Backpacks are off-axis loading systems. There is no way to safely carry weight in a backpack, no matter how light the load, because it disrupts our body mechanics by design. Even an empty backpack can distort posture, which may lead to fatigue and pain.
Why ergonomic backpacks fail
When posture is distorted, our joint and muscle mechanics become distorted as well. Even light loads applied to disrupted mechanical systems produce damaging forces in the joints and muscles. The greater the distance of the load from the center axis of the body and the greater the postural distortion, the greater the damaging forces on the musculoskeletal system.