Fill a back pack with as many books as you want to use as weight.
2. Flex your abdominal muscles to keep your spine neutral and place your feet shoulder-width apart and your toes slightly pointing out.
3. Slowly lower your body by hinging at your hips while keeping your chest up and stop when your thighs are parallel to the ground.
4. Immediately start coming back up by extending your knees but stop before your legs are completely straight.
5. Keep moving in this range until fatigue.
Working muscle/muscles:
