Check ScotteVest products. I don't carry any bag/pack anymore. Everything I EDCing fits into the SeV vest (or SeV jacket the other time) pockets. The vest distributes the weight evenly over your shoulders, I believe, even better than a backpack. Everything is at hand, unnoticeable to me and to others, and the weight of the "bag" itself is precisely zero. The shoulder bag is plain dangerous as your spine is never straight because of the load's asymmetry. One day I looked in the mirror and noticed that one of my shoulders is higher than another. It took 5 years of special spine exercises to revert that. In the long run that may turn into other, more serious spine problems.

For the stairs - sure thing it's not translating, as there are different muscles working on ascend and especially on descent. Last weekend I've been hiking for 10.6 miles, with the 800 feet gradual climb. The way up felt like nothing at all to me, but descending back to 0 I had to use my hiking stick for every step after just 10 minutes of walking down. At some point I figured that I can walk without any pain turning back first, and looking over the shoulder, but that was not really safe on that trail. So, I had to invent a special long stride step involving the stick to speed up the descent while minimizing the pain. Fortunately it was a steeper trail (not the same as the ascending one), so it took not too much time and I was back to the parking lot shortly before dark. The GPS tracker showed my ascending trail speed at 3 mph, descending - just 1.2... Only by yesterday's evening I started to descend on the stairs normally without the severe muscle pain.