Shulchan Aruch, Choshen Mishpat 421:5 says (in my own translation):
Two who wrestled together, and one knocked the other to the ground so that he fell and got blinded, he's not liable.
The S'ma explains:
The reason for this is: Since they both wrestled willingly, each intending to knock down his fellow, and each knowing that it's impossible to be precise so as to knock someone down without injury, it is as if they gave up and give in to one another so that, even if one of them got damaged, whatever it may be, since the other would not intend to damage him but it would occur through a fall, he would not be liable. This is similar to what it says about two who run in a public thoroughfare and damage one another while running: that the damaging party is not liable, because anyone running in a public thoroughfare knows that others also have the right to run in a thoroughfare and that, when two such runners meet, damage is likely.
…When two are running, it's not because of forgiveness [of liability] alone [that there's no liability] but because both did an act of damage. Thus, even without there being a public thoroughfare, the same: any two running at each other.…
The same would seem to apply here, but, of course, for practical halacha, consult a competent authority.