The large, cord-like structure you can feel at the back of your ankle is the Achilles tendon, the thickest and strongest tendon in the human body. It connects your two main calf muscles to the heel bone (calcaneus) and is responsible for pushing your foot downward every time you walk, run, or jump. But the Achilles isn’t the only tendon running behind the ankle. Several smaller tendons pass behind the inner and outer ankle bones, and knowing which is which helps you make sense of ankle pain when it shows up.
The Achilles Tendon
The Achilles tendon forms where the two muscles of the calf, the gastrocnemius and the soleus, merge into a single thick band that attaches to the back of the heel bone. It inserts across three distinct zones on the heel: an upper, middle, and lower facet. The soleus and the outer head of the gastrocnemius attach to the middle zone, while the inner head of the gastrocnemius attaches to the lower zone. This layered design distributes force across a wide area of bone rather than concentrating it at a single point.
What makes the Achilles remarkable is the load it handles. During running, it absorbs roughly 4 to 8 times your body weight with every stride. That capacity makes it essential for explosive movements like sprinting and jumping, but it also means that when the tendon is irritated or weakened, even routine walking can become painful.
Tendons Behind the Inner Ankle
If you place your fingers just behind the bony bump on the inside of your ankle (the medial malleolus), you’re touching the path of the posterior tibial tendon. It’s the largest tendon on the inner side of the ankle and makes a sharp, nearly 90-degree turn as it wraps around that bump before attaching to bones on the inner side of your foot. Its main job is supporting the arch. It also helps point the foot downward and turn the sole inward.
When this tendon becomes inflamed or degenerates over time, the arch gradually collapses. This condition, called posterior tibial tendon dysfunction, is the most common cause of adult-acquired flatfoot. Pain typically shows up along the inside of the ankle and into the arch. In early stages, the tendon is inflamed but intact. In later stages, it stretches and weakens to the point where it can no longer hold the arch up, and the foot begins to visibly flatten and roll inward.
Tendons Behind the Outer Ankle
Two tendons run behind the bony bump on the outside of the ankle (the lateral malleolus): the peroneus brevis and the peroneus longus. They sit together in a groove on the back of the fibula, held in place by a band of tissue called the superior peroneal retinaculum. The brevis is flat and runs directly against the bone, while the longus is rounder and sits just behind it.
These tendons are the primary muscles that turn your foot outward (eversion) and act as stabilizers for the outer ankle. Problems with them tend to fall into three categories. Peroneal tendonitis causes a gradual onset of pain and swelling along the outer ankle, sometimes with a crackling sensation when you move the foot. Peroneal subluxation happens when the retinaculum that holds the tendons in place tears or loosens, allowing them to snap in and out of their groove. You’ll feel a painful clicking or popping right behind the outer ankle bone, especially when turning the foot outward. Chronic subluxation can eventually cause longitudinal tears in the tendons as they repeatedly slide over the edge of the bone.
How to Tell Which Tendon Is Causing Pain
Location is the simplest clue. Pain centered directly at the back of the heel or a few inches above it, right along the thick cord you can pinch between your fingers, points to the Achilles. Pain along the inside of the ankle and into the arch suggests the posterior tibial tendon. Pain behind or below the outer ankle bone, especially with a snapping sensation, points to the peroneal tendons.
For a suspected Achilles rupture, clinicians use the Thompson test: you lie face down and the examiner squeezes your calf. In a healthy leg, the foot will point downward. If the foot doesn’t move, the tendon is likely torn. This test is highly accurate, with a sensitivity of 96% and specificity of 93%. A complete Achilles rupture often feels like being kicked in the back of the leg, followed by sudden difficulty pushing off the foot.
Common Achilles Tendon Problems
Achilles tendinopathy is the broad term for pain and dysfunction in the tendon, usually from repetitive overload rather than a single injury. It can affect the middle portion of the tendon (midportion tendinopathy) or the spot where it inserts into the heel bone (insertional tendinopathy). You’ll notice stiffness first thing in the morning, pain at the start of activity that may ease with warming up, and tenderness when you squeeze the tendon.
The most well-studied treatment for Achilles tendinopathy is eccentric loading, which means strengthening the tendon while it lengthens under tension. The classic protocol involves 3 sets of 15 repetitions of heel drops off the edge of a step, performed twice a day, seven days a week. Load and speed are adjusted based on your pain response rather than a fixed weight target. The goal is to work at a moderate level of discomfort without sharp pain. Most programs run for 12 weeks before significant improvement is expected, though many people notice changes sooner.
What Recovery Looks Like After Surgery
If the Achilles tendon ruptures completely, surgical repair is one treatment option. Recovery follows a predictable but lengthy timeline. For the first two weeks, you bear no weight on the foot and wear a protective boot. Around week 2, partial weight bearing with crutches begins. By week 4, most people can ditch the crutches and start gentle resistance exercises with a band, along with balance training on flat surfaces. Pool therapy can begin around this time if the wound has healed.
Week 8 is a turning point: you transition out of the boot and into a supportive shoe, often with a small heel wedge for comfort. Standing heel raises start around this time. By weeks 12 to 16, running and jumping can be introduced if strength benchmarks are met, including the ability to do more than 10 single-leg heel raises with height close to the uninjured side. Full return to sport typically falls between 6 and 9 months after surgery.
Why These Tendons Are Vulnerable
The tendons behind the ankle share a common vulnerability: they all change direction around bony landmarks. The Achilles pulls straight down but absorbs enormous loads. The posterior tibial tendon makes a near-right-angle turn behind the inner ankle. The peroneal tendons curve through a groove behind the outer ankle. These direction changes create friction, compression, and concentrated stress at predictable spots, which is why tendon problems in the ankle tend to occur at these exact locations rather than along the straight portions of the tendon higher up in the leg.
Age, sudden increases in activity, and footwear that doesn’t support the arch all raise the risk. The posterior tibial tendon is especially susceptible in middle-aged adults, while peroneal tendon problems are more common in people with a history of ankle sprains that left the outer ligaments loose. The Achilles affects both recreational athletes who ramp up training too quickly and sedentary adults whose tendons have lost capacity from underuse.