Chin tuck, sit up ("Crunch" Core/abdominal strengthening, arms crossed; 02)

Lie on your back with your knees bent and your feet flat on the floor.
Cross your arms over your chest and flatten the small of your back against the floor.
Gradually tuck your chin in, so that you feel the muscles on the back of your neck lengthen.
Keep your chin tucked in as you sit up.
Lower yourself back down to the floor, maintaining this position with your head.