Hollow Rock

Lie down on an exercise mat. Have your hands to your sides, feet together, and legs straight.
Get a feel for the movement by contracting your abs as if you’re trying to initiate a crunch. Doing so will put your lower back against the exercise mat.
Squeeze your thigh muscles as your legs are together and lift your feet a couple of inches off the floor. Keep your legs straight.
Lift your head a couple of inches off the floor and simultaneously extend your arms back behind your head and slightly up. Your lower back should contact the floor during the setup and as you hold in position afterward.
Hold the position for as long as you can and breathe regularly. Aim for 20 to 30 seconds initially, and slowly work up to minute-long holds.

Hollow rocks are challenging and force you to maintain a neutral spine as you hold the position. As a direct benefit, hollow holds build a rock-solid core, elevate your performance on many strength exercises, and allow you to keep your spine in a healthy position.