Pain at the back of the pelvis most often traces to the sacroiliac joint, the large, irregularly shaped connection between your sacrum (the triangular bone at the base of the spine) and each ilium (the wing-shaped bones you feel when you put your hands on your hips). But the posterior pelvis is a crowded neighborhood of bones, ligaments, nerves, and muscles, and several other structures can produce similar pain. Sorting out which one is responsible takes some detective work, because the symptoms overlap and imaging alone can be misleading.
The Sacroiliac Joint and Why It Causes So Much Trouble
The sacroiliac joint (SIJ) bears the weight of your entire upper body and transfers it into your legs every time you stand, walk, or climb stairs. Unlike a ball-and-socket joint that moves freely, the SIJ is designed for stability. It relies on a network of thick ligaments, fascia, and surrounding muscles to create what clinicians call “force closure,” a compressive squeeze that locks the joint in place during loading. When you take a step, the sacrum tilts slightly forward (a motion called nutation), which tensions the dorsal ligaments and presses the back of the ilia together, stiffening the joint just enough to handle the incoming force.
1PubMed. Form and Force Closure of the Sacroiliac JointsWhen this system works well, you never think about it. When it doesn’t, pain shows up in the back of the pelvis, and the exact location depends on which part of the joint is involved. Irritation of the upper portion tends to produce pain along the iliac crest, while problems in the middle portion cause pain near the dimples on your lower back, and lower-joint issues send pain into the mid-to-lower buttock.
2PubMed. Referred pain location depends on the affected section of the sacroiliac jointSIJ pain can arise from too much or too little movement at the joint, from compression or shearing forces, from soft-tissue injury, from inflammation, or from changes in adjacent structures like the lumbar spine. Leg-length differences and prior lumbar fusion surgery also increase the load on the SIJ. And women tend to have more SIJ mobility, higher ligament strains, and greater pelvic loads than men, which partly explains why posterior pelvic pain is more common in women.
3PubMed Central. Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of PainAs the ligaments that stabilize the SIJ lose stiffness, joint stress and angular motion increase, putting extra strain on the remaining ligaments. The interosseous sacroiliac ligament, buried deep between the bones, takes the biggest hit.
4Spine. A Finite Element Analysis of Sacroiliac Joint Ligaments in Response to Different Loading ConditionsInflammatory Sacroiliitis and Axial Spondyloarthritis
Not all SIJ pain is mechanical. Sacroiliitis, or inflammation of the sacroiliac joint itself, can be part of a group of autoimmune-related conditions known as axial spondyloarthritis. This is a chronic, rheumatic disease that targets the SIJ, the spine, and the points where tendons and ligaments attach to bone.
5PubMed Central. Clinical Manifestations and Diagnosis of Axial SpondyloarthritisThe distinguishing feature of inflammatory SIJ pain is its pattern. It tends to start before age 40, comes on gradually rather than after a specific injury, feels worse with rest and better with movement, and often wakes you in the second half of the night. Mechanical SIJ pain, by contrast, usually worsens with activity and eases when you stop moving. The distinction matters because inflammatory sacroiliitis responds to different treatments, including medications that target the underlying immune process, and early diagnosis can change the long-term trajectory of the disease.
6PubMed Central. Sacroiliitis – early diagnosis is keyIf your posterior pelvic pain has been hanging around for more than three months, is worse in the morning or after prolonged rest, and you’re under 45, it’s worth asking your doctor to evaluate for inflammatory causes rather than assuming it’s a simple muscle or joint strain.
Pregnancy and Postpartum Pelvic Pain
Pregnancy is one of the most common triggers for posterior pelvic pain, and the reasons are mechanical, hormonal, and anatomical all at once. As pregnancy progresses, the growing uterus shifts the center of gravity forward, increasing the curve of the lower back and changing how forces travel through the pelvis. Body weight increases. Hormones like relaxin and estrogen loosen the ligaments connecting the pubic symphysis and the sacroiliac joints, making the pelvis more mobile in preparation for delivery.
7PubMed Central. Sacroiliac Joint and Pelvic Dysfunction Due to Symphysiolysis in Postpartum WomenThat increased mobility is useful during birth but can be a source of ongoing pain afterward, especially if the symphysis separates more than usual. Women already have wider pubic angles and less SIJ curvature than men, giving them more baseline mobility to begin with. When you add the hormonal and mechanical changes of pregnancy on top of that, the SIJ can become hypermobile, leaving the surrounding ligaments and muscles struggling to stabilize the joint during everyday activities like walking, rolling over in bed, or standing on one leg.
