Maintaining the half sit-up position while performing any movement with the arms will challenge the abs, hip flexors, and even the neck musculature.

Anchor your feet under something stable and lean back until you feel your abs turn on. Try to keep your spine and neck as neutral as possible. You can go light and do the exercise for 60 seconds, or use heavier weights and shoot for 10-15 reps.

Related:  4 Challenging New Ways to Do Sit-Ups

Related:  Big, Thick, Chunky Ab Training