An overview for

COMFORT

I had rather forgotten about ‘comfort’ as it had been shot down by the scientific community as far back as 2003 (again see  sittingsafely.com/comfort/ .   It should be, as in the Hippocratic oath –  Do no harm.   After last week (2015)  I have added “This is important ……with ‘comfort’ which misleads, being that treacherous guide  which only turns up truthfully when the bio-mechanics (ergonomics) are fully correct.   At present there is no chair on the market that does not have this potential.  Simply, prolonged sitting results in backache. Stop.  At least potentially for about 70% of users and health problems for 100%.

The search for comfort may have misled chair designers resulting in no ergonomic optimum chair on the market.  ☛ Comfort→

ADJUSTMENTS

A plethora of adjustments is confusing and can be adverse if adjusted wrongly.  Ideally they should be avoided as far as is possible.  This should benefit both design and cost  ☛ Adjustments→

MOVEMENT & EXERCISE

• Spinal movement & change of position are important for a number of reasons including IV disc nutrition. ☛  Movement & exercise→
•  ‘Dynamic Seating’.Recently there has been interest in continuous small amplitude movement for upright chairs, the chair re-aligning with the users centre of gravity.  ☛ Various chairs. How do they measure up?
• 2T exercise and movement systems   ☛ The unstable TRANSITIONAL MODE. It’s importance.
• Sit/Stand. ☛ SIT/STAND (& stools)→ .  Bio-mechanically superior to the use of existing upright chairs and can be an adjunct to the 2T system which then becomes a ☛ 4M work-station→ concept (and a ‘full’ solution).

SLEEP

Seep of between 30 and 90 minutes in the afternoon could help companies improve productivity.  ☛ Sleep→

FAMILIARITY BIAS

Chair designers say  “I agree but this concept is ahead of it’s time.  Familiarity bias will strike in and uptake will be poor.”    PR people say the opposite.  “This is a ‘paradigm change’.    Once people understand that it is safe and possibly even safer than any existing chairs, they will embrace them. There will be large media interest.  This will ensure automatic PR,”  Big uptake will occur rapidly and firms without a 2T product in their range will miss out, possibly terminally (to use medical jargon).   They may experience ’ Normalcy Bias’    as did the good people of New Orleans as hurricane ‘Katrina’ approached.  See  ☛ Familiarity bias→

SPECIAL NEEDS

People with ‘special needs’ are universal and these needs require to be addressed,  particularly in a reclined mode.  The 2T concept is particularly suited for this.  See ☛ special needs→

←Return to  ☛ 2T CONCEPT a full solution

For an overview of the bio-mechanics  see  Bio–mechanics that determine safe sitting↑

Effect of sitting for prolonged periods.

Sitting for prolonged periods  is obvious to most patients and clinicians who treat them  as an association with LBP with and results in a high reporting of LBP by sedentary workers and was recognised in the early work as a risk factor (Bendix 1994).    Sitting in the ordinary way is not associated with spinal pathology unless prolonged for about half a workday (Lis AM, 2007) and a higher  prevalence rate has also been reported in those occupations that require the worker to sit for the majority of a working day and is significantly higher than the prevalence rate of the general population (Papageorgiou 1975) particularly among those aged 35 years and older who have had sedentary jobs for several years (Kelsey JL. 1971) and if combined with  awkward postures or Whole Body Vibration (Lings S, 2000)  which occurs with driving a car (Kelsey 1975), tractor or, worst, a helicopter.  This association was refuted in a later study with 45 pairs of identical twins (Battie et al., 2002).

It is not a general view but long-term, low-level chronic stressors that occur with prolonged sitting (Wood and McLeich, 1974.)  may be as important as acute impact forces such as falls and lifting strains of heavy manual work.   Sitting for more than 6 hours daily increases mortality by 37% for females and 17% for men. The gender difference is unexplained (Patel.2013).

There is a strong presumption that prolonged sitting in adverse ergonomic circumstances, especially in childhood, is fundamental to the explosive rise in the prevalence and incidence of spinal pathology and LBP.   Demographic studies to determine differences in the incidence of LBP in populations that use Western style upright chairs and other ‘natural’ styles of sitting suggest the adverse effect of the former. Non-Western peoples, who do not use chairs, tend to have a low incidence of LBP.    In Japan, this increases as elements of the population adopt chairs (Schlemper7).