In many women, this resolves within a few months after delivery as hormones normalize and core strength returns. Targeted pelvic stabilization exercises during the postpartum period have been shown to reduce pain and improve both core strength and functional ability.
8Journal of Endometriosis and Pelvic Pain Disorders. Effect of pelvic stabilization exercises in postnatal women with pelvic girdle painDeep Gluteal Syndrome and Nerve Entrapment
If your pain extends from the back of the pelvis into the buttock, hip, or down the back of the thigh, the problem may not be the joint at all. Deep gluteal syndrome is an umbrella term for conditions in which the sciatic nerve or other nerves get compressed in the space behind the hip joint. The most familiar version is piriformis syndrome, where the piriformis muscle squeezes the sciatic nerve, but the same space contains other muscles and structures that can do the same thing.
9PubMed Central. Deep gluteal space problems: piriformis syndrome, ischiofemoral impingement and sciatic nerve releaseDeep gluteal syndrome is considered underdiagnosed. It can involve the gemelli and obturator internus muscles, the hamstring origin, or the gluteal nerves themselves. Symptoms typically include pain in the buttock that may radiate, along with numbness, tingling, or a feeling of the leg “not working right.” Prolonged sitting often makes it worse, and activities that stretch or compress the deep buttock structures, like crossing your legs or climbing stairs, can flare it up.
10PubMed Central. Deep Gluteal syndrome: An underestimated cause of posterior hip painSacral Stress Fractures
A less obvious but potentially serious cause of posterior pelvic pain is a stress fracture of the sacrum itself. These come in two varieties. Insufficiency fractures happen when normal forces act on bone that has been weakened, usually by osteoporosis. Fatigue fractures happen when abnormally high or repetitive forces act on normal bone, as in runners or military recruits who ramp up training too quickly.
11PubMed. Sacral insufficiency fractures: current concepts of managementSacral insufficiency fractures typically appear in older adults, especially postmenopausal women, and may come on without any clear injury. They are frequently missed because the symptoms overlap with common lumbar spine problems. Risk factors include osteoporosis, pelvic radiation therapy, rheumatoid arthritis, and long-term corticosteroid use. For fatigue fractures, the strongest risk factors are a recent jump in training intensity and an inadequate diet.
12PubMed Central. Risk factors associated with sacral stress fractures: a systematic reviewThe pain from a sacral stress fracture tends to be deep, achy, and hard to localize. It often worsens with weight-bearing and can be confused with SIJ dysfunction or lumbar disc disease. If you have risk factors for weakened bone and develop new-onset posterior pelvic pain, especially without an obvious mechanical cause, a stress fracture should be on the list of things to rule out. Standard X-rays often miss them, and MRI or a bone scan is usually needed.
13PubMed Central. Sacral insufficiency fracture, usually overlooked cause of lumbosacral painIschiofemoral Impingement
The ischium is the bone you sit on. Directly adjacent to it is the lesser trochanter, a bony prominence on the inner part of the upper femur. Between these two landmarks sits the quadratus femoris muscle. When the space between the ischium and the lesser trochanter narrows too much, the muscle gets pinched, producing pain in the deep posterior hip and buttock.
14PubMed Central. Ischiofemoral Impingement Syndrome: Clinical and Imaging/Guidance Issues with Special Focus on UltrasonographyIn one study, all patients with quadratus femoris abnormalities were women between 30 and 71, and they had significantly narrower ischiofemoral spaces compared to controls. Every affected patient showed swelling in the muscle, and about a third had partial tears.
15PubMed. Ischiofemoral impingement syndrome: an entity with hip pain and abnormalities of the quadratus femoris muscleCertain pelvic shapes, including wider ischial angles and increased femoral neck angles, predispose people to this condition regardless of age or sex.
16PubMed. Pelvic morphology in ischiofemoral impingement The pain typically worsens with hip extension and external rotation, movements like taking a long stride backward or turning your foot outward. Because the location overlaps with hamstring and SIJ pain, ischiofemoral impingement often goes unrecognized until imaging shows the narrowed space and muscle changes.
Tailbone Pain
If your pain is lower and more central, right at the very bottom of the spine, the coccyx (tailbone) is the likely suspect. Coccydynia, or tailbone pain, is most often linked to abnormal mobility of the coccyx. When you sit, the coccyx should flex slightly. In some people, it flexes too much (hypermobility, defined as more than 25 degrees) or shifts out of position entirely (subluxation, where the coccyx displaces more than 25%).
