Many jobs carry occupational risks and sport is no different. For pro cyclists, crashing hard on tarmac is the obvious one, but when it comes to health, iliac artery endofibrosis (IAE) is another.

It’s a health condition that’s rare in the general population, but increasingly common among professional cyclists who ride 10s of thousands of kilometres every year.

IAE is a vascular condition that causes a narrowing of the external iliac artery, which carries blood from the pelvis to the legs. It’s not just seen in professional cyclists in the sporting world, but they are especially afflicted by it.

Pro riders who’ve suffered from issues with their iliac arteries are many, including Ineos Grenadiers’ Bob Jungels and recent Tour de France Femmes winner Pauline Ferrand-Prévot. It also curtailed the careers of Zdeněk Štybar and Tayler Wiles.

The exact number of cyclists with IAE is impossible to pin down because many cases go undiagnosed or are misattributed to other physiological or performance reasons, though it’s been reported that up to 20 per cent of elite road cyclists may develop IAE over time.

In fact, a 2022 paper published in the online medical journal Cureus called for IAE to be classified as an occupational disease, particularly in the case of athletes and cyclists. Whether it’s on the increase or awareness is growing remains to be seen.

So, what’s going on? And why might you, as a recreational rider, be affected?

Narrowing of the vessel

“The term endofibrosis means that somewhere within the wall of a blood vessel is an increase in fibrous tissue, which is essentially scar tissue,” says Professor Nick Cheshire, head of vascular surgery at Royal Brompton Hospital.

“Normally it’s flexible and soft, especially in young people, but if an area has been irritated over a long period of time, the scar tissue builds up and starts to narrow the blood vessel. This is called ‘stenosis’, which restricts the blood flow and is a secondary effect to the fibrosis.”

When a cyclist is resting, all is good. But when they work hard, their muscles demand more oxygenated blood to feed working muscles. This narrowness of the blood vessel reduces blood circulation of the limb, causing pain, weakness or numbness during movement.

The most common symptoms are a cramping or burning sensation in one leg, usually the thigh but also the calf or buttocks. Generally, it occurs during high-intensity efforts, like out-of-the-saddle ascents or sprinting to the line, but soon settles down once intensity is lowered.

Another symptom is losing power. This is how it manifested itself in Jungels, who’s had two IAE operations.

“Symptoms for me were fatigue and a loss of wattage,” says Jungels. “I could feel that I just hadn’t been able to hit those high, intensive numbers for a fair while. My body really struggled when we hit race pace.”

It was a similar tale for Štybar, who said at the time of his problems in 2023, “I was training well, hitting good numbers and should have been in the best shape of my life, but I just couldn’t perform in races.

“My legs were so tired, so sore. When we hit a climb, I was immediately dropped and could no longer do the numbers. I didn’t understand. It was such a weird feeling. I felt lost.”

What are the causes of iliac artery endofibrosis?

For the general population, IAE is often down to the usual high-risk factors such as smoking and diabetes, resulting in narrowing blood vessels and painful calf muscles. For professional cyclists, it’s also down to lifestyle factors, albeit the usually positive ones.

“It’s down to repeated movements,” says Cheshire. “The arteries supplying the leg begin at the umbilicus level [commonly known as the naval or belly button]. This is where the main blood vessel exiting the heart divides into two. They’re called the iliac arteries.”

Each iliac artery divides into an external iliac artery, which continues down the limb and becomes the femoral artery that supplies most of the lower limb, and internal iliac artery, which supplies blood to the pelvic organs, gluteal region and parts of the reproductive organs. It’s the external iliac artery that’s often affected in IAE, especially in cyclists.

“The artery comes around the pelvis and into the leg via the groin,” says Cheshire. “To enter the groin, it has to come under the inguinal ligament. As you’re pedalling, the artery is irritated by the ligament.”

This can cause kinking and damage to the iliac artery wall, leading to a build-up of fibrous tissue within the artery. This not only narrows the artery but prevents it from dilating during exercise, impairing bloodflow and oxygen supply to the leg.

The large swathes of blood associated with intense efforts passing through the narrow or kinked artery can cause further microtraumas and endofibrotic changes.

