https://pubmed.ncbi.nlm.nih.gov/6230074/
I read the actual report years ago but can’t find it now. But I recall the conclusion clearly - posture not vibrations.
Personally, I have just over 11,000 hours rotary and all my back problems relate to foolishness as a teenager and inattention to posture while sitting, in later life. Pretty sure my flying isn’t a factor.
My favourite seat was the 212 - nylon netting stretched over a well formed metal frame!