3 Reasons Why You Should Stop Doing Crunches and Sit-ups

1. These ab exercises will not make your belly flat, nor will they give you a 6-pack

Working a certain muscle will not make the fat around it melt. That is, sadly, just not the way the body works. Every body is different, and a personal trainer cannot tell you for sure where the fat to fuel your workout will come from. Working a muscle can only make a muscle stronger and, often, bigger. So, if you want a smaller belly, doing exercises that will make your rectus abdominus muscles bigger might not be the solution. What you really want is to pull your belly in, but that's mainly what the transverse abdominus muscles do, and crunches do not work these muscles. To make the matter worse, sit-ups also work the hip flexors, which, when tight, pull your pelvis in an anterior tilt...and make your belly stick out!

Goal of doing sit-ups vs. What actually happens

What you want to is to work on your postural muscles, to flatten your back and pull your belly in and the best way to do so is through isometric core exercises, balance training, and postural correction stretches.

2. They do not always work the right muscles

When doing crunches and sit-ups, you expect to be working out your abs, but the truth is that even if you feel the burn there, it might not actually be the main muscles getting a workout. Instead, you might be working out muscles that do not really need strengthening for most people since just sitting around all day is already making them quite tight: your hip flexors. Indeed, the hip flexors are muscles that bring your thighs and torso closer together, and what do V sit-ups do? Bring your thighs closer to your torso!

The hip flexors connect your lower back to your thigh bones.
If your lower back ever hurts or feels tight after doing sit-ups,
now you know why!
If you really wanted to work your abs without working your hip flexors, you would have to make tiny movements, like lying pelvic tilts, or challenge your balance and tighten your core muscles isometrically.

3. They do not train the muscles the right way

If your goal is practical and sports-related rather than aesthetic and you want to strengthen your abs to be better at a certain sport, crunches and sit-ups will work some of your abdominal muscles, mainly the rectus abdominus muscles (the "6-pack muscles"), but not in the most efficient way
What you really want to do is strengthen your core to stabilize your spine and pelvis. Most sports will not require you to repeatedly curl your spine or twist from left to right at the waist, so why train your body to do so?  Additionally, the movements in crunches and sit-ups are very unnatural for your spine and actually dangerous. A recent study done at the University of Waterloo in Ontario found that they put a strain on the back and increase the risk of disk bulge and herniation. 
If a posture is considered bad to have when sitting, how could it be good to practice it at the gym?

In most sports, your pelvis has to remain relatively stable while you move your arms, legs, or torso.  Your core muscles' role is basically similar to the role of the wide back support belts you see Olympic weightlifters wear: it should brace your spine and keep it long and decompressed to allow you to do more powerful movements. So train it to be good at what it is meant to do: challenge your balance and get your core to work in ways similar to the sport you are training for