Trainee: "I want to build my lower abs."
Trainer: "Ok! We're going to do leg lifts three days this week!"
Has your trainer said this before? If so, you may want to inform him or her that abdominals don't work like that. We can't just do a ton of leg lifts in an effort to target the lower ab muscles.
You see, the "abs", or more precisely, the rectus abdominis, work as a whole; they operate as a single unit -- from top to bottom, and from bottom to top. It's the obliques that can be activated individually, and it's the hip flexors that aid the stomach area in a number of hip-associated exercises, such as during leg lifts.
According to Kinesiology, The Mechanics & Pathomechanics of Human Movement, "It appears that while the obliques are regionally activated (and have functional separation between upper and lower regions), all sections of the rectus are activated together at similar levels during flexor torque generation. It appears that there is not a significant functional separation between upper and lower rectus in most persons."
Another study, titled Electromyographic comparison of the upper and lower rectus abdominis during abdominal exercises, states: "Eight healthy, adult volunteers completed 6 random abdominal exercises: curl up, Sissel ball curl up, Ab Trainer curl up, leg lowering, Sissel ball roll out, and reverse curl up. Action potentials were recorded and analyzed from the upper rectus abdominis (URA) and the lower rectus abdominis (LRA) using surface electromyography (EMG) during a 2-second concentric contraction. The average normalized data were compared between the URA and the LRA in order to determine the behavior of the different muscle sites and between exercises in order to determine which exercises elicited the highest EMG activity. There were no significant differences (p > 0.05) between the EMG activity of the URA and LRA during any exercise."
So what gives? Why don't you have lower abs? Actually, you have them! It's most likely a result of seeing them. Why is it more difficult to see the lower abs? Well, one of the main reasons is because of belly fat, in particular lower belly fat; this area of the body is one of the last areas to shed adipose tissue (fat).
If you want to build muscle where the 6-pack lies, continue cranking out sit-ups, curl-ups, and leg lifts! If you want to see said 6-pack, try burning away fat through increased conditioning.