I've posted the below several times. Might be something you already know but, I didn't until I had to learn to heal my back.
* Crunches shorten the rectus abdominus.
* The lumbar spine has only 3 degrees max rotation per segment while the thoracic spine has up to 7 degrees rotation per segment.
* Shoulders that round forward or have some C shape to them, greatly limit thoracic rotation.
The shortening of rectus abdominus can lead to the C shaped shoulders to some extent. When RA shortens, it flattens the lordotic curve in the low back which greatly weakens it as so much strength derives from the structure of the curve. If the shoulders are rounded and greatly limiting thoracic rotation, then the rotational limits are exceeded in the low back. The low back is now flatter, weaker and subjected to rotational forces that exceed natural limits. If we add to this a lack of hip mobility and short hamstrings the low back is doomed. The rotation is to be derived from the hips and thoracic spine not lumbar.
The lumbar spine needs to be trained for stability not movement. Planks, side planks, walking while carrying one kettlebell overhead etc. can provide superior core training.
@EFW I did write it. However, there is not an original thought in it. When I hurt my back, I researched a lot. Read two of the McGill books and lots of other info. The above is my distillation of the information into something hopefully useful and actionable by skiers. The absolute best at putting the info into an actionable format is Mike Robertson. Has a masters degree in biomechanics. Ignore the hokey marketing, the guy is really knowledgeable. Not promoting it; just citing it as my main source of what I believe to be great information.
http://robertsontrainingsystems.com