Popsugar Health & Fitness Intermediate Workouts 100-Rep Core Workout This 100-Rep, Core-Busting Workout Takes All of 4 Minutes to Complete 13 February 2018 by Jenny Sugar View On One Page Photo 4 of 4 ADVERTISEMENT () Start Slideshow Share This Link Copy ← Use Arrow Keys → 4 Reverse Crunch Lying on your back, lift your legs in the air with your knees bent. Place your hands on the floor beside you. Without momentum, use your lower abs to slowly curl your hips off the floor and into your chest. Slowly lower them back to the starting position. This counts as one rep. Complete 20 reps. Previous Next Start Slideshow Intermediate Workouts5-Minute WorkoutsWorkoutsAb Exercises