Category Archives: Design

Design details

‘Two Tilt, (2T), ‘BiModal’ or (3M) chair

INTRODUCTION

OK. So you are interested in designing a chair which is ergonomically optimised. This involves a certain knowledge of ergonomics and the 2T solution to the problems. An overview is given here but you are advised to trawl through the main corpus of posts and pages.

So, what is wrong with an upright chair? See →The upright seated posture.

These adverse effects need to be addressed for effective REMEDIATION →

Written for designers, first see → Office Chair DESIGN, Ergonomics & Low Lumbar Backache→

Screen Shot 2018-12-03 at 17.20.27 2T so called as there are only 2 stable positions, each at either end of range.

Reclined mode. A fully supported reclined work position for prolonged work. .

40° from horizontal is assumed to be the most supine reclination as a work mode. Less becomes impractical, (except for the 4M workstation) and more loses some of the axial compression advantage.
For more details, see ☛The 2Tilt RECLINED MODE for fully safe sitting.→

Upright mode for certain short activities and quitting the chair and is the default mode when the chair is unoccupied. ☛2T Upright modes→
- UPRIGHT MODE & TRANSITION for the 2Tilt concept→
An unstable mid range for quick & easy transition between modes.

DETAILS & OPTIONS

The 2T chair consists of (essentially) 2 parts :-

Carcass shell
A base, on which the shell is mounted.
A tilt hinge mechanism.

THE CARCASS/SHELL

Of fundamental importance to achieve ergonomic optimisation with resulting comfort.
Reclined mode configuration the’shell’ components are intended to describe a seat’s supporting interface with the human body.
When completed the Shell Chair forms could be employed to assist in creating an actual shell-based seat in various possible ways:
Shows that a slightly flexed thoracic spine + the upper lumbar joints is not adverse.
An office version would be narrow with a small telescopic headrest to address height variation and to reduce footprint.
Iliac (Pelvic) support at correct level and shape to prevent a lumbar ‘sag’.
Incorrect configuration can engender discomfort (as can be experienced in certain dentists chairs) and more important is potentially dangerous.
Head support that ensures a forward vision field.
Variability at head and feet only.
Leg/foot support.
a foot rest of some sort is required in the reclined mode .
Ability to change rapidly to the upright mode.

The alternative upright mode

This alternative upright mode is for certain short tasks only. Relatively unimportant if the user can be relied on to avoid using this mode for prolonged use.
Better use iliac support or a Forward Tilted Seat .
- FTS (forward tilted seat, 20°) ☛ The FORWARD TILTED SEAT (FTS).→
- Iliac support . ☛ BACK-RESTS. Pelvic support v. Lumbar.→
- ☛ ISCHIAL OFF LOAD system→
With the 2T chair the upright mode is not maintained for long periods and so the feet can be easily tucked under the seat, making height irrelevant…as in the Balans chairs.

Iliac support is included to prevent lumbar spine sagging in the relaxed mode or if used in the upright mode.

Materials

Moulded alone in a thin rigid material (as with Aaro Aalto’s ply furniture or the Knoll Saarinen). Graphene is obviously relevant.
Moulded to form a base shell carrying relatively thin textile/foam skin padding (Eero Saarinen Knoll and Eames Vitra collection).
As a final form around which a tensile mesh or textile ‘hammock’ design could be generated (Herman Miller Aeron and Knoll Generation chair).
As a final form around which various upholstered designs could be generated (Knoll Eames or Herman Miller ranges)
As a final form around which other associated furniture or workstation accessories can be generated (modular office systems)
Consider Gordon Murray’s system consisting of tubes + composite giving extra strength

http://www.bbc.co.uk/iplayer/episode/b01f11hp/How_to_Go_Faster_and_Influence_People_The_Gordon_Murray_F1_Story/ (Note by Peter Bessey )

CHAIR SIZE

Largely determined by the the head-rest and foot-rest.

Shoulder width to allow people with broad shoulders to have full movement without lateral constraint.
In this assay the headrest is wide to include audio speaker components for home use.
Also ? size models. See http://www.hermanmiller.com/MarketFacingTech/hmc/solution_essays/assets/se_The_Anthropometrics_of_Fit.pdf . In spite of all their adjustments, bells and whistles, Hermann Miller still has 3 sizes. We should follow with something similar.

