The Jack Knife is an intermediate core exercise. Start in the top of the sit-up position with your hands behind you and feet off the ground. Bring your knees up towards your chest and chest towards your knees. Once you reach the top of a comfortable range of motion slowly lower back down to the start position so that your feet are extended out and upper torso drops towards the ground.
Core| Intermediate| Rectus Abdominus (Abs)