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Preventing back injuries from sitting at work
E.N. Corlett, Phd
The act of sitting
down is so simple that we don’t think about it very much. After all, we
just bend our knees and there we are, sitting. Taking the weight off our
legs is also an immediately comfortable feeling, so almost any seat feels comfortable
in the short term.
But if you work at a job that requires
extended periods of sitting, for instance at a computer, then in the longer term “almost
any seat” can do serious damage. A recognition that this is so has
given rise to several reports on back pain in school children, A recent
publication by BackCare, (2005), reported that one
in five secondary school pupils experienced back pain, of which 23% visited a
medical practitioner, results similar to the adult population. Their report
draws attention to the important role of the chair and desk combination as a
major item for improvement and gives recommendations for better furniture.
Studies of seated work in industry and commerce
has produced
many recommendations for improvements in the layout of workplaces, whilst research
has revealed the reasons for such problems as upper limb disorders. However,
although there is much research into the effects of sitting, in most of the workplace
recommendations this research fails to influence recommendations on the choice
of seat. Whilst there is emphasis on lumbar support, seat padding and a
curved front edge, the basic problem discussed by Mandal, (1974) and repeated
by him and others many times since, is not addressed.
This problem,
which has been studied in detail by many researchers
in the last thirty years, concerns the shape of the spine when sitting. The (lumbar) curve, present in the lower back when standing, allows the transfer of the weight of the upper body to the legs through the spinal discs
with the least deformation of these discs. The effect of sitting
on a horizontal seat is that the pelvis is rotated backwards, which flattens
the lumbar curve, deforming the discs and adding extra pressure to them as a
result. The flattened curve also causes tension in the back muscles to
maintain the sitter upright, loading the discs still more. These continuous
extra loads reduce the available capacity of the discs to transmit external work loads.
These adverse
effects can be avoided, and the lumbar curve substantially
retained, if the seat is sloped to allow the thighs to remain at about 20 degrees
below the horizontal. A sloping seat will not remain comfortable for long
because the gravity load will tend to cause the sitter to move down its surface,
displacing lower garments upwards and adding extra load to the legs to hold the
sitter on the seat. But if the seat is curved from front to back, sitters can
sit on a horizontal surface at the top of the curve whilst their legs remain
at the desired slope. Furthermore, if, when the chair is
adjusted for height, the seat is arranged to tilt, the seat angle will
be altered but the sitters still sit on a horizontal segment at the top of the
curve, whilst their feet can remain on the floor.
Seats to this
design are available and have been used in offices,
checkouts, factories and elsewhere. Research evidence shows that the spinal
discs are under much less loading with this design than
when compared to a conventional horizontal seat. These results support
the contention of earlier researchers and show that this design is more favourable
for retaining a healthy spine than the use of conventional seating. The
risk of spinal damage is reduced whilst the chances
of keeping a healthy back are much increased.
BackCare (2005) Back
pain in children and young people. Publ. by BackCare. Elmtree Road, Teddington, TW11 8ST, UK
Mandal, A C (1974) The seated man, (Homo Sedens) (3rd.Edition
1985) Dafnia Publications, 2930, Klampenborg, Denmark
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