Weighted Hanging Knee Raise
Rest your forearms on the pads of a vertical bench (aka Roman chair), hold onto the handles and secure a light dumbbell (10 to 15 pounds) between your feet with your legs extended toward the floor. Keeping your upper back flat against the bench pad throughout, bend your legs to lift your knees straight up until your thighs are past parallel with the floor. At the top of the rep, crunch your pelvis upward so your glutes come off the pad. 3 sets, 10 to 12 reps
Decline Reverse Crunch
Lie reversed on a decline bench set to around 30 degrees, grasping the ankle pads with your hands. With your legs and hips bent 90 degrees, contract your abs to bring your knees toward your chest and lift your glutes off the bench. Squeeze the abs for a count or two before lowering your legs back down. 3 sets, 10 to 15 reps
Lie on the floor with your hands behind your head and legs extended and raised a few inches off the floor. Simultaneously bring your knees to your torso while crunching your upper body toward your legs. Squeeze in the middle, then lower back down without letting your feet touch the floor between reps. 2 sets, 20 reps.