The bio-mechanical evidence shows that conventional mid upright sitting causes backward tilting of the pelvis resulting in reduction, or even reversal, of the protective disc wedge angles at the lowest 2 lumbar joints (L4/5 & L5/S1) where mechanical spinal breakdown is commonest.  This effect is augmented by lumbar support directed to above the L5 vertebra (Gorman ).  The predicted effect is retropulsion of the disc contents (NP) and is confirmed by pMRI studies (Smith, 2006).    Posterior disc protrusion was put forward by Cyriax (1945) as the main cause of backache and later posterior structures of the motion segment, Zygapophyseal (facet) joints and ligaments have also been shown to be involved in pain production (See Anatomy/nerve supply). Irreversable stretching of the lumbo-sacral and supraspinous ligaments can result in pain and joint instability leading to CTD and earlier degeneration (see Anatomy/Ligaments).This has not been confirmed by epidemiological studies.    A study by the US Department of Health and Human Resources (NIOSH 1997) reviewed a number of factors and the general conclusions seem to suggest that the evidence was contradictory and confusing. There was an emphasis on non-physical psycho-social factors and heavy load handling.   Much of the research into the causation of spinal breakdown has concentrated on violent or inappropriate spinal usage afflicting manual workers today and evidence is shown in skeletons from the prehistoric past.

 The   pMRI evidence

An investigation using Whole-body Positional MRI (pMRI), by FW. Smith, Bashir W (2007) who found that the upright position, at 90°, caused disc contents to move the most, while the relaxed position (135°/45° reclined) caused disc contents to move the least. This confirms that the upright position is the worst for the back, while the relaxed position is the best.

The above effect has been shown incontrovertibly to occur by pMRI scans (Smith FE 2006).  From  which the following pictures are derived (arrows, etc, are added).  This confirms the bio-mechanical evidence.

pMRI scan in reclined, relaxed, sitting mode shows the NP in a safe mid-position.  The hip angle is at 135°.    the NP is in the safe  mid-position.   This is practical and preferable and is advocated for the 2Tilt principle in the reclined mode.

pMRI scan in an upright sitting mode shows the NP has translated posteriorly which can culminate in protrusion.   Hips are at an angle of 90° with the seat-pan horizontal.

This is visual confirmation of the bio-mechanical evidence.

  For full  remediation see: The 2T concept⟶

Next see  ☛ Loading→

Ligaments

Adams et al. (1980) showed that the supraspinous-interspinous ligaments segments are the first ligamentous tissues to become stressed with forward bending of the lumbar spine.

Inter-vertebral disc

Thus, the disc is capable of withstanding the large compressive forces that result from muscular recruitment. Hutton and Adams (1982) found that cadaver discs from males between the ages of 22 and 46 could, on average, withstand single loads of over 10,000 N before failure occurred. In most cases, the failure was in the thin bony membrane that forms the boundary between the disc and the vertebral body (vertebral endplate) rather than through nuclear prolapse. Since the disc is an avascular structure, the health of the endplate is critical for nutrient exchange, and even small failures may hasten the degenerative process.

Researchers have found that prolapsed discs occurred more frequently when the vertebral segments were wedged to simulate extreme forward bending of the spine (Adams and Hutton, 1982). In this position, the anterior portion of the annulus fibrosis undergoes compression while the posterior portion is under tensile stress. Over 40 percent of the cadaver discs tested by Adams and Hutton (1982) prolapsed when tested in this hyperflex posture, and with an average of only 5,400 N of compression force applied. This finding shows that the disc is particularly susceptible to bending stresses. In a later study in which Adams and Hutton (1985) simulated repetitive loading of the disc, previously healthy discs failed at 3,800 N, again mostly through trabecular fractures of the vertebral bodies. Taken together, these studies show that the disc, especially the vertebral endplate, is susceptible to damage when loading is repetitive or when exposed to large compressive forces while in a severely flexed posture.

Muscle action

From a biomechanical perspective, co-contraction is a way in which joints can be stiffened, stabilized, and moved in a well-controlled manner. Cocontraction, however, also has the potential to substantially increase the mechanical loads (compression, shear, or torsion) or change the nature of the loads placed on the body’s articulations during an exertion or motion. This is because any co-contraction of fully or partially antagonistic muscles requires increased activation of the agonistic muscles responsible for generating or resisting the desired external load. Thus, the co-contraction increases the joint loading first by the antagonistic force, and second by the additional agonist force required to overcome this antagonistic force. Therefore, work activities in which co-contraction is more common impose greater loads on the tissues of the musculoskeletal system.

