That firm, round sensation under the skin of your lower back is almost always one of a handful of benign causes: a small fatty nodule called an episacral lipoma (sometimes nicknamed a “back mouse”), a standard subcutaneous lipoma, a taut band of muscle commonly called a trigger point or muscle knot, or a superficial cyst. Less commonly, nerve-sheath tumors, fascial herniations, and other soft-tissue growths can produce the same feeling. The specific character of the lump and whether it moves, hurts, or has grown over time all help narrow down what you’re dealing with.
Episacral Lipomas and “Back Mice”
If the ball you feel is small, slightly mobile, and sits near the bony ridges at the top of your pelvis, there’s a good chance it’s an episacral lipoma. These are small herniations of fat through the tough connective-tissue layer (fascia) that covers the muscles of your lower back. Clinicians sometimes call them “back mice” because they can seem to scoot around under the skin when pressed. On ultrasound, they show up as small subfascial fat herniations near the posterior superior iliac spine, the bony prominence you can feel at the top of each buttock.1SpringerLink / Skeletal Radiology. Subfascial fat herniation: sonographic features of back mice
What makes back mice interesting is that they can cause a surprising amount of pain for their size. People sometimes describe sharp, localized tenderness right over the nodule, and the pain can radiate into the buttock or even the leg, mimicking sciatica. That radiating quality leads to frequent misdiagnosis: patients get worked up for disc herniations or sacroiliac joint problems when the culprit is a small fatty lump sitting right under the surface. A clinical series found that injection of local anesthetic and corticosteroid into the nodule provided lasting relief for roughly nine out of ten patients, with high satisfaction rates and no reported side effects.2PubMed. Treatment of low back pain associated with back mice: a case series The injections resolved both local and radiating symptoms, though they often didn’t eliminate the nodule itself. In other words, the lump stayed but the pain went away.
Subcutaneous Lipomas
Lipomas are the most common benign soft-tissue tumors, and they can show up virtually anywhere fat exists, including the lower back.3International Journal of Surgery Case Reports. Approach and management of a giant lipoma in the left lumbar region A typical lipoma feels soft, doughy, and rubbery, and it moves easily when you push it. Doctors sometimes test for a “slip sign,” where the lump slides out from under the fingers when pressed at the edge. Most lipomas are painless, grow slowly over months to years, and stay small enough that they’re more of a cosmetic nuisance than a medical problem.4PubMed Central. A Giant Lipoma Presenting With Neurological Symptoms: A Rare Clinical Presentation
That said, lipomas occasionally reach large sizes, and when they do, they can press on nearby nerves, restrict movement, cause inflammation, or trigger compression symptoms depending on exactly where they sit.3International Journal of Surgery Case Reports. Approach and management of a giant lipoma in the left lumbar region A lipoma that was painless at two centimeters may start causing trouble at six or seven. If your lump has been stable for years and doesn’t hurt, your doctor will likely recommend monitoring it. If it’s growing, painful, or interfering with function, surgical removal is straightforward and recurrence rates are low.
One way to distinguish a lipoma from a back mouse: lipomas tend to be softer, more superficial, and more mobile. Back mice are often smaller, more tender, and anchored closer to the fascial layer. Both are benign, but they behave differently and respond to different treatments.
Muscle Knots and Trigger Points
Sometimes what feels like a ball in your lower back isn’t a discrete lump at all but a taut, contracted band within the muscle itself. These myofascial trigger points are extremely common in the erector spinae and multifidus muscles that run along either side of the spine. They feel like firm, marble-sized nodules deep in the muscle, and pressing on them often reproduces your pain or sends it radiating into the buttock or hip.
Trigger points differ from lipomas and cysts in a few telling ways. They’re usually tender to pressure rather than painless. They can change in size and tenderness from day to day, especially with activity or stress. And they don’t have the smooth, well-defined borders of a true mass; they feel more like a dense area within the muscle. If you press hard and the “ball” seems to release or soften after sustained pressure, it’s very likely muscular.
Dry needling, a technique where a thin needle is inserted directly into the trigger point, has shown moderate evidence for reducing low back pain intensity when combined with other therapies. A meta-analysis found that dry needling of trigger points was more effective than comparison treatments for alleviating pain intensity and functional disability at the time of treatment.5PubMed. Evidence for Dry Needling in the Management of Myofascial Trigger Points Associated With Low Back Pain: A Systematic Review and Meta-Analysis A randomized trial using shear wave elastography confirmed that dry needling reduced resting muscle stiffness in the erector spinae after one week compared with sham treatment.6PubMed Central. Effect of dry needling on lumbar muscle stiffness in patients with low back pain: A double blind, randomized controlled trial using shear wave elastography However, a systematic review of trigger-point needling in the lower body found that while short-term pain improvements were consistent across studies, improvements in function, range of motion, and quality of life were not.7PubMed Central. Dry Needling in Subjects With Muscular Trigger Points in the Lower Quarter: A Systematic Review The practical takeaway: dry needling can help with pain but works best as part of a broader rehabilitation plan, not as a standalone fix.
