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.