The best ab exercises for strengthening your core include movements for both the upper and lower abdominals. By performing an ab workout on a regular basis, you can also increase the definition of your abdominal muscles. The actual number of exercises and repetitions to perform during each ab training workout depends on your own personal goals and level of fitness.1. Crunches
This is one of the main exercises to use for both strengthening and defining your upper abdominal muscles. To perform crunches, lie down on the floor and bend your knees. Keep both feet flat on the floor at approximately shoulder width. Place your hands behind your head but do not use them to assist with the exercise. To begin the movement, lean up until the middle of your back is off of the floor. Squeeze your ab muscles for a count of one, then lower yourself back down to the floor. Do three sets of at least 10 repetitions.2. Side Crunches
Side crunches are an exercise to use for your oblique muscles, which are on the sides of your midsection. To perform the exercise, lie down on the floor on your right side. Slide your legs up on the floor slightly toward your chest so they are bent at the knees, then put your right hand on your left hip. Put your left hand behind your head but do not use it to help pull your body up during the exercise. To begin the side crunch movement, lean up and try to touch your left elbow to your left hip. Squeeze and hold your abs for a count of one, then lower yourself back down to the floor. Do 10 repetitions and then switch positions to work the other side. Try to do three sets of 10 repetitions for each side.3. Leg Raises
Leg raises are an exercise to use for your lower abdominals. To perform this exercise, lie down on the floor with your legs fully extended and your feet pointing up. Place your arms by your sides with your palms on the floor. To begin the movement, slowly lift your legs six inches off the ground and then squeeze your abdominal muscles. Hold the position for a count of one, then lower your legs back down to approximately one inch off of the ground. Do not let your feet touch the floor until after you finish the last repetition. Try to do three sets of 10 repetitions each time you train your lower abs.4. Knee-Ups
Knee-ups are another exercise for the lower abs. To perform this exercise, sit on the end of a bench and place your hands on the sides for support. Extend your legs out in front of you with your feet together and your heels on the floor. To begin the movement, bend your knees and pull them up toward your chest. Squeeze your abs for a count of one, then extend your legs back in front of you but do not let your feet touch the floor. Try to do three sets of 10 repetitions for this exercise.