Here's a pdf of a Express Assumption of Risk Form for Personal trainers. This is a basic form letting the client know about the risk and your policies and a waiver for you. .
Reference: From NSCA, 201A2, NSCA’s essentials of personal training, 2nd ed., J. Coburn and M. Malek (eds.), (Champaign, IL: Human Kinetics).
Express Assumption of Risk Form