Working the lower abs helps define the abdominals as they fuse into the groin, but doing so can put the hip flexors at risk for strain. Some lower ab exercises are thus no-nos when it comes to avoiding low back and groin pain:
Leg lifts where the hip extends beyond 90 degrees is a surefire way to strain the hip flexors. Do them at your own risk.
Bicycles are, unfortunately, not much easier on the hip flexors. Do them regularly and risk low back and groin pain from strained hip flexors.
Right! The correct way to work the lower abs is to pull the hip flexors (particularly the psoas) out of the equation. Notice how she maintains a 90 degree angle at the hip joints–all movement should occur at the lower abs. Bravo!