What Is Achilles Tendinopathy? Causes and Treatment

Achilles tendinopathy is a painful, degenerative condition of the Achilles tendon, the thick band of tissue connecting your calf muscles to your heel bone. Rather than the acute inflammation that the older term “tendinitis” implies, tendinopathy involves a gradual breakdown in the tendon’s internal structure, with disorganized collagen fibers, abnormal blood vessel growth, and altered gene expression related to tissue remodeling. It is one of the most common overuse injuries in runners and recreational athletes, but it also affects sedentary people, and the reasons it develops reach beyond simple overtraining into metabolic health, medication side effects, and the tendon’s own unusual blood supply.

Why the Achilles Tendon Is Uniquely Vulnerable

The Achilles tendon is the strongest tendon in the human body, capable of withstanding forces several times your body weight during running and jumping. That strength, though, comes with a trade-off. The tendon’s midsection receives considerably less blood flow than its upper and lower ends. A cadaver study quantifying blood vessels along the tendon found that the middle portion had roughly half the vascular density of the regions closer to the calf muscle or the heel bone.¹ This relatively starved zone gets its supply mainly from the peroneal artery, while the upper and lower segments tap into the posterior tibial artery.²

That blood-supply gap matters because tendons depend on circulation to deliver oxygen, nutrients, and the growth factors needed for everyday tissue maintenance. When the midsection is repeatedly loaded without adequate recovery, the repair process can fall behind the damage, setting the stage for the structural deterioration characteristic of tendinopathy. This is why the midsection is the most common site for both chronic tendinopathy and spontaneous rupture.

Midportion Versus Insertional Tendinopathy

Clinicians distinguish two forms based on where the problem sits. Midportion tendinopathy affects the body of the tendon, typically two to six centimeters above the heel. It is the more common of the two and is closely linked to the hypovascular zone described above. People with midportion tendinopathy usually feel a stiff, achy pain that warms up with activity but worsens afterward, especially the next morning.

Insertional tendinopathy occurs right where the tendon attaches to the calcaneus (heel bone). Ultrasound studies show that the injured side in people with insertional tendinopathy tends to have a thicker tendon at the bone attachment, along with a significantly higher rate of calcification or bony deformity compared to those with midportion disease.³ This distinction is not just academic: the two forms respond somewhat differently to exercise programs, shoe modifications, and surgery, so getting the location right early on shapes the treatment plan.

What Actually Happens Inside a Degenerating Tendon

Older textbooks framed Achilles problems as straightforward inflammation, but tissue samples tell a different story. Histological and molecular analysis of tendinopathic tissue shows significantly altered expression of genes related to collagens and the enzymes that remodel the tendon’s structural matrix. Inflammatory cells are present but at lower levels than in chronic ruptures, suggesting that the process is more about failed healing than ongoing acute inflammation.⁴ The collagen fibers, which are normally arranged in tightly parallel bundles, become disorganized. New, poorly formed blood vessels grow into areas that are usually avascular. The net effect is a tendon that is thicker but structurally weaker, with reduced capacity to handle load.

Causes and Risk Factors

No single factor explains why some people develop Achilles tendinopathy and others do not. The causes span a spectrum from training decisions to systemic metabolic conditions.

Training Load and Biomechanics

A sudden spike in running volume, intensity, or hill work is the classic trigger. But the picture is more nuanced than “too much, too soon.” Research on runners returning to sport with existing Achilles tendinopathy found that running on consecutive days amplified the relationship between asymmetric ground contact time and pain. In other words, the more days people ran in a row, the more their compensatory movement patterns predicted pain on the symptomatic side.⁵ Rest days are not just about total volume; they give the body time to recalibrate gait asymmetries that accumulate with fatigue.

Metabolic and Systemic Factors

This is where the condition diverges from a purely athletic injury. People with Achilles tendinopathy tend to have a lipid profile consistent with insulin resistance: higher triglycerides, lower HDL cholesterol, and elevated apolipoprotein B.⁶ Diabetes appears to be a particularly strong risk factor. A recent meta-analysis estimated that people with diabetes have roughly seven times the odds of developing Achilles tendinopathy compared to those without.⁷ The likely mechanism involves advanced glycation end products, elevated inflammatory signaling, and impaired collagen turnover, all of which degrade the tendon’s ability to repair itself. If you are a non-athlete with persistent Achilles pain, these metabolic connections are worth discussing with your doctor.

Medications That Harm Tendons

Fluoroquinolone antibiotics (ciprofloxacin, levofloxacin, and related drugs) carry well-documented tendon toxicity. A population-based study found that current fluoroquinolone use tripled the risk of Achilles tendon rupture, and the risk persisted for about 60 days after the last dose. When fluoroquinolones were combined with oral corticosteroids, the risk jumped dramatically.⁸ The danger is greatest in older adults. One study found odds of Achilles rupture more than 20 times higher in patients over 80 who were currently using quinolones.⁹ If you are prescribed a fluoroquinolone and have existing tendon pain, that is a conversation worth having with your prescriber, because alternative antibiotics exist for most indications.