Neurological pathways

Investigations have identified pain pathways for joint pain, pain of disc origin, longitudinal ligaments, and mechanisms for sciatica. In the case of facet pain, several mechanisms were identified including an extensive distribution of small nerve fibers and endings in the lumbar facet joint, nerves containing substance P, high-threshold mechanoreceptors in the facet joint capsule, and sensitization and excitation of nerves in the facet joint and surrounding muscle when the nerves were exposed to inflammatory or algesic chemicals (Dwyer, Aprill, and Bogduk, 1990; Ozaktay et al., 1995; Yamashita et al., 1996). Evidence for disc pain was also identified via an extensive distribution of small nerve fibers and free nerve endings in the superficial annulus of the disc and small fibers and free nerve endings in the adjacent longitudinal ligaments (Bogduk, 1991, 1995; Cavanaugh, Kallakuri, and Ozaktay, 1995; Kallakuri, Cavanaugh, and Blagoev, 1998).

Several studies have also shown how sciatic pain can be associated with mechanical stimulation of spine structures. Moderate pressure on the dorsal root ganglia resulted in vigorous and long-lasting excitatory discharges that would explain sciatica. In addition, sciatica could be explained by excitation of dorsal root fibers when the ganglia were exposed to the nucleus pulposus. Excitation and loss of nerve function in nerve roots exposed to phospholipase A₂ could also explain sciatica (Cavanaugh et al., 1997; Chen et al., 1997; Ozaktay, Kallakuri, and Cavanaugh, 1998). Finally, the sacroiliac joint has also been shown to be a significant, yet poorly understood source of low back pain (Schwarzer, Aprill, and Bogduk, 1995). Hence, these studies clearly show that there is a logical and well demonstrated rationale to expect that mechanical stimulation of the spinal structures can lead to low back pain perception and reporting. How these relate operationally to clinical syndromes is less certain.

There also appears to be a strong temporal component to ligament status recovery. Ligaments appear to require long periods of time to regain structural integrity, and compensatory muscle activities are recruited (Solomonow et al., 1998; Stubbs et al., 1998; Gedalia et al., 1999; Solomonow et al., 2000; Wang et al., 2000). The time needed for recovery can easily exceed the typical work-rest cycles observed in industry.

Further points :-

• IVD & joint angle.
  • IV discs prolapse occurred more frequently when the vertebral segments were wedged to simulate extreme forward bending of the spine (Adams and Hutton, 1982) in addition to when loading is repetitive or when exposed to compressive forces while in a flexed posture.   See ☛ IV Dscs→
  • Lordosis occurs at two levels of the human spine, cervical and lumbar.  The spinal joints subject to internal derangement are the 4th, 6th  & 7th cervical and the 4th & 5th lumbar” (Cyriax JH. 1946).
  • Both these spinal levels are where mobile segments meet a solid mass, the skull and the pelvis, and where mechanical spinal pathology mostly occurs (Harrison DD 1998)and differences are found when comparing LBP patients with healthy patients (Jackson RP, 1994).
• • Approximately two-thirds of total lumbar lordosis occurs at the inferior two segments (L4-L5-S1) (Kamali, 2003).
  • This configuration occurred as a result of hominins adopting an upright (orthograde) stance for efficient bipedalism (see pages on  Paleo-anthropology).
  • The lordotic configuration involves a large wedge angle (16-24° at L5/S1) at the lower lumbar joints.  The point of the wedge lies posterior. (See ‘Angles & Lordosis)
  • The wedge angle reduces any tendency for retropulsion of the disc contents.
  • Retropulsion can lead to impingement on pain sensitive structures ( ) and protrusion and extrusion of disc contents.
  • Retropulsion occurs on axial loading with joint flexion (See ‘Effects on sitting posture’).
  • The wedge angle is reduced when the pelvis is tilted backwards which occurs with upright sitting.
  • This adverse effect is augmented by reduction of the hip flexion angle (occurs with a seat parallel to the floor) and pelvic support set too high.
  • A large wedge angle is protective and occurs in people with a low incidence of LBP and where there is no tendency for reversal of the angle (ie. to go to 0° or below).
  • Retropulsion is corrected (reduced) in a reclined position or with hip extension (Smith 2006).
  • Excessive extension (lordosis) transfers pressure to the posterior pain sensitive structures.  In moderation this is not evident clinically in the lumbar joints (but is in the cervical region).
  • Movement to ensure intra-discal pressure changes is essential to provide dis nutrition (See ‘Disc nutrition’).
• LIGAMENTS
  • Ligaments limit joint movement in a specific direction and provide stabilisation (See Anatomy/ligaments ).
  • Ligaments are visco-elastic and can be lengthened by excessive, prolonged stretching.
  • Excessive joint range and instability can lead to CTD (Solomonow 2003).
  • Excessive joint range can disable the normal protective action of muscles (See Anatomy/muscles).
  • Adams et al. (1980) showed that the supraspinous-interspinous ligaments segments are the first ligamentous tissues to become stressed with forward bending of the lumbar spine.     Ligaments appear to require long periods of time to regain structural integrity, and compensatory muscle activities are recruited (Solomonow et al., 1998; Stubbs et al., 1998; Gedalia et al., 1999; Solomonow et al., 2000; Wang et al., 2000). The time needed for recovery can easily exceed the typical work-rest cycles observed in industry.   See ☛Ligaments→

