Office chair

An office chair around 1995

The development of the office chair led from the simple and rigid wooden chair for the workplace at the desk to the modern swivel chair with backrest and, depending on the requirements, also with armrests mounted on chair castors or carpet glides. It is optimized for use on desks and is height adjustable for ergonomic reasons . The height is adjusted with a gas pressure spring . A height and inclination adjustment of the backrest is usually provided, often also an inclination adjustment or a sitting ball-like, i. H. horizontally rocking and / or swinging mounting of the seat. Due to the storage on castors, the chair can be easily moved while sitting.

history

Office chair made of wood for staff, in use from around 1910 to 1960

Office chair for supervisors (change in height by turning), in use between 1920 and 1960

The hour of birth of the office chair is unknown. In the 18th century, seated work in the office was likely to have replaced the standing work of an accountant. The change will also have been associated with increasing industrialization , also justified by the spread of administrative activities in the face of growing bureaucracy , which led to an increase in workplaces with desks and office chairs.

The first office chairs were simple wooden constructions without armrests, which were quickly followed by a construction made of metal with a wooden seat. The first office chairs were not upholstered; later they were upholstered by a simple horsehair-filled cushion attached to the chair . Around 1840 there were office chairs with suspension, which were followed by more flexible designs around 1850. At least one American office chair model from 1849 has been photographed that was already equipped with armrests and rotatable. The model can also be rolled, but it is a very small iron roller that is not very gentle on the floor covering and carpets. At that time, office chairs only had four legs or a base with four castors.

In Germany, the introduction of a standardized wooden office swivel chair without castors is likely to have been around 1912 . The height was adjusted by turning the central axis designed for this up or down (model Federdreh from Stoll / Sedus).

The cantilever chair from 1928 by Marcel Breuer occupies a special position among office chairs . In later years, the front vertical of the cantilever chair was sloped slightly backwards in order to prevent the chair from tipping forward.

In the 1970s and 1980s, the requirements for office chairs changed significantly. The employers' liability insurance association , in particular the administrative trade association (abbr.VBG), and the Federal Institute for Occupational Safety and Health (abbreviated BAuA) placed the focus on safety aspects (accident and tipping safety) and ergonomics for health reasons. The office swivel chair received countless changes in the mechanics of rotating and tilting, the suspension and lumbar support , as well as the more breathable upholstery, but above all the introduction of the safe five-legged base with plastic castors or plastic carpet glides.

design

In 1969 the designer Rainer Bohl constructed an adjustable backrest that became part of the Relaxoflex synchronous office chair developed by D-Team in 1970–74. The designers Klaus Franck and Werner Sauer developed a flexible office chair with synchronous automatic that adapts to the user: the FS line (1980), which has received numerous awards. Even Herman Miller Embody chair (2008), Maarten Van Severen 04.chair (2000) and Konstantin Grcic's office chair 360 (2009) are among the award-winning office chair designs.

Normalization

• DIN EN 1335 - Office furniture - Office work chair
• DIN EN 1728 Furniture - Seating furniture - Test method for determining the strength and durability
• DIN 4550 office furniture - Self-supporting seat height adjustment elements with energy storage for office work chairs - Safety requirements, testing
• DIN SPEC 1133 office furniture - office work chair
• DIN 4551 office furniture; Office swivel chair with adjustable backrest, with or without armrests, height adjustable
• DIN 68877 industrial swivel chair - safety requirements, testing
• DIN 68878 chairs for the living area - functional properties - requirements and test methods
• ANSI / BIFMA X 5.1 - General-Purpose Office Chairs - Tests

Chair castors

Castor wheels on an office chair

With some office chair models, the castor wheels are braked when the chair is unloaded. This prevents the chair from rolling away with a slight bump before sitting down. When the chair is loaded, the chair castors are unbraked. Hard floor rollers are softly coated and always two-colored for easy identification; The coating prevents dark lines from being created by abrasion on hard floors (parquet, tiles). However, since such a cover can become so soft on hot summer days that when the chair is used by very heavy users (100–120 kg) it can “stick” to comparatively “hard” carpeting and then pull threads out of it when rolling still (always one-colored) soft floor rolls made of uniformly solid plastic. In order to reduce the rolling resistance on carpet, chair castors with a diameter larger than the usual 50 mm are advantageously used, e.g. B. with 65 mm diameter. Larger rollers press less into the carpet, flex it less and are easier to align in a new direction of movement. In the case of chairs for light people (children), the role type is usually not differentiated; the softly coated rollers are then used universally for all floors.

