Did you figure out what that was? I think Sarah probably knows.
This, my friends, is the result of "absorption drills", the fancy name Krav Maga gives to what I would call getting the s*it kicked out of you. You are meant to stand still whilst someone kicks you over and over and over again in the same spot. The "exercise" is intended to build up tolerance and the aftermath shows up particularly well on Bren's lily white skin.
Still, it does make you seem pretty bad-ass, doesn't it?