The vertical leg crunch

The vertical leg crunch is another effective move for the rectus abdominis and the obliques. It's similar to a regular crunch, but your legs are straight up, forcing you to use your abs to do all the work and adding intensity to the exercise.

How to:

1. Lie on the floor and extend the legs straight up with knees crossed.
2. Place your hands behind the head for support, but avoid pulling on the neck.
3. Contract the abs to lift the shoulder blades off the floor, as though reaching your chest towards your feet.
4. Keep the legs in a fixed position and imagine bringing your belly button towards your spine at the top of the movement.
5. Lower and repeat for 1-3 sets of 12-16 reps.

View this exercise in the Best Ab Workout.