Lactic acid is fitness enemy No. 1. It builds up in your muscles, makes them ache, and, for the sake of everything that is fit and pure, needs to be flushed out of your system and massaged from your muscles. Lying on your back with your legs up against the wall will surely help.
Or, at least, that's what a lot of people—including some trainers, physiologists, and academics—will tell you.