As far as spinal health is concerned, Sit-ups don’t work. Three major reasons why this is so:
First, they reverse the forward curve of the low back, loading the disc and creating pressure to load the vertebrae to subluxate rearward.
Second, they reverse the forward curve of the neck loading the discs of the neck and upper back, and aggravating any subluxations that may be present.
Thirdly, after the initial movement of the crunch, the abdominal muscles are already contracted, and so the additional motion to an upright position doesn’t use the abdominal muscles, but rather the Psoas muscle. This muscle is attached to the leading edge of the lower lumbar vertebrae and inserts into the femur. The sit-up is effectively bending the lumbar spine in the reverse direction, as it’s pulling on the same vertebrae. Brutal on the spinal discs, particularly if subluxation of a vertebra already has the tissues inflamed, and the nerve response compromised.
Better to do crunches or leg lifts to exercise the abdominal muscles. With crunches, it is best to reach the fingers down as far as can be reached between the shoulder blades. Then bring the elbows close together to lock the wrists beneath the angle of the jaw, and perform the crunch be reaching the elbows towards the ceiling. By doing so, you’ll protect the curves of the spine as they transition from the rearward curve of the Thoracic spine to the forward curve of the neck. You’ll also increase the load on the abdominal muscles by keeping the weight of the head and arms out on the long arm of the exercise lever. Adding a 30 degree deviation of the elbows can also be used to isolate exercise to the Oblique muscles.