Base

The base, the "foot" of the chair to which the chair castors are attached, is called the base. Cross, because the base used to have four arms and therefore looked like a cross. For reasons of stability, the base now has five or six arms. The center of the base accommodates a gas pressure spring with which the seat height of an office chair can be adjusted. A chair mechanism with seat, armrests and backrest is mounted on the upper end of the gas pressure spring. In the case of purely plastic footsteps, a sagging of the gas pressure spring down to the floor is observed over a longer period of time. Reason: the central conical hole in which the gas pressure spring for height adjustment is located, gradually widens - especially when there is a sudden load when sitting down. Better plastic star bases also contain a metal ring in the central conical bore, which prevents this sagging because it cannot be widened. Alternatively, aluminum base crosses are used, the surfaces of which are polished or coated with a color. A chrome plating is usually omitted from price and environmental concerns today.

Mechanics and their effects

Rocking mechanism and synchronous mechanism

Office chair with rocker mechanism, functional principle

With a rocking mechanism , the backrest and seat are firmly coupled so that the seat always tilts when you lean back. The angle between the backrest and seat remains constant.

Lowering the seat when leaning back prevents the backrest from giving the pelvis / back an impulse forwards every time you sit up. This, in turn, prevents the lumbar spine from resting against the backrest after a short period of time, resulting in a poor posture.

When swinging back and forth in a chair with such a mechanism, a rocking chair effect can arise, for which there are essentially two causes: Either the axis of rotation is fairly in the middle of the seat, then the knees come up when leaning back. Or the contact pressure of the backrest (generated by a gas spring, springs or torsion bar) is too weak, so that the seated person hardly experiences any back support from a medium recline position up to the maximum backrest incline. Modern seats therefore rotate at the front edge and have an individual adjustment of the backrest counterpressure.

The rocking chair effect is avoided by means of a knee rocker mechanism , as the axis of rotation around which the seat and backrest move is shifted forwards by a knee joint. The disadvantage of rocking mechanisms is that the angle between the seat and the back is rigid, as this unnecessarily restricts the movement of the seated body. This is avoided with a synchronous mechanism:

The synchronous mechanism, like the rocking mechanism, couples the backrest with the seat surface, but an inclination of the backrest typically causes the seat surface to incline somewhat less in a ratio of 3: 1 to 2: 1. For example, if the backrest is tilted backwards by 10 °, the synchronous mechanism ensures that the seat surface tilts downwards by typically 5 °. This coupling increases the angle between the upper body and thigh when leaning back, so that the joints move, the body stretches and the blood circulation is facilitated. In addition, the support point of the pelvis should be better preserved.

The great advantage of the rocker and synchronous mechanisms compared to mechanisms with static or only oscillating backrests (see below) is that the seated person can move around according to the work requirements and yet the back is permanently protected against incorrect posture. Since the mobility when sitting on a synchro-mechanism office chair is enhanced by a pendulum seat, the European Patent Office assessed the "class of synchro-mechanism office chair with pendulum seat" as an invention, regardless of the special construction.

Asynchronous mechanics and permanent contact mechanics

With the asynchronous mechanism , the backrest and seat are decoupled. The seat and backrest can be adjusted separately, which enables the angle of inclination of the backrest and seat to be individually adjusted. If the backrest is unlocked, it swings freely and is kept permanently in contact with the user's back by a spring. If such a mechanism does not allow the seat to be adjusted, it is called a permanent contact mechanism (often abbreviated to “PC”). The previously mentioned “rocking chair effect” is missing.

Shirt pull-off effect

If the backrest slides along the back when leaning back, it tends to push the shirt of a user upwards. This so-called "shirt pulling effect" should always be as small as possible. This requirement applies in particular to asynchronous and permanent contact mechanisms, in which the angle changes between the seat surface and the backrest and thus the shirt pull-off effect are particularly large. The shirt-pulling effect is less when the pivot point for the backrest pivoting movements is far to the front - if possible before the position of the gas pressure spring, which is used for height adjustment.

Pelvic contact of the backrest when leaning back

There are technical requirements for office chairs that want to receive the so-called "ergonomic seal". This list says u. a .: " Backrest : Loss of pelvic contact when leaning back: as little as possible." With the golden rule "butt behind" - d. H. slide towards the backrest - the loss of pelvic contact when leaning back can be minimized. Some office chair manufacturers avoid any loss of pelvic contact when leaning back - and thus permanent support for the lumbar vertebrae area, which is so often affected by herniated discs - by using two swivel joints for the backrest. One is at the bottom of the mechanism, another behind the backrest halfway up. This achieves “active lumbar support”, because when you lean back with your upper body, the lumbar vertebral area is supported more or less, depending on the pressure of the upper body against the backrest.

