The loins are the area on each side of the spine between the lowest ribs and the top of the pelvis, roughly where your hands land when you put them on your hips and slide them toward your back. In modern medical language this region is usually called the lumbar area or the flank, but “loin” remains a perfectly valid anatomical term and still appears in clinical descriptions of pain patterns. Because the loins sit over several important muscles, nerves, and organs, pain in this zone can come from a surprisingly wide range of causes, from a simple muscle knot to a kidney stone to rarer emergencies that need immediate attention.
Where Exactly Are the Loins
If you picture the torso from behind, the loins occupy the soft area lateral to the lumbar spine. The upper boundary is roughly the twelfth rib, the lowest floating rib you can feel near the back. The lower boundary is the iliac crest, the bony ridge at the top of your pelvis. The spine itself forms the inner edge, and the side of the abdomen forms the outer edge. That wedge-shaped territory is the loin, and you have one on each side.
Beneath the skin and a layer of fat lies a region packed with structures. A well-defined pad of fatty tissue called the lumbo-gluteal adipose body sits in the triangular zone where the loin meets the upper buttock and flank, cushioning deeper structures and sometimes becoming relevant in imaging studies when doctors are trying to figure out what is causing pain.1PubMed. The lumbo-gluteal adipose body Deeper still, two key muscles dominate: the erector spinae group running vertically along the spine, and the quadratus lumborum, a broad, flat muscle connecting the lowest rib to the pelvis on each side. The quadratus lumborum is especially relevant to loin pain because it stabilizes the spine during almost every movement you make, from walking to bending to simply sitting upright.
Organs That Sit Behind the Loins
One reason loin pain gets so much clinical attention is that the kidneys sit directly beneath this area. Each kidney is tucked against the posterior abdominal wall, roughly at the level of the twelfth rib, protected partly by the lower ribs and partly by the muscles and fat of the loin. The adrenal glands perch on top of the kidneys. Because these organs are retroperitoneal, meaning they sit behind the membrane that lines the abdominal cavity rather than inside it, pain originating from them tends to be felt in the back and flank rather than in the front of the belly.
The ureters, the thin tubes that carry urine from each kidney down to the bladder, run through this region as well. When a kidney stone gets stuck somewhere along a ureter, the pain often starts in the loin and radiates downward, sometimes all the way into the groin. Parts of the colon also curve through the retroperitoneal space near the loins. On the right side, the ascending colon and the cecum (where the appendix attaches) sit nearby. On the left, the descending colon runs along the back wall. In women, the ovaries and fallopian tubes are close enough to the lower loin that problems there can present as flank pain too.
Musculoskeletal Causes of Loin Pain
The most common reason people feel pain in the loin region is musculoskeletal, not organ-related. Muscle strain from lifting, twisting, or prolonged sitting accounts for a large share of complaints. The quadratus lumborum is a frequent culprit. This muscle can develop painful trigger points, tight knots that refer aching or sharp pain across the loin and sometimes into the hip or buttock. Myofascial pain syndrome in the quadratus lumborum is common enough that researchers have compared different treatment approaches for it, including trigger point injections and shockwave therapy.2PubMed Central. Extracorporeal Shock Wave Therapy Versus Trigger Point Injection in the Treatment of Myofascial Pain Syndrome in the Quadratus Lumborum
The lumbar spine itself is another major source. Disc bulges, facet joint arthritis, and degenerative changes in the lower spine all generate pain that people describe as being “in the loins.” Because the lumbar plexus, the network of nerves arising from the lower thoracic and lumbar spine, exits through this area, irritation of those nerves can send pain radiating into the hip, groin, or thigh.3Folia Morphologica. Lumbar plexus — review This is why a doctor hearing “loin pain” will often check for signs of nerve involvement, like numbness, tingling, or weakness in the leg, alongside checking the organs.
A practical clue for you: musculoskeletal loin pain usually changes with movement or position. It gets worse when you bend, twist, or press on the area, and it often eases when you find a comfortable position and stay still. Organ-related pain, by contrast, tends to be constant or colicky regardless of how you move, and pressing on the muscles does not reproduce it.
Kidney Stones and Renal Colic
When a stone forms in the kidney and starts to move into the ureter, the resulting pain, called renal colic, is among the most intense pain people experience. It typically hits suddenly, with no warning, and settles in the loin on the affected side. About half the time nausea or vomiting comes along with it, and in some cases the drop in blood pressure can cause fainting.4PubMed Central. Pathophysiology and symptoms of renal colic in children – a case report The exact location of the pain shifts depending on where the stone is lodged. A stone high in the ureter tends to cause pain in the loin and back. As it moves lower, the pain migrates toward the lower abdomen and groin. A stone near the bladder can even cause urinary urgency or burning.
