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Lie or brace in the starting position with the lower back gently pressed toward the floor.
1. Lie on your back, arms up, knees bent at 90°.
2. Lower one arm and the opposite leg toward the floor without arching the back.
3. Return and repeat on the other side.
Breathe steadily throughout; never hold your breath during the hardest part.
Yanking the neck, using momentum instead of the core, and letting the lower back arch.
Stop if you feel it in the lower back rather than the abs; reduce range of motion first.
Beginner: Move only the legs, arms stay up.
Intermediate: As described.
Advanced: Add a light dumbbell/band for resistance.