Backrests

Padded

The shape of the padded backrest should be ergonomically adapted to the back. For control purposes, the backrest is locked in an almost upright position: when the seated person sits down with their pelvis on the back of the backrest and then leans against the backrest, the upholstery should replicate the shape of the back. Fixed upholstery forms a standard back according to DIN regulations, which, however, rarely corresponds to the individual curvature of the back. The resulting punctual pressure pain tempts the seated person to avoid the backrest by sliding forward, which makes incorrect posture possible. Adjustable lumbar supports (in height, e.g. above the height of the backrest, in strength via adjustment wheels, air pumps, etc.) avoid pressure pain. There are many variants with different effects of passive, adjustable lumbar supports that are located on or in the backrest upholstery in the lumbar spine area. A permanently locked backrest is also not recommended for individual lumbar spine support, as an ergonomic office chair should restrict the body's urge to move as little as possible.

With mesh covering

The inexpensive backrest contains a mesh covering. In order not to allow the back to freeze with light clothing in summer, it should be provided with a windproof cover. Like all fabric coverings, nets are particularly flexible and - in contrast to upholstered backrests - no point elasticity. Like tennis rackets, they have to be particularly tight in order to maintain their shape under load. A mesh-covered backrest should also support the lumbar vertebrae well; At the same time, however - because of the lack of padding - it must not press hard against the back with the lower edge of the mesh frame. It is difficult to meet these two requirements at the same time; for example, the mesh-covered backrest could have a very strong curvature against the back, so that the lower edge of the frame is several centimeters away from the lumbar spine.

Two-part

The vertically split backrest supports the back over 50 percent according to the expert opinion of Prof. Peters, Dr. Bird. The principle consists of two movable oscillating elements and the arrangement of the backrest, which, by leaning against it, lift the lower back area using the leverage. In addition, the vertically split backrest follows every movement.

There are also horizontally split backrests. Since both halves of the backrest can be moved separately, the mobility of the body is increased while leaning against it and the back support is improved.

Dynamic sitting

An office chair can significantly influence the dynamic of sitting. While a rigid chair induces static sitting, an office chair can support more dynamic sitting through its mechanisms and adjustment options. A distinction is made between active-dynamic sitting (where the person sitting changes their sitting position on their own initiative) and passive-dynamic sitting (where the change in position is dictated by the chair).

Active-dynamic sitting

In order to alleviate or even remedy back pain that is triggered by not relaxed, static sitting, doctors recommend active-dynamic sitting as the simplest measure. The sitting position is not rigid, but is constantly changing. This is made possible by movable seats and backrests. On the one hand, a user trains the supporting muscles of his spine, which prevents back diseases, and on the other hand, frequent changes between load and relief improve the nutrient supply of the intervertebral discs. Chairs with the above-mentioned types of mechanics promote active-dynamic sitting, predominantly one-dimensional, forwards and backwards.

The so-called exercise ball promotes active, dynamic sitting; However, it has no backrest (so it is not a “chair”) and requires the user to constantly balance, which many find too strenuous when sitting for long periods of time and some as reducing concentration when doing office work (especially computer work ). As a result, exercise balls have almost disappeared from the offices.

Various modern pendulum and balance mechanisms are based on the so-called "foot-controlled movement". The seat is suspended in them like on a swing and / or is mounted so that it can be moved all around. These seats swing and / or swing minimally, as there are e.g. B. on a swing it is impossible to keep the upper body, legs and feet in absolute calm with one another; so these movements are generated unconsciously. In the case of chairs with swinging seats, they can be triggered by arm movements; This unrest in posture is dampened by placing the arms on a desk. The movements are intended to control the muscles to counter-movements and promote the circulation of blood in the bloodstream .

Passive-dynamic sitting

Alternatively, a passive-dynamic (externally controlled) movement can also be useful, as has been successfully used in medicine for years (e.g. motorized rail for joints ). In chairs, the basic principle is an externally driven movement of the seat surface by an electric motor - e.g. B. their microrotation - to force the constant movement of the seated person and thus the alternating stress on the spine, which is important for the nutrition of the intervertebral discs . However, there is still disagreement in sitting research as to whether passive-dynamic movement makes sense in view of the superior prophylaxis of alternating sitting, standing and moving.

Seat dynamics as a decisive factor for ergonomics

At least since the temporary triumph of the Sitzball in the early 1990s, it has become clear to the public that sitting as dynamically as possible brings great health benefits. Consequently, the “Institute for Hygiene and Work Physiology of the Swiss Federal Institute of Technology Zurich” (ETHZ) defined “What is an ergonomic chair?” In 1995. This document is based on the knowledge that long periods of sitting still lead to pain, i. H. every body needs exercise. Accordingly, the ETHZ calls for ergonomic office chairs etc. a. that they restrict the freedom of movement of a seated person as little as possible. This font is more relevant today than ever.

In general, sitting promotes efficient work, but even long periods of sitting on an ideal office chair is only the second best solution. It is better to organize work that leads to a change of posture in between: For example, standing desks / tables as well as cupboards and printers lead to health-promoting standing up outside of the immediate reach.