Pelvic (Iliac) support

Pelvic support, developed for upright chairs by John Gorman, an engineer and chiropractor, was in contradistinction to the prevailing ‘lumbar support’. Precise support, shaped to the curve of the iliac crest is applied to the posterior iliac spine and iliac crest of the pelvis. A slight forward
nudge at this point is mechanically efficient in extending the two lowest joints and prevents the pelvis rotating backwards. Pelvic support was designed for upright chairs. As a component of the cacass ‘shell’ it need only be minimal and not adjustable. Intended to Screen Shot 2017-05-09 at 17.12.05 prevent backward pelvic tilting when supine lying. It cease to be ‘pelvic’ and becomes the adverse ‘lumbar’ support above 20 cm from the seat -pan. Do not allow adjustment! See http://sittingsafely.com/pelvic-support/ →

The Forward tilted seat might be preferable.
See ☛ The FORWARD TILTED SEAT (FTS).→

ANGLES & Backrest variability

CAD drawings suggest that the backrest should be hinged to the seat pan to obtain optimal configuration in the 2 modes. Adds to manufacturing cost. These were explored by Peter Bessey as a theoretical exercise in 3D CAD, based on anthropometric (US) data from the well known Henry Dreyfuss book. It commenced around a simple 50percentile male mannequin which was readily available. The information from this investigation, should first be used to generate a totally adjustable Rig device, that should be used for trialling and true research purposes, before any final design concepts are prepared.

With the adoption of the 4M deskless, work-station model the constraints imposed by these angles become largely irrelevant See ☛ OFFICE WORK-STATIONS→

THE SEAT-PAN

2T reclnd Torso/seat interface pressure studies show that most of the body weight is concentrated at and around the ischial tuberosities. Very little at the thighs, except in the Forward TS mode. Only a ⅓ the thighs require support.

“stool height.” The term “sitting height” is reserved for the height to the top of the head when seated.
For men, the median popliteal height is 16.3 inches and for American women it is 15.0 inches.
The popliteal height, after adjusting for heels, clothing and other issues is used to determine the height of the chair seat. Mass produced chairs are typically 17 inches high.
For someone seated, the buttock popliteal length is the horizontal distance from the back most part of the buttocks to the back of the lower leg. This anthropometric measurement is used to determine the seat depth.
Mass produced chairs are typically 15-17 inches deep.

Diagram annotated from Mandal. The Seat height based on Mandal’s ‘Forward tilted seat’ (FTS),by 15-20°.

2 holes should be cut near in the wood base to limit slippage with a FTS seat. Position of ‘Butt-pockets’ showed by Okamura research (above).

Peripheral details

HEAD-REST

Essential for full torso support
Variability is required.
The headrest needs to be minimalised fo reduce the overall footprint.
onl.y the base of the skull requires support and the upper neck.
This only requires a height of 3-4” and a breadth of 9”
Measurements are shown below.

The head-rest should be adjustable in both

Longitudinal
A-P directions

Arm-rests

Are helpful for typing. They might limit some movement.
- How should they be mounted?
- On the carcass. This moves with the user and may be an advantage when modes are changed.

FOOT-REST

Optional only if the heels do not rest on the floor or desk bar. This can be arranged if the chair is lowered when in the reclined mode (as in the Okamura chair). I regard this as only acceptable if performed easily without complicated manual adjustments. ( Is it possible to have a mechanism that does this when the user leans backward into the reclined mode?) A SPRUNG SOLE MUST BE OPAQUE FOR ISLAMIC SUSCEPTIBILITIES.

A monococque design is probably unsuitable for an office work chair, but may be possible in a deskless design. Ideally the leg/foot-rest should retract or fold back when in the upright mode. A simple system is shown in the garden chair. A similar model can be bought cheaply at B&Q.

There is no need for the calf support. This could result in DVT. The lower leg bones (tibia& fibula) give the only support required between the thigh at the seat pan and the heel at the foot- rest, However variability of the leg-support is essential.

TILT-HINGE

To enable the transition between the stable end-ranges.
To allow range from 7-135º,
and that can be used for short periods as a form of comforting exercise, particularly for rehabilitation for an internal derangement at the low lumbar levels. (See ☛ http://sittingsafely.com/exercise/ ).
The intermediates are unstable.
Controls are avoided because fixed Mid ranges reproduce the conventional adverse sitting posture.
Movement is activated by user’s change of posture.

