To get a true six-pack—one where even those hard-to-define lower abs look nice and cut—you need to work your abs in more ways than one. Standard flexion exercises, like Crunches and Sit-Ups, aren’t going to cut it. While these movements are effective to an extent, any exercise where the upper body is the primary mover is going to mostly work your upper abs.
To make your lower abs pop like a soda can, you need to focus on exercises in which your legs are the primary mover. Lower-body movements develop your lower abs and increase overall core strength, which is essential for preventing lower back pain.
To develop the ripped lower torso you’ve always wanted, try the following workout three times per week on non-consecutive days. Perform four sets of 15-20 reps of each exercise.
For more information on fueling and training schedules for a six-pack, check out 6-Pack Abs for Athletes.

Physioball Reverse Crunches

• Lie on back with arms to sides, and drape legs over small physioball
• Squeeze ball between calves and back of thighs
• Raise knees toward chest to perform a crunch
• Slowly return to start, then repeat

Leg Drops

• Lie on  back with arms to sides and legs together
• Raise legs until they’re perpendicular to ground
• Drop legs quickly until heels are six inches above ground, then pull them back up to perpendicular
• Repeat for specified reps

Hip Raises

• Lie on back with arms to sides and legs in air, perpendicular to ground
• Raise hips off ground, pushing feet into air
• Lower with control to starting position, then repeat

Physioball Knee Tucks

• Assume push-up position with toes dug in to top of physioball
• Drive knees to chest, rolling ball towards hands; keep back flat and core stable as you move
• Extend at hips and knees to roll ball back to starting position
• Repeat

V-Ups

• Lie on back with arms overhead
• Keeping arms and legs straight, fold up until hands and feet meet above belly button
• Lower yourself with control, then repeat

Bicycles

• Lie on back with hands gently touching back of head
• Pull right knee up to left elbow
• Repeat on opposite side, pulling left knee up to right elbow
• That’s one rep. Continue alternating between those two movements until you complete the set

Of course, as an athlete, you shouldn’t train just for six-pack abs. Round out your core training with exercises in STACK’s Core Guide.