When the Leg Goes Numb: Iliac Artery Endofibrosis in Triathletes

Every triathlete knows the feeling of legs that won't go any harder late in a race. That is normal fatigue, and it affects both legs more or less equally. Iliac artery endofibrosis feels different. One leg, usually the same one each time, starts to hurt, cramp, weaken or go numb when you push hard, and the problem disappears within a few minutes of easing off. Then it comes back the next time you go hard.

At the 2026 IRONMAN World Championship in Kona, Sam Laidlow withdrew after the bike. TRI247's live report said he described restricted blood flow in his left leg due to iliac artery fibrosis, and that he planned to have surgery. This article doesn't speculate on his case. It uses it as a prompt to explain what the condition is, who gets it, how it's diagnosed and what treatment involves, because the condition is often missed in age-group athletes.

The research base is small: case series, retrospective reviews, diagnostic studies and an expert consensus. There are no randomised trials. That means most of what follows describes patterns seen by specialists rather than precise, well-measured risks.

What iliac artery endofibrosis is

The common iliac arteries branch from the aorta in your lower abdomen. Each one divides, and the external iliac artery continues down into the leg, becoming the femoral artery at the groin. It carries almost all of the blood that your working thigh and calf muscles need.

In endofibrosis, the inner lining of the artery thickens with fibrous tissue. That narrows the channel. It is different from the fatty plaque of atherosclerosis seen in older, sedentary people. A patient information page in Vascular Medicine by Martinez and Wells (2022) noted that more than 90% of cases involve the external iliac artery and that the left side is affected more often than the right.

Two other problems often occur alongside it:

• Lengthening and kinking. The artery can become longer than it should be and kink when the hip is flexed.
• Vasospasm. Some vascular surgeons have reported the artery narrowing further immediately after maximal exercise.

At rest, even a narrowed artery can deliver enough blood. The problem appears when flow needs to rise many times over during hard exercise. The narrowed or kinked section limits that rise, so the leg muscles run short of oxygen.

Who gets it

Most reported cases are in competitive cyclists, but the condition isn't limited to them.

• Cyclists dominate the case series. A 2023 review by Mazurová and colleagues in the Revista da Associação Médica Brasileira, which screened 233 articles and reviewed 20, reported that around 80% of cases are performance cyclists and that men are affected 8 to 10 times more often than women.
• Prevalence in top athletes is uncertain. Some authors have reported figures of 10% to 20% in elite or competitive cyclists, but the same review noted that the true prevalence is unknown. Those figures come from specialist referral settings, not population screening, so they shouldn't be applied to age-group triathletes.
• Mileage seems to matter. Martinez and Wells reported that cyclists with the condition had ridden an average of around 120,000 km before symptoms began, at more than 14,500 km a year.
• Women and runners get it too. Shalhub and colleagues, in the Journal of Vascular Surgery in 2013, described eight patients treated surgically between 2001 and 2010. All eight were women, two had both legs affected, and their sports were cycling (three), running (one) or both (four). Their median age was 42.5 years, with a range of 39 to 60.

That last point matters for triathletes. The condition is often described as a young male cyclist's problem, and many patients are. But a woman in her forties or fifties who swims, bikes and runs can develop it too, and may be even less likely to have it considered.

Why cycling, and why position matters

No single study has proven what causes endofibrosis. The leading explanation, summarised by Martinez and Wells, is repeated mechanical stress from hip flexion, combined with the very high blood flow that endurance cycling demands for hours at a time.

When you sit on a bike, especially in a low aero or time-trial position, your hip is deeply flexed. The external iliac artery runs over the front of the hip and pelvis, close to the psoas muscle. Each pedal stroke flexes and extends the hip thousands of times an hour. The hypothesis is that this repeated bending, stretching and high flow damages the artery wall over years.

Inference: triathlon puts many athletes in exactly the position the hypothesis points to. Long hours on the aero bars, a closed hip angle and sustained high power are part of the sport. That doesn't mean an aero position causes endofibrosis on its own, and there are no data comparing rates in triathletes and road cyclists. But it's a reason to take one-sided leg symptoms seriously if you spend many hours each year in a low position. TriForward's "Aero Until It Hurts" covers the wider trade-offs of a closed hip angle.

The symptoms

The pattern is more useful than any single symptom. Clinical descriptions, including Martinez and Wells, describe:

• Pain, cramp or a tight, 'empty' feeling in the thigh, calf or buttock of one leg, often described as the leg 'giving out' or losing power.
• Numbness, tingling or weakness, sometimes making the leg feel 'dead' or heavy.
• Symptoms only at high intensity. They typically appear during hard efforts, climbs, time trials or races, and not on easy rides.
• Rapid recovery. Symptoms usually settle within a few minutes of easing off, then return with the next hard effort.
• One leg more than the other. Around 85% of cases are reported as one-sided. Some athletes notice they can't hold the same power on the affected side.
• Normal at rest. There's usually nothing to see or feel when you're not exercising.