  MUSCLES & NEUROLOGY

  • Muscle cocontraction, can substantially increase the mechanical loads (compression, shear, or torsion) or change the nature of the loads placed on the body’s articulations during an exertion or motion.   See ☛ Muscles→
  • Neurological pathways.   Pain pathways have been identified for joint pain, pain of disc origin, longitudinal ligaments, and mechanisms for sciatica.   See ☛ neurology→
• Requirements for optimising the bio-mechanics of prolonged sitting.
• • Reduction of axial loading→
  • Preserve the lumbar lordosis→
  • avoid backward tilting of the pelvis→
  • Ensure disc nutrition by pressure changes with movement→

  Combined these can be difficult to achieve, for office and home, but can be done using the 2 Tilt concept→.

  Systems in use at present, partial remediation

  • Pelvic support→
  • Forward tilted seat (FTS)→
  • Reclined work mode (Okamura)→

  See Origins of lumbar vulnerability (Paleoanthropology)→

This is not just a quick transition from the reclined to the upright modes (2T) but provides physiological benefits and is itself a mode.  The concept is renamed the Tr-Modal (3M).  2T = 3M.

2Tilt Principle 3  Requirement 9.. The adverse intermediate upright positions should be unstable.  This allows an easy and rapid transition from one mode to the other and no adjustment is allowed to maintain an intermediate position

This may seem counter-intuitive.  Why shouldn’t the user be able to sit easily in any position that seems comfortable?   It worries chair designers dreadfully although they may not be bothered by the ‘dynamic seating’ concept..

Why not?

•   It is, equivalent to the mid upright position of most office chairs, carrying an ergonomic penalty.
  A user, used to a mid-upright chair,  could use an adjustment to maintain this position for long periods which would add an unnecessary adverse effect to an otherwise optimal system.
• The intermediate positions being unstable require muscular effort to be sustained.   On moving back a few degrees from the stable forward upright mode the occupant enters this upright unstable position which provides proprioceptive feedback and frequent small amplitude pressure changes.
• A user has a choice and may find a short episode of this low amplitude exercise pleasant.  Longer periods are liable to be tiring.
• There is also an added advantage  in providing therapeutic exercise following an acute LBP episode with it’s resultant muscle and reflex atrophy. (See ☛Anatomy/muscles).
• Addition of a sit/stand mode in the 3M version allows the user to walk around.  ☛ SIT/STAND (& stools)→
• Why add to extra cost?
• Spinal movement & change of position are important for a number of reasons including IV disc nutrition. ☛  Movement & exercise→
• Dynamic Seating’.Recently there has been interest in continuous small amplitude movement for upright chairs, the chair re-aligning with the users centre of gravity.  ☛  Movement & exercise→
• 2T exercise and movement systems   ☛  Movement & exercise→
• Sit/Stand. Bio-mechanically superior to the use of existing upright chairs and can be an adjunct to the 2T system which then becomes a ☛ 4M work-station→ concept (and a ‘full’ solution).☛ SIT/STAND (& stools)→

A specific illustration

The ALTMARK Chair has both a reclined and upright mode which makes it interesting and ahead of the market.   The webpage shows a fixed intermediate  position in both the reclined and semi upright modes.    At best it gives no biomechanic advantage and was probably intended to add comfort → .   Misled by ” that treacherous guide  which only turns up truthfully when the ergonomics are fully correct”.   If this mode becomes a part of the unstable intermediate mode it allows a faster transition and also gives the user the choice for dynamic motion and rehabilitation.  A win-win.   Scrapping a fixed intermediate mode reduces manufacturing costs. A win-win-win!

Return to  ☛ 2T CONCEPT a full solution →    or

Next ☛ 2T CONSIDERATIONS →

Recliners are not intended for office use.  However, using a reclined mode and a tilting mechanism, they have certain similarities to a 2Tilt chair.  The boundaries between contract and home furniture  are decreasing and the division may be disappearing entirely. 

See http://www.onofficemagazine

Recliner chairs are only suitable for home use.   The full stringent requirements for prolonged sitting are not necessary.  The tilting mechanism is not required to be unstable and can stop in mid range without an adjustment.  Until an efficient system for this appears on the market even an electric mechanism is used which is surely the last refuge of inadequate design!

