Those aren't folds they are creases. Example.. ( Folds:
)
( Creases:
)
To explain, creases are lines shaping the body, these are mainly found on the connecting parts of clothing touching the body or surounding it.
Folds are layered so depending on the postition of the body they can form the motion of pull, push, stretch, crumple or overlap. Sweat pants would no doubt have little if not any wrincles or folds depening on the size and shape of the body. Folds are much more visable as compaired to creases, they can either begin at the out side of clothing and form around joints or muscles, or they can continue lines that stop at certin positions and form connections with other folds thus giving the illusion of being pulled, pushed, stretched, crumpled or overlaped.
I think you get the idea so i'll just shut up now.