Diagnosis

In most cases, a thorough physical exam is enough. The hallmarks are localized tenderness, thickening of the tendon that you can feel by pinching it, and pain that worsens with loading, especially first thing in the morning or at the start of exercise. A clinician may use the “Royal London Hospital test,” squeezing the tender area with the ankle in different positions to reproduce symptoms.

Imaging comes into play when the diagnosis is uncertain, when symptoms do not respond to initial treatment, or when surgery is being considered. Ultrasound and MRI perform comparably for detecting structural abnormalities, with neither modality showing a clear advantage over the other for diagnostic accuracy across both insertional and midportion tendinopathy.¹⁰ An older comparison against surgical findings confirmed that both modalities give similar information, though MRI picked up abnormalities in slightly more cases.¹¹ The practical takeaway: ultrasound is faster, cheaper, and widely available in sports-medicine clinics. MRI is reserved for complex cases or pre-surgical planning.

One important caveat about imaging is that a normal scan does not always rule out the condition, and an abnormal scan does not always confirm it. One study found that ultrasound identified abnormal morphology in only about two-thirds of symptomatic tendons, while also flagging abnormalities in some tendons that felt perfectly fine. Baseline ultrasound findings did not reliably predict clinical outcome a year later.¹² Treatment should be guided by symptoms and function, not by what a scan looks like.

Treatment

Exercise-based rehabilitation is the first-line treatment and the one with the strongest evidence. Other interventions layer on top of it or come into play when exercise alone falls short.

Loading Programs

Two styles of therapeutic exercise dominate the literature. Eccentric training, which emphasizes the lowering phase of a heel raise, has been the standard since the late 1990s. Heavy slow resistance training, which uses both the lifting and lowering phases at higher loads and slower speeds, arrived more recently. A randomized controlled trial directly comparing the two found that both produced equally good, lasting improvements in pain and function at one year.¹³ A scoping review suggested some differences in what each approach does to the tendon itself: heavy slow resistance appeared better at reducing pathological tendon thickness and abnormal blood-vessel formation, while eccentric training increased tendon stiffness more.¹⁴ In practice, many clinicians use elements of both, tailoring the program to the patient’s tolerance and preferences.

A loading program needs at least three months of consistent work before you can judge whether it is helping. Clinical guidelines recommend sticking with exercise for that full period before moving on to other treatments.¹⁵ That can feel like a long time when you are in pain, but the tendon’s slow biology demands patience.

Shockwave Therapy

Extracorporeal shockwave therapy (ESWT) uses focused or radial pressure waves delivered through the skin to stimulate a healing response. A systematic review concluded that shockwave therapy reduces pain and improves function in midportion Achilles tendinopathy, and that combining it with eccentric exercises and stretching may work better than shockwave alone.¹⁶ The treatment is typically given in three to five sessions, spaced about a week apart. It can be uncomfortable during the procedure but is well tolerated overall and carries minimal risk.

Injections

Platelet-rich plasma (PRP) injections have generated a lot of interest, but the evidence is underwhelming. While short-term pain scores tend to improve after PRP injection, broader outcome measures such as function scores, tendon thickness, patient satisfaction, and time to return to sport do not show clear benefit.¹⁷ Higher-quality randomized controlled trials in athletes have similarly failed to demonstrate clear advantages over placebo.¹⁸ A large systematic review and meta-analysis was even more blunt, finding that PRP was not more effective than placebo for Achilles tendinopathy.¹⁹ Corticosteroid injections around the Achilles are generally avoided because of the risk of weakening or rupturing the tendon.

Surgery

Surgery is reserved for people who have completed a full course of conservative treatment, typically six months or more, without adequate improvement. For insertional tendinopathy, the standard procedure involves debriding (cleaning out) the damaged tendon tissue and removing a bony bump at the back of the heel. In more advanced cases where more than half the tendon is degenerated, surgeons sometimes transfer the nearby flexor hallucis longus (FHL) tendon as reinforcement. A randomized study found that debridement alone and debridement plus FHL transfer produced similar improvements in pain, function, and patient satisfaction, with the transfer appearing to add some ankle strength without sacrificing big-toe strength.²⁰ A more recent study using MRI to guide the decision found that tailoring FHL transfer to cases with over 50% tendon degeneration was associated with higher function scores.²¹ The general message: surgery helps, but it does not need to be aggressive in every case.

