How to build muscle in my abdominals
If your abs are visible, keep your diet clean (limit starchy carbs...fats are ok) and do ab exercises (floor crunches, twisting slant board crunches, cable rope crunches, etc.) 3–5 days per week. You may see a lot of lifters doing Roman Chair leg raises with straight legs, thinking that it is an ab exercise, but is isometric (static) in nature, as the "job" of your abdominal wall is strictly to move your chest and spine towards your lower body. Roman Chair leg lifts, without moving your upper body towards your lower body is simply a hip flexor exercise, with your abs contracting in order to protect your spine. So, sure, do Roman Chair leg lifts but do it with bent knees and crunch your upper body forward as your knees come up toward your chest.