Adjustment options for ergonomic office chairs

Modern office chair with a large backrest, armrests and adjustable headrest

An ergonomic chair can be "personalized" with numerous setting options:

• The seat height is adjustable (with a lever that acts on a gas pressure spring)
• The armrests are height adjustable, also in the width between them and the direction is adjustable. There are also armrests that can be turned backwards (behind the backrest)
• The seat inclination of many synchronous mechanisms can be adjusted in two stages in addition to the synchronous mechanism.
• It is particularly important for smaller or larger people to be able to move the seat back and forth or to have seats of different lengths ("deep"). The seat depth should be set so that the backrest can be touched, but the hollows of the knees are free so that the blood circulation is not obstructed there.
• The height of the backrest should be adjustable so that the back can be supported from the top of the pelvis.
• There is usually a locking lever that can be used to prevent the chair from swinging. The lock should be used occasionally in order to stimulate / encourage the holding muscles of the body by not leaning on the chair when the chair is locked.

Office chairs in special designs

Several manufacturers offer so-called saddle chairs . The seat of these chairs is curved upwards. They therefore press on the buttocks and thighs and do not comply with the European office chair standard DIN EN 1335 1-3. The need to revise the office chair standard is supported by many studies. These studies show that the joint angles of the body when sitting in a chair according to the DIN EN 1335 office chair standard cause health risks for the musculoskeletal system .

Another special design is the 24h chair or control room chair. These are chairs that have been specially developed for shift workplaces where mainly monitoring activities are carried out (e.g. control centers , control rooms , security centers, air traffic control ). These chairs are particularly robust and have a reinforced mechanism, which is designed for permanent loads (24 hours / day, 7 days / week) and frequent adjustment processes (the chair is readjusted with almost every shift change). The cover is made of a particularly abrasion-resistant material. Due to its lower breathability, leather is only used on the side and back of the chair. A lumbar support is also part of the standard equipment of a control room chair. Control room chairs are often based on seats from the automotive sector, as these inherently have a very robust basic construction.

Recommended sitting time

Office chairs are provided by the manufacturers with the indication of the recommended sitting time. This value is specified with hours per day and describes how long the maximum time you can sit on the model without causing a disruption of wellbeing or possibly long-term damage to health. Office chairs for private use have a recommended sitting time of two to four hours a day. Models that are used in office workplaces that are used all day should be recommended for a sitting period of eight hours per day. A list of the ergonomic requirements for an office swivel chair that is qualified in terms of occupational medicine can be found at the " Federal Institute for Occupational Safety and Health ".

literature

• Elke Trappschuh: The office swivel chair by Klaus Franck and Werner Sauer. Verlag form, Frankfurt am Main 1998, ISBN 3-931317-24-2 .
• Klaus-Stephan Otto, Thomas Speck von Gabler (ed.): Darwin meets Business: Evolutionary and bionic solutions for the economy. Gabler-Verlag, Wiesbaden 2011, ISBN 978-3-8349-2443-8 .

Web links

Commons : Office chair  - collection of pictures, videos and audio files

Wikisource: Announcement regarding the establishment of seating for employees in open sales outlets (Germany, 1900)  - sources and full texts

Wiktionary: Office chair  - explanations of meanings, word origins, synonyms, translations

Wiktionary: Swivel chair  - explanations of meanings, word origins, synonyms, translations

• What is an ergonomic chair? ETH Zurich , September 6, 2000, archived from the original on November 30, 2001 ; Retrieved August 27, 2010 . 
•  ( Page no longer available , search in web archives )

Individual evidence

1. ^ Vita Klaus Franck , accessed on August 24, 2011
2. ↑ Maarten Van Severen in the English language Wikipedia
3. ↑ DIN EN 1335
4. ↑ ANSI / BIFMA X 5.1
5. ↑ Representation and explanation of seat mechanics for office chairs with permanent contact, synchronous mechanism or rocker mechanism, accessed January 23, 2020
6. ↑ Computer and office workstations: guidelines for design. (PDF; 3.6 MB) (No longer available online.) VBG and BAuA, St. Augustin, archived from the original on June 13, 2013 ; Retrieved May 14, 2012 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
7. ↑ Christian Rothe: OSH of A-Z . ISBN 978-3-448-10084-6 , p. 171.
8. ^ AC Mandal: Balanced sitting posture on forward sloping seat. Retrieved August 23, 2010 . 
9. ^ Hanns Schoberth: Sitting posture, seat damage, seating furniture . Axel Springer Verlag, 1962, ISBN 3-540-02904-4 .
10. ↑ List of ergonomic requirements for office furniture and work equipment 3. Ergonomic requirements for office swivel chairs. (No longer available online.) Federal Office for Occupational Safety and Health, archived from the original on July 8, 2013 ; Retrieved June 27, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.