I'm not 100% sure if I understand your question. I think you are questioning how to bill for the PT assistant or the excercise physiologist. If so, you would bill the services out under the PT's name, since they would be overseeing the services by the assistant. Of course there are laws regarding what the PT assistant can and can't do, and whether the supervising PT needs to be on sight or not. As long as those laws are followed, you would bill out the services under the PT's name.
If that's not specifically what you were asking, let me know.
Thanks
Michele