A properly written waiver would specifically include a provision which exculpates the gym for any negligence, which would include any failure to provide a safe environment. In fact, for sports like ours where you are obviously at risk of being injured just by participating, the law provides what's called "Implied Assumption of the Risk", which is a defense that the gym could invoke which basically says that by participating in the activity, you are knowingly accepting the fact that you could potentially be injured, and therefore you can't sue in tort when those injuries occur. However, in that situation, negligence on the part of the gym or the instructors (agents of the gym) becomes actionable IF there is no waiver signed. But again, a waiver that doesn't cover negligence is basically useless.