The Achilles tendon is the thick band of tissue running down the back of your lower leg, connecting your calf muscles to your heel bone (the calcaneus). It is the largest and strongest tendon in the human body, and it makes walking, running, jumping, and pushing off the ground possible. Despite its strength, it is also one of the most commonly injured tendons, particularly among active adults.
Where It Is and What It Does
The Achilles tendon begins in the middle of your calf, where the two major calf muscles merge into a single, rope-like structure. It travels down the back of your leg and attaches to the heel bone just above your ankle. You can feel it easily by running your fingers along the back of your ankle: that firm cord beneath the skin is the tendon.
Every time you point your toes, rise onto the balls of your feet, or push off to take a step, the calf muscles contract and pull on the Achilles tendon, which in turn pulls on the heel. This simple mechanism is behind nearly every movement that propels you forward or upward. During running, the peak force transmitted through the Achilles tendon can reach roughly 12.5 times your body weight, or about 9 kilonewtons in an average adult. That makes it one of the most heavily loaded structures in the entire body.
Size and Structure
In a healthy adult who isn’t especially athletic, the Achilles tendon measures about 4.5 to 5 millimeters thick at its midpoint. Elite athletes tend to have slightly thicker tendons, averaging around 5.4 millimeters, likely as an adaptation to repeated high loads. The tendon runs roughly 15 centimeters from the calf to the heel, though this varies with height.
Inside, the tendon is built almost entirely from type I collagen, the same protein that gives bones and skin their tensile strength. These collagen fibers are arranged in tight, parallel bundles with a slight wave pattern, which allows the tendon to absorb and release energy like a spring. Scattered between the fibers are specialized cells called tenoblasts that produce and maintain the collagen. As you age, the number of these cells decreases, and the tendon gradually becomes less resilient.
Blood Supply and Vulnerability
One reason the Achilles tendon is injury-prone has to do with its blood supply. A section in the middle of the tendon, a few centimeters above the heel, was historically described as a “watershed zone” with relatively poor blood flow. For decades, researchers believed this low-flow area made the tendon more vulnerable to degeneration and rupture. More recent studies using live blood flow measurements have challenged that idea, finding that blood flow is actually fairly uniform throughout the tendon. Still, the midsection remains the most common site of both chronic pain and full tears, so other factors like repetitive stress and age-related collagen changes likely play a role.
Common Achilles Tendon Problems
The two main categories of Achilles injury are tendinopathy (chronic pain and degeneration) and rupture (a partial or complete tear).
Tendinopathy develops gradually. You might notice stiffness at the back of your ankle first thing in the morning, or a dull ache that worsens during or after activity. The older term “tendinitis” implies inflammation, and that can be present early on. But in most long-standing cases, the tissue shows degeneration rather than active inflammation. This distinction matters because treatments that target inflammation, like anti-inflammatory medications, may help an acute flare but are less effective for a tendon that has been symptomatic for months.
A full Achilles rupture is more dramatic. It typically happens during a sudden push-off or change of direction, and people often describe hearing a pop or feeling like they were kicked in the back of the leg. The incidence in the general population is about 5 to 10 per 100,000 people per year, and it is increasing overall. Among competitive athletes the rates are much higher: roughly 8 percent of competitive athletes will experience a tendon rupture at some point. Sprinters, decathletes, and soccer players are hit hardest, with lifetime rupture rates around 17 to 18 percent.
How a Rupture Is Diagnosed
If your doctor suspects a full Achilles tear, they will likely perform a simple physical exam called the Thompson test. You lie face down on an exam table with your feet hanging off the edge. The doctor squeezes your calf muscle and watches your foot. In a healthy tendon, the squeeze causes your foot to point downward. If the tendon is completely torn, the foot barely moves or doesn’t move at all. Your doctor may test both legs to compare the response. Imaging, usually an ultrasound or MRI, can confirm the diagnosis and show the exact location and size of the tear.
Treatment and Recovery
For tendinopathy, the first line of treatment is almost always a structured exercise program. Eccentric exercises, where you slowly lower your heel off the edge of a step, have strong evidence for reducing pain and strengthening the damaged tendon over a period of weeks to months. Temporary activity modification, heel lifts, and physical therapy round out the approach for most people.
A complete rupture is a bigger decision. Some people undergo surgical repair, where the torn ends are stitched back together. Others are managed without surgery, using a boot or splint for about six weeks while the tendon heals on its own. Both approaches have trade-offs: surgery carries a slightly lower re-rupture risk but comes with surgical risks like infection, while nonsurgical management avoids the operating room but requires careful, prolonged immobilization.
Regardless of the path, early movement matters. Doctors typically allow gentle heel movement within two to six weeks of the injury, depending on how things are healing. Physical therapy ramps up gradually from there. Most people return to normal daily activity within four to six months, though getting back to competitive sport can take longer, often nine months to a year.
What Puts You at Higher Risk
Several factors increase your chances of Achilles problems. Age is the biggest one: tendon cells decline in number and function over time, and most ruptures occur in adults between 30 and 50. Sudden increases in training volume, especially running or jumping, stress the tendon faster than it can adapt. Tight or weak calf muscles transfer more load to the tendon. Certain antibiotics in the fluoroquinolone class are known to weaken tendons and raise rupture risk, particularly in older adults. Obesity adds chronic load, and prior tendon pain that was never properly rehabilitated leaves the tissue in a weakened state.
Keeping the Achilles healthy comes down to a few practical habits: warming up before explosive activity, increasing training intensity gradually (a common guideline is no more than a 10 percent increase per week), maintaining calf strength and flexibility, and paying attention to early warning signs like morning stiffness or tenderness just above the heel. A sore Achilles that gets worse over days rather than better is worth addressing before it becomes a bigger problem.