The flank is the fleshy region on each side of your torso between the lower edge of your ribcage and the top of your hip bone. It sits behind the front of your abdomen but in front of your spine, wrapping around the side of your body in a zone that most people casually call their “side.” Although the word sounds vague, it refers to a well-defined anatomical area packed with layered muscles, tough connective tissue, major organs, and nerves, all of which help explain why doctors pay close attention when someone reports pain there.
Where the Flank Begins and Ends
Your flank occupies the space between two bony landmarks you can feel with your hands. The upper border is the lower margin of your twelfth rib, the last and shortest rib in the set. The lower border is the iliac crest, the curved rim of your pelvis that you can trace from your hip bone around toward your spine. In front, the flank blends into the lateral part of your abdomen, roughly where the muscles of your belly give way to the muscles of your side. In back, it transitions into the lumbar region alongside your lower spine. Clinically, when a doctor taps or presses the angle where your twelfth rib meets the muscles running along your spine, they are testing what is called the costovertebral angle, a spot right at the rear edge of the flank.
Because the flank covers a generous swath of your side, it sits over both superficial and deep structures. Skin and subcutaneous fat form the outermost layer. Beneath that, several muscles stack on top of each other, separated and reinforced by sheets of dense connective tissue called fascia. Deeper still, the kidneys sit in their fat-padded compartment, and sections of the colon curve through the region. Understanding these layers from the outside in makes it easier to see why so many different conditions can produce “flank pain.”
Muscle Layers of the Flank
The flank gets its structural strength from muscles that overlap each other like plywood sheets, with the fibers in each layer running in a different direction. The outermost is the external oblique, whose fibers angle downward and forward. Just beneath it is the internal oblique, running upward and forward, roughly perpendicular to the external oblique. The deepest of the three flat abdominal muscles, the transversus abdominis, wraps horizontally like a corset. Together, these three muscles compress and support the abdominal contents, but they also let you twist, bend sideways, and flex your trunk.
Deeper and closer to your spine, the quadratus lumborum fills the gap between the twelfth rib and the iliac crest on each side. It is a thick, roughly rectangular muscle that stabilizes the lower spine and assists with side-bending. When the quadratus lumborum develops trigger points, the result is often a deep, aching pain along the flank that gets mistaken for kidney trouble or a disc problem. Both manual therapy techniques, such as muscle energy technique and strain-counterstrain, have been shown to reduce pain and improve function when the quadratus lumborum is the culprit.1Pakistan Journal of Medical and Health Sciences. Effectiveness of Muscle Energy Technique Versus Strain Counter Strain Technique on Trigger Points of Quadratus Lumborum among Low Back Pain Patients
Covering the upper portion of the flank from behind is the latissimus dorsi, the broad “lat” muscle that sweeps from the lower spine and pelvis upward to the arm. Its tendinous attachment, or aponeurosis, merges into the connective tissue layers that reinforce the entire area, linking your trunk muscles to the fascia that stabilizes the spine. During trunk rotation, the obliques and the latissimus dorsi work together in a coordinated pattern: the internal oblique and latissimus dorsi on the side you are turning toward act as primary drivers, while their counterparts on the opposite side control the movement as brakes.2BioMed Research International. Angular velocity affects trunk muscle strength and EMG activation during isokinetic axial rotation
The Thoracolumbar Fascia
Running through and around the muscles of the flank is one of the most structurally complex sheets of connective tissue in the body: the thoracolumbar fascia (TLF). This is not a single membrane but a composite of aponeurotic sheets and fascial layers that form a tube-like sheath around the paraspinal muscles, the deep muscles that run alongside your vertebral column. The rear wall of this sheath is made up of a posterior layer, which itself has been described as having superficial and deep portions. A middle layer forms the front wall of the sheath, and on the side of the body these layers converge at a thickened seam called the lateral raphe, where the aponeuroses of the abdominal muscles join in.3PubMed Central. The thoracolumbar fascia: anatomy, function and clinical considerations
Recent anatomical work has added nuance to this picture. While some researchers have treated the posterior layer as having neatly separable sub-layers, dissection studies have found that the aponeuroses of the latissimus dorsi, internal oblique, and other muscles blend into a continuous structure that resists being peeled apart into distinct sheets, at least in most of the lumbar region. Collagen bundles running diagonally from the spinous processes toward the iliac crest appear to be inseparable elements woven throughout the posterior layer rather than belonging to any one lamina.4PubMed Central. Inferolateral collagen bundles as inseparable elements in the posterior layer of the thoracolumbar fascia
On the microscopic level, the deep fascia of the flank and lumbosacral region has a three-layer sandwich structure: a thick, collagen-rich middle layer flanked by thinner layers of looser connective tissue. In the fascia lata of the thigh, the dense middle layer is roughly 210 to 260 micrometers thick, while in the thoracolumbar fascia it runs about 300 to 340 micrometers, reflecting the greater mechanical demands placed on the lower back and flank.5PubMed Central. 3D Microstructural Characterization of Human Deep Fascia Using Optical Projection Tomography, Digital Light Sheet Microscopy, and Magnetic Resonance Microscopy The upshot for your everyday life is that this fascia acts as a tension-transmitting web: when you lift, twist, or brace your core, forces travel through it from muscle to muscle and from one side of the trunk to the other.
