Wonderful article about the anatomy of a comfortable back carry written by Dana McKeen (Babywearing International of Greater Burlington).
“The elusive comfortable ruck – is it a myth? Does it require a magical comfy wrap? Tough shoulders? There are of course many factors, but once you nail down the concept of how the carry functions, you can develop your technique to maximize comfort from that factor first, then figure out the rest.
Certain carries can be comfortable higher or lower on the back. For most people, a ruck is not one of those carries. This has to do with the direction of the pass and how it moves across baby in relation to your tightening. Here are some photos as an example – please note, working with an opinionated toddler watching her favorite show as a bribe, but the concepts are still illustrated!”
More information on the link.