I don't think the backpack "keeps your back in shape". If anything, it's delivering lots of compressive force to your spine. When you walk with a backpack, you have built in shock absorbers---your big leg muscles. Unless you're riding a double boinger MTB, your bike doesn't provide that. Every bump you hit results in that force traveling down your spine to your tailbone. Plus, if you happen to crash, any rigid items in your pack can damage your spine.

Heavy loads call for on the bike carry, perhaps even a backpack that converts to a pannier.