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 when your butt touches the bench placed behind you, immediately return to the starting position. (You are not allowed to sit and rest on the bench).
Working muscle/muscles:
