Great question. I've been looking into this also. Here's from wiki:
A number of certifications are available in United States of America, although a number are not accredited. Most require a high school diploma, cardiopulmonary resuscitation (CPR) and automated external defibrillator (AED) certification, and some type of examination.
A 2002 investigation evaluated a random sample of 115 personal trainers using the Fitness Instructors Knowledge Assessment (FIKA) (which measures knowledge in nutrition, health screening, testing protocols, exercise prescription, and special populations). The study described that:
70% of those surveyed did not have a degree in any field related to exercise science.
Those who did not have a bachelor's degree in exercise science-related field scored 31% less than those with a bachelor’s degree or higher in the field.
Those holding either an American Council on Exercise (ACE), American College of Sports Medicine (ACSM) or the National Strength and Conditioning Association (NSCA) certification scored 83% of the questions correctly.
Those holding any other certification (not ACE, ACSM or NSCA) answered 38% of the questions correctly.
Years of experience was not predictive of personal trainer knowledge.
In partnership with the fitness industry, the International Health, Racquet & Sportsclub Association (IHRSA) (which represents over 9,000 health and fitness facilities) started an initiative in 2002 to improve standards for both its own clubs and the industry as a whole. In January 2006, IHRSA implemented a recommendation that its facilities only accept personal trainers with certifications recognized by the National Commission for Certifying Agencies (NCCA) or an equivalent organization. IHRSA considers other accreditation agencies if recognized either by the Council for Higher Education Accreditation (CHEA) and/or the U.S. Department of Education (USED). As of January 2010, the ACE, ACSM and NSCA certifications are among the 15 accredited certifications recognized by IHRSA, two of which are accredited by an agency other than NCCA (the Distance Education Training Council (DETC)).
There remains no national legal restriction on the industry to date.http://en.wikipedia.org/wiki/Personal_trainer