On Ergonomics and Chairs for Computer Geeks
I got fed up of getting numb buttocks, itchy anus, stiff wrists and elbows and aching neck muscles when I sat in front of my computer for more than a few hours. My symptoms would ease as soon as I relaxed in a easy chair, and I can sit for eight hours a day behind a steering wheel with few problems.
I wanted to replace my mid-priced standard task chair with something more relaxing and supporting, like the seats in my van. However while more money brings more and fancier adjustment controls, more expensive looking fabric, and thicker cushions, there is but one basic design.
Standard task chair design and recommendations
The standard chair has a level seat and a low back support, a swivel base with adjustable height resting on five castors arranged in a circle. Armrests and upper back support are optional.
This chair is designed to
- encourage an upright posture to minimise load on the back muscles
- ensure stability regardless of the orientation of the base.
- make access to and from a desk knee well easy.
- face the work in hand wherever it is on the desk, without having to shift the castors.
Recommendations for use aim to
- keep the head upright and balanced on the neck to avoid muscle strain,
- keep the wrists straight to avoid carpel tunnel syndrome
- avoid supporting the upper body on the arms.
Chair arms are intended to raise the arms and encourage good hand position when typing,. A footrest is recommended for comfort and to prevent the lower body sliding forward (although the chair can slide backward on its castors to the same effect).
The traditional task
The task for which this chair is designed is basically copy typing. The emphasis is on keyboard use and the horizontal surface of the desk, and the assumption is that the operator uses touch typing, and so the large angle between the screen and keyboard in unimportant.
The new task
The task I am addressing is primarily reading the screen, with sporadic bursts of keyboard or mouse input. For example ‘surfing’ the world wide web mostly involves about four mouse clicks a minute. In the 1970’s remote terminals with limited bandwidth used a channel for output to a screen which was 16 times faster that the one for the keyboard, because most computer users output far more data than they enter
The ergonomics of the display.
The display must be close enough for the operator to resolve all detail, which for standard screens and normal vision means under about 1 metre away, and far enough away for the whole screen to be seen with minimal eye movement. Wearers of bifocal and varifocal spectacles have especial constraints in this respect. Changes in focus between screen, copy and keyboard also need to be minimised.
The screen should be positioned so that the lower edge is at least as close to the eye as the upper edge, the eyes prefer to focus in as they move down.. Studies have shown that while setting the monitor 15degrees or more below the horizontal and angling it to face the operator results in low postural strain, eye strain and eye dryness, it also results in excessive glare from the room lights, destroying the benefits.
However this assumes that he head is held in a balanced vertical position, the angle can be achieved without tilting the monitor up at all if the head is tilted back and supported on a head rest.
The ergonomics of the mouse.
The mouse is held under the dominant hand (i.e. right, if right handed) on a horizontal flat, smooth, clean pad about 20cm wide while clicking buttons with two or three fingers. If this is positioned badly then the operating arm will carry its own weight on the wrist, plus possibly somewhat supporting the upper body. To minimise the continuous load on wrist and elbow, the pad should be at elbow height (making the forearm horizontal) and the wrist should be supported about 2cm below the height of the top of the mouse, taking the forearm weight off the wrist and making the latter a straight as possible to minimise pressure in the carpal tunnel.
The ergonomics of the keyboard.
The keyboard should be presented with the home keys (g & h) around the body centreline. The standard ASCII keyboard extends 32 cm to the right of this line, and the numeric keypad overlaps the ideal mouse pad position.
Unlike the mouse, the keyboard cannot carry the arm weight and so the outstretched arms will quickly tire unless the wrists are supported. The load can be minimised by sitting well forward with the upper arm vertical, but even