There are no lower abs guys. The rectus abdominus is either stimulated or it isn't. The segments are the result of whatever connective tissue design your parents gave you. Any sort of crunch works the abs. If you want them to be thicker, add weight to your crunches (machine at the gym). 500 crunches won't make abs thicker just like walking 3 miles or going up stairs all day doesn't make your legs bigger like squatting heavy weight will.
When people are doing "lower ab" exercises, the prime mover is the psoas muscle, which originates in the lower back and inserts into the superior femur. The rectus abdominus is just a stabilizer muscle (not a synergist either) for "lower ab" exercises, as lay people consider them. Furthermore, if you want your adonis belt to stand out, then you need to 1) have low body fat and 2) lift heavier weight while doing the hip flexor machine. Just like marching in place or running won't make your cum gutters bigger, you have to lift heavy weight for that structural change (thickness) to occur. Body weight isn't heavy weight unless you are starting from scratch.
If you want to see the connective tissue of your lower rectus abdominus, you must have extremely low body fat, which comes from not eating/dieting. People with body fat that low do not have much muscle mass in most cases (look at runners--ripped but low muscle mass) or just have deep connective tissue thanks to their parents such that they can maintain higher body fat levels while keeping that look. Of course, there are some drugs that also can artificially give you the vacuum packed look also that are probably illegal without a prescription. Additionally, magazines airbrush and use people who do have great parents for those shapely connective tissue designs AND models will dehydrate themselves for photoshoots just like bodybuilders do for shows. People don't look like that in real life when they are healthy.