Fitness Photography is one of those ill-defined terms that tends to change meaning, depending on the listener.
In most cases, it is the generic term used for any photos where the model, doesn’t wear a lot of clothes. Swimwear, calendar, some promotional shots, and a lot of other work fit into this category.
I started shooting as a hobby, and ended up shooting portfolios on the side while I was working in NYC in the entertainment and advertising industries. Gradually I added web and graphic design, and have become a full service operation for those who want to promote themselves into this field.