My observationI've taken to walking from class to class wearing my backpack backwards. With the pack in front of me instead of behind.
It's nice, it means the weight distribution is different and so I feel like I use different muscles (or the same muscles in a different way). I can't say for certain obviously, as I'm not educated in anatomy.
I've spent the last 15 or so years of my life carrying a backpack, and it's been a great help, but I'm also guessing that it has done some interesting things to my posture. This change won't fix things, but it does seem to help.
Because I spend so much time in front of a computer, I spend a lot of time sitting down and hunching forwards. This would seem to catch the general muscle-use of wearing a pack on my back. In both cases I am contracting my front or bending forwards. This overlap increases the need for me to do daily activities that work other muscles.
For any designers or pack-makers out there
It would be really neat to see a pack specifically designed for wearing on the front. My current pack isn't too bad but it's not ideal. The shape of the shoulder straps seems the most in need of reworking.