They seem to have focused on damage from the pressure of the shoulder straps to neck and shoulder nerves.
Unlike every other pack I've ever used, my last two have been designed so that the hip belt supports almost all of the weight, with the shoulder straps just helping to keep the pack upright. Here are some links I collected while I was choosing them and learning to use them:
Backpacks: Adjusting the Fit (REI; includes video clip)
Adjusting your Hiking Backpack to Fit Right without Pain (lookingforadventure.com; includes video clips of fitting, packing)
How to Adjust a Backpack for Hiking (David Camp, Northwest Backpack Specialties)
Backpacks - Only the Fitted Survive (Hudson Trail Outfitters, Ltd.)
How to Choose a Backpack (REI)
If someone has better links, I'd love to check them out.
I figure the hip belt can cause problems of its own but it wouldn't affect the neck and shoulders much. My first hipbelt pack wasn't very well designed. I found that the hipbelt gave me itchy red spots on my hips after a few miles. I ended up wearing two squares of sheepskin inside my underwear, fleecy side toward me. The hipbelt on my second one is much better padded and has never given me any trouble. I've retired the sheepskins for now.