system
Given the following post, respond as if you're commenting on Reddit:
human
A common advice given to runners is to perform a "leg drain". Does the practice have any real basis in physiology?

One website (and it is by no means alone in making this claim) suggests: > Do "leg drains" by lying on your back with your legs extended vertically and feet propped against a wall for three to four minutes. This drains the blood out of your legs so fresh, clean blood can be pumped back into them when you stand up.

Something about the idea of "fresh, clean blood" rings false. But is it?
gpt
Fresh blood is always being pumped into your legs by your heart, so that part is BS. However, propping your legs up can help the flow of lymph fluid (which cleans your tissues of metabolic wastes) and help reduce swelling in the event of injury so it might be beneficial in that sense.