Average dimensions of the human back (anthropometry)
This doesn't really answer the question and I only post it at the asker's request. 

NASA has measured the dimensions of the back in 40 year old American males and 40 year old Japanese females in the year 2000 at 1G. Of course gravity matters to NASA…



From figure 3.3.1.3-1 (American male). All data in centimetres:


921 Waist back


5th percentile: 43.7
50th percentile: 47.6
95th percentile: 51.6

506 Interscye


5th percentile: 32.9
50th percentile: 39.2
95th percentile: 45.4