The sit-up is a strength training exercise commonly performed 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 muscles and spine, 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.

Strength exercises such as sit-ups and push-ups do not cause the spot reduction of fat. Gaining a 'six pack' requires both abdominal hypertrophy training and fat loss over the abdomen — which can only be done by losing fat from the body as a whole.

Modern research suggests that the abdominal muscles are responsible for only the first 30° of lift in a sit up — effectively the part of the motion where the shoulders only leave the ground. The hip flexors are responsible thereafter. This diversion of effort from the abdominals reduces the effectiveness of training for purposes of abdominal isolation and makes the sit-up a test of combined spinal and hip flexion rather than spinal flexion alone.