CONCEPTS
- - Concept1. Fulcrum at point of balance under seat, with damping to avoid jolt at extreme of range. Simplest,
  - Concept2. Hinge at the anterior border of the seat-pan, using a constant force spring.
  - This makes the fwd tilt the default mode. The damped spring allows the chair to fall back and to be lowered to the reclined position.
  - Spring tension would have to be adjusted so that user movements would activate the alternative range,
  - Also to allow for an user of unusual weight.

Thoughts on MONOCOCQUE solutions for home use

- 1/ A cushion for headrest. Fixed variably,
- 2/ Needs to be 2 holes (butt pockets) about 3″ from front edge.
- 3/ See detail notes for foot-rest
- 4/ 5Star base looks wrong for an iconic chair.
- 5/ The drinks flap could rotate to become a computer table.

Screenshot 2019-09-18 at 16.19.47 Details of the p3 prototype.

Screenshot 2019-09-16 at 12.07.58

To incorporate a heel-rest to the Aaron monocoque design

The tilt-in-space feature allows the whole chair to tilt up to 30 or 60 degrees, while maintaining hip and knee angles. Applies mainly to wheelchairs but note that also pertains in the 2T monocoque concept.

THE 2t BASE

All the 2T prototypes used a standard 5 star base as shown above. A 4 wheel base is easily designed and inexpensively manufactured to various standards. The subject is considered further in the post on the Base→

Screenshot 2019-09-29 at 17.04.48

For user ‘special needs’. A patient would have a shell moulded for the torso that would then be mounted on a suitable version of a 2T chair instead of the standard shell. This addresses the axial compression and pelvic tilt in the reclined mode. In the upright mode the backrest stand away from the torso with a Forward Tilted seat and so becomes irrelevant. But support might be needed and then we would have to think about Gorman’s Iliac version. This is already built into the standard backrest shell, in a reduced form. Plaster-of-Paris’ (POP) casts are commonly made in orthopaedic and physio departments and can be used to form a shell for the individual from fibreglass or other material at no great expense. 3D printing could be relevant. ☛ ‘Special needs’→

For some ideas & options see :-
Office Chair DESIGN, Ergonomics & Low Lumbar Backache
Various chairs. How do they measure up?
WORK-CHAIRS, a new breed with a reclined mode.
ROORKEE CHAIRS→

HOSPITAL VERSIONS

Hospital applications

I was warned in Cambridge to avoid discussion of medical applications of the 2T concept as this might detract from it’s major office use. However it could change the hospital experience for the better. It might also improve the NHS finances!

There seem to be 5 areas with overlapping needs for which a 2T Concept has a perfect application :-

Mass emergency. With the distressingly increasing frquency of international outrages which involve high volume of casualties which are tended while lying on the floor, a light emergency bed is required.
A Hospital resuscitation unit.
Hospital A&E trolley
Hospital bed
For specific hospital requirments such as neck traction.

OPTIONS & DETAIL NOTES

A lightweight bed for emergency use.

Screenshot 2019-09-26 at 15.54.24 It certainly has advantages over the beds illustrated, BLU-MED’s Portable Ward Beds for Mobile Hospitals has 1-inch aluminium tubing, Weighs only 17-pounds, including IV pole, mattress and decking and Folds to 32-inches x 42.5-inches x 4-inches. This is impressive but, in addition, a 2T version would be more versatile with a wheelchair mode giving easy mobility. The 2T prototypes also used 1″ tubing, slightly heavier, using steel, but probably stronger and more stable.

2. A 2T version of a trolley for A&E patients (gurney). A design similar to the p/1 prototype monocoque version is suggested. It is mentioned in the press (30/10/2019) that there might be a big demand for the new patients admitted to the NHS who might be left lying around for long periods on uncomfortable trolleys. A multiple use version is described below.

3. A 2T version of a bed. See Hospital beds→ for a plethora of models. A 2T version would be a major advance on most of them in cost, versatility and comfort.

4. Hospital resus. A similar model to the above but of more robust design. The ability to attend to the patient at a desired height and tilt is an advantage. Rapid position change, particularly the Trendelenberg position, needed quickly in acute episodes, is a major advantage.

