Flutter Kicks

Lie on the end of a flat bench or the floor and stabilize your body by placing your hands under your rear and pressing your palms firmly against the ground; extend your legs out.
 
With your legs in a fixed, slightly bent position throughout the movement, keep one leg stationary while raising the other leg up in a semicircular arc until they are about 75 degrees above the level of the floor.
 
Reverse the movement and return to the starting point. Repeat with other leg.
 
The entire motion should be done in a smooth, controlled manner that lasts about 4 seconds, with the downward movement no faster than the upward.