The bicycle is a classic abdominal exercise that has been shown in research to stimulate the most abdominal muscles at one time.
Begin by laying supine with one leg and hip flexed at 90 degrees and one leg flat on the floor. Your hands should be behind your head. Inhale, then exhale, as you rotate and crunch your abdomen to touch the right elbow to left knee. Pause briefly, then straighten your left leg, lower your upper body to floor, and then bend your right leg as you crunch up and touch left elbow to right knee. Return to the starting position. Repeat this exercise for time or set repetitions.