Taking responsibility is key. Even if it wasn’t your fault. (Which it probably wasn’t completely – it’s not the straw that broke the camels back. Taking responsibility will help avoid any larger problems. When you step up and take the blame, they are likely to take some of it also. It is a shared responsibility.
Don’t be hard on yourself though…
What people do with the other hundreds of hours outside your session is not within your control. The way they sit, sleep and the other activities they participate in contributes to the potential for injury. There are a lot of people out there who need to be careful with exercise, because their body is already chronically over-stressed. It’s hard for the trainer to know all of the areas that are under tension without being a mind/body-reader!
Hopefully this was a good lesson for you.
It’s not always “XYZ exercise that caused the injury”. Many times it’s much deeper than we can really figure out.