different lineages refined this arrangement in different directions. In humans, the result is a forearm with exceptional rotational freedom and a shoulder with extraordinary range of motion, at the cost of joints that are inherently less stable than those of four-legged animals that use their forelimbs primarily for weight bearing. That trade-off, mobility for stability, is written into the shapes of every bony surface and every ligament in the arm.