To make the movement more difficult, you can keep your feet raised in between reps, keeping them from touching the ground.
Lower abdomen, from just above your belly button to your pubis. The lower half of the six pack abs.
Outer sides of your abdomen.
Muscles spanning the front of your thigh.
Upper abdomen, from just above your belly button to your sternum. The upper half of the six pack abs.
Bodyweight Only
This means that the only thing you need to successfully perform the exercise is your body. No other equipment is needed.
Exercises that target the same primary muscle groups and require the same equipment.
Exercises that target the same primary muscle groups with different equipment