The 2023 Cureus case report by Carfagno and colleagues illustrates how varied this can be. A 38-year-old male cyclist developed sudden right-leg weakness and pins and needles within 5 to 10 minutes of cycling, which resolved with rest. A 50-year-old female triathlete had about four years of left-leg pain during exercise, radiating to her thigh and pelvis, with an early foot drop. Before her vascular diagnosis, she had a fasciotomy (an operation for compartment syndrome) and treatment for a calf muscle tear, but her symptoms persisted. Imaging finally showed a blocked section of her left external iliac artery.

Why diagnosis takes so long

Martinez and Wells reported that diagnosis can take between 12 and 41 months. In the Shalhub series, the women had had symptoms for a median of 5.5 years, ranging from 2 to 15 years.

There are several reasons:

• Resting examination is normal. Pulses, blood pressure and strength can all look fine in the clinic.
• The symptoms resemble common problems. Muscle cramp, chronic exertional compartment syndrome, nerve entrapment, a back problem or simple fatigue are all more likely explanations on paper.
• It's rare in general practice. Many clinicians will never have seen a case.
• Athletes adapt. They back off on hard efforts, change their position or accept lower performance on one side.

For a triathlete, the practical lesson is that a problem which repeatedly fails to respond to physiotherapy, massage, stretching or a bike fit deserves a different line of questioning, not more of the same treatment.

How it's diagnosed

The exercise ankle-brachial index

The ankle-brachial index (ABI) compares blood pressure at the ankle with blood pressure in the arm. At rest it is usually close to 1.0 in healthy people, including those with endofibrosis. The key is measuring it immediately after hard exercise, when the narrowed artery can't keep up.

Bruneau and colleagues, in the Clinical Journal of Sport Medicine in 2009, looked back at 22 patients with one-sided, surgically confirmed endofibrosis and 53 athletes without symptoms. They compared the ABI at rest, after a simple bedside exercise test and in the first minute after a maximal cycle ergometer test.

• Resting ABI distinguished patients from controls only moderately (area under the ROC curve 0.76).
• After the bedside test, accuracy was not significantly better than at rest (0.83).
• After maximal cycle ergometry, accuracy was almost perfect (0.99).
• A post-maximal ABI below 0.48 was 100% specific and 80% sensitive for endofibrosis.

That's the strongest diagnostic data available, but it comes from one retrospective study with 22 patients in a specialist centre. The authors' main point is practical: the test needs a maximal effort on a bike, not a few squats or a short jog in the clinic.

Imaging

Once an exercise test suggests a problem, imaging is used to confirm and locate it. Options include duplex ultrasound, often done with the hip flexed into a cycling position, magnetic resonance angiography (MRA) and CT angiography (CTA). Martinez and Wells list all of these as part of the workup.

Expert consensus

In 2016, an international group of vascular surgeons, sports physicians, sports scientists, radiologists and vascular scientists published a Delphi consensus in the European Journal of Vascular and Endovascular Surgery, led by Robert Hinchliffe. Over three rounds, with 18 responses in the second round and 14 in the third, and a 70% agreement threshold, they agreed that:

• There is a typical set of presenting symptoms that should raise suspicion.
• An exercise test should be part of the diagnosis.
• Duplex ultrasound is useful for diagnosis.

They didn't agree on the exact diagnostic thresholds for non-invasive tests. That's an honest reflection of how small the evidence base is.

Treatment options

Conservative management

Martinez and Wells describe a conservative first step for some athletes:

• Changing bike position, for example raising the handlebars, moving the saddle forward and avoiding pulling up hard on the pedals, to open the hip angle.
• Reducing training intensity.
• Changing sport.

These approaches may reduce symptoms, but they don't remove the lesion. The Delphi panel agreed that continued exercise is likely to allow the condition to progress, while stopping exercise may prevent progression. For an athlete who wants to keep racing hard, conservative management is often a holding measure rather than a solution.

Surgery

The Delphi panel agreed that endarterectomy with a vein patch, where the thickened lining is removed and the artery widened with a patch of the patient's own vein, is the preferred operation. Other surgical options reported in the studies below include releasing a kinked artery or bypassing the affected section.

The panel also agreed that endovascular treatment, such as balloon angioplasty and stents, is not advisable. Martinez and Wells make the same point. Inference: the reason commonly given is that a stent in an artery that flexes with every pedal stroke is exposed to repeated bending, but that rationale is clinical reasoning, not trial evidence.

Outcomes

The most detailed prospective study comes from the Netherlands. Schep and colleagues, in The Lancet in 2002, assessed 80 endurance athletes referred with symptoms suggesting flow limitation in the iliac arteries. Six had both legs affected, giving 92 legs.