This repetition of movement is at peak volume in the professional ranks with male pros accumulating over 30,000km in training and racing each year. If there’s an issue, every pedal stroke will only exacerbate the situation.

Although inconclusive, the cycling position has been cited as a major reason, too, with the extreme hip flexion seen in the professional peloton combining with heavy bloodflow during hard efforts to stimulate these endofibrotic changes. That’s how Jungels saw it.

“Cycling is a relatively unnatural activity for your body, so where you bend your leg from the hips, scar tissue forms inside,” he said. “After nearly 20 years of training, this can build up and prohibit bloodflow.”

Excessively long iliac vessels could also increase the likelihood of kinking during hip flexion, while there are suggestions that systemic factors could also play a part, like some metabolic disorders and autoimmune disease such as Raynaud’s, but this requires further exploration.

“There are very few studies on endofibrosis and athletes,” says Cheshire. “It’s a mixed bag. Some sufferers have a single point narrowing; some have a long area of narrowing; some it’s directly underneath the inguinal ligament; some people it’s closer up to where the iliac artery begins.

“Like any arterial disease and narrowing of any form, whether it’s smoking and diabetes related or repetitive injury related, there are often personal anatomical factors that mean one person suffers and another doesn’t.”

Treatment options

If IAE is left untreated in cyclists, it can lead to the performance decline mentioned, plus significant vascular damage and, in some cases, permanent functional impairment.

While it’s not typically life-threatening, the impact on an athlete’s career and quality of life can be severe. Which is why treatment is the only option.

“The ideal treatment is something that avoids damaging the muscles,” says Cheshire. “So, the ideal treatment is ‘balloon angioplasty’. This is where a catheter with a small inflatable balloon is inserted into the narrowed section of the artery, which is then inflated to stretch the vessel and improve bloodflow.

“The problem is that this isn’t very effective when faced with the tough fibrous scarring in the elite athlete population.”

This is supported by research including a 2016 study that suggested balloon angioplasty, a minimally invasive procedure, failed in most cases, unlike surgery, which led to a 99 per cent return-to-sport rate.

“Surgery is often the only solution for athletes, which is a really big deal because they might return to the sport, but at what level?” says Cheshire.

“It often involves an abdominal incision before opening up the artery and then applying a patch.” More specifically, once the artery wall’s been opened, the narrowed section is removed and replaced with a sewn-in patch of tissue, which is either synthetic or from the rider’s own vein.

Like arterial disease in general, it’s more common in men. “A ratio of about four to one, I think,” says Cheshire, “albeit that could be heavily down to the fact there are more male cyclists. But the main problem is in identifying the problem.

“You see, it depends on the narrowing. If it’s less than 50 per cent, the pulse may be normal. And in fit athletes, it might not even show up on ultrasound. But there’s a much greater chance it will if you increase the blood flow, as if you were exercising.

“It’s why our investigation labs feature a treadmill and static bike. It’s called the ‘Stress Doppler’ and the ultrasound is done before and after exercise. It’s not routinely done but it can be invaluable for cyclists.”

Cheshire says that recreational riders can suffer from this condition, too, and if you have concerns that you might be suffering from IAE, mention this stress test to your doctor.

There’s also a diagnostic test called the ABI, or ankle-brachial index. The test measures blood pressure at the ankle and arm both at rest and immediately after exercise, and detects a drop in the ABI caused by reduced blood flow to the legs.

To ensure accuracy it should be performed in the cycling position, with exercise intensity gradually increased until symptoms appear, making the test sensitive enough to identify the condition.

How can you prevent IAE in the first place? That’s a tricky one. You might presume bike fit could play a part, but there is no research into its effectiveness.

In theory, you’d think the trend for shorter crank length would help by decreasing hip flexion and opening up the hips. Throw in a raised front end and you’d think the less-aggressive position might help. But again, there’s no research to support this idea.

“It’s hard to prevent because it’s down to lots of repetitive movements,” says Cheshire. “It’s a slow-burner of a condition.

“If you’ve had surgery, it might never come back, especially as a professional’s career is relatively short, so they won’t be racking up such enormous miles for decades upon decades. Ultimately, if you have any concerns, seek out an expert who can diagnose the problem or put your mind at rest.”

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