The fitness industry has a dirty little secret. Lower abs exercises are like unicorns, the tooth fairy and big foot. There are no exercises that will isolate the lower half of the rectus abdominis or abs/six pack abs as this muscle is commonly referred to. The rectus abdominis is one muscle and it works together as a whole.
Some of the confusion when it comes to "abs" is that there is a mix up and misunderstanding of the abdominal muscles which consist of more than just the rectus abdominis or what people know is six pack abs. There are four muscles that make up the abdominal muscle group. These are the transverse abdominis, internal obligues, external obliques and the rectus abdominis.
When most people think of abs, they are usually referring to the rectus abdominis, the muscle that looks like a six pack on someone with a low body fat percentage. There has been a misconception with this muscle. Some people think that there is an upper and lower area of this muscle that can be worked independently of each other. This is a myth. To illustrate this, put one hand on the upper part of the rectus abdominis and one hand on the lower part and try to contract each section independently.