system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it uncomfortable to stand with both legs straight for extended periods of time?
gpt
Muscles in the legs massage the veins when you're walking. This helps the veins return blood back to the heart.

If you're standing still all your muscles are tensed to keep you upright. This means they're not in a cycle of contraction and relaxation, like when you're walking. No cycle, no vein massage, which in turn leads to less back flow of blood to the heart. This causes your feet to swell with fluid that pools in your legs (if you've ever wondered why the elastic bands on your socks leave imprints on your skin after a long day, it's the same phenomenon).

Besides, your muscles don't particularly enjoy being contracted for very long amounts of time, because when they contract and tense up they tend to shut off parts of their own blood supply.

All of this contributes to the uncomfortable feeling you're describing.