A couple of weeks ago, David Bradford from Cycling Weekly emailed me asking my opinion on this extraordinary picture (courtesy of http://www.cyclingweekly.com/fitness/heres-science-behind-pawel-poljanskis-legs-veiny-343017).
The pic is of the professional cyclist Pawel Poljanski, and was taken midway through this year's Tour De France. It had attracted some attention in the mainstream media, although the premise that 'we'd all have legs like his if we were skinny enough' was a bit off key. https://www.thetimes.co.uk/edition/sport/wed-all-get-poljanskis-veiny-legs-if-our-body-fat-was-as-low-as-his-dpd32pf5x Here's my response to David:
The first line of approach to rebuff this is the known and exceptional vascularisation of the athlete. That's evidenced in their greater (but non-visible) capillarisation within the muscle, ie around the fibres, but also their greater vascularisation at an arterial and venous level, and that latter aspect is visible, partly. Not in having more piping as such, but in having possibly greater arterial diameters and definitely greater flow and dilatation - ie how the artery responds (dilates) to the muscle's demand for blood and oxygen. That's well established for the arterial side - the delivery of blood and oxygen, which are key limitations to endurance performance - although for the venous aspect it is less well studied. But fundamentally the main reason for the difference (to the average Times reader, for ease of comparison), is the well trained athlete has far greater blood volume as a whole; those veins are simply 'more full' mostly because of that - a hypervolemia.
That's the key difference between me and my legs and Poljanski (not just that I'm hairy, pale and a bit more portly!) My blood volume is just over 7 L, which is about 90 mL per kg of body mass (ie 9% of body mass). We can directly establish this through the carbon monoxide rebreathing method, which is utilised to measure total haemoglobin mass. A sedentary person may be more like 5 to 7%.
I don't know if Poljanski has had his own blood volume measured, but over the years our work and others has shown that a world class endurance athlete could be as high as 150 mL/kg, ie 15% of their body mass is blood - twice as much as the average Joe. Combined with the exceptional conditioning and vascularisation of the cyclist's leg musculature, along with the heat of the day, then much of that blood will simply be most visible there. When it's hot, your veins work hard to cool your blood. And if you're this skinny, you see them easily. But the main issue is the greater total blood volume.