Upper Abdominals
According to University of New Mexico exercise scientist Dr. Len Kravitz, unlike other muscles, the abdominals rarely fatigue to the point that you'll need an extra day of recovery. More is better with abs, and Kravitz recommends you work them every day. If you can't get to the gym that often, at least three per week should do the trick.
For your upper abs go to the cable crunch machine. Begin by grabbing the rope behind your neck and then kneeling. Round your back and contract your abs while you pull the rope down toward your knees. Do three sets of 10 to 12 reps.
On an incline bench, cross your arms across your chest and curl up high while tightening your ab muscles. Make sure that when you go back down your lower back barely touches the pad. Adjust the bench angle or grab a plate to increase the difficulty. Do three sets of 10 to 12 reps.
Obliques
To work the outermost muscles on your trunk—the obliques—you'll have to do a bit of twisting.
For the crossover crunch, lay down on your back on an exercise mat with your hands behind your head and your arms locked out to the sides. Move one shoulder toward the opposite knee, return to your back and then do the other side. Do three sets of 20 to 30 reps.
Grab an exercise ball to do the Swiss ball crunch. Keeping the ball stable with one arm, cross your other arm across your chest and pull your elbow towards your hip, crunching as high as you can. Switch and work the other side. Do three sets of 20 to 30 reps.
To work your obliques on the cable crunch machine, assume the same position you did for the upper abs except, this time, instead of pulling the rope straight down, cross over to your opposite knee. Do three sets of 10 to 12 reps.
Lower Abdominals
The reverse crunch is a great way to work your lower abdominals. With your hands and back on the floor, use your lower abs to lift your hips up and pull your knees slowly over your chest. For added difficulty and even better results use an incline board. Do three sets of 20 to 30.
The hanging knee raise is another great exercise for your abs. Using straps or a pull-up bar, bring your knees up as high as can while trying not to swing. Do three reps of 10 to 12.
For the best overall results with all of your ab workouts, focus on slow, controlled movements and contraction of the abdominal muscles.