Cysts in the Lower Back
Several types of cysts can form in or near the lower back and produce a palpable lump or, more often, pain that makes you feel like something solid is pressing on your spine.
Epidermoid and dermoid cysts in the spinal region are uncommon but well-documented. They can develop congenitally or be acquired after procedures that introduce skin cells into deeper tissue. One well-described pathway involves lumbar puncture: if skin cells are carried inward on the needle, they can slowly form a keratin-filled cyst over years or even decades. In one case, a woman who had received spinal anesthetic injections more than 70 times over 14 years developed an epidermoid cyst that eventually caused leg pain and sensory changes.8Neurologia medico-chirurgica. Acquired Lumbar Epidermoid Cyst in an Adult A younger patient with a remote history of a single lumbar puncture presented with over a year of low back and radicular leg pain from an epidermoid cyst at the L4-5 level that showed characteristic diffusion restriction on MRI.9PubMed. Epidermoid Cyst of the Lumbar Spine After Lumbar Puncture: A Clinical, Radiographic, and Pathologic Correlation
These spinal cysts usually aren’t something you can feel with your fingers at the skin surface. They produce symptoms by pressing on neural structures inside the spinal canal, so you’d feel pain, numbness, or weakness rather than a palpable bump. More superficial cysts, like sebaceous cysts or pilonidal cysts, can absolutely form lumps you can touch. A sebaceous cyst feels like a round, firm marble under the skin, sometimes with a central pore. Pilonidal cysts tend to form right at the top of the gluteal cleft and can become red, swollen, and painful if they get infected.
Nerve-Sheath Tumors and Other Rare Growths
Schwannomas are benign tumors that arise from the cells wrapping around peripheral nerves. They’re slow-growing, and because of that gradual pace, diagnosis is often delayed. A case of a 20-year-old man with a left lower back mass illustrates the typical pattern: he noticed the lump four years before seeking evaluation, and it had grown slowly before stabilizing. On exam it was firm, well-defined, and contained under the skin with no drainage or retraction. Ultrasound showed a solid, heterogeneous mass measuring roughly five centimeters, sitting in the subcutaneous tissue superficial to the paraspinal muscles at L3-L4, with no extension toward the spinal canal.10PubMed Central. Cutaneous schwannoma presenting as a lower back mass
Spinal schwannomas can also form deeper, within the spinal canal itself, where they press on nerve roots and cause pain, numbness, or weakness rather than a visible lump. About three in ten spinal nerve root tumors are schwannomas.11Annals of Medicine and Surgery. Large lumbosacral schwannoma in a young female- a case report from Afghanistan Retroperitoneal schwannomas, which grow in the deep abdominal space behind the organs, are extremely rare but can present with vague, creeping lower back pain as their main symptom.12PubMed. Retroperitoneal giant schwannomas: report on two cases and review of the literature These deeper tumors wouldn’t produce a ball you can feel at the surface; they’d manifest as persistent pain that doesn’t respond to typical treatments.
Other rare possibilities include ligamentum flavum hematomas, which are blood-filled cystic lesions that form within the ligament connecting adjacent vertebrae. One documented case involved a 61-year-old woman with no prior medical history who developed low back pain and leg symptoms from a hematoma at the L4-5 level. Surgical removal resolved her symptoms completely.13PubMed Central. Ligamentum Flavum Hematoma Presented with Low Back Pain: A Case Report and Review of the Literature These are curiosities rather than common diagnoses, but they illustrate why persistent, unexplained lower back symptoms sometimes deserve imaging.
When the “Ball” Isn’t a Physical Lump
Not everyone who feels something ball-like in their lower back can actually point to a visible or palpable mass. Sometimes the sensation comes from the fascia, the thick sheet of connective tissue that envelops the lower back muscles. Research has identified at least three ways the lumbar fascia can generate pain: tiny tears irritating pain-sensing nerve endings in the fascia, tissue remodeling from chronic overuse or immobility that disrupts the body’s position-sensing signals, and sensitization of spinal-cord neurons from pain input arriving from other structures in the same spinal segment.14PubMed Central. The Lumbodorsal Fascia as a Potential Source of Low Back Pain: A Narrative Review Any of these mechanisms can create a localized, deep sensation that your brain interprets as a solid object pressing against your spine, even when imaging shows nothing discrete.
