1. Camp Trails external frame pack (the old favorite) typically 35 lbs total on my back. At least 1500+ miles with this pack.
2. EMS "Long Trail" internal frame pack, same load (35 lbs.)
3. Granite Gear Vapor Trail, 23 lbs total on my back.
With scenario #1, I never had back pain. With scenarios #2 and #3, some back pain. (Admittedly, less so in #3 than in #2.) Any idea what's going on?