5. Cervical traction. At present the patient lies supine and flat with traction from the skull to a weight attached over the end of the bed. This is prolonged and the position becomes uncomfortable, the constraint is a disadvantage and reading is difficult. With a 2T design the patient can be tilted to a more comfortable, less constrained, semi-reclined position without altering the traction.

6. Patient fitting. A version of the shell for specific needs for individual patients has already been described. A patient would have a shell moulded for the torso that would then be mounted on a suitable version of a 2T chair instead of the standard shell. This addresses the axial compression and pelvic tilt in the reclined mode. In the upright mode the backrest stand away from the torso with a Forward Tilted seat and so becomes irrelevant. But support might be needed and then we would have to think about Gorman’s Iliac version. This is already built into the standard backrest shell, in a reduced form. Plaster-of-Paris’ (POP) casts are commonly made in orthopaedic and physio departments and can be used to form a shell for the individual from fibreglass or other material at no great expense. 3D printing could be relevant. ☛ ‘Special needs’→

A multi-purpose version of a 2T trolley for A&E patients which provides a low cost product to help the NHS finances and, being ergonomically optimised, provides the greatest comfort , which can also be used

in a mass emergency,
for emergency resuscitation
a wheelchair
Temporary bed.

The Cambridge monocoque 2T Prototype 4, above, was optimised for prolonged office sitting. It also shows a good configuration for an ambulant or semi-ambulant patient. If visualisation of printed matter is not needed then ideally the reclination can be greater.

It can be used to illustrate of the variatons need for a trolley/emergency bed.

The 2T principle 8, for an office chair, has to be modified to allow Fixed adjustment stable positions in the transitional mode at the hinges.
- Requires hinges at
  - Back-rest/seat pan.
  - Seat pan/Leg-rest
This is needed to provide
- An upright (wheel chair) mode,
- A straight horizontal mode for a user to lie prone or on side.
- Also Retain the position ergonomically optimised (as above).
Length adjustment is required for headrest and leg rest (as in the 2T concept)
Rotation of, say, 20°, to allow a user to quit the trolley without having to move it away from the wall.

Advantages A 2T version has advantages
- Simple and inexpensive
- For storage and logistics it can be easily disassembled, sterilised and stacked.
- Allows an upright (wheel chair) mode,
- Semi-reclined mode. As for the 2T chair version, for comfort.
- The patient can lie comfortably supported when lying supine and can be in a sitting position..
- Strait, full, supine mode modification without semi-reclination is also needed.
  - Lying on side, fully prone or in the recovery position would then be possible.
  - The slight ‘iliac’ support would be lying above the pelvic brim and limit slippage if the head end is lowered.
  - The slightly elevated legs aids venous return. It can be addressed by increasing the seat/back-restangle or having these components hinged.
- The patient can be effortlessly put in the Trendelenburg position. ( the head is lower than the feet).
- The 2T principle 8 has to be modified to allow stable positions in the transitional mode.
- Manufacture can be inexpensive.
- - The BASE mounting
  - The 2T prototypes all used a standard 5 star base.
  - This is not essential but if used needs modification.
    - Rotation should be limited, for stability, to 35º in the forward direction.
    - The 2 back legs should be extended for stability when reclined.
    - They can be used to support a shelf.
    - the-2t-base/

From Peter Bessey :-

For a trolley to take most benefit from 2T, I would suggest that it not be fixed, but permit rotation when required. Achieving that and allowing for it safely, within the frame detailing, is the kind of functionality that is likely to take time to perfect. (I need persuading! Extra expense and unneccessary)

Attempting to also value engineer the design, so that it can be manufactured optimally for low budget purposes too, is also the kind of activity that takes time too. But I would anticipate that is needed equally for the NHS, as for disaster-relief instances in low-income countries.

The latter situations could also benefit from a stacking, or folding design, so that minimum volume is taken up during storage, particularly awaiting actual disaster instances, as well as for off-the-shelf use in standard hospital environments too. (The base has to be disconnected from the shell)

Once those types of considerations become desirable and part of the brief’s goals, then the task is somewhat more complex than ‘just a simple gurney’.

- See Hospital beds→ for design & a plethora of models.