A  chair using the 2T (or 3M) concept, which has bio-mechanic optimisation, is different only because it can be used as a work chair.   Will people buy recliners when a cheaper and better designed dual purpose chair comes on the market?  This may also be particularly relevant to the growing ‘gamer’ market.   Only small modifications are required to achieve 2T optimisation which gives a huge oportunity to manufacturers of ‘recliners’.  For a further discussion see MANUFACTURERS of ergonomic chairs→

BERG Unik recliner

Not designed as an office or work chair, nevertheless has features similar to the 2T concept  It is helpful to note the differences.

• The mid ranges between full reclined and upright do not have the instability that is basic to the 2T concept.  This  adjustment, although potentially adverse, is mentioned as a selling point.
• Adopting the fully reclined position is a slight struggle. This may be due to a failure to understand the adjustments and a reason to scrap them..
• In the fully upright position, equivalent to the upright mode, the seat is parallel to the floor mitigated by a degree of pelvic support.  This is allowed by the 2T concept although the FTS is preferred.
• There is unnecessary, even adverse, adjustment of the height of the pelvic support.   For the 2T this is the ‘Iliac’ version and fixed at 20 cms.
• The leg rest extension works well.  That it only supports and compresses the calf is worrying.  The 2T requires a foot-rest which is adjustable to the users leg length.   This and the head-rest are the only 2 components that require adjustment.  The torso should be fully supported by correct design.
• Upholstery is usual with recliners but allows build up of heat and moisture.  Arguably it adds comfort and conceals design mistakes but also can reduce correctly designed support.
• Have a look  at Berg→

METRONAPS

Another interesting product.   Again, this is not a work chair and looks like a recliner.  It is intended for short naps in an office environment to which a user can retire if desired.  It now recognised to have benefits ☛sleep→ at 2T, early concept.

• It costs $1300.
• Reclined seems to be the only mode and this precludes it from being a 3M chair or 4M workstation.
• The hood allows it to become a semi Pod.  It is of interest as it could be incorporated into a 2T chair and contain the monitor etc.
• It should be noted that a 2T, whether 3M or 4M, can easily be slept in.    Indeed this was an early criticism by a CEO “This is much too comfortable.  My workforce will go to sleep!” He failed to notice that he was lying on correctly shaped plywood with no upholstery whatever.

Work-chairs which recline

These chairs have been designed for the convenience of the operator and not  the client.

Dentists chairs

I have recently experienced 3 dentists chairs.   The discomfort was worse than that inflicted by the dental procedure.  Although expensive with a reclined mode and full length support, the configuration was misapplied,  the reclined support being lumbar and not pelvic.  A truly disgraceful error of design.   Trawling through other models, some do not look so bad.

American barber’s chairs

I have only just become aware of this extraordinary genre having visited  the Oulton antique shop.   They have an upright and reclined mode with variation between.   This model from the 1920 looks alike an instrument of torture (and probably was).   At least it had height variability with the head-rest and the 2 modes.

Return to    ←Various chairs.

OKAMURA,   ATLAS chair & Cruise concept.

Okamura is a major Japanese company and produced a reclined work-chair that was ahead of the field in addressing the biomechanical factors that are necessary to avoid or limit  LBP.

“Looks familiar, Henry?” was the message from John Jukes about this chair.  Sure enough it showed the semi  reclined work position that I had been advocating since 1998 on the web.  In my reply to Jukes I made the following points :-

• The upright mode is in an adverse mid-upright position in its worst form, and the less adverse intermediate positions are allowed.
• Improvements could be made, particularly to the positioning of the headrest which requires manual adjustment to provide the necessary support in the reclined mode.  As can be seen in the brochure photographs this is not done.
• The method of changing from reclined to upright mode involves raising or lowering the seat.
• There is a plethora of unnecessary controls.

As the 2T concept was considered ‘outlandish’ back then, this chair was a considerable leap forward by Okamura.  It has been suggested that Okamura designers had seen my web page.  I doubt this as they had failed to apply the essential requirements need to make this a more user friendly (and cheaper) work-chair.

However, congratulations to Okamura.  Also their Atlas model allows some insights into where their design went wrong in relation to the 2Tilt (2T) concept.

• It has a reasonably good reclined position.  But then things become complicated as it differs from a chair designed along 2T principles.
• Problems occur with the further design and is unintentionally shown in the photograph, above, as the users head is not being supported although there is a possible 40° forward adjustment.   This illustrates the point that adjustments are usually maladjusted and have confusing controls.

