Sit Ups

"Put your feet under a piece of furniture, a bed or a couch, and bend your legs at a 45-degree angle...Hold your hands in front of your waist with your fingers knit together and go up and down. It is not necessary to lie back fully - only about three quarters of the way - but the movement should be very smooth and rhythmical." (Arnold: The Education of a Bodybuilder p. 167)

Modifications:
To increase intensity, hold a weighted plate or medicine ball, or wear a weighted vest.