Your 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 your body, and you rely on it every time you walk, run, jump, or push off the ground. Despite its strength, it’s also one of the most commonly injured tendons, partly because of a quirk in its blood supply that makes one section especially vulnerable.
Where It Is and What It Does
The Achilles tendon begins in the middle of your calf, where it forms from the two muscles that make up the bulk of your lower leg (the gastrocnemius and soleus). From there it narrows as it travels down the back of your leg and attaches to the calcaneus, the bone that forms your heel. You can feel it easily by pinching the area just above your heel, where the cord-like structure sits close to the surface.
Its primary job is transmitting force from your calf muscles to your foot. When your calf contracts, the Achilles pulls your heel upward, pointing your toes downward in a motion called plantarflexion. This is the movement that propels you forward with every step. But the tendon does more than just relay muscle contractions. It also acts like a spring, storing elastic energy when it stretches under load and releasing that energy to help power your next stride. This energy-recycling ability is one reason human walking and running are so efficient.
Why the Middle Section Is a Weak Spot
Not all parts of the Achilles tendon are equally resilient. The midsection, located roughly 4 to 7 centimeters above where the tendon attaches to the heel, has significantly less blood flow than the rest of the tendon. Less blood means fewer nutrients and oxygen reaching the tissue, which slows repair after everyday micro-damage. This poorly supplied zone is where ruptures most commonly occur and where healing tends to be slowest after injury or surgery.
Common Achilles Tendon Problems
The most frequent issue is Achilles tendinitis, which comes in two distinct forms depending on where the irritation develops.
Noninsertional tendinitis affects the fibers in that vulnerable midsection. Over time, those fibers can develop tiny tears, causing the tendon to swell and thicken. This type is more common in younger, active people, especially runners who ramp up their training too quickly.
Insertional tendinitis targets the lower portion of the tendon, right where it anchors to the heel bone. Bone spurs often form on the back of the heel alongside this type. It can show up at any age and any activity level, though tight calf muscles are a frequent contributor because they place extra stress on the attachment point.
Both types share a similar set of symptoms: pain and stiffness along the tendon first thing in the morning, pain that worsens with activity, swelling that builds throughout the day, and tenderness when you press on the back of your heel. Many people notice their worst pain the day after a hard workout rather than during it.
Achilles Tendon Ruptures
A rupture is a partial or complete tear of the tendon, often described as feeling like being kicked in the back of the leg. It typically happens during a sudden push-off movement, like sprinting or jumping. One simple way clinicians check for a rupture is the Thompson test: you lie face down with your feet hanging off the edge of a table, and a provider squeezes your calf muscle. If the foot doesn’t move downward in response, that strongly suggests the tendon is torn.
When surgery is needed, it’s ideally performed within two weeks of the injury. Recovery after surgical repair follows a gradual timeline. For the first two weeks, you’ll be in a splint or cast and unable to bear weight. By week two to four, you transition to a walking boot and start putting weight on the leg with crutches. Around week eight, you begin walking in regular shoes. Running and jumping typically start between weeks 12 and 16, and a full return to sport takes 6 to 12 months depending on the demands of your activity.
Risk Factors Worth Knowing
Age is one of the biggest risk factors for Achilles problems. As you get older, the cells that maintain tendon tissue become less effective at repair. Obesity adds to the risk because of the higher mechanical load on the tendon. Pre-existing conditions like autoimmune disorders and kidney disease also increase vulnerability.
Several common medications can weaken the Achilles tendon. Fluoroquinolone antibiotics (a class that includes ciprofloxacin) are the best-known culprit. In reported cases, the Achilles tendon is affected about 90% of the time, and roughly 40% of those cases progress to a rupture. The damage can appear as early as 48 hours after starting the medication, or it can show up months after you’ve stopped taking it. In some cases the damage is irreversible. Long-term use of corticosteroids (typically after at least three months) and cholesterol-lowering statins have also been linked to tendon problems, though these associations tend to develop more slowly.
Keeping Your Achilles Tendon Healthy
Because the tendon’s midsection has limited blood supply, giving it time to adapt to new demands is one of the most effective protective strategies. If you’re starting a running program or increasing your training volume, doing so gradually lets the tendon tissue remodel and strengthen without accumulating damage faster than it can repair. Eccentric calf exercises, where you slowly lower your heel off the edge of a step, are one of the most well-supported ways to build tendon resilience. These exercises load the tendon while it lengthens, which stimulates the tissue to become thicker and more organized over time.
Maintaining flexible calf muscles also reduces the pulling force on the tendon’s attachment point. If you notice morning stiffness or a nagging ache along the back of your lower leg that worsens with activity, addressing it early with relative rest and targeted strengthening gives you the best chance of avoiding a more serious problem down the line.