I may challenge individual clients with exercises that they believe to be out of their reach as long as I am convinced that they can accomplish them. The reward is the accomplishment of it in itself. I would only do that with a client to whon this is a true motivation and who would not end up hurting himself in trying.
Other than that, I would not consider it because with every challenge comes the possibility of not meeting it, and I try to keep the atmosphere as positive as possible. Nobody wants to fail or even come in second.
I constantly challenge my clients to reach just a little bit higher, and we celebrate every individual accomplishment.