Also see design-details/→

UPRIGHT MODE & TRANSITION for the 2Tilt concept

A 2Tilt concept Upright Mode is subsidiary but essential to allow the reclined mode to be fully and easily used in an office or work environment. An unstable Transition Mode has health benefits and it’s importance is described.

Upright mode

The 2T upright mode is subsidiary but required for certain short activities and quitting the chair and is the default mode when the chair is unoccupied.

The upright mode is for short tasks only and the configuration is unimportant providing that the intermediate, transitional range is unstable.

Maintaining the wedge angle of the lower two lumbar joints, in the upright mode, can be achieved by a ☛forward tilted seat (FTS)
or by correct use of ☛ iliac support. The latter cause additional design problems in this case and so is not recommended for an original design but may be convenient for modification of an existing model.
also see ☛the Ischial off load system→.

An unstable transitional mode.

2T Principle 3.Requirement 8. Mid ranges should be unstable and easily negotiated. It is necessary to be able to switch rapidly and easily from a reclined to the upright mode2T mid range.

This is mainly for convenience but the switch also results in a change of pressure on the disc and this has the added advantage of providing a pumping action which aids disc nutrition.
Their instability can be used for short periods as a form of exercise ( ☛dynamic seating→)
This is not just a quick transition from the reclined to the upright modes (2T) but the instability provides physiological benefits and is itself a mode. The concept is renamed the Tr-Modal (3M). 2T = 3M.

2Tilt Principle 3 Requirement 9. No adjustment is allowed to maintain an intermediate position. The adverse intermediate upright position 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?

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.

Spinal movement & change of position are important for a number of reasons including IV disc nutrition. ☛ Movement & exercise→
Particularly for rehabilitation of the Multifidus (☛ muscles→) following an internal derangement at the low lumbar joints.
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.

‘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→

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! Further described in WORK-CHAIRS, a new breed with a reclined mode→.

Next, return to The 2Tilt RECLINED MODE for fully safe sitting. → or check the effects of The upright seated posture. →

ADJUSTMENTS?

A plethora of adjustments is confusing and can be adverse if adjusted wrongly. Ideally they should be avoided as far as is possible.

Chair designers place emphasis on variability and adjustability for greater comfort.
“One size does not fit all. The amazing diversity of human form requires a chair that can adapt to different shapes and proportions as well as different heights and weights.”

The variability of the torso shown in these body types is mainly due to the soft tissues. There is surprisingly little variability in the underlying supporting skeleton of the torso. A greater understanding of spinal anatomy shows that variation occurs at the periphery – the long (limb) bones and the neck.

Adjustments should be avoided, as far as possible, except for the head and feet and possibly height of seat. As variation in size of the skeleton of a human torso is not great except for the soft tissues and to the long (limb) bones there is no need for variability to be built into the chair parts that support the torso. With most chairs these are adjusted into the wrong position! This flies in the face of chair designers who are searching for the Holy Grail of infinite adjustability an entirely misplaced endeavour and merely another example of ‘familiarity bias‘. If a manufacturer prefer a plethora of adjustments, although increasing costs and degrading the green footprint, this does not entirely negate the 2T principles. Although as far as possible, adjustments should be avoided, the footrest position must adjust to leg-length.

Scrapping adjustments

A number of adjustments can be scrapped. They make the whole system confusing and fiddly, not to mention, expensive. As shown under the 2T concept the only adjustments required with an anatomically correct design are for the length of the foot-rest, the height of the seat, and vertically and A-P for the head-rest. Other adjustments should be viewed with scepticism and not as an advantage. `Scrapping adjustment controls would make any chair more friendly to the user. Win-Win for everyone including office managers! Manual adjustments are equivalent to double declutching in motoring. Days long past! Misconceived regulations may constrain this concept in some respects but can usually be circumvented.

3D printing in the manufacturing of a 2T chair would allow a ‘ bespoke, tailor-made’ version for individual users and is particularly relevant for those with ‘special needs’.

Variation at the head and neck

In the reclined mode a line of sight to the desk top is required which must accomodate axial variations in neck length and a degree of cervical flexion. The last can be obtained by reducing the reclination to a more upright configuration which loses some bio-mechanic advantage. The upper lumbar/thoracic kyphosis can safely be increased as it spans some 14 joints. The thoracic joints, splinted by the ribs, is regarded as unable to flex but may be able to achieve about 5° overall and more at the upper lumbar joints. This increase in flexion allows less cervico-thoracic flexion to ensure a horizontal line of sight.

