Fitness is much like fashion in the sense that it is always changing and evolving. Often times it repeats itself but always comes back to the tried and true classic techniques. As in fashion, there are always new fads in fitness, some work most don't. The fitness industry is always introducing new exercise programs, a majority of these programs take you over your targeted heart rate, using your muscle for your energy supply as opposed to burning fat. Other classes have you doing exercises that stress your connective tissues and joints leaving you injured.
When embarking on a new Fitness regime, make sure that the program is one that executes good form and proper technique, working with your body type and physical limitations. The workouts should be something that is safe for you and is effective in showing you the results that you want to achieve.