Endofibrosis (EF) of the iliac arteries is a non-atheromatous flow-limiting condition specific to highly trained athletes. Although the pathophysiology is not well understood, haemodynamic injury and mechanical stress are considered to play a key role in developing EF. Recurrent exposure to these factors could trigger an arterial remodelling process that results in an intravascular lesion such as EF that more resembles an "adaptive intimal thickening". Although the natural history of this condition is not known, it is believed to be progressive with increasing numbers of reports of limb threatening ischaemia due to dissection and/or thrombosis of the arterial lumen. Therefore, it is imperative to understand the pathophysiological mechanism behind this condition in order to develop accurate diagnostic and screening criteria/tools for an early identification and characterization of EF. As there are no data in the literature supporting this hypothesis, the investigators have designed this pilot study to evaluate this hypothesis as to whether athletes with EF have evidence of vascular dysfunction in their arteries. There are currently no data on whether athletes who develop EF have underlying arterial disease or abnormalities. It is quite possible that people who develop EF have an underlying systemic endothelial dysfunction and that this may represent a risk factor for the development of the disease. Therefore the investigators will study non-invasively endothelial function (measured as flow mediated dilatation) as well as arterial stiffness (measured as pulse wave velocity and augmentation index) to improve the understanding about the structural and mechanical characteristics of the vascular tree in this cohort of patients.