17PubMed. Standardized radiologic protocol for the study of common coccygodynia and characteristics of the lesions observed in the sitting positionEither pattern can set off a chronic inflammatory process that degrades the joint between coccygeal segments over time.
18PubMed Central. Coccydynia The classic story is pain that worsens with sitting, especially on hard surfaces, and improves when you stand. A fall onto the tailbone, prolonged cycling, or childbirth can trigger it. Coccydynia is annoying but rarely dangerous, and most cases improve with cushioning, postural changes, and physical therapy.
Myofascial Trigger Points and Pelvic Floor Muscles
Sometimes the back of the pelvis hurts because a muscle there has developed a trigger point, a hyperirritable spot in a taut band of muscle that can produce local and referred pain. The gluteus medius, which wraps around the outside and back of the pelvis, is a common offender. Research mapping its nerve-entry points to the classic trigger-point locations described in clinical texts has confirmed that the posterosuperior and anteroinferior regions of the muscle are especially prone to developing these painful knots.
19PubMed Central. Anatomical Investigation of the Gluteus Medius Muscle Innervation and Its Topographical Correspondence With Myofascial Trigger PointsThe pelvic floor muscles also play a role that is often overlooked. In men with chronic pelvic pain, researchers have found clear relationships between myofascial trigger points both inside and outside the pelvic floor and the locations where patients report pain.
20PubMed. Painful myofascial trigger points and pain sites in men with chronic prostatitis/chronic pelvic pain syndrome Women with conditions like endometriosis can develop similar patterns, where the original visceral pain rewires neural pathways and generates myofascial trigger points that persist even after the original condition is treated. In these cases, the trigger points become their own pain generator, independent of the initial source.
Why Imaging Can Be Misleading
One of the biggest traps in diagnosing posterior pelvic pain is putting too much weight on what shows up on imaging. In a study of people with no pelvic pain at all, about two-thirds showed degenerative changes in their sacroiliac joints on CT scans, and roughly 30% had substantial degeneration. By the ninth decade of life, over 90% of asymptomatic people had visible SIJ degeneration.
21PubMed. The prevalence of sacroiliac joint degeneration in asymptomatic adultsSimilarly, when MRI is ordered to look for inflammatory sacroiliitis, the most common findings are actually non-inflammatory changes like disc degeneration and facet joint wear, which were present in over 40% of patients suspected of having sacroiliitis.
22PubMed. MRI of the SI joints commonly shows non-inflammatory disease in patients clinically suspected of sacroiliitisThe practical takeaway is that finding degeneration or wear on a scan does not mean that is what is causing your pain. Age-related changes in the SIJ are nearly universal, and blaming them for symptoms can lead to unnecessary procedures while the actual pain generator goes untreated. A good clinician uses imaging to confirm or rule out specific suspicions, not as a fishing expedition.
How Clinicians Sort It Out
Diagnosing sacroiliac-origin pain is genuinely difficult. Pain referral patterns, symptom history, hands-on maneuvers, and imaging all contribute, but none individually is definitive.
23PubMed. Challenges in Diagnosing Sacroiliac Joint Pain: A Narrative ReviewThe standard approach involves a cluster of physical provocation tests. A clinician will perform several maneuvers designed to stress the SIJ and see which ones reproduce your pain. Individually, the tests are imperfect. But a meta-analysis found that when multiple provocation tests come back negative, clinicians can conclude with about 92% certainty that the SIJ is not the pain source. The flip side is less reassuring: a positive cluster gives only about 35% certainty that the SIJ is actually the problem.
24PubMed. Diagnostic Accuracy of Clusters of Pain Provocation Tests for Detecting Sacroiliac Joint Pain: Systematic Review With Meta-analysisIn other words, the physical exam is much better at ruling the SIJ out than ruling it in. Among individual tests, the FABER test (where the clinician positions your hip in a figure-four and applies pressure) and the thigh thrust test have the best combination of accuracy, and pairing them improves diagnostic ability beyond any other test combination.
25PubMed Central. Accuracy of the Diagnostic Tests of Sacroiliac Joint DysfunctionOther tests with reasonable track records include the distraction test, compression test, and sacral thrust. Practitioners often use a battery of three to five of these; if a minimum number come back positive, the SIJ becomes a leading suspect.
