The Hollow Body Rock is one of the most difficult bodyweight abdominal movements in your arsenal. It forces you to stabilize your core while in motion and works both your rectus abdominis, transverse abdominis, and obliques. To begin the exercise, find yourself a comfortable mat and place it on the floor. Lie with your back on the mat with your legs extended out and heels touching the ground. Extend your arms up past your head in line with your ears. They should still be parallel to the ground. Inhale into your belly and then simultaneously lift both your legs and your arms up 12 inches off the floor so your body has now formed a ‘U’ shape. Your lower back and glutes are still flat against the mat. Tighten your core and now shift your body weight forward to your legs, rocking your torso upwards on the axis of your buttocks while maintaining the stiff ‘U’ shape of your body. As your feet get close to the floor, slow the descent down and reverse the motion, so that you are now rocking backwards until your shoulder blades touch the mat and your legs are rising higher into the air. At no point should your head, hands, or feet come in contact with the floor. One complete back and forth motion equals one Hollow Body Rock. Repeat this rocking back and forth motion as long as you can. If done correctly, you should feel a strong burn in your abdomen that will quickly push you to the limits of your endurance. If you find this movement too challenging in the beginning, try keeping your arms extended out at your sides instead.