E-Series Sit Up BenchE Series Sit Up Bench your Ultimate Strength and Comfort Training Tool "Decline crunches are sit ups that you perform on a surface with a negative incline instead of on a horizontal plane. You probably know classic sit ups as a strenuous exercise, but one that quickly proves its worth in terms of fitness and muscle building. However, if you want to intensify the exercise, you should perform it as inclined sit ups, i. e. so called "decline crunches".