Having tight and defined lower abs is the key to ensuring you have a strong core and an aesthetically pleasing stomach. However, the lower abs is one of the hardest areas to target and often involves leg-raising exercises. Not done with proper form, lower back and hip pain can result with no results to show.
The bicycle crunch develops strong overall abdominal muscles, especially lower abs and develops a good level of core strength and balance. It is leg raise exercises though, that isolate your lower abs in particular, so sitting on a chair to perform bent knee leg raises will increase the load on the lower abs and hip flexors. This exercise will also target the obliques and your front and outer thighs.
Lying leg raise isolate the lower abs, but also work all the major abdominal muscles, hip flexors and obliques. These are good for developing a strong and leaner mid section, whilst improving coordination and balance. This exercise can be performed with ankle weights to further increase the load on the abs.
A declined bench can be used to perform decline crunches to target the lower abs. Lay on your back in a declined position and crunching up concentrating on using your abdominals, will isolate the lower stomach muscles, but will also target the upper abs too.