Bicycle crunches / Air bikes is one of the most common abdominal exercises that can easily done at home, and requires no equipment.
This is a Compound exercise that work on several muscle groups. It primarily works on the rectus abdominis muscle (the six pack) and also the obliques. and also engages the thighs and glutes as secondary muscles.
Make sure you are not using your hands to pull your head or neck up. This may lead to neck problems and pain.
Use a steady motion that does not cause your body to rock as you exercise to make sure that the focus stays on your abs and that the crunches do not become a meaningless motion carried out by momentum.
This exercise puts stress on your lower back. it is advised not to do it if you suffer from lower back problems.