Prolonged sitting can make this worse. Finite element modeling of the lumbar disc under sustained sitting loads has shown that flexed postures produce greater height loss, fluid loss, and mechanical stress in the disc compared with neutral or extended sitting positions.15PubMed Central. Biomechanical response of lumbar intervertebral disc in daily sitting postures: a poroelastic finite element analysis If you spend hours slouched at a desk and then notice a persistent “ball” feeling in your lower back when you stand up, the sensation may be coming from a combination of disc loading, muscle guarding, and fascial irritation rather than from an actual lump. The hallmark of this type of sensation is that it tends to fluctuate with position and activity level, unlike a true mass, which stays palpable regardless of what you’re doing.
How Doctors Figure Out What It Is
If you show up with a palpable lump in your lower back, the first step is usually a physical exam. Your doctor will assess the size, texture, mobility, and tenderness of the mass. A soft, freely mobile, painless lump that’s been stable for years is almost certainly a lipoma. A small, tender nodule near the pelvic brim suggests a back mouse. A firm, well-circumscribed mass that’s been slowly growing warrants imaging.
Ultrasound is the typical first-line imaging study for superficial soft-tissue masses. It can characterize most lumps based on appearance alone and is inexpensive, fast, and radiation-free.16PubMed Central. Practical approach to ultrasound of soft tissue tumors and the added value of MRI: how I do it When ultrasound leaves questions about exact tumor location or relationship to nerves and blood vessels, MRI adds complementary information. However, a study examining cases where MRI was ordered after a diagnostic ultrasound for soft-tissue masses found that the MRI did not change the working diagnosis in about three-quarters of cases and didn’t narrow the differential diagnosis in roughly two-thirds.17PubMed. Does Magnetic Resonance Imaging After Diagnostic Ultrasound for Soft Tissue Masses Change Clinical Management? That doesn’t mean MRI is useless; it means ultrasound handles the heavy lifting for most surface-level lumps, and MRI earns its role when the mass is deep, large, or has worrisome features.
Contrast-enhanced imaging can help distinguish benign from malignant masses. Homogeneous enhancement patterns on both ultrasound and MRI were highly specific for benign tumors, while inhomogeneous patterns were moderately specific for malignancy.18PubMed. Soft-Tissue Tumor Contrast Enhancement Patterns: Diagnostic Value and Comparison Between Ultrasound and MRI If there’s still uncertainty after imaging, a needle biopsy can provide a definitive tissue diagnosis.
When to Be Concerned
The vast majority of palpable lumps in the lower back are benign. Still, certain features should prompt a timely medical evaluation rather than a wait-and-see approach:
- Rapid growth: A mass that doubles in size over weeks rather than months or years needs prompt imaging to rule out a sarcoma or other aggressive process.
- Neurological symptoms: New numbness, tingling, or weakness in one or both legs alongside the lump suggests something may be pressing on a nerve root or the spinal cord.
- Fixed and hard: A lump that doesn’t move when you push on it and feels rock-hard is less likely to be a lipoma or cyst and more likely to need further workup.
- Night pain or weight loss: Pain that wakes you from sleep or unexplained weight loss alongside a new mass are red flags that warrant imaging and possibly referral.
- Size over five centimeters: Most clinical guidelines recommend imaging and possibly biopsy for any soft-tissue mass larger than about five centimeters, regardless of how benign it feels on exam.
If your lump has none of these features, the odds strongly favor something harmless. But even a benign lump can be worth addressing if it’s causing you pain, limiting your movement, or just making you anxious. A quick ultrasound can settle the question in most cases and spare you months of unnecessary worry.
Practical Self-Assessment
While you wait for a medical appointment, a few observations can help your doctor narrow the diagnosis faster. Try pressing on the area with moderate finger pressure. If the lump is soft and slides easily under the skin, think lipoma. If it’s tender and sits near the bony ridge of your pelvis, think back mouse. If pressing on it reproduces your pain and the area feels more like a dense band than a discrete marble, think trigger point. If there’s a visible skin pore at the center of the lump, think cyst.
Note whether the sensation changes with position. A feeling that’s worse when slouching and better when lying flat or walking is more consistent with fascial or disc-related irritation than a mass. A lump that’s equally palpable in every position is more likely an actual physical structure. Track whether the area has grown, shrunk, or stayed the same. Take a photo with something for scale, like a coin, so you have a baseline for comparison. All of this information saves time and helps your clinician steer toward the right next step without unnecessary testing.