• Flow limitation was confirmed in 58 of 92 legs (63%).
• Kinking was the main cause in 40 of those 58 legs (69%).
• Twenty-three legs were operated on, mainly by releasing the kinked artery.
• All operated athletes reported improvement, and maximal cycling workload and post-exercise ankle pressure improved significantly.
• Twenty athletes (87%) returned to their desired competitive level.

In the Shalhub series of eight women, all had vein patch angioplasty, and over follow-up of 1 to 10 years all had normal exercise tests and returned to their sport.

Both studies are small, from specialist centres, and selected the athletes most likely to do well with surgery. They show that good outcomes are possible, not that every athlete will return to their previous level.

A triathlon scenario

The following example is illustrative, not drawn from a case study. A 46-year-old age-group triathlete has raced long-course for a decade and spends around 10,000 km a year on the bike, mostly on aero bars. Over two seasons, they notice their left leg 'switching off' on hard climbs and in the last hour of race-pace rides. It feels like cramp in the thigh and calf, sometimes with tingling in the foot. Easing off for two or three minutes makes it go away. Easy rides and runs feel fine.

They see a physiotherapist, who finds tight hip flexors and a slightly weaker left glute, and gives them exercises. A bike fitter raises the bars a little. Things improve a bit, but the problem keeps coming back on hard efforts. Because their resting examination is normal, it's another year before a sports physician arranges a maximal cycling test with ankle pressures measured afterwards.

The lesson isn't that every one-sided leg problem is vascular. Most aren't. It's that when symptoms are reproducible, effort-dependent, one-sided and quick to resolve, and normal treatment hasn't fixed them, a blood-flow cause should be on the list.

Common mistakes

• Assuming it's cramp. Exercise-associated cramp is common and usually has nothing to do with blood flow, as TriForward's "Cramping at Kona" discusses. But cramp-like symptoms that always affect the same leg, only at high intensity, and settle within minutes deserve a different explanation.
• Relying on a normal resting check. Normal pulses and a normal resting ABI don't exclude the condition.
• Testing with a submaximal effort. The best diagnostic data come from a maximal cycling test.
• Repeated fits and physio with no change. Position changes are worth trying, but if they don't fix a reproducible pattern, ask about vascular assessment.
• Assuming you're the wrong demographic. Older athletes and women are reported in the case series.

What we don't know

• How common it is in age-group triathletes. All prevalence figures come from specialist referral settings or elite cyclists.
• Whether aero position increases risk. The hip-flexion hypothesis is plausible, but no study has compared triathletes with road cyclists or tested whether position changes prevent it.
• The natural history. Specialists believe continued hard training allows progression, but long-term data are limited.
• The best non-invasive diagnostic thresholds. The Delphi panel couldn't agree on them.
• Long-term surgical outcomes in age-group athletes. The published series are small and mostly elite.

References

• Schep G, Bender MHM, van de Tempel G, Wijn PFF, de Vries WR, Eikelboom BC. Detection and treatment of claudication due to functional iliac obstruction in top endurance athletes: a prospective study. The Lancet, 2002;359(9305):466–473. doi:10.1016/S0140-6736(02)07675-407675-4)
• Bruneau A, Le Faucheur A, Mahé G, Vielle B, Leftheriotis G, Abraham P. Endofibrosis in athletes: is a simple bedside exercise helpful or sufficient for the diagnosis? Clinical Journal of Sport Medicine, 2009;19(4):282–286. doi:10.1097/JSM.0b013e3181b20456
• Hinchliffe RJ, D'Abate F, Abraham P, Alimi Y, Beard J, Bender M, et al. Diagnosis and management of iliac artery endofibrosis: results of a Delphi consensus study. European Journal of Vascular and Endovascular Surgery, 2016;52(1):90–98. doi:10.1016/j.ejvs.2016.04.004
• Shalhub S, Zierler RE, Smith W, Olmsted K, Clowes AW. Vasospasm as a cause for claudication in athletes with external iliac artery endofibrosis. Journal of Vascular Surgery, 2013;58(1):105–111. doi:10.1016/j.jvs.2012.12.060
• Carfagno VF, Rouintan J, Rucker MA, Carfagno D. External iliac artery endofibrosis: a discussion on two unique cases. Cureus, 2023;15(9):e44839. doi:10.7759/cureus.44839
• Martinez A, Wells BJ. Vascular Disease Patient Information Page: External iliac artery endofibrosis. Vascular Medicine, 2022;27(2):207–210. doi:10.1177/1358863X221076565
• Mazurová T, Sengul I, Toman D, Pelikán A, Sengul D, Mazur M, et al. Endofibrosis as a cause of peripheral artery disease: a comprehensive review and proposal of two novel algorithms for diagnosis and treatment. Revista da Associação Médica Brasileira, 2023;69(2):352–356. doi:10.1590/1806-9282.20221374
• TRI247. IRONMAN World Championship Kona 2026: live updates. 10 October 2026. tri247.com