The 20-Minute Workout
Why it works: Keeping your legs in the air challenges your entire core.
How to do it:
1. Lie on the floor with knees bent, legs in air, hands behind head, and elbows out.
2. Contract your abs and lift your head, shoulders, and upper back off the floor for two counts; lower for two. Repeat eight times. On the last rep, hold for eight counts, then pulse up for eight counts before lowering your upper body.