ELI5 Standing Still - why is it harder than walking?

Why is the world record for time walking longer than time standing? People can walk many miles without discomfort, but when we stand for more than a few minutes or so, we get uncomfortable? What is Postural orthostatic tachycardia syndrome or POTS?   ... We explain like i'm five!     Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: mynameispineapplejoe, faykin,ieatfrosties, freshcoconutty, neubourn & syymo   To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]