• 
• 1. Good.  Easily adjusted for users of differing height.  Integral and essential for the 2Tilt concept
• 2. Pelvic support should  be at 200 mm. and not adjustable.  Usually adjusted to an adverse lumbar height.
• There is no need for adjustment of the depth of the seat with a 2T design.
• 3. Also no need for this adjustment.  If the FTS upright system is used, the headrest  remains well behind the users head,  no adjustment being required.  This becomes more complicated if a pelvic support system is used for the upright mode.  See note below.
• 4. In the reclined mode the head-rest needs to be at about 6″ forward from the back-rest.    This depends on the degree (up to 40°) of thoraco/upper lumbar kyphosis designed into the back-rest.
• 5.  Good.  The reclined back-rest angle is 45°.   Better at 40°.
• 6.  Good.  The backward extended legs give greater stability in the reclined mode.
• From the diagram it can be seen that there are some controlled adjustments which are sub-optimal and would be avoided in the 2T concept.
• The upright mode is in the usual mid-upright position which may incurs a 40% axial compression penalty (Nachemson, Sato³⁴ but not Wilke¹) over standing and 500% over lying supine.   The less adverse intermediate positions are allowed.  The 2T concept allows intermediate positions but these are unstable and can allow rocking exercise.
• The mid upright mode could be partially mitigated by correctly placed iliac support.  The emphasis here must be on ‘correctly placed’ and this can be easily adjusted into an adverse position which is not allowed in the 2T concept.  (Pelvic support →).
• The seat is convex and can allow the user to sit back and take advantage of the pelvic (or better, ‘iliac’) support. In this case the headrest has to be adjusted back from it’s position that is required when reclined.  (See‘Ischial Off Load system’).  (see •3 above)
• The user can also sit on the front edge where the seat is tilting downward as a FTS and where the head rest becomes irrelevant.  Feet would have to be braced against the floor to prevent slipping (See below).
• Fixed at a low level,  the seat allows the feet to rest on the floor in the reclined mode, as shown.  I am told that this causes problems for tall people who have tried it and an adjustable work-table may be required (no bad thing).
• I found the shift from the reclined mode to be awkward and difficult but can probably be eased by adjustment and familiarity.  Ideally, for the ‘2 Tilt concept’ this should be easy and without adjustment with the intermediate positions being unstable and can allow rocking exercise.
• Ways may be found for this to be acceptable in a 2T solution, where there is a specified requirement for support from head to feet.  One solution is a leg/footrest that is retracted in the upright mode.
• The 2T prototypes are monocoque and the leg-rest is straddled when in the upright mode which is probably impractical for an office model

• It has plenty of room for improvement but the Atlas was the only chair designed for work in a reclined mode.   As combined with the specific desk it is awkward but a formidable (but expensive)  buy, and especially for those working in a trading environment.
• Failure to take account of the necessary requirements for a reclined work mode (the 2T concept) has resulted in a clunky model resulting in, I think, a poor user uptake.  A pity! (See Requirements →).
• See their website

With the system shown below Okamura demonstrates the problem when the floor is used as the footrest in the reclined work position.  The work top would have to be very low and would be too low when in the upright mode.  The solution involves either automatically raising the work-top when entering the upright mode with a related variable height desk.  The problem is avoided with a desk-less work station.

The Okamura website describes this “Ankle tilt reclining is a mechanism by which the chair seat sinks backward in sync with the reclining of the chair back. When reclining, the gradual opening of the ankle, knee, and hip joints without moving the ankle itself serves to create a relaxed state and promote better blood circulation. In addition, the fact that the chair seat does not push the body upward makes this a body-friendly reclining mechanism which reduces the likelihood of leg swelling and numbness. Since the chair moves in unison with the body, even if reclining is repeatedly done, your concentration will not be broken by feelings of discomfort”

A similar account of kinematic reclination in the Aeron chair can be seen on the HM website→

Okamura work chairs are good of their sort but have failed to produce a 2Tilt model. but nearly there ….

The Contessa has pelvic support and a domed seat (ischial off-load) and the Leopard has FTS which might have a problem with slippage.

To see Okamura web→

The Luxos is sort of reclined with a stool for a leg-rest.   Good pelvic support and a forward tilt which are alternatives and not complementary but equivalent to Ischial offload system.   In this photograph the headrest is well out of the way in the upright mode.

