Weighted Bicycle Crunch
Lie on your back with your knees bent and feet flat on the floor. Hold a medicine ball above your torso with both hands. Lift your legs off the floor so that your knees and hips are bent at 90-degrees.
Then, pedal your legs in the air as if you are riding a bicycle. As you do, raise your upper body off the floor and twist your torso as you bring the medicine ball towards each alternating knee.
Squeeze your abs as you bring your elbows towards your knees.
Don't strain your neck. Keep it in line with your spine at all times.
Don't round your lower back as you bring your knees to your chest.