To get the best-looking abs, forget the 15-minute ab programs in favor of a combination of sprint and strength training workouts.
Strength training and high-intensity intervals produce metabolic adaptations, making it easier to get lean abs, whereas programs that just train the abs have little pay off. For example, one study in the Journal of Strength and Conditioning Research compared the effect of a 6-week abdominal training program in which participants performed 7 ab exercises (2 sets, 10 reps) 5 days a week. Unfortunately, none of the exercises used weights, meaning the metabolic and mechanical effect on the body was minimal.
The exercises used were the Bent-Knee Sit-Up, Lateral Trunk Flexion, Leg Lifts, Oblique Crunch, Stability Ball Crunch, Stability Ball Twist, and Abdominal Crunch. Results showed NO changes in body weight, body fat, abdominal fat, or waist circumference. Participants did improve abdominal endurance with a one-minute curl up test.
For the amount of time and effort invested in that ab program, participants didn’t get much benefit. A better training program would include deadlifts, squats, bench press, chin-ups, lunges, rows, and overhead presses, with one ab exercise a day rounding out the training program.
The metabolic disturbance would be much greater since these lift use larger muscle groups than the abs, while the squats, deads, chins, and lunges all require significant recruitment of the abdominal muscles. Plus, the added benefit of lifting heavy weights will help you build "definition" in the muscles that make up the "six-pack."
Another solution is to do sprint intervals to burn excess fat around the middle. High-intensity intervals have been shown to be even more effective than strength training for getting lean fast.
A recent study showed that participants who did a 6-week sprint interval program (4 to 6 sprints for 30 seconds each with 4 minutes rest) lost an impressive 12.4 percent body fat and an average of 3 kg of fat mass. Total training varied from 18 minutes when participants performed 4 sprints to 27 minutes when they did 6.
If your primary goal is lean abs, a high-volume total body program with sprints is the way to go. You’ll gain strength and muscle mass in the obliques, rectus abdominus, and erector spinae, giving you better trunk stabilization and a rock hard stomach.
Vispute, S., Smith, J., LeChiminant, J., Hurley, K. The Effect of Abdominal Exercise on Abdominal Fat. Journal of Strength and Conditioning Research.2011.25(9), 2559-2564.