Answer to Question 7
The answer is civilised forms of sitting, particularly sitting with a backrest and particularly when driving. A backrest may limit the total flexion of the spine but the effect is always to increase the flexion of the lowest spinal joints (L 4-5 and L 5-S.) in comparison with unsupported sitting. This effect was shown in the tabulations in the paper “The influence of backrest inclination and lumbar support on lumbar lordosis” (1) in Spine 1979. See “analysis of lumbar support paper” for explanation of this point which was not recognized by the authors of the paper. Critically, it needs to be understood that a backrest flexes the lowest lumbar joints in comparison with unsupported sitting.
The significance of this for back pain is difficult to detect statistically as it is so universal. However, the effect of car driving was noted as early as 1972 by Kelsey and Hardy (2), was confirmed in 1986 in Spine (3) and was strongly emphasised by the work of Professor Mark Porter, Professor of Ergonomics at Loughborough University who observed "-- -- a six-fold increase in -- -- low back trouble by car drivers who spend more than half the working day behind the wheel"(4)
Would we expect this mechanically? The answer is definitely yes. Car seats tip the thighs and therefore the pelvis further back than almost any other seat. Car seats incorporate far more direct support for the lumbar spine than any other seats. We sit still in a car seat for far longer than we ever sit anywhere else. This mechanical situation could not be better designed to cause the problem outlined above.
For a mechanical analysis of all forms of sitting see the booklet "The Evolution of Low Back Pain" by John Gorman which is a free download on this website. (5)
(1) Anderson GBJ, Murphy RW, Ortengren R, Nachemson AL The influence of backrest inclination and lumbar support on lumbar lordosis. Spine 4: 52-58, 1979 (For comments on this paper see ‘one pager’ at Analysis of Lumbar Support )
(2) Kelsey JL, Hardy AJ Driving of motor vehicles as a risk factor for acute herniated lumbar inter-vertebral disc. Amer J Epidemiol 102: 63-73, 1975
(3) Biering-Sorensen F, Thompsen, C Medical, social and occupational history as risk indicators for low-back trouble in a general population. Spine 11: 720-725, 1986
(4) Seating Design. Current Problems and Future Strategies Dr J. Mark Porter. Automotive Interiors International. Pages 6 to 19. 199?
(5) "The Evolution of Low Back Pain" by John Gorman. Booklet 57 pages. pdf free download.