Okmaura Sabrina Chair has a “Synchro-Reclining” function enabling the chair backrest and seat to be synchronized by operating the lever located at the bottom of the seat.  The functions include a forward-tilting seat and adjustable armrests.    Okamura have seen the 2T details but fail to respond.   From the text thay may be   begining to accept my views   HAS

Only a 2Tilt reclined mode addresses all the described adverse ergonomic effects of prolonged sitting which are described below.  For a correct fully remediated sitting position …

See  Reclined mode advantages→

• This has to be correctly configured and begs further prerequisites for a truly practical work chair.
• Support is given to the whole length of the body, from head to feet.
• Pressure is distributed widely, mainly from the seat-pan to the Backrest
• Even with the reduced axial loading the lumbar lordosis should be maintained and is best effected by some degree of pelvic (not lumbar) support. This avoids any tendency to backward tilt of the pelvis with stretching of the posterior components.   The support should only be sufficient to avoid lumbar flattening when supine, and should preferably not be adjustable so as to avoid excessive lordosis and wrong positioning.
• The kyphotic curve  of the thoracic spine is allowed to be exaggerated being  a stable area and fully supported. Included with the upper lumbar joints, it extends,over 14 segments and the slight extra flexion at the upper lumbar joints is allowable.
• A correctly placed head/neck-rest is required so that the occupant has no need to move the neck to establish a comfortable task-related visual field.  With the pronounced fully supported thoracic curve (kyphosis) the headrest needs to be  forward from the backrest. This involves some neck flexion, over several joints, which is supported and and the weight of the head can be off-loaded to the head-rest if the force of the head-rest is directed to the base of the skull and not the back if the head.  Freedom of easy movement away from the headrest is easy, if so required.
• In the reclined work position the key board and mouse should rest on or near the lap.  This is an ergonomically correct position for the arms and wrists.
• Arm-rests are not an essential requirement of the 2T concept but depend on design and usage.
• The head and leg-lengths are the most variable parts of the human body,  so vertical, axial, adjustment of these is essential in the reclined mode.   A spring foot exercise system is an advantage for exercise of the calf muscles.
• Apart from this, adjustments should be avoided as far as possible,
• For a fully supported position a leg & foot-rest are essential.   In the Okamura solution the chair is lowered so that the heels can rest on the floor or a bar under the desk.  In the early 2Tilt ‘prototype1’→ the leg-rest/ seat / backrest/ headrest configuration remains constant (monocoque) and in the upright mode the legs are astride the leg-rest with the feet on the floor.  This is probably not practical for an office desk and the leg-rest may have to be retracted or folded down.
• The calf length leg rest, seen in recliners, should be avoided in a 2T chair as compression of the calf can result in Deep Vein Thrombosis (DVT).  This may be obviated with recliners which have upholstery and when the legs  are in a position at or above the level of the heart.
• In a reclined mode heat and moisture build-up can be a problem.  Solutions include a mesh construction  or padded slats.

Prototypes

A prototype (p5) was constructed, in the MfI Dept of Engineering in Cambridge, to show the practicality of the 2T concept.

This was the simplest form, based on the original monocoque configuration to illustrate angles and construction.The essential requirements were determined and additional requirements were mentioned.  It is possible, and likely, that complications and variations can, and will, be made and it is hoped that this study will be a reminder of the essential principles. Students working on this (P5) model found it very comfortable and used it whenever possible. I was amazed to see how they bounced about on what was only a somewhat flimsy prototype.

About an earlier (P4) prototype, I wrote  ‘I found this reclined position very comfortable and  adopted it whenever possible.  There is no tendency to go to sleep!  When viewing the screen I used the head rest but sometimes brought my head slightly forward when typing. Ideally the footrest should be sprung. The thoraco-upper lumber curve could be exaggerated further to allow less flexion at the neck.  The 2T chair in the reclined mode is relevant to users who have unusual spinal configuration.    HAS.  See      ☛ ‘Special needs’→

The ‘Tilt in Space’ v ‘reclined systems’ debate

This largely relates to wheel chair patients with major conditions such as spinal cord injuries,  muscle diseases, cerebral palsy  and head injuries and is hardly relevant to office workstations.  The difference is that the TiS has a fixed seat-to-back angle being monocoque as in my p1-5 prototype versions. Cambridge studies showed this to be easily feasible although I prefer the leg-rest retracted in the upright mode unless used in a desk-less system.  The reclined systems mode is achieved by opening up the seat-to-back angle and is claimed to increase shear, discussed by Lange, M. L. 2000.

• ☛ 2T office chairs→
• ☛ Adjustments?→
• ☛ Comfort→
• ☛ Exercise & movement→

Finally check

• NEXT for the associated FULL concept see ☛
• A full ergonomic solution the 2T CONCEPT→
• 2Tilt concept, the UPRIGHT MODE. →
• The FORWARD TILTED SEAT (FTS).
• The unstable TRANSITIONAL MODE. It’s importance.→ 
• WORK-CHAIRS, a new breed with a reclined mode.→
• Design details→

 The bio-mechanical factors and their effects that may determine spinal pathology and lead to backache (Low Back Pain. LBP & further pathology) are discussed.

