Healthy tendons are glistening, white to off-white cords of tissue with a smooth, slightly shiny surface. They look and feel like tough, flexible rope, with a pearly sheen that comes from densely packed fibers running in parallel lines. If you’ve ever handled a raw chicken drumstick and noticed the firm white band connecting muscle to bone, you’ve seen a tendon up close.
Shape and Size Vary by Location
Not all tendons look the same. The most familiar shape is a rounded cord, like the Achilles tendon at the back of your ankle, which is the thickest tendon in the body at roughly 15 centimeters long. The tendons in your fingers and wrists are thinner, almost string-like, and you can sometimes see them move under the skin when you wiggle your fingers.
Some tendons are flat and broad rather than cord-shaped. These sheet-like versions, called aponeuroses, are thin, delicate layers of tissue rather than thick ropes. The abdominal wall, for instance, is reinforced by flat tendon sheets rather than round cords. Whether round or flat, all tendons share that characteristic white, fibrous appearance.
Why Tendons Look White
The color comes down to blood supply, or rather the lack of it. Tendons have very few blood vessels running through them. Muscles are red because they’re packed with blood vessels and oxygen-carrying proteins, but tendons get most of their limited blood supply from a thin outer sleeve called the paratenon. This covering is highly vascularized and delivers nutrients to the tendon, but the tendon tissue itself is nearly avascular. A healthy tendon on ultrasound shows no internal blood flow at all.
That low blood supply is also why tendons heal slowly after injury. The Achilles tendon, for example, is most vulnerable to damage in its midsection, precisely because that zone has the least blood flow of any part of the tendon.
The Fiber Pattern Up Close
If you could zoom in on a cross-section of tendon, you’d see a remarkably organized structure. Tendons are built like a cable: tiny fibers called microfibrils are bundled into larger fibers, which are grouped into fascicles, which are bundled together to form the whole tendon. Thin layers of connective tissue separate each bundle, and a tougher outer sheath wraps the entire structure.
About 60% of a tendon’s dry weight is type I collagen, the same structural protein found in skin and bone. That collagen accounts for roughly 95% of all the collagen in the tendon. The rest is water, specialized proteins, and a small number of cells. This collagen-heavy composition is what gives tendons their distinctive white, fibrous, almost silky look and their remarkable tensile strength.
On ultrasound imaging, this internal organization shows up as a striking “fibrillar pattern,” a series of fine, bright, parallel lines running the length of the tendon. Radiologists use this pattern as the hallmark of a normal, healthy tendon. When that crisp parallel appearance starts to blur or disappear, it signals damage.
What Damaged Tendons Look Like
A healthy tendon is firm, white, and neatly organized. A damaged one tells a different story. In the early stages of tendon disease (tendinopathy), the tissue loses some of its pearly whiteness and becomes slightly gray or dull. The surface may appear less smooth, and the tendon often looks swollen or thickened compared to its healthy counterpart.
As damage progresses, the changes become more dramatic. The collagen fibers, normally arranged in tight parallel lines, become disorganized. In severe cases, a process called mucoid degeneration replaces the structured collagen with a jelly-like substance. When surgeons see this during an operation, the affected tendon (or ligament) looks bulky, yellowish, and gelatinous, sometimes described as having a “celery stalk” or “noodle-like” appearance because of the mix of preserved fibers running through soft, degraded tissue.
On ultrasound, damaged tendons lose their characteristic bright, striped fibrillar pattern. Instead, the image shows dark, swollen areas where the organized collagen structure has broken down. Blood vessels may also become visible within the tendon itself, something that never shows up in healthy tissue.
How to See Your Own Tendons
You can observe several tendons without any special equipment. Hold your hand flat, palm up, and gently flex your wrist. The tendons running along the inside of your forearm will pop up as visible ridges under the skin. Make a fist and extend your fingers, and you’ll see the extensor tendons on the back of your hand fan out toward each finger.
Your Achilles tendon is easy to find, too. Run your fingers along the back of your ankle just above the heel bone, and you’ll feel a thick, firm cord about the width of your index finger. That firm, rope-like texture under your skin is exactly what the tendon looks like beneath the surface: a dense, white, fibrous cable built to transfer the force of your calf muscles into the movement of your foot.