I do not know the laws of your country. In the U.S. we have every candidate fill out his health history, and state that he has had a physical from a doctor within a year, and knows of no prior injuries, health conditions, or disease which may pose a danger to his training. We also require medical physicals every two years and a copy on file.

He then has to sign a "Hold Harmless Agreement" holding the trainer and the gym harmless in case of injury or death. He further agrees that any question of liability or neglegance, will be settled by arbitration instead of a law suit.

Your next line of defense is to carry liability insurance on your gym.

I also carry personal liability coverage under my Homeowners policy and Business License.
Now all of this may sound like over kill. But if the worst happens, I wont lie awake at night worrying about losing my home and assets.

Stick to accepted training standards, take a certified First Aide course for coaches, and think safety at all times.