Your clients should understand that they are making a commitment, not only to themselves but also to you, to work hard and make every effort to help themselves achieve something worthwhile; it is not sufficient for them to simply attend sessions with you, but that they must make a conscious effort to try their best at every session.
When explaining the agreement to your client make the above clear to them. Make sure they know that they must put in the effort and that they cannot rely solely on you for their fitness gains.
I always get my clients to sign the document, to demonstrate that they will comply with it. From your point of view it also gives you leverage; a get-out if they claim that it’s your fault that they haven’t had any increase in fitness/weight loss etc. I’ve not had any cause to use this threat, though have mentioned it on occasion when a particular client doesn’t appear to by trying their best, it quickly gets an extra 20% out of them!
You can find a commitment attitude agreement template in the Personal Trainer Business Forms and Templates pack for you to customise to your needs.