One thing missing in this survey is the arrival of shoulder strap lift straps. If you go back to the 1970s, backpacks more or less uniformly had a shoulder strap that started at the top of your back, sat directly against the body and wrap up and over the shoulders. This transferred a significant amount of weight directly to the top of the shoulders.
At some point, people figured out that it made sense to run an adjustable strap from the very apex of the shoulder strap back to the pack "frame" (whatever that was), designed to generally run at about a 45 degree angle upwards. This massively reduces the load on the shoulders, and converts the shoulder straps more into balance/position elements than load bearing.
At some point, people figured out that it made sense to run an adjustable strap from the very apex of the shoulder strap back to the pack "frame" (whatever that was), designed to generally run at about a 45 degree angle upwards. This massively reduces the load on the shoulders, and converts the shoulder straps more into balance/position elements than load bearing.