We talk about six-packs, but the real number is four—your abdominal muscles are divided into four distinct groups. These six exercises hit them all, while giving you enough variety to keep things interesting. After the crunch, pick one exercise from each section: A (upper and lower rectus abdominis, or a portion of the rectus abdominis and the obliques) and B (upper and lower rectus abdominis, plus obliques). Remember to pull your stomach in as you exercise and you'll incorporate the transverse abdominis. To really isolate the transverse abdominis, add exercise C. Whichever moves you choose, you'll hit your abs from several angles to build a strong, flat midsection that works hard and looks even harder.

The right workout for you

Sculpt amazing abs with this easy-to-use checklist

Your level: BEGINNER

• Work your abs three times a week
• Sets of each exercise: 2-3
• Repetitions per set: 10-15
• Speed of each repetition: 3-4 seconds up, 3-4 seconds down
• Rest between sets: 30-60 seconds

Your level: INTERMEDIATE

• Work your abs twice a week
• Sets of each exercise: 2-4
• Repetitions per set: 10-15
• Speed of each repetition: 3-4 seconds up, 3-4 seconds down
• Rest between sets: 60-90 seconds

Your level: ADVANCED

• Work your abs twice a week
• Sets of each exercise: 3-5
• Repetitions per set: 10-15
• Speed of each repetition: 3-4 seconds up, 3-4 seconds down
• Rest between sets: 90-240 seconds

Know your muscles
Your abdominals are composed of four muscle groups: the rectus abdominis, the external and internal obliques, and the transverse abdominis

Together, these four muscle groups support the torso and assist it in various movements—bending the body to either side, twisting right and left, and lowering and raising the upper body.

The rectus abdominis, which is responsible for pulling your torso toward your hips, attaches at your sternum and your fifth through seventh ribs, and connects to your pubic bone. The external obliques run diagonally down from the lower ribs and connect to the pelvis and the pubic bone.

The internal obliques, which lie underneath the external obliques and run diagonally to them, start on the iliac crest and connect to your lower three ribs. The internal and external obliques are responsible for torso rotation and lateral flexion (bending to the side). Lastly, the transverse abdominis muscle, which runs underneath your obliques, stretches from your lower ribs to your pubic bone. Its main job is to pull your abdominal wall inward, protecting your internal organs and helping you expel air.

Weighted Crunch (entire rectus abdominis)
Lie flat on your back with your knees bent and your feet flat on the floor. Place a light medicine ball between your knees and squeeze it so it stays in place throughout the exercise. Hold a light weight plate (five to eight pounds to start) in your hands. Slowly draw your knees up toward your chest while simultaneously curling your head and shoulders off the ground. Pause, then slowly lower your legs and upper body back to the floor, or just above the floor to keep constant tension on your rectus abdominis.

Get more: Instead of a weight plate, hold a medicine ball at your chest, keeping your elbows out to your sides. Have your workout partner stand in front of you. As you curl up, throw the ball to your partner. Ask him to lightly toss the ball back at your chest so you can catch it, pull it back to your body, and then curl back down.

Cable Pulldown (upper rectus abdominis, obliques)
Kneel in front of a high-pulley cable and grab a rope attachment with both hands. Position your hands either by your ears (palms facing in) or just below your chin (palms touching the top of your chest). Keeping your hands locked in place, slowly curl yourself down and forward, starting by drawing your chin toward your chest, then letting your shoulders and back follow. Curl down as far as you comfortably can, then slowly reverse the motion. After each repetition done in this way, curl yourself down while twisting either to the left or to the right. Alternate sides on each repetition to work your obliques.

Get more: Stand with your right shoulder facing the machine. Reach your left arm across your face and grab the handle, then place your right hand on top of your left hand. Rotate your torso to your left as you draw your arms across and down. Once your hands are above your left thigh, slowly return to the starting position. Switch sides and repeat.

Pulse Twist (lower rectus abdominis, obliques)
Lie on your back and tuck your hands under your pelvis, along the sides of your tailbone. Keeping your legs straight and feet together, raise them so the soles of your feet point toward the ceiling and your buttocks lift a few inches off the floor. At the top of the move, twist your hips to the right so that your feet point to the left. Lower your legs back to the starting position and repeat the move, this time twisting your hips to the left.

Get more: Instead of twisting your hips at the top of the move, lower your butt to the floor, then slowly roll both legs to one side. Go as far as you comfortably can without losing your balance. Rotate your legs back up until they're above your hips and repeat the exercise, lowering your legs to the other side this time.

Twisting Legs-Up Crunch
Lie on your back and raise your legs so that the soles of your feet point toward the ceiling. Place your hands lightly behind your ears, elbows pointing out. Keeping your legs upright, slowly curl up and to the left. Lower yourself and repeat to the right. Alternate from left to right throughout the set.

Get more: Start the move with your legs straight and suspended at a 45-degree angle to the floor. As you curl your upper body off the floor, simultaneously raise your legs until your feet point toward the ceiling. As you bring your head and shoulders back down to the floor, lower your legs back to a 45-degree angle.

Twisting Crunch
Lie on your back with your legs bent at 90 degrees and your feet on the floor. Touch your hands lightly to the sides of your head. Slowly lift your shoulders off the ground and twist your body to the left so your right elbow points between your knees. At the same time, draw your knees up and in to meet the elbow. Lower yourself back down and repeat the move, this time twisting to the right so that your left elbow points between your knees.

Get more: Attach a bar to the end of a low-pulley cable and lie with your feet pointing toward it. Wrap your feet underneath the bar and raise your legs till they point straight up. The bar should rest along your insteps, with the cable between your feet. Now do twisting crunches, feeling the constant tension on the lower portion of your abs.

Swiss-Ball Stability Pose
Grab two Swiss balls and lie facedown across them. Your body should be straight, with just your chest lying on the first ball and your knees and shins resting on the other. With your feet spaced about 12 to 18 inches apart, place your hands on the floor for balance and hold this position for 60 seconds.

Get more: As the stability pose becomes easy, place your hands on the sides of the ball. For a greater challenge, try reaching your arms out to the sides or straightening them in front of your head. Moving your feet in closer together so they touch also increases the difficulty.