[Lower Abdominal Crunch] mainly exercises the lower abdominal muscles. Lie flat on the ground, draw in your abdomen and lift your thighs so that your thighs are 90 degrees to your body and 90 degrees to your thighs. Tighten the abdominal muscles above the navel, roll up the body, move slowly, and maintain the angle between the thighs. Do 4 sets of 15-20 reps every other day.