If you go by the original anime/manga series, what Yoko's wearing is a modified version (belt loop, zipper) of the "bun huggers" worn by many female athletes: they're brief, flexible and form-fitting; and they're not hot pants nor short shorts (neither of which are very flexible). Material-wise, they're akin to Lycra Spandex. Artwork outside the original anime/manga series, except for some dōjinshi and fan art, tends to give Yoko baggier pants/shorts - as can be seen in this drawing.
