If you want great looking abs, the first thing you need to do is reduce the layer of fat covering your abdominal muscles. The only way to burn fat is to have a calorie deficit: either burn more calories than you eat or eat less calories than you burn. You can do this through diet, exercise, or a combination of both. Resistance training of your large muscles - legs, chest and back - will maximize your calorie burn and may also help increase your resting metabolism as you gain lean muscle.
Abdominal Exercises strengthen your muscles but are not effective in reducing abdominal fat. You can have very strong abdominal muscles covered by a layer of fatty tissue. It is more efficient to spend the majority of your limited exercise time burning the fat than strengthening the abs. You cannot spot reduce - working the muscle in an area does not reduce the fat in that area, fat does not "turn into to muscle" they are two different types of tissue.
Most people will see definition in the abdominal area at around 9-12% body fat for men and 10-14% for women. This will vary from person to person, depending on where their body stores fat (sometimes called your body type). There are some theories about cortisol, a stress hormone, leading to increased storage of fat in the abdomen, but abdominal fat tends to be dictated mainly by your body type.
You don't need machines to effectively work your abs. Some of the most effective abdominal exercises don't use machines at all and some machines are actually less effective than plain old crunches. (See the next post for a report on the most and least effective ab exercises from the American Council on Exercise).
Most people focus on the rectus abdominus (6 pack muscle) and the obliques, but it is more important to work on the inner abdominal core. In fact, some people should avoid the most popular ab exercises altogether because it can exacerbate problems with posture and low back pain. Simple exercises that focus on activating the deepest abdominal muscle (the transversus abdominus) include a drawing in maneuver, balancing on one leg, planks, and quadrupeds with arm and leg raises. These core exercises can improve posture and pull the abdomen in, giving the appearance of a smaller and tighter stomach. You can also incorporate core training into your everyday exercise routine, click here for a sample program.
There is no conclusive evidence that you can work so called upper and lower abdominals separately. See ACE's study (sidebar) which showed that the abdominal muscles function as a single sheath.
Quality is more important than quantity. You don't need to do hundreds of crunches a day to have good looking abdominal muscles, and you don't need to use heavy weights. The abdominal muscles work slowly and work isometrically, they respond very well to being trained the same way. Increase the level of difficulty by varying the speed, adding isometric holds, and decreasing the stability of your abdominal exercises. You can get results by working for 15-20 minutes 3-4 times a week.
Not every abdominal exercise is right for everybody. If you're working with a personal trainer, your fitness assessment will help determine the best selection and any necessary modifications based on your own personal personal abilities, physical condition and goals. Just because you see someone else doing an exercise that looks interesting, or it is part of the routine in a group class, that doesn't necessarily make it right for you. Avoid any exercises that are painful to your lower back and exercises where you are unable to maintain proper form. In classes, if you have concerns or problems keeping up with the class, speak with the instructor or a personal trainer to determine whether you need to modify the exercise or perform an alternate instead. For example, a member recently complained to me about back pain and inability to work full range of motion while doing leg raises in abs class. Modifications could include bending knees during the exercise, placing hands beneath the tailbone or an alternate position.