26PubMed Central. Specificity, sensitivity, and predictive values of clinical tests of the sacroiliac joint: a systematic review of the literature When clinical suspicion is high but provocation tests are ambiguous, a diagnostic injection, where an anesthetic is placed directly into the SIJ under imaging guidance, serves as the reference standard. If the injection temporarily eliminates the pain, the joint is confirmed as the source.
Treatment and Rehabilitation
Treatment depends entirely on the underlying cause, but for the most common scenario, mechanical SIJ dysfunction, the first line is almost always exercise-based rehabilitation. A systematic review of randomized trials found that motor control exercises alone didn’t reduce pain much compared to other therapies, but when combined with other approaches like manual therapy or general strengthening, the combination produced large improvements in both pain and disability.
27PubMed. Effectiveness of motor control exercises versus other musculoskeletal therapies in patients with pelvic girdle pain of sacroiliac joint origin: A systematic review with meta-analysis of randomized controlled trialsThe logic is straightforward. The SIJ needs muscles to stabilize it, and the key players are the erector spinae along the back, the deep abdominal muscles, the glutes, and the hamstrings. Strengthening exercises targeting these groups can restore the force closure mechanism. In one case study, a sedentary worker with SIJ pain went from significant discomfort to almost no pain on provocation tests after just three weeks of individualized strengthening focused on the erector spinae, rectus abdominis, and hamstrings.
28PubMed Central. Effects of individual strengthening exercises for the stabilization muscles on the nutation torque of the sacroiliac joint in a sedentary worker with nonspecific sacroiliac joint painWhen conservative measures aren’t enough, the treatment ladder includes anti-inflammatory medications, steroid injections into the joint, and, for persistent cases, radiofrequency ablation (RFA). RFA uses heat to disrupt the nerves that carry pain signals from the SIJ. A multicenter randomized trial of 210 patients found that cooled radiofrequency ablation reduced pain scores by an average of 2.5 points on a 10-point scale at three months, compared to only 0.4 points with standard medical management. Over half the ablation group met the threshold for meaningful pain relief, compared to about 4% of those receiving standard care.
29Regional Anesthesia & Pain Medicine. Cooled radiofrequency ablation versus standard medical management for chronic sacroiliac joint pain: a multicenter, randomized comparative effectiveness studyMeta-analytic data supports RFA’s effectiveness at both three and six months, though the benefits tend to diminish over time as nerves regenerate.
30PubMed. The role of radiofrequency ablation for sacroiliac joint pain: a meta-analysisSitting, Posture, and Everyday Aggravators
If you spend long hours sitting, the way you sit matters. When you slump into a posterior pelvic tilt, sometimes called “sacral sitting,” you shift load off the buttock bones and onto the sacrum and lower back. Research measuring contact pressures in wheelchairs found that sacral sitting reduced peak pressure on the buttocks but increased shear force on the ischial region and raised contact pressure on the back.
31PubMed Central. Influence of sacral sitting in a wheelchair on the distribution of contact pressure on the buttocks and back and shear force on the ischial region That forward-sliding shear is the kind of force the SIJ ligaments are least equipped to handle, and over hours it can aggravate an already irritable joint.
Sitting on an overly soft surface, driving for extended periods, or crossing your legs habitually can all contribute. Simple changes like using a firmer chair, placing a small rolled towel behind your lower back to encourage a slight lumbar curve, and getting up to walk every 30 to 45 minutes can make a measurable difference for people with SIJ-related pain.
When Pain Rewires the Nervous System
For some people, posterior pelvic pain starts with an identifiable problem, a strained ligament, an inflamed joint, endometriosis, but persists long after the original issue has healed or been treated. This happens because chronic pain can change how the nervous system processes signals. The brain and spinal cord become more sensitive to input from the affected region, amplifying ordinary sensations into painful ones. Researchers have found evidence of this central sensitization in women with chronic pelvic pain, including changes in brain structure and altered sensory processing, though it remains unclear whether these nervous-system changes are a cause or a consequence of ongoing pain.
32PubMed. Central sensitization in urogynecological chronic pelvic pain: a systematic literature reviewConditions like endometriosis can accelerate this process by remodeling neural networks and generating myofascial trigger points that continue to produce pain independently. In practical terms, this means treating the original diagnosis may not be enough. If pain has been present for months or years, addressing the nervous system’s sensitization, through graded exercise, pain-science education, manual therapy, or sometimes medication, becomes as important as addressing the tissue-level problem. This is one reason posterior pelvic pain that has lingered warrants a clinician who considers both the joint and the broader pain system, rather than focusing exclusively on what the MRI shows.