Bicycle Crunches

Lie on the floor with your knees bent at about 90 degrees and your lower legs elevated in a position parallel to the ground.
 
Place your hands behind your head without interlocking your fingers.
 
Simultaneously pull your knees in toward your hips one after the other and bring your opposite elbow towards the pulled in knee; your leg motion should be similar to that of a person riding a bicycle.
 
Do not pull head with hands.
 
The entire motion should be done in a smooth, controlled manner that lasts about 3 seconds, with the inward movement no faster than the outward.