Qualifications
Uhh...legally speaking, in most states, none.
However, there are several hoops you need to jump through before you can become a practicing physiologist. First, even for entry-level jobs, you need a bachelor's degree. If you're hoping to advance to supervisory positions or if you want to go into clinical exercise physiology, you're going to want to scoop up a master's degree in kinesiology, too. While you're in school you'll be exposed to a variety of things, as physiologists generally takes classes in exercise physiology, sports psychology, business, nutrition, as well as specializations in youth or elderly exercise.
Want a Ph.D.? That will only be necessary if you want to spend your career conducting research in a lab.
On top of your degree you'll probably want to add some fitness certifications. Certification-granting organizations worth their salt include the American College of Sports Medicine, the American Council on Exercise (ACE), and the National Strength and Conditioning Association (NSCA).