This is an effective exercise which helps to strengthen those lower belly muscles. I'm using a fitness block just to make it more challenging, but you leave it out.
How to: Start off on the floor with your back and use your legs to anchor the fitness block between your ankle. You may want to put your hands underneath your tailbone to avoid creating a curve underneath your back (but I have kept my arms out).
At the same time, use your ab muscles to lift your head, shoulders and legs off the ground and then slowly lower to the floor.
Now you: Perform 3 sets of 12-15 repetitions. It is a tricky move, so you can keep your legs bent until you feel stronger.