Sit-up

The sit-up is a strength training exercise commonly performed with the aim of strengthening the abdominal muscles and hip flexors. It 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 vertebra from the floor until everything superior to the buttocks is not touching the ground. Stuart McGill of the University of Waterloo has found that situps can be dangerous due to high compressive lumbar load. He suggests replacing it with the curl-up in exercise programs.

Criticism

Although still common in military training, martial arts, and mass exercise classes, the conventional sit-up is sometimes considered dangerous by modern experts, and has largely been replaced by the crunch, since the publication of a research paper on the subject in 1999, for the following reasons:

Risks to vertebral column

Full sit-ups involve the hip flexors, as well as the 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.

According to the National Institute of Occupational Safety and Health, a straight leg sit-up generates approximately of force on the spine, and a bent-knee sit-up, both levels above the 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.