Recovery Timelines and Returning to Sport

Recovery from Achilles tendinopathy can take up to a year, and the condition has a high rate of recurrence, especially during the transition back to sport.¹⁵ That timeline surprises people. They often expect weeks, not months. But tendon tissue turns over slowly compared to muscle, and pushing back to full activity before the tendon has adapted is the most common reason people relapse.

What “ready to return” actually means is poorly standardized. A systematic review of return-to-sport criteria found wide variation across studies. The most common definitions involved reaching pre-injury activity level, being pain-free, and recovering muscle strength. But many of the criteria used in published studies lacked clear benchmarks.²² In practice, a gradual, staged progression that monitors symptoms and adjusts volume accordingly tends to work better than a strict calendar-based approach. A useful self-monitoring rule many physiotherapists use: pain during or after exercise that stays below a four on a ten-point scale and settles by the next morning is generally acceptable. Pain that does not settle, or that gets progressively worse session to session, is a signal to pull back.

When It Is Not Tendinopathy

Several conditions produce pain in the same neighborhood and can mimic Achilles tendinopathy or coexist with it. Retrocalcaneal bursitis, an inflammation of the small fluid-filled sac between the tendon and the heel bone, is one of the more common look-alikes. Research measuring pressure inside the bursa found that people with chronic retrocalcaneal bursitis had higher bursa pressure, which can impinge on the front surface of the Achilles tendon and create symptoms that overlap with insertional tendinopathy.²³ Other mimics include plantaris tendon entanglement, stress fracture of the calcaneus, posterior ankle impingement, and, in children and adolescents, calcaneal apophysitis (Sever’s disease), which involves the growth plate at the back of the heel rather than the tendon itself. If symptoms are not responding as expected, revisiting the diagnosis is reasonable.

The Role of Pain Sensitization

For some people with chronic Achilles tendinopathy, the pain seems out of proportion to any structural damage visible on imaging. Research into pain processing has found signs of central sensitization, a state where the nervous system amplifies pain signals, in a subset of people with the condition. One study detected increased mechanical pain sensitivity not just at the Achilles but at distant sites such as the shin muscle and the opposite Achilles, suggesting that the brain and spinal cord had turned up the volume on pain processing in general.²⁴

That said, this does not appear to be a universal feature of the condition. In a larger study of people with midportion Achilles tendinopathy, only about 5% scored above the clinical threshold for central sensitization on a standardized screening questionnaire, and all nine who did were female. Duration of symptoms was not correlated with sensitization scores, meaning that simply having the condition for a long time did not predict this kind of pain amplification.²⁵ For the minority who do show sensitization, treatments that address pain neuroscience education, graded exposure, and psychological coping strategies may be helpful additions to a loading program.

How Footwear Affects Tendon Load

Shoe choice can meaningfully shift how much force your Achilles absorbs. A study testing different shoe heights found that high-top shoes with laces tied reduced peak Achilles tendon tension by about 10% compared to low-top shoes, largely by limiting how far the ankle could flex forward.²⁶ On the other end of the spectrum, minimalist shoes (thin, flat, flexible) increase Achilles loading in rearfoot-striking runners, raising peak tendon force, loading rate, and tendon stress compared to conventional cushioned shoes.²⁷

None of this means minimalist shoes are bad or that high-tops cure tendinopathy. But if you are managing active symptoms, a shoe with a modest heel-to-toe drop (the kind most traditional running shoes have) will offload the tendon somewhat during daily activity. That small mechanical advantage can make the difference between tolerable and miserable mornings. Conversely, transitioning abruptly into flat or minimalist footwear while your tendon is irritated is likely to provoke a flare. Any footwear change should be introduced gradually, just like training load.

Why Evolution Made the Achilles a Weak Link

It might seem paradoxical that the body’s strongest tendon is also among the most injury-prone. The answer has to do with what the Achilles was optimized for. The tendon evolved as an energy-storage spring: it stretches under load, stores elastic energy, and then snaps back to amplify the power of the calf muscles during walking and, especially, running. This elastic recoil significantly reduces the energy cost of locomotion, and the benefits are greatest at faster speeds, consistent with the hypothesis that running ability exerted strong evolutionary pressure on Achilles tendon length.²⁸ The Achilles tendon was integral to the transition from tree-dwelling ancestors to bipedal humans who could cover long distances on foot to hunt and forage.²⁹

The catch is that a long, compliant, spring-like tendon is different from a short, stiff one built purely for force transfer. A spring needs to deform repeatedly under high load, and the midsection’s relatively low blood supply is partly a consequence of its design: dense vasculature would interfere with the smooth gliding and elastic behavior the tendon needs. So the same architecture that lets you run efficiently also means the tendon is slower to heal when things go wrong. Modern humans compound the problem by sitting for hours (which keeps the tendon shortened and unstimulated) and then suddenly loading it with intense weekend activities, a usage pattern our ancestors’ tendons were not designed for.