This work is intended to explore the bio-mechanical factors that may determine spinal pathology that leads to backache (Low Back Pain. LBP) and serious spinal pathology.  Once identified, corrective measures can be developed, which are described.  It is hoped that it will provide a resource for chair designers so that  the common misconceived  views can be avoided.   A fully optimised system is also described.

The present position.  How did we get here?

Mandal 1As long ago as 1985 AC Mandal explained that the upright sitting posture, advocated as ‘correct’ was seriously flawed  (See  ☛ Why? Mandal’s Homo sedens.→).   This has been validated by subsequent research but is still advocated as ‘correct’.  Mandal advocated a forward tilted seat (FTS) which is incorporated into the    Ischial off-load system now used by some high-end office chairs.    (See  ☛ . mid-upright sitting.→).

Early Intra-discal pressure studies suggested that upright sitting pressure was higher than when standing.   This resulted in the adoption of Stand/Sit systems.  Later studies showed little or no difference between sitting and standing pressures.   However the large pressure difference between the standing and a reclined modes were confirmed.

Cyriax JH in the 1940s Suggested that the lumbar flexion on sitting resulted in  backward translation of the inter-vertebral disc (IVD) contents.   To this was added the effect of stretching of the ligaments posterior to the lumbar joints.   Gorman JD, an engineer turned chiropractor, showed how this effect could be remediated by correctly placed back support at the level of the Iliac crest which he called ‘Pelvic Support’ as distinct from ‘Lumbar Support’.   (☛BACKRESTS. Pelvic support v. Lumbar.→).

This work

As a consultant in Orthopaedic (Musculo-skeletal) Medicine, many of my patients associated their problems with sitting and questioned me accordingly.   With this in view I re-examined the basic pathology and bio-mechanics in relation to sitting and in 1998 arrived at the 2T CONCEPT a full solution.   It was regarded by the chair industry as weird but not by patients.   I concluded that a reclined position for prolonged work provides the only system that corrects all the adverse effects of conventional prolonged upright sitting.    reclined mode →.   For a reclined office work-chair to be practical a number of requirements are essential including a secondary upright mode for short tasks and an easy transition between these two modes.  Designated the (2T)  this is now designated 3M (tri-modal) since the health benefits of  the transitional mode, providing it was unstable, were recognised.   A   Sit/Stand facility can be incorporated to become a 4M   ( ☛OFFICE WORK-STATIONS→.).   A 2T prototype was tested for practicality in the Cambridge University MfI department.       Although originally considered outlandish in 1998, recently design is at last  moving in this direction.  (☛The FUTURE is GOING THE 2T (4M) WAY→).

Requirements for optimising the bio-mechanics of prolonged sitting

Partial remediations.

Partial solutionsSystems in use at present,    See ☛ REMEDIATION

2T direction

The big story here, however, is that it was found that a reclined mode is the only position that avoids all the adverse effects of prolonged upright sitting.    For a reclined office work-chair to be practical a number of requirements are essential.  

Combining these requirements can be difficult to achieve but is possible in a suitably supported reclined mode at 40-45° which conforms to spinal morphology (configuration). This alone is impractical as a work position unless a number of secondary requirements are incorporated to make this simple concept suitable and practical for a work chair.

  1. A  reclined mode which has a correctly supported, supine, stable position  for prolonged work    ☛2T reclined mode→
  2. An upright mode required for certain jobs.    ☛2T Upright mode→
  3. An unstable intermediate mode to enable a quick, easy transition between the 2T modes.    ☛2T transition mode→     With the recognition that the transition movement has physiological advantages, this has been upgraded to a ‘mode’.   dynamic seating→

These 3 modes constitute the basis of the 2T CONCEPT .

From Jim Platts. MfI Department.   University of Cambridge

There is a small detail that you might like to add in to your forward thinking, which takes the notion of some form of certification for ‘good’ chair shapes a step further than we discussed yesterday.

What is to me so delightful is Dr Henry has very elegantly identified the core detail of ‘good sitting’ as maintaining the angle of the pelvis, so that the first few vertebrae above it are sustained in their right geometric relationships and carry their loads easily, without the geometry becoming distorted and the surrounding tissue strained, causing back problems. The notion of some form of certification process hinges around the understanding of the notion of ‘good sitting’.

There is a step beyond this. Once the notion of ‘good sitting’ becomes not only well understood but also well accepted – and I think we have enough clarity from all the work that Dr Henry has pulled together, and enough medical agreement about that work, for this to happen – there comes a point where it becomes professionally unacceptable to sell chairs that cause ‘bad sitting’ and will in due course cause back ache. At this point the position of the global chair industry shifts, because you cannot sell large numbers of office chairs that are known to induce back problems and open yourself up to what the Americans would call a class case in the law courts, accusing you of knowingly selling a harmful product.

I guess the example would be seat belts in cars. It is not only that they save lives for those in the car. They save an inordinate amount of accident and emergency time and effort in hospitals on Saturday nights. So we don’t wear seat belts ‘because they are a good thing’ we wear them because not to do so is a bad thing.

There are of course many, many ways of producing ‘good sitting’. But where I think there is real value in the simple clarity that Dr Henry has achieved in his work, is that the essentials of ‘good sitting’ are made very clear. And THAT – the spreading of that message – in the end has the power to ban ‘bad sitting’.

Best regards,     Jim



‘Sitting disease’

Compared to people who sit the least, those who spend most time in a chair have a 112 % higher risk of developing diabetes, a 147% higher risk of suffering “cardiovascular events” such as strokes and a 49%t increased risk of death from any cause.  (   Alarming but not entirely an effect of the bio-mechanics and not examined here.  However some posts are relevant.

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