Sit-up (exercise)

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.

Criticism
Although still common in military training, martial arts, and mass taught exercise classes, the conventional sit-up is sometimes considered dangerous by modern experts, and has largely been replaced by the crunch, for the following reasons:

Risks to vertebral column
Full sit-ups involve the hip flexors, as well as abdominal muscles. This can cause the back to arch, with the risk of spinal damage. This is a particular risk for individuals with weak abdominal muscles, but also for individuals who train aggressively, exhausting their abdominal muscles in a training session. Even if these risks are avoided, the leverage exerted by the hip flexors risks compression of the lumbar intervertebral discs. Most branches of the US armed forces have ceased using sit-ups in training because of the problems resulting from these factors, which can include direct back damage and referred pain or numbness due to pinching of the spinal cord.

According to the National Institute of Occupational Safety and Health, a straight leg sit-up generates approximately 3500 newtons (790 lbf) of force on the spine, and a bent-knee sit-up 3350 newtons (750 lbf), both levels above the 3300 newtons (740 lbf) that correlates highly with lower back injury. 

Hip flexor involvement
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.

Abdominal muscular hypertrophy
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.