There is no shortage of opinions on this topic. People can succeed (and fail) with all sorts of abdominal training programs. Besides, great abs have been found on many people that train them religiously each day, but better abs have been found on athletes that do no formal abdominal training at all!
The most important factor in developing eye-catching abdominal muscles (the "6-pack") is decreasing the body fat in that area, plain and simple. As for training techniques, almost everyone should train this area as you would any muscle group. Why spend 20 minutes per day doing endless crunches when 5-10 minutes every other day (or less!) will accomplish the same results? Only athletes (or back pain rehabilitation patients) may need additional abdominal work to assist their core stability in sport (or everyday function).
While most individuals in the gym believe that daily, high-volume training is optimal, there is nothing special about the abdominal muscles to suggest that a greater training frequency is required. Muscle biopsies have determined the rectus abdominis to be composed of 46% slow-twitch fibers, not unlike the vastus lateralis of the quadriceps (Johnson et al., 1972). As mentioned earlier, a more important aspect in developing a well-muscled midsection that can be shown off at the beach, is the reduction of as much body fat as possible.
To be precise, in order to have visible abs you must decrease your body fat to 10% or less. Test your body fat monthly. Have the same skilled professional do the testing each time in the same manner under the same conditions (i.e. same time of day, pre-workout, same day of the week, same diet pre-test). You would be surprised how small fluctuations in these variables can influence the outcome of body fat estimation tests.
Fortunately, the greatest body fat reductions are not achieved through multiple sets of high repetition abdominal exercises and therefore there is no need to waste your precious time performing endless sets of abdominal crunches. In fact, your time is better off spent including compound resistance exercises (exercise using many of the body's large muscle groups; i.e. the squat, bench press, pull-ups, deadlifts, etc) and performing strategic anaerobic interval and aerobic training.
When you perform compound exercises and interval training you will realize that more muscles are working and more metabolic effort is necessary in comparison to a simple abdominal crunch. Remember that when you head back to the gym!
Compound exercises should be the best choice for helping to maintain muscle mass while shedding body fat. Anaerobic interval training promotes the expenditure of energy (fat) long after you have left the gym, thus helping to shed the body fat while you are no longer exercising. Aerobic exercise is beneficial for individuals with a lot of fat to lose (i.e. if you test at greater than 15% body fat). However, if you only have a little fat to lose before the muscle stands out, then dietary manipulations and more anaerobic training (intervals and resistance training) are the priority changes.
As for specific abdominal training recommendations, isolated abdominal training 1-3 times per week is sufficient. Abdominal training should be brief and to the point (i.e. 2 exercises, a handful of sets, and a high-intensity of training). This will require passing on the basic lying abdominal crunch and instead use exercises that incorporate resistance. Like any other muscle group, the abdominals can be trained in a rep range of 6-15 per set. This will help build optimal abdominal strength for sport and daily function and will be of greater benefit in attaining abdominal musculature "definition".
Craig Ballantyne is a Certified Strength & Conditioning Specialist and writes for Men's Health, Men's Fitness, Maximum Fitness, Muscle and Fitness Hers, and Oxygen magazines. His trademarked Turbulence Training fat loss workouts have been featured multiple times in Mens Fitness and Maximum Fitness magazines, and have helped thousands of men and women around the world lose fat, gain muscle, and get lean in less than 45 minutes three times per week. For more information on the Turbulence Training workouts that will help you burn fat without long, slow cardio sessions or fancy equipment, visit via one of the ads on this blog.