They look like any other shin guards to me. Maybe for martial arts, since sports ones are usually made of hard plastic. It looks like almost all the characters this time around have clothes made of all manner of different materials. Just in this pic you can see Johnny has cloth wraps, spandex pants, cushioned shin guards, and sneakers. Compared to Scorpion who's got metal, woven cloth, leather, and bone in his outfit.
I think that the variety of materials issue has been thoroughly squashed with MK9.