Sitting down is a natural human posture but it has its share of disadvantages. The posture causes slackening of abdominal muscles, curves the spine and impairs the functioning of certain internal organs.

But let’s face it ? an average office employee spends 50 to 90 per cent of his working hours on a chair, which is probably why scientific studies of furniture design have largely focused on chairs, especially office chairs.

Basic principles

The seat should be so designed that the posture can be changed frequently, providing enough free movement for upright and forward (spine slopes forward) postures to be changed periodically to a backward reclining posture that is supported by a backrest.

To avoid kyphosis (convex curvature of spine at the back that eventually leads to a hunched back) and back strain, work seats must be provided with backrests for the lower back to support the spine.

Too upright a posture with lordosis (convex curvature of spine at the front) will lead to muscular fatigue.

Office chair design

The chair must have good stability, and its four feet must be at least as far apart as the width and depth of its seat.

The office chair must permit any movement of the arms as may be required. Height of armrests should hence be optimised ? they should be low enough so that the forearms can rest easily on the armrests without having to raise the shoulders painfully and yet high enough so as to feel a gentle upward push from the armrests onto the forearms.

Height

Small to medium-sized adults are comfortable with a 16-inch (average) seat height.

But at this level, taller persons are compelled to push back their posterior and create an unhealthy stronger curve on the spine. That is why several readymade chairs have their seat height fixed at 18 inches with the presumption that shorter people can always use a footrest. This is not right.

We should insist on adjustable seats (not very expensive) that can be fixed at any position between 15 and 20 inches.

The height of the seat must be considered in conjunction with its work table. The vertical distance from the top level of the seat to the tabletop should be somewhere between 11 and 12 inches to provide proper elbow support when the arms are resting on the table.

A seat that is too low (or a table too high) will force the worker to raise the upper arm over long periods, creating static muscular fatigue in the arms and shoulders. If the seat is too high (or the table too low), the worker is obviously compelled to stoop forward.

Measurements

Seat measurements are important. Its width (side-to-side measure) should be kept at 15 to 16 inches, which may be increased for armless chairs only.

The seat depth (front-to-back measure) should be between 14 and 16 inches; less than 14 inches will not give sufficient support to the buttocks, while more than 16 inches will make you shift backwards and raise your feet off the floor.

Shape

Seats should be flat or slightly concave from side to side. Front to back, they should have a backward tilt of about 3 to 5 degrees that prevents a tendency to slide forward off the chair.

Traditionally, offices go for low backrests for junior staff and real high ones for seniors. If such hierarchy can be dispensed with, a backrest that is 22 to 24 inches vertically above its point of contact with the seat is ideal for everyone.

Tilt

The backrest that can be tilted backwards from 0 to 7 degrees is best suited to the shift from a normal sitting position to a sensible reclining position.

A tilt greater than 7 degrees is a no-no as it is bound to raise the feet off the floor and give a feeling of instability.

Backrest

The shape of the backrest is vital for orthopaedics. Just above the surface of the seat, the base of the backrest should either be open or at least so strongly concave that while sitting upright the buttocks can be rotated backwards easily.

Somewhere at 7 to 8 inches above the seat, the backrest should become convex to provide lumbar support; and then again, somewhere at the chest height, it should again become concave so that one can lean back into it comfortably.

Padding

Any work seat used for more than an hour at a time must have a light padding ? this is more conducive to comfort than any sophisticated profile.

All upholstery on the seat and back should allow the body to sink in to a maximum extent of 1 to 1? inches; very soft cushions are not recommended.

Upholstery should be covered with a material that absorbs body transpiration (water vapour and sweat) ? while polypropylene fabric and leather is unsuitable (especially in non air-conditioned spaces); ribbed PVC fabrics are fine. The fabric must be sufficiently permeable to allow all moisture to evaporate when the chair is not being used.

Revolving chairs

They must have a five-prong base, each fitted with a castor (wheel) and the castors could be lockable. Never use such revolving chairs over thick-pile carpets; the bare floor is best.

The author is a senior faculty member at J.D. Birla Institute (Calcutta), where she has been delivering lectures, guiding research and conducting projects in housing and interior design for over 20 years. An interior design consultant, she specialises in ergonomics at home and work. She can be contacted at kusumsmail@yahoo.com