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From the Escape Collective article:
> While the medical literature isn’t yet settled on the exact cause of EIAE – it’s an uncommon and still relatively new condition – there is a prevailing theory. The thought is that the repeated stretching, compression, and deformation of the external iliac artery as a result of the hunched-over bike position, plus the massive volumes of blood passing through an elite athlete’s arteries during intense efforts (20 litres/minute or more), causes damage to the arterial wall.
> But that’s not the full story.
> “There seems to be a positional component, but we don't have any rigorous causal experiments that show, ‘Oh, if you change your on-the-bike position, it's increasing risk,’” says Jem Arnold, a physiotherapist, Escape member, and PhD candidate in exercise physiology at the University of British Columbia who’s doing research on this very topic. “Also all the athletes in the professional peloton are in an aggressive position, but not all of the athletes develop FLIA. So bike position is a factor, but it's not the only factor.”
> The over-developed hip flexors of pro cyclists might play a role too. The external iliac artery and the psoas major muscle are connected by fascia and with repeated flexion of the hip during cycling, the enlarged psoas muscle can pull on the external iliac artery, further contributing to arterial damage.
> Other possible risk factors seem to include previous injuries to the hip area, plus individual anatomical variations. That is, if an individual’s arteries are more prone to kinking or twisting, that might increase their likelihood of EIAE.
> There’s another possible cause; one that hasn’t yet been spoken about much.
> “I think the most current theory that I agree with is that there's a failure of the endothelium [the single layer of cells that line blood vessels],” says Dr William Campbell, a vascular surgeon at Melbourne’s Epworth Centre who’s operated on several dozen cyclists with EIAE, including Gigante. “So the cells that line the artery are just not producing the substances that cause blood vessels to dilate, increase in size, and so they have an opposite effect, and cause vasospasm, so decreasing in size.
> “Sometimes you see a bit of elongation of the artery because of the position they're in,” Dr Campbell tells Escape. “And that was often a theory that this was an elongation and kinking, but I'm not so sure about that. That may play a part. But I think the thing that I see mainly is the dynamic change in the blood vessel at peak exercise, where, instead of dilating, it goes into a spasm, and that cuts off the blood supply to the leg.”