Done properly, the air bicycle abdominal exercise is a very effective movement because you are engaging your upper and lower abdominal areas at the same, while simultaneously exercising your hip flexors.
Although there is really no such thing as the “upper” and “lower” abdominal area, there are certain exercises where you will feel the burn in one area more than the other, which is what makes the air bicycle movement so effective. By keeping your shoulders elevated, you are tightening up the part of your abs that are closest to your chest while at the same time moving your legs which targets the hip flexors and the lower abdominal area.
The keys to this movement are:
Don’t stop until you feel the burn, and even then try to go for more reps
Support your head with your hands, but do not pull on your head
Bring your knees up as far as you can on each repetition
Focus on every repetition rather than just getting into a “groove” of back and forth movements