None, on what you say. 'Volenti non fit injuria' as lawyers say, 'to the willing there is no actionable injury'. If I do something voluntarily which has an inherent risk. which I ought to see, then I'm assumed to have accepted that risk. You must have seen some risk in what you did, or ought to have.
You may find that there's a clause in the terms for use of the gym which seeks to exclude liability. Some terms are unfair and not enforceable, or otherwise prevented by statute, but nevertheless such a clause may be reasonable and binding in your case and put an end to any claim.
It might be different if the instructor had undertaken to ensure you could not fall and said he'd hold you, you only took part because of his undertaking and he negligently failed to hold you properly, with the consequence that you were injured.