The adjustment for the head-support should be at a lower thoracic level which would allow flexion to extend over these extra joints. It has an added advantage by ensuring a correct level for support of the shoulder girdle.

With a FTS in the upright mode the headrest becomes redundant.

Pelvic support

The initial requirement of the 2T concept is ‘a reclined mode with correct support’. The word ‘correct’ should be emphasised. There is an unnecessary and unfortunate tendency, in more expensive chairs, to enable pelvic support to be varied in height. This is unnecessary as there is little variation in the height of the iliac crest above a flat seat. Adjustment should be scrapped so that it cannot be adjusted upwards into an adverse height above 20cm when ‘pelvic’ support changes to adverse ‘lumbar’ support. In my experience adjustments are usually made in the wrong direction. This was the case with the HM Aeron chair where the pelvic support should be only used at it’s lowest position. Even this is high for a smallish person. This height might only slightly reduce the mechanical efficiency for an unusually tall man but will not harm a small woman Anecdotally, this is certainly my experience when looking at many ‘ergonomic’ chairs. Invariably the adjustment allows support above 20cm and is commonly so mis-adjusted by the user. In some chairs the depth of pelvic support can be adjusted using an inflatable cushion similar to a sphygmomanometer. Like any other it can be wrongly adjusted by the user searching for ‘comfort’.

The design concept appears misconceived. Only a nudge is required to prevent backward tilting of the pelvis and thereby comfort and, more important, the preservation of lordosis is ensured. Manufacturing costs are reduced. So scrap it!

(by HAS). (by Wicketts D, after Reynolds et al. 1982153)

For pelvic (Iliac) support the pressure is directed to the posterior superior iliac spine at 168-146 mm. above the seat (+ 15mm for the soft tissues). This distance does not vary greatly between individuals, +/- 22 mm. among the adult population and for this small amount of variability no adjustment is required. 168 mm. iliac support would be optimal for a large man but adverse for a small woman. The importance of this measurement for chair design has not been appreciated and further assessments would be welcome. Three models may be required to cover the top percentile of very large men, the middle percentiles and the bottom percentile for a very small woman.

In the reclined mode of the 2T (3&4M) solution→ , iliac support is only required to prevent the lumbar lordosis from flattening. In the upright mode it plays no, or little, part with a FTS but is important and has to be correctly configured if the seat is flat & horizontal or in the ‘ischial off-load’ system.

The height of a chair is important for an upright chair, but less so for a fully reclined chair. Anthropometric tables for popliteal (below knee) height has includes work by Pheasant (1990) and in a more homogenous group of women in the US forces of whom 35% required seats lower than 16 inches (Gordon 1988). A study (Takeoka 1991 ) of 200 Japanese women concluded that office chairs should adjust down to a height of 141⁄2 inches. Mandal, for a 671⁄2” (172 cm) girl, found that a tilted seat height of 25” gave a hip angle of 42° and required a table height of 36” (91 cm).

Much of this becomes redundant in the context of the 2 Tilt principles. In the reclined mode the variability in the human torso can be contained in a single construct, provided that there is vertical adjustment for the head and feet. Adjustments may be required for individuals of unusual shape or have deformities, for example permanent adjustment for the width between armrests. Shifting the position of the tilting fulcrum may be required and this adjustment should be effected with an Allan key or spanner by someone who understands anatomy. My observation that most chairs are adjusted into the wrong direction is confirmed by an article in Hermann Miller (research). Three overall sizes can be allowed! An account of kinematic reclination in the Aeron chair can be seen on the HM website→

Tension control & adjustment

Commonly advertised as a selling point. I am not sure why. The monocoque P3 at MfI, Cambridge had a simple fulcrum tilting method. I was expecting this to need adjustments but a trial with a large available porter made no difference to the movement. Would an engineer kindly explain?

And other considerations. Next see, ☛ Comfort→
☛ Exercise & movement→

Sitting Safely?

The spinal considerations. SOME ESSENTIAL NOTES ON THE BIO-MECHANICS for CHAIR DESIGNERS, OFFICE MANAGERS, USERS & BACKACHE SUFFERERS.