A knot under the skin usually appears as a visible or palpable bump that can range from pea-sized to several centimeters across. What it looks like depends entirely on what’s causing it. Most are benign, but they vary widely in texture, movement, color, and depth. Here’s how to tell the common types apart by their appearance and feel.
Lipomas: Soft, Movable Lumps
Lipomas are one of the most common causes of a noticeable knot under the skin. They’re made of fatty tissue and feel soft and “doughy,” almost like a small rubber ball sitting just beneath the surface. Most start around 1 centimeter but can grow to 5 or even 10 centimeters over time. The skin over a lipoma looks completely normal, with no color change or visible opening.
The hallmark of a lipoma is how easily it moves. If you press on it from one side, it slides a little under your finger. It’s painless, grows slowly (sometimes over years), and has smooth, well-defined edges. You can usually feel exactly where it starts and stops. They show up most often on the neck, shoulders, back, arms, and thighs, and many people have more than one.
Epidermoid Cysts: Firm With a Visible Center
Epidermoid cysts (sometimes called sebaceous cysts) look different from lipomas in a few key ways. They tend to feel firmer and rounder, like a marble under the skin rather than a doughy lump. They’re often anchored in place and don’t slide around as freely when you press on them.
The most distinctive visual feature is the punctum: a tiny dark dot or opening on the skin’s surface directly over the cyst. Not every cyst has one, but when it’s there, it’s a strong clue. If a cyst ruptures or gets squeezed, it can discharge a thick, white, toothpaste-like material made of keratin (the same protein in your hair and outer skin). This discharge often has a noticeably foul smell. Cysts can become red, swollen, and tender if they get inflamed or infected, which changes their appearance from a calm, skin-colored dome to an angry, warm bump.
Swollen Lymph Nodes: Small, Tender, and Location-Specific
Swollen lymph nodes feel like small, oval-shaped lumps, typically 1 to 2 centimeters across. They’re often described as pea- or bean-shaped, and they tend to feel rubbery and somewhat tender to the touch. Unlike lipomas and cysts, which can appear almost anywhere, swollen lymph nodes show up in predictable spots: the sides of your neck, under the jaw, in the armpits, or in the groin.
A reactive lymph node (one that’s swollen because your body is fighting off an infection) is usually movable, tender, and symmetrical. It often appears alongside other signs of illness like a sore throat, cold, or skin infection nearby. Most shrink back to their normal size within two to four weeks once the infection clears. A lymph node that is hard, fixed in place, painless, and continues growing without any obvious infection is a different situation and worth getting checked promptly.
Muscle Knots: Deep, Tender, and Invisible
What people call a “muscle knot” is technically a trigger point, a tight, hyperirritable spot within a band of muscle. These feel very different from the lumps described above. A muscle knot sits deeper in the tissue, embedded within the muscle itself rather than floating just under the skin. It feels like a small, hard nodule or a taut rope of muscle fiber, firmer than the surrounding tissue.
Muscle knots don’t create a visible bump on the skin’s surface in most cases. You find them by pressing into the muscle and feeling for a spot that’s harder than normal and painful when compressed. Pressing on one can sometimes cause the muscle to twitch visibly. The key difference: a muscle knot moves with the muscle when you flex, and it hurts. Lipomas, cysts, and lymph nodes sit in the layer above or beside the muscle and don’t change with movement.
Ganglion Cysts: Firm Bumps Near Joints
Ganglion cysts are fluid-filled lumps that appear near joints or tendons, most commonly on the wrist. They account for 60 to 70 percent of all soft tissue masses in the wrist area, making them extremely common. They feel firm and smooth, like a round pebble under the skin, and they don’t move freely because they’re attached to the joint capsule or tendon sheath below.
One distinctive feature: ganglion cysts are translucent. If you hold a bright flashlight directly against a large ganglion in a dark room, light passes through it because the cyst is filled with clear, jelly-like fluid rather than solid tissue. A solid lump won’t light up this way. Ganglion cysts can change size over time, sometimes shrinking or growing depending on joint activity, and they may ache or cause discomfort if they press on a nearby nerve.
Dermatofibromas: Small, Colored, and Dimpling
Dermatofibromas are small, firm bumps that usually appear on the lower legs. They’re typically under a centimeter across and sit partly within the skin itself rather than purely beneath it. Their color varies: they can be pink, red, brown, gray, or purplish, and the color may shift over the years.
The signature test is the “dimple sign.” If you pinch the skin on either side of a dermatofibroma, the top of the bump dimples inward rather than popping outward. This happens because the fibrous tissue is tethered to the layers below. They feel hard, like a small pebble embedded in the skin, and they’re painless in most cases. Dermatofibromas are harmless and don’t require treatment, but their color and firmness can sometimes cause concern if you’re not sure what you’re looking at.
Abscesses: Red, Warm, and Painful
An abscess is a pocket of pus caused by infection, and it looks noticeably different from other under-the-skin lumps. Early on, it appears as a firm, red, swollen area that’s warm and painful to the touch. The surrounding skin is often visibly inflamed.
As the abscess matures, the center softens. Doctors describe this as “fluctuance,” a spongy, fluid-filled feeling when you press on it, as if there’s liquid moving beneath the surface. The overlying skin thins out and may develop a yellowish or whitish point where pus is close to breaking through. Unlike a lipoma or cyst, an abscess develops over days rather than weeks or months, and it’s accompanied by escalating pain, heat, and redness that spreads outward.
Warning Signs Worth Noting
Most lumps under the skin are harmless, but certain characteristics raise the risk that a mass could be something more serious. Size is the single biggest factor: lumps larger than 5 centimeters (about 2 inches) carry the highest risk of being malignant. Research from the University of Washington’s orthopedic oncology program found that cancerous soft tissue masses in the arms or legs are usually larger than 5 centimeters and deep-seated, often within the muscle rather than just under the skin.
Other features that warrant evaluation include rapid growth over weeks, a lump that feels fixed to the tissue beneath it and doesn’t move at all, irregular or poorly defined borders (sometimes described as “fingerlets” or satellite bumps around the main mass), and a hard, non-tender texture. A painless lump that keeps growing is more concerning than a tender one that appeared suddenly, since pain often signals infection or inflammation rather than malignancy.