Achilles tendonitis is inflammation or degeneration of the Achilles tendon, the thick band of tissue that connects your calf muscles to your heel bone. It’s one of the most common overuse injuries in runners and recreational athletes, but it can affect anyone who puts repetitive stress on their lower legs. The condition causes pain at the back of the leg or heel that typically starts mild and worsens over time if left unaddressed.
Where the Pain Shows Up
There are two distinct types, and the difference comes down to location. Non-insertional Achilles tendonitis affects the middle portion of the tendon, roughly 2 to 6 centimeters above where it attaches to the heel bone. This is the more common type, especially in younger, active people. The tendon fibers in this zone begin to break down, swell, and thicken.
Insertional Achilles tendonitis occurs right where the tendon meets the heel bone. It can involve inflammation of the small fluid-filled sac (bursa) that sits between the tendon and the bone. On imaging, the fibers on the side of the tendon closest to the bone are most commonly affected. This type can strike regardless of activity level and sometimes develops bone spurs at the attachment point.
Inflammation vs. Degeneration
The term “tendonitis” implies inflammation, and that is what happens early on: micro-tears develop when the tendon is suddenly overloaded with more force than it can handle. But most people who seek help have actually progressed beyond that initial phase into something closer to chronic degeneration, sometimes called tendinosis.
In a healthy tendon, collagen fibers are neatly aligned and designed to bear heavy loads. In a degenerating tendon, those organized fibers are replaced by immature collagen that doesn’t link together properly. The tendon loses its firm, white, glistening appearance and becomes soft, dull, and brownish. New blood vessels grow into the damaged area, but they don’t function normally and don’t promote healing. Notably, inflammatory cells are rarely present at this stage, which is why anti-inflammatory treatments alone often fall short for long-standing cases.
This distinction matters because the approach to a fresh, inflamed tendon (rest, ice, short-term anti-inflammatories) differs from what a chronically degenerated tendon needs (progressive loading and rehabilitation).
What It Feels Like
The hallmark symptom is a mild ache at the back of the leg or just above the heel after physical activity. In early stages, you might only notice it after a run or a long walk. As the condition progresses, the pain can become a sharper burning sensation triggered by stair climbing, sprinting, or pushing off while walking. Eventually, it can hurt even at rest.
Morning stiffness is extremely common. The tendon tightens overnight, so the first few steps out of bed are often the worst. That stiffness and soreness typically ease with gentle movement, only to return after more intense activity. You may also notice tenderness when pressing on the tendon, visible thickening or swelling along the back of the ankle, and difficulty flexing the foot normally.
What Causes It
Overuse is the primary driver. The tendon can handle enormous forces, but when loading outpaces recovery, damage accumulates. A large prospective study of runners published in the British Journal of Sports Medicine found that for every additional 12.5 kilometers of weekly running distance, the odds of developing Achilles tendonitis increased by 67%. That makes training volume one of the strongest modifiable risk factors.
Biomechanics also play a role, though not always in the ways people expect. The same study found that runners whose ankles didn’t rotate and roll inward enough during the stance phase were actually at higher risk, suggesting that some degree of natural ankle motion helps absorb impact forces. Interestingly, footfall pattern (whether you land on your heel, midfoot, or forefoot) did not significantly influence risk, challenging a common recommendation to change your running strike.
Other contributing factors include tight or weak calf muscles, sudden jumps in training intensity, running on hills or uneven surfaces, and wearing shoes with poor heel support. Age is relevant too: the tendon becomes less flexible and less resilient over time.
Medications That Raise Risk
A class of antibiotics called fluoroquinolones (prescribed for urinary tract infections, respiratory infections, and other conditions) is strongly linked to Achilles tendon problems. A meta-analysis of 15 studies found that fluoroquinolone use nearly quadrupled the odds of developing Achilles tendonitis and more than doubled the risk of tendon rupture. The risk is highest in people over 60 and those also taking corticosteroids. If you’re prescribed one of these antibiotics and notice new Achilles pain, that’s worth raising with your doctor promptly.
How It’s Diagnosed
Diagnosis is primarily clinical. A provider will press along the tendon to locate the painful area, check for swelling or thickening, and assess your ankle’s range of motion. One important step is ruling out a tendon tear. The Thompson test involves lying face down with your feet hanging off the exam table while the provider squeezes your calf. In a healthy tendon, your foot will move downward. If it doesn’t move, the tendon may be torn, which is a different and more urgent problem.
Imaging isn’t always necessary for a straightforward case. Ultrasound or MRI may be ordered if the diagnosis is unclear, if symptoms haven’t improved with treatment, or if a partial tear is suspected.
Treatment and Recovery
The good news is that most people recover without surgery. Non-operative treatment succeeds in roughly 80 to 85% of cases, even for the insertional type, which was historically considered harder to treat conservatively.
In the first two to three weeks, the focus is on calming symptoms: reducing activity, icing the area, and sometimes using a short period of immobilization with a walking boot or heel lift. Anti-inflammatory medications may help during this acute window but aren’t useful as a long-term strategy, especially once the problem has shifted toward degeneration rather than inflammation.
The cornerstone of rehabilitation is eccentric exercise, which means loading the tendon while it’s lengthening. The most widely studied approach involves standing on the edge of a step and slowly lowering your heel below the level of the step. A standard program calls for three sets of 15 repetitions, performed twice daily, seven days a week, for 12 weeks. It sounds simple, and the volume is high, but this protocol has strong evidence behind it. The loading stimulates the tendon to remodel and lay down healthier collagen fibers.
For cases that don’t respond to exercise alone, shockwave therapy is an option. This non-invasive treatment delivers acoustic energy pulses to the tendon. A systematic review found that shockwave therapy reduces pain and improves function, and the combination of shockwave therapy with eccentric exercises and stretching outperformed either approach alone. Results were mixed across individual studies, so it’s not a guaranteed fix, but it’s considered safe and worth trying before considering surgery.
How Long Recovery Takes
Expect a timeline measured in months, not weeks. The initial rest and immobilization phase lasts two to three weeks. The eccentric exercise program runs for 12 weeks at minimum. Many people start feeling meaningful improvement around weeks 6 to 8 of rehabilitation, but full recovery from a chronic case can take three to six months. Tendons have a limited blood supply compared to muscles, which is a major reason they heal slowly.
Returning to activity too quickly is the most common mistake. The pain may subside before the tendon has fully remodeled, creating a false sense of recovery. A gradual return to running or sport, with incremental increases in distance and intensity, reduces the chance of relapse. If you were running 40 kilometers a week before the injury, plan on building back to that volume over several weeks, not jumping back in where you left off.