• A six-pack is a result of developing your rectus abdominal muscles.abs image by nutech21 from Fotolia.com
  
  The abdominal muscles of your stomach help you to maintain a sitting position and a healthy posture. They also protect your internal organs in the abdominal cavity, which lacks a bone covering like the ribcage. Toning and strengthening your abdominal muscles will improve your posture. Tightening your abs can be done with a few simple exercises, but as with any exercise regimen, you have to stick to the workouts and supplement them with a low-fat diet.
  
  

Rectus Abdominals Exercise

• The rectus abdominals are the muscles commonly thought of as abs. They stretch vertically from the pectorals to the groin. The rectus abdominals are actually a single paired muscle separated into eight sections by crisscrossing tendons.
  
  These muscles can be worked with crunches--not sit-ups. According to CriticalBench.com, sit-ups use your hip flexors rather than your abdominals. To do a crunch, lie down flat on your back with your knees bent at a 90-degree angle. Lift your upper body off the ground, tensing your abs as you approach your knees and exhale slowly. Don't lift all the way; instead, lift only a few inches off the floor and focus on tensing your abs. Do a set of 20 each day.
  
  

Oblique Abdominals Exercise

• The internal and external oblique abdominal muscles wrap around the sides of your torso and compress your abdomen. Your obliques run diagonally from your ribcage to your hips. The external obliques run up from the center of your pelvis to your ribcage, and the internal obliques run beneath them and to right angles. To work the obliques, you should do oblique crunches.
  
  Lie on your side with your arms crossed over your chest. Keep your legs straight and feet together. Lift your legs as you lift your upper body, bringing your elbow to your hip. The oblique crunch doesn't have a wide range, but it's effective. According to "Men's Health," you should do 10 repetitions per side.
  
  

Transverse Abdominals Exercise

• The transverse abdominals run around your abdomen like a girdle. They attach to the erector spinae--muscle along the spinal column--and pull your abdomen in so you have a straight posture. Because the transverse abs are the innermost and least-visible muscles, they are often neglected in workouts, according to PregnancyInfo.net.
  
  One effective transverse abdominal exercise is the pelvic tilt. Lie flat on your back with your feet and upper body touching the floor. Slowly lift your pelvis off the floor without arching your back. Slowly lower to the ground. Do 15 repetitions of this exercise.