Sit-ups

The sit-up (or curl-up) is an abdominal endurance training exercise commonly performed to strengthen and tone the abdominal muscles. It is similar to a crunch (crunches target the rectus abdominus and also work the external and internal obliques), but sit-ups have a fuller range of motion and condition additional muscles. Sit-ups target the hip flexors, rectus abdominus and also work the iliopsoas, tensor fasciae latae, rectus femoris, sartorius, and, to a very small degree, the obliques.

It begins with lying with the back on the floor, typically with the arms across the chest or hands behind the head and the knees bent in an attempt to reduce stress on the back muscles and spine, and then elevating both the upper and lower vertebrae from the floor until everything superior to the is not touching the ground. Some argue that situps can be dangerous due to high compressive lumbar load and may be replaced with the crunch in exercise programs.

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

The movement can be made easier by placing the arms further down away from the head. Typical variations to achieve this include crossing the arms to place the palms on the front of the shoulders and extending the arms down to the sides with palms on the floor. The 'arms on shoulders' variation is also used to make the incline sit-up easier.

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Wikipedia