The severity depends partly on individual pain tolerance and partly on how much pressure builds up behind the blockage. If a stone sits in one spot for a while, the body activates mechanisms to reduce that pressure, which can actually lessen the pain temporarily, sometimes fooling people into thinking the stone has passed when it has not.4PubMed Central. Pathophysiology and symptoms of renal colic in children – a case report Kidney stones are often discovered incidentally on imaging done for unrelated reasons, meaning you can have stones without pain until one decides to move.
Kidney infections, or pyelonephritis, are another renal cause of loin pain. The pain tends to be more constant and dull compared to the sharp waves of colic, and it is almost always accompanied by fever, chills, and painful urination. Pressing firmly over the affected kidney, a clinical test called costovertebral angle tenderness, usually reproduces the pain sharply.
Loin Pain Hematuria Syndrome
A much rarer kidney-related condition worth knowing about is loin pain hematuria syndrome, or LPHS. It causes severe, recurring flank pain alongside blood in the urine, either visible to the naked eye or detectable only under a microscope. The syndrome was first described in 1967 and remains poorly understood even today.5Clinical Kidney Journal. Loin pain hematuria syndrome Estimates put its prevalence at roughly one in eight thousand people.6PubMed. Loin pain hematuria syndrome
What makes LPHS frustrating for both patients and doctors is that standard imaging and blood tests often come back normal. The kidneys look fine on scans, and there is no stone, no infection, and no obvious structural problem. The pain can be unilateral or bilateral, intermittent or persistent, and severe enough to be disabling. Several theories exist about its cause, including small blood vessel abnormalities within the kidney, but none has been proven definitively. Treatment options range from pain management to, in extreme cases, surgical interventions on the kidney’s nerve supply. Because the diagnosis is essentially one of exclusion, people with LPHS often go through extensive testing before the label is applied.
When the Problem Is Not the Kidney
One of the trickier aspects of loin pain is that organs you would not immediately associate with the flank can send pain there. On the right side, a retrocecal appendix, meaning an appendix that points upward behind the cecum instead of hanging down in the classic position, can cause right flank or even right upper abdominal pain instead of the textbook lower-right-quadrant pain most people expect from appendicitis. In one study of patients with this anatomical variant, only about half had pain in the lower right abdomen. Roughly a quarter presented with right flank pain, and nearly one in five had pain in the upper right abdomen.7PubMed. Ascending retrocecal appendicitis: clinical and computed tomographic findings About half of these cases involved a perforated appendix with abscess formation, likely because the unusual pain location delayed the correct diagnosis.
Retrocecal appendicitis can even mimic a kidney problem so convincingly that it presents as a perinephric abscess, a pocket of infection around the kidney, before anyone suspects the appendix.8PubMed. Right perinephric abscess: a rare presentation of ruptured retrocecal appendicitis CT scanning reliably identifies the inflamed appendix and any retroperitoneal inflammation, even when the clinical picture is misleading.9PubMed Central. Ascending retrocecal appendicitis presenting with right upper abdominal pain: utility of computed tomography
In women, ovarian torsion, where an ovary twists on its blood supply, is a gynecological emergency that can present as sudden flank or lower abdominal pain. It should be considered in any female patient with acute abdominal pain, because delay in untwisting the ovary can lead to loss of the organ.10PubMed Central. Twisting Ovaries: Three Cases of Ovarian Torsion Ovarian cysts that rupture or hemorrhage can produce similar symptoms, though the pain is usually less dramatically sudden.
Vascular Emergencies That Mimic Loin Pain
Perhaps the most dangerous mimic of loin pain is a ruptured abdominal aortic aneurysm. The aorta, the body’s largest artery, runs down through the abdomen just in front of the spine. When an aneurysm in this vessel ruptures or begins to leak, the pain can radiate into the flank and feel remarkably similar to renal colic, complete with nausea and sometimes urinary symptoms. In older patients with risk factors like high blood pressure and a history of smoking, this possibility needs to stay on the radar. Early recognition and rapid imaging are critical because a ruptured aortic aneurysm is fatal without emergency surgery.11Australasian Journal of Clinical Nephrology. When Renal Colic Isn’t Renal Colic: Ruptured Abdominal Aortic Aneurysm Masquerading as Flank Pain
The lesson here is that loin pain in someone over sixty, especially if they have vascular risk factors and the pain does not follow the typical pattern for kidney stones, warrants urgent evaluation. A kidney stone is by far the more common diagnosis, but the consequences of missing an aortic emergency are so severe that doctors are trained to keep it on their differential list.
