While spinal flexion will activate more upper and middle fibers, ab exercises that emphasize a posterior tilt of your pelvis preferentially activate the lower fibers of your rectus abdominis. Knee raise and knee tuck variations are some of the best. Try this variation:

Rack Knee Raise

  1. Use a Smith machine. A bar in a power rack will work, but a Smith machine would be better. It'll stop the gym police from alerting the sirens, and you can easily adjust the height to be just above your underarms.
  2. Use pads. The locked-in position over the bar has a different feeling to it than your standard hanging knee raises. It feels like a more targeted ab exercise and really helps you focus on posteriorly tilting your pelvis at the top. You'll just need to throw on a bar pad for comfort.
  3. Get your elbows to your knees. This exercise is self-limiting. Full range of motion is when your knees raise higher than your hips and your pelvis posteriorly tilts. Your abs aren't hip flexors. In fact, the only important part of any knee tuck is the final portion at the top. So getting your elbows to your knees every time should be the main focus.

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