Organs That Sit Behind the Flank
The kidneys are the headlining organs of the flank. Each kidney lies in the retroperitoneal space, behind the membrane that lines the abdominal cavity, roughly at the level of the twelfth rib. They are cushioned by a layer of perirenal fat and held in place partly by the surrounding fascia. The right kidney usually sits a bit lower than the left because the liver pushes it down. This retroperitoneal position is the reason a doctor will tap your back at the costovertebral angle when investigating possible kidney problems. Pain or tenderness there raises suspicion for kidney stones, infections, or other renal issues.
The ascending colon on the right side and the descending colon on the left also pass through the flank region, sitting just in front of the kidneys. On the left, the lower tip of the spleen tucks under the ribs near the upper boundary of the flank, while on the right, the lower edge of the liver extends into the same neighborhood. Injuries to the flank, whether from a fall, a car accident, or a direct blow, can therefore involve abdominal organs, kidney damage, or both, which is why emergency physicians assess the flank carefully during trauma evaluations.
Why Doctors Focus on Flank Pain
Flank pain is one of those complaints that sits at a crossroads. It could come from the muscles, the fascia, the nerves, the kidney, the colon, or even referred pain from the spine. That ambiguity is exactly why clinicians take it seriously. Among the most common acute causes is a kidney stone working its way down the ureter. The classic presentation involves severe, colicky pain radiating from the flank toward the groin, sometimes accompanied by nausea and blood in the urine.
To help distinguish kidney-related pain from muscular or other causes, doctors often check for costovertebral angle (CVA) tenderness by thumping the flank just below the twelfth rib near the spine. However, the test is far from definitive. A study evaluating CVA tenderness in patients with suspected ureteral stones found a sensitivity of about 65% and a specificity of only 50%, meaning it correctly flagged roughly two-thirds of people who had a stone but incorrectly flagged half of those who did not.6PubMed Central. Evaluation of the usefulness of costovertebral angle tenderness in patients with suspected ureteral stone In practice, that makes it a useful initial clue rather than a definitive test. Imaging, urinalysis, and the full clinical picture still drive the diagnosis.
Other causes of flank pain include pyelonephritis (kidney infection), which typically adds fever and malaise to the picture; muscular strains from sports or heavy lifting; rib fractures; shingles affecting a thoracic dermatome; and, less commonly, conditions like an aortic aneurysm or a retroperitoneal bleed. Because the list is long and the stakes vary from trivial to life-threatening, persistent or severe flank pain warrants medical evaluation.
Nerve Entrapment in the Flank
A lesser-known cause of chronic flank pain involves the lateral cutaneous branches of the intercostal nerves, the small sensory nerves that exit between the ribs and travel through the abdominal wall muscles to supply sensation to the skin of the flank. When one of these nerve branches becomes trapped or pinched as it passes through the muscle layers, it can produce a sharp, burning, or stabbing pain in a well-localized spot on the side. This condition has been termed Lateral Cutaneous Nerve Entrapment Syndrome, or LACNES.7PubMed. Lateral Cutaneous Nerve Entrapment Syndrome (LACNES): A previously unrecognized cause of intractable flank pain
LACNES can be tricky to diagnose because the pain mimics kidney or musculoskeletal conditions, and standard imaging often comes back normal. The hallmark finding is a small, exquisitely tender point on the flank that becomes more painful when the abdominal muscles are tensed, a positive Carnett’s sign. In one reported case, a patient developed LACNES after performing abdominal exercises, and ultrasound revealed a small mass in the internal oblique muscle at the site of nerve compression. A trigger point injection at that spot provided dramatic relief.8PubMed Central. Successful Nerve Block Therapy for Lateral Cutaneous Nerve Entrapment Syndrome (LACNES) Triggered by Exercise For patients who do not respond to injections, surgical neurectomy, removing the entrapped nerve segment, is another option that has been explored.9PubMed. Success rates of a surgical neurectomy for lateral cutaneous nerve entrapment syndrome (LACNES)
The broader lesson from LACNES is that the flank’s layered anatomy creates natural pinch points. Small nerves have to thread through tight fascial and muscular tunnels, and anything that swells the muscle, shifts the fascia, or changes the mechanics of the trunk can turn one of those tunnels into a trap.
Weak Spots and Lumbar Hernias
Most people think of hernias as a groin or belly-button problem, but the flank has its own vulnerable areas. Two triangular gaps in the posterior abdominal wall, known as the superior and inferior lumbar triangles, are spots where the overlapping muscle layers thin out enough to leave a relative weak point. The superior lumbar triangle (Grynfeltt-Lesshaft triangle) sits just below the twelfth rib, bordered by muscles and the edge of the erector spinae group. The inferior triangle (Petit’s triangle) lies just above the iliac crest.
A lumbar hernia occurs when abdominal contents, usually fat or a portion of bowel, push through one of these weak spots. It is rare, but when it does happen it produces a soft, sometimes reducible bulge in the flank that can be mistaken for a lipoma or a muscle strain.10PubMed Central. Primary superior lumbar hernia: a rare cause of lumbar swelling The superior triangle is the more common site, likely because the inferior triangle’s floor is partly reinforced by the external oblique aponeurosis. Surgical repair is the standard treatment, and awareness of these triangles matters for surgeons planning incisions in the flank to avoid inadvertently widening an already weak area.
The Flank as a Surgical Window
Because the kidneys and ureters sit directly behind the flank, surgeons have long used lateral and posterolateral approaches through this region to access them. In open surgery, a flank incision running along or just below the twelfth rib allows the surgeon to reach the kidney without entering the abdominal cavity proper, reducing the risk of disturbing the bowel. The muscle layers are split or divided along their fiber direction where possible, then repaired in layers during closure.
Minimally invasive techniques have adapted this same principle. In retroperitoneoscopic surgery, small ports are placed through the flank to reach the kidney and its surroundings from behind the peritoneum. This approach avoids the abdominal cavity entirely, which can mean less postoperative pain and a faster recovery. Getting the anatomy right is critical: cadaver dissection studies have been used to refine port placement and avoid injury to structures like the renal vessels and the peritoneum itself.11PubMed Central. Technical and Anatomical Aspects of Retroperitoneoscopic Renal Surgery: A Summary of Tribulations and Resolutions Encountered at a Tertiary Care Institute of North India The flank’s layered, mostly muscular wall makes it a forgiving corridor for surgeons, but the trade-off is limited working space compared to a front-of-the-abdomen approach.
Fat Distribution and the “Love Handle”
If you have ever heard someone complain about their “love handles,” they are talking about the subcutaneous fat that accumulates over the flank. This area is one of the body’s preferred fat-storage depots, particularly in men, though women store fat there too. The fat in the flank sits in two planes: a superficial layer just beneath the skin and a deeper layer between the muscles. Cosmetic surgeons performing liposuction in this region have noted that fat is distributed fairly evenly between those planes, which affects how the procedure is done. Ultrasound-assisted techniques address both layers, while suction-based liposuction is typically confined to the superficial plane to preserve structural support and encourage the skin to retract smoothly.12PubMed Central. Muscle Delimitation by Segmental Liposuction: A New Anatomical Approach
The stubbornness of flank fat has a biological basis. Subcutaneous adipose tissue in the trunk tends to have a higher density of alpha-adrenergic receptors compared to other body regions, which makes the fat cells there less responsive to the hormonal signals that mobilize stored fat during exercise. This is why the love-handle area is often among the last places people lose fat even on a calorie deficit, and why spot-reduction exercises targeting the obliques do not preferentially shrink the overlying fat pad.
How the Flank Develops Before Birth
The flank does not simply appear as a continuous wall; it is built during embryonic development by the folding and fusion of the lateral body wall. During roughly the fourth week of development, lateral folds of tissue sweep inward from each side to close the ventral body wall and form the abdominal cavity. When this process goes smoothly, the result is the seamless muscular and fascial tube that becomes your trunk. When it fails, the result can be ventral body wall defects of varying severity.13Seminars in Ultrasound, CT and MRI. When Closure Fails: What the Radiologist Needs to Know About the Embryology, Anatomy, and Prenatal Imaging of Ventral Body Wall Defects
The lateral body wall folds are responsible for closing the thoracic, abdominal, and pelvic portions of the ventral wall. That means the muscles, fascia, and connective tissue of the flank are derivatives of this folding process. The complexity of the layered muscle arrangement, three flat muscles with crisscrossing fiber directions, is set up early in embryogenesis by the migration and differentiation of mesoderm-derived cells. Understanding this origin also explains why congenital defects in the abdominal wall, while most visible at the front, reflect disruptions in the same developmental process that shapes the lateral wall.
The Flank and Trunk Shape in Human Movement
The proportions of the flank and lower trunk have functional consequences beyond aesthetics. Research on how trunk shape relates to running performance found that individuals with a narrower lower trunk, essentially a narrower pelvis and less lateral bulk through the flank and waist, tended to reach higher running speeds. The associated body shape also included a flatter ribcage and a taller trunk overall.14PubMed Central. Variation in human 3D trunk shape and its functional implications in hominin evolution These findings line up with what is observed in elite distance runners, who tend to have slim flanks and narrow hips, and they echo broader patterns in human evolution. As our ancestors became endurance runners on the African savanna, changes in pelvis width, trunk length, and lateral trunk shape were part of the trade-offs between efficient upright locomotion and other functional needs like childbirth and load carrying.
In everyday movement, the flank muscles contribute to virtually every activity that involves rotating or stabilizing the trunk: throwing a ball, swinging a golf club, carrying a child on one hip, or simply staying upright on a moving bus. The coordinated firing of the obliques, transversus abdominis, quadratus lumborum, and latissimus dorsi transfers force between the upper and lower body, and the thoracolumbar fascia distributes that force across the full width of the trunk. When any link in that chain is injured, weakened, or restricted, the flank often becomes the place where people feel it first.