The sit-up is a strength training exercise commonly performe with the aim of strengthening the abdominal muscles and hip flexors.This begins with lying with the back on the floor, typically with the knees bent in an attempt to reduce stress on the back, and then elevating both the upper and lower spine from the floor until everything superior to the buttocks is not touching the ground. Some now consider it dangerous and relatively ineffective, replacing it with the crunch in many training programs as an abdominal exercise.