Jack knife sit ups are effective at working both the upper and lower abdominal muscles at the same time.
Begin by laying flat on your back with your arms and legs extended. To begin the exercise, bend at your waist and raise your legs while you also raise your torso. You should continue to lift your legs until they make approximately a 45 degree angle with the floor. You also want your arms to be parallel with your legs; your entire upper torso should be off the floor. Try to hold this position for a second and then return to the starting position (flat on the floor). Repeat this movement until you have completed your set.
It can be difficult to hold this movement at the top position, but try to do it by squeezing your abs as hard as you can.
You can make this movement more difficult by holding a medicine ball in your hands or you can hold one between your legs.