If you have a genuine interest in becoming a Personal Trainer then you realy need to know your stuff or you will end up being lumped together with all those other people who went into it for all the wrong reasons. As you have said, looking the part goes a long way. It is the best form of salesmanship. But there also has to be a great deal of knowledge underneath all that muscle. Although your future clients will be coming to you to learn how to get into shape ... they are not idiots ... they can spot a fake. So it is important to take it seriously.
As to credentials, there are so many available from genuinely recognised certifications, right on down to send me $49 and I'll print you up a nice cerificate on my printer type of credential. The following lists just some of the more well known fitness certification agencies. Within each of these agencies, there are differenct types of certifications (ie: Personal Trainers, Nutritional Specialists, etc.) so you will have to see which one is more inline with what you wish to pursue.
ISSA International Sports Sciences Association
ACSM The American College of Sports Medicine
NSCA National Strength and Conditioning Association Certification Commission
NCSF National Council of Strength & Fitness Certifying Agency
NESTA National Endurance Sports Trainers Association
ACE American Council on Excercise
NFPT National Federation of Professional Trainers
NSPA National Strength Professionals Association
Also, have a look on the fitness job boards in your area to see what experience and credentials prospective employers are looking for so it will give you an idea of where you should be heading. Just a bit of caution though: The fact that an employer is asking for a type of certification does not mean it carries more weight than others. I know a couple gyms who continually ask for the same type of certification simply because it is the cheapest and quickest for employees to obtain.
You will also have to have CPR and First Aid training. The CPR is usually a necessity before you sit your final exam.
Back to the salesman issue again ... That really depends on the type of work and the environment you are in. If you are self-employed then it plays a huge part. If you are working in a gym franchise setting then it may only play a small part depending on how much initial contact you have with clients. Some gyms have people who take care of all the selling and paperwork side of things and then pass the new member over to the trainers. Other gyms though will have you doing the whole thing. So it is important at your interview to ask alot of questions with regards to your role in the gym. When I first started, I did the selling side of it but now, thankfully, I just concentrate on training.
If you want to test your current knowledge on fitness to see if you are ready to take the next step, then try the Pretest at the following link:
NSCA CSCS Certification Preparation Test
Hope this helps you with your career mate.
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KP -- Fitness Basics