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unknown user
02-02-1998, 07:57 PM
I'm glad that Dr Siff has succeeded in starting a wide-ranging
discussion. I'd like to give my opinion on a number of points.

1: He says that
"... epidemiologically ... it appears that lumbar flexion under
loading poses special risks to the intervertebral discs, so that it is
generally advised that we avoid lifting with forward lumbar flexion
..."

The problem here is that the epidemiology is extremely poor,
especially in terms of work related factors. I also suspect the model
employed: it was observed that prolapsed intervertebral discs could
cause sciatic pain via nerve root entrapment and therefore assumed
that excessive loading on the disc would be likely to cause back pain.
This ignores the musculature which, I submit, is much more likely to
be damaged and to be the immediate cause of acute low-back pain.

2: I'd be grateful if Dr Siff would point us to the studies he's
referring to in regard to injuries suffered by weightlifters and
powerlifters. Data on incidence rates, typical injury sites and
severity would be interesting.

3: I submit that, at the loads and velocities Dr Siff quotes for
specific Eastern European lifters, the differential loading of the
spine in the flexed and extended postures will have a very small
effect on the total spinal load. In other words, for those lifters at
those loads and speeds, back posture is largely irrelevant.

4: It seems to me that the comments of Dr Hewett and Dr Voo in effect
combine to say that we cannot generalise from a very small number of
elite lifters to the whole of the workforce involved in manual
handling.

5: A little history is in order: The following is from T. McClurg
Anderson, "Human Kinetics and Analysing Body Movements", Heinemann
Medical Books, 1951, Chapter XVII, "Bag Lifting', p 246:

"... the load carrying efficiency of the back muscles is reduced by
the flexibility of the spinal lever upon which they operate. When the
spinal column is flexed and has to be extended against strong
resistance the back muscles are more likely to be injured because they
are simultaneously fulfilling two conflicting functions, i.e. (1)
moving the spinal bones upon each other and (2) at the same time
trying to stabilize the spine so that it may act as an effective
lever. Further, in the flexed position the stresses imposed upon the
spine are transmitted through the thick, soft discs of cartilage
interposed between the spinal bones. Lifting with a straight back
means that the extensor muscles of the back are essentially concerned
in stabilising the spine and that stresses are transmitted more
directly from bone to bone, with only thin plates of shock absorbing
cartilage between the bones. ...with a bent back, the spinal joints
are 'unlocked' and the thick discs of cartilage between the bodies of
the spinal bones are compressed. [In a straight back] the joints
between the posterior processes of the spinal bones are 'locked',
pressure on the discs of cartilage is reduced, and stresses are
transmitted more directly through stabilised bony structures. In the
latter case the spinal muscles are more efficiently employed in moving
the spinal lever as a whole."

I've always thought the term 'straight' inappropriate for the back,
but it is clear that advice to lift like this goes back at least as
far as 1951. However, McClurg Anderson went on to weigh the relative
benefits and limitations of the two methods. Other criticisms of the
straight back advice go back at least as far as 1972. J.R. Brown
wrote, in "Manual Lifting and Related Fields: an Annotated
Bibliography", p 427:

"The suggestion that lifting should be carried out at all times by
means of a straight back and bent knees appears as a continuous thread
running through the majority of works cited. Investigations by the
present author and others have shown that the systems advocated in the
official weight lifting manuals are incorrect, and that in many
instances, methods advocated are likely to do more harm than good."

6: It strikes me (I largely agree with both Ben Weitz and Jan-Paul van
Wingerden), that we should avoid rigid prescriptions on "how to lift"
but encourage workers to know their limits and lift in ways that they
find safe and comfortable.

Andrew.Pinder@hsl.gov.uk
Ergonomics and Work Psychology Section, Health and Safety Laboratory
Broad Lane, Sheffield, S3 7HQ, UK.
Tel +44 114 289 2594, Fax +44 114 289 2526