Lying Leg Raises
Lie on the ground with back flat and legs fully extended. Raise your legs off the ground and then lower them close to the ground and back up.
The distance you raise them depends on your physical capability. You can try raising all the way up but if you cannot just raise up to 6 inches.
It is important to remember not to bounce your legs off the ground and use that momentum. Stop short of the ground for each rep.