Nerve Pain Masquerading as Something Deeper
Not all loin pain comes from organs or muscles. The intercostal nerves, which run along the lower ribs, and the cutaneous branches that penetrate the abdominal wall can become irritated or entrapped. When they do, the result is a sharp, burning, or aching pain in the flank or abdomen that can convincingly mimic visceral organ pain. These peripheral nerve syndromes have been recognized for over a century but remain under-diagnosed because the pain feels like it is coming from inside the body.12American Journal of Emergency Medicine. Peripheral neuropathic mimics of visceral abdominal pain: Can physical examination limit diagnostic testing?
One example is slipping rib syndrome, where a lower rib moves abnormally and irritates the intercostal nerve above or below it. Another is anterior cutaneous nerve entrapment, where a small nerve gets pinched as it passes through the abdominal muscles. Both can cause pain in the loin or flank area that gets worse with certain postures or movements. The good news is that a careful physical examination, including pressing along the rib margins and testing for localized tenderness, can often identify these problems and spare you from rounds of imaging and blood work aimed at organs that are perfectly fine. A local anesthetic injection at the point of tenderness can confirm the diagnosis by eliminating the pain temporarily.
This is one reason why a thorough hands-on exam still matters despite all the scanning technology available. The lumbar plexus alone gives off six peripheral nerves, each with anatomical variations from person to person, and any of them can be a source of pain when compressed or irritated.3Folia Morphologica. Lumbar plexus — review
How Doctors Sort Through the Possibilities
Given how many different structures share the loin region, figuring out what is actually causing the pain requires a systematic approach. History is the first filter: when did the pain start, is it constant or comes and goes, does it radiate anywhere, is there fever, blood in the urine, or changes in bowel habits? The physical exam adds more information, checking for tenderness over the kidney, muscle spasm, signs of nerve irritation, or an abdominal mass.
Imaging usually comes next. For suspected kidney stones, a CT scan without contrast dye is the standard first-line test in most emergency departments. But when the pain turns out not to be from a stone, both CT and ultrasound perform reasonably well at identifying alternative causes. In a comparison study, ultrasound caught non-stone causes of flank pain with about 78% sensitivity, while non-contrast CT managed about 74% sensitivity for those same alternative diagnoses.13PubMed. Acute flank pain: comparison of unenhanced helical CT and ultrasonography in detecting causes other than ureterolithiasis Both had very high specificity, meaning that when they identified a cause, they were almost always right. The two tests had slightly different blind spots: ultrasound missed a case of appendicitis and an ovarian torsion, while CT missed a complicated ovarian cyst and a case of pleuritis. Neither is perfect, which is why clinical judgment guides which test is ordered and whether additional imaging is needed.
Blood and urine tests fill in more of the picture. A urinalysis showing blood suggests a kidney or urinary tract problem. White blood cells in the urine point toward infection. Elevated inflammatory markers in the blood can signal appendicitis, an abscess, or pyelonephritis. Normal results across the board, paired with reproducible tenderness on physical exam, might redirect attention toward musculoskeletal or nerve-related causes.
Why “Loin” Persists in Medical Language
You might wonder why doctors and medical texts still use the word “loin” when “flank” and “lumbar” seem more precise. Part of the answer is historical inertia. The term has been embedded in clinical vocabulary for centuries, appearing in condition names like loin pain hematuria syndrome that are now too established to rename. Another reason is that “loin” captures a slightly different anatomical emphasis than “flank” or “lumbar” alone. “Flank” tends to refer more to the side of the body, while “lumbar” emphasizes the spine. “Loin” comfortably bridges both, covering the posterior and lateral territory between the ribs and pelvis without committing to a purely spinal or purely lateral focus.
In everyday English, “loins” carries other connotations, from biblical language about “girding your loins” to butcher shop terminology for cuts of meat from the same anatomical region in animals. The pork loin and beef tenderloin come from this same area of the animal’s back, between the last rib and the hip. That overlap is not coincidental; human and animal anatomy names developed in parallel, and the same Latin root gave us both the medical and the culinary term. The word traces back to the Latin “lumbus,” meaning the lower back, which also gave us “lumbar.” So when your doctor writes “loin tenderness” in a chart and you see “pork loin” at the grocery store, the same piece of anatomy is being described in two very different contexts.