Defense Crunches are not only safe and effective but also the best exercise for developing the abs. Prosecution Spinal flexion (bending the spin forward), as in crunches, has been shown to place a lot of stress on the disks and should be replaced with core exercises that do not flex the spine, such as the plank.
The research showing that flexion of the spine places a lot of stress on the spinal disks was done in isolated pig cervical spines, so it has little relevance to the human lower back in real life.
Research actually shows that flexing the spine is good for the disks and spine because it boosts nutrient delivery to the disks, increasing flexibility and reducing low-back pain.
The best way to develop the ab muscles is by performing exercises that involve spinal flexion, such as crunches.
Some fitness “experts” try to make a mountain out of a molehill. The truth is that doing crunches or other exercises that involve spinal flexion is actually safe, healthy and effective.
So keep doing crunches. There are only two times of day when there’s a slight chance they could hurt your back. First, because your disks absorb fluid while you sleep, the pressure inside them when you wake is about 250% higher than before going to bed, thus increasing bending stresses at the disks by about 300%. As the day goes on, the disks become more elastic and flexible. So, if you train first thing in the morning, do your ab work at the end and at least one hour after waking up. Second, when you sit for a prolonged amount of time, disks gain height and decrease lumbar range of motion, which can increase the risk of injury. So if you sit at a desk all day and then drive to the gym, do not train abs as soon as you get there. Do a warm-up on a treadmill for 5 or 10 minutes or train abs last.