What's the meaning of "rearward-declined"?
This type of analytic study indicates the importance of supporting the back by leaning it on a rearward-declined backrest and by maintaining the natural shape of the spine.

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In this context, it appears to mean that the backrest is not at 90 degrees to the seat, but is tilted backward a bit so that the person sitting in the seat leans backward slightly into the backrest.