These adverse effects need to be addressed for effective REMEDIATION →

 The final common pathway …

 to IV Disc pathology at the joints of the lumbar-sacral junction (L3/4, L4/5, L5/S1) is reduction of the L/S joint angle resulting in posterior movement of the intervertebral disc (IVD) contents and can be seen, on pMRI scan (Smith F 2006).    This retropulsion of disc contents can progress to protrusion and extrusion (see ☛The intervertebral disc→).   Also CTD→

1. Axial loading.

With  the currently advocated upright sitting posture there is an increase of compression on the L3 disc is x2.5 above that when lying supine (Nachemson).   This has been questioned by later work and determined by  Wilke (Wilke1999) as 0.10 MPa for standing and  sitting  0.55 MPa.  With  the currently advocated upright sitting posture there is an increase of spinal loading 500% above that of lying supine which is relevant to the 2T reclined mode.   For more detail see ☛ Loading→

  The intra-discal pressure is augmented In the flexed position, largely due to tension of the posterior ligaments,  while the anterior portion of the annulus fibrosis (AF) undergoes compression   In full flexion this can be as much as 100%.   The pressure gradient increases anteriorly,   tending to retropulsion of the nucleus pulposus (NP).

2. Axial compression + flexion. 

Disc prolapse  occurred more frequently when the vertebral segments were wedged to simulate extreme forward bending of the spine (Adams and Hutton, 1982). In this position, the anterior portion of the annulus fibrosis undergoes compression while the posterior portion is under tensile stress. Over 40% of the cadaver discs tested by Adams and Hutton (1982) prolapsed when tested in this hyper flexed posture, and with an average of only 5,400 N of compression force applied. This finding shows that the disc is particularly susceptible to bending stresses. In a later study in which Adams and Hutton (1985) simulated repetitive loading of the disc, previously healthy discs failed at 3,800 N, again mostly through trabecular fractures of the vertebral bodies. Taken together, these studies show that the disc, especially the vertebral endplate, is susceptible to damage when loading is repetitive or when exposed to large compressive forces while in a severely flexed posture.

3. Backward pelvic tilt. An effect of upright sitting.

When sitting there is backward tilting (anat; forward rotation) of the pelvis, which reduces or reverses the protective wedge angle of the lower lumbar joints  ☛(See Lumbar & spinal support→).

←Flexion  The axial loading force comes to lie posterior to the pivot point at the Ischial Tuberosities. A backward turning movement results in backward tilt in of the pelvis.

• The  backward pelvic  tilt flattens the lumbar lordosis and reduces the protective wedge angle of the lower intervertebral discs.  Resulting in  Posterior translation of the disc contents (NP),, see below→

• The loss of low lumbar lordosis reduces or even reverses the protective wedge angle of the lower lumbar joints
•  

4. Posterior elements

• Stretching of the posterior elements (including ligaments) which can become permanent and allow instability. (☛Ligaments & CTD→) & (see ☛ ‘Liagament integrity & creep’)→ 
• The likely forces that must be resisted by the ilio-lumbar and the supraspinous ligament (shown in blue) when sitting in the usual lumbar support seat, which allows backward tilting of the pelvis, with a bodyweight 40kg (excluding the legs) can be calculated.  Simple moments about the centre of the L5 disk suggests a ligament tension of about 70 kg (700 Newtons).  This is probably a worst-case estimate.   (JD Gorman)

5. Constrained upright sitting

The adverse effects have been long and variously described     Prolonged constrained static postures are uncomfortable and deleterious for both spinal and general health.   ☛Importance of MOVEMENT→.  Recently there has been interest in continuous small amplitude movement for upright chairs, the chair re-aligning with the users centre of gravity,  and termed  ‘Dynamic Seating’.     Exercise is required to maintain this position. This provides proprioceptive feedback and frequent small amplitude pressure changes which may be comforting for short periods and helps multifidus muscle action. Rani  Lueder  gives a review account (Lueder R 2002) and the referenced evidence→is considerable.

Recapitulation :-

For the scientific research  details, see ☛ bio-mechanics→

    See ☛ Remediation→ 

The mid-upright mode is the worst possible position. How did it happen?   ☛ Why? Mandal explains→

Advocates a Forward Tilted Seat (FTS).   The FORWARD TILTED SEAT (FTS).

The upright seat  is still recommended….Some of these diagrams suggest that iliac back support is incorporated.  This is a partial solution providing it is correctly configured. See ☛ Lumbar support→

The pMRI evidence

An investigation using Whole-body Positional MRI (pMRI), by FW. Smith, Bashir W (2007) who found that the upright position, at 90°, caused disc contents to move the most, while the relaxed position (135°/45° reclined) caused disc contents to move the least. This confirms that the upright position is the worst for the back, while the relaxed position is the best.   See  Positional MRI →

See  ☛  Biomechanics research for safe sitting.→