The Achilles tendon is the thick band of tissue that runs down the back of your lower leg, connecting your calf muscles to your heel bone. It’s the largest and strongest tendon in the human body, and you rely on it every time you walk, run, jump, or push off the ground. Despite its strength, the Achilles tendon is also one of the most commonly injured tendons, partly because of the enormous forces it handles and a limited blood supply that makes healing slow.
Where It Is and What It’s Made Of
The Achilles tendon starts where your two calf muscles (the larger, visible one and a smaller, deeper one beneath it) merge into a single, cord-like structure. That cord travels down the back of your ankle and attaches to your heel bone, called the calcaneus. You can feel it easily by pinching the back of your ankle just above the heel.
The tendon itself is built from two proteins: collagen, which provides tensile strength, and elastin, which gives it a degree of stretch. In a healthy adult, the tendon measures roughly 4 to 9 millimeters thick, with an average around 6 millimeters. It’s surrounded by a thin sheath called the paratenon, which reduces friction and supplies some of the tendon’s blood flow.
What the Achilles Tendon Does
Your Achilles tendon acts like a powerful spring. When your calf muscles contract, they pull on the tendon, which pulls on the heel bone, pointing your foot downward. That motion is what propels you forward when you walk and launches you off the ground when you jump. Without a functioning Achilles tendon, standing on your toes or pushing off during a stride becomes nearly impossible.
The forces involved are remarkable. During running, the peak force transmitted through the Achilles tendon reaches roughly 9 kilonewtons, equivalent to about 12.5 times your body weight. Even normal walking loads the tendon at several times body weight with every step. This combination of constant use and extreme load is part of what makes the tendon vulnerable to injury over time.
Why It Heals Slowly
One of the Achilles tendon’s defining characteristics is its poor blood supply. Blood vessels reach the tendon from the surrounding sheath, but the middle portion of the tendon, roughly 2 to 6 centimeters above where it attaches to the heel, receives the least blood flow. This zone of relative avascularity is sometimes called a “watershed zone,” and it’s exactly where most injuries occur. Limited blood flow means less oxygen and fewer nutrients reach damaged tissue, which slows repair compared to injuries in more vascular parts of the body.
Common Problems
Achilles tendon problems fall along a spectrum from mild irritation to complete rupture.
Tendinopathy and tendinosis. The most common issue is a gradual breakdown of the tendon’s internal structure, called tendinosis. Rather than a sudden injury, this develops over time from repetitive microtrauma, reduced blood supply, or aging. The collagen fibers degenerate and become disorganized. This is different from true tendinitis, which involves actual tearing and an inflammatory healing response. Many cases that people call “tendinitis” are actually tendinosis, a degenerative process rather than an inflammatory one. You’ll typically notice stiffness and aching at the back of the ankle, especially first thing in the morning or after sitting for a long time.
Paratenonitis. Sometimes the problem isn’t the tendon itself but the sheath surrounding it. The paratenon becomes inflamed and thickened, and can stick to the tendon underneath, causing pain with movement.
Rupture. A complete tear of the Achilles tendon often feels like a sudden snap or a kick to the back of the leg. It can happen during a quick change of direction, a sudden sprint, or a jump landing. The incidence of Achilles tendon rupture repairs in the United States has been rising, reaching about 7.3 per 100,000 patients in 2024. Ruptures are most common in men between their 30s and 50s, particularly those who play sports intermittently (the “weekend warrior” pattern).
Risk Factors for Injury
Several factors raise the odds of Achilles tendon problems. Age is one of the biggest: the tendon loses elasticity and blood supply over the decades. Sudden increases in training intensity, running on hard surfaces, and tight or weak calf muscles all contribute. Being overweight adds mechanical load to a tendon that already handles enormous forces.
Certain medications also play a role. A class of antibiotics called fluoroquinolones has a well-documented link to tendon damage. A large study of over one million U.S. Medicare beneficiaries found that one widely prescribed fluoroquinolone more than doubled the risk of Achilles tendon rupture (a 120% increase) within 30 days of use. If you’re prescribed this type of antibiotic, it’s worth knowing that tendon pain is a recognized side effect, and the risk is higher for people over 60 or those also taking corticosteroids.
How a Rupture Is Diagnosed
Doctors can usually diagnose a complete Achilles rupture with a physical exam. The most reliable bedside test is simple: you lie face down and the examiner squeezes your calf muscle. In a healthy tendon, your foot will point downward. If the tendon is torn, the foot barely moves. This squeeze test has a sensitivity of 96 to 100 percent and a specificity of 93 to 100 percent for complete ruptures, making it one of the most accurate physical exam tests in orthopedics. Ultrasound or MRI can confirm the diagnosis and show the exact location and extent of a tear.
Recovery After a Rupture
Whether a ruptured Achilles tendon is treated with surgery or managed with bracing and rehabilitation alone, recovery is a long process. Most people spend the first several weeks in a boot or cast, then gradually progress through physical therapy over many months.
Returning to sports after a rupture is highly variable. Studies of elite athletes show return-to-sport rates ranging from 61 to 100 percent, depending heavily on the demands of the sport. But even after returning, most people have lasting changes in how the tendon performs. Calf strength deficits of 10 to 30 percent compared to the uninjured leg are common at one year after surgery. Endurance losses of 20 to 30 percent in heel-rise tests persist beyond 12 months. Many patients regain significant function, but some experience long-term reductions in strength, elasticity, and range of motion that may affect high-level athletic performance permanently.
Keeping the Achilles Tendon Healthy
The best strategies for protecting your Achilles tendon are straightforward. Eccentric calf exercises, where you slowly lower your heel off the edge of a step, are one of the most studied and effective ways to both prevent and treat early tendinopathy. Gradual progression in training volume matters: sharp increases in running distance or intensity are a classic trigger for tendon problems. Maintaining a healthy weight reduces the baseline load on the tendon with every step, and wearing supportive footwear that doesn’t place excessive strain on the heel helps as well.
Because the tendon’s blood supply is limited, giving it time to adapt to new demands is more important than it is for muscles, which heal faster. If you notice persistent stiffness or pain at the back of your ankle, addressing it early gives you a much better chance of resolving the problem before it progresses to a more serious injury.