Roll Out
A good exercise for the abdominal muscles, if you have the upper body strength to support the movement.

Rest your knees and feet on the floor, holding a suitable freewheeling bar, with both arms kept straight throughout the movement.
Contract your abdominal muscles, whilst slowly rolling out to your front, aiming to take both arms out straight, whilst taking your back parallel to the floor.
Once in the extended position, keep your core muscles contracted for 1 - 3 seconds, prior to returning back to your starting point.
Gradually build up both the amount of reps that you can perform, the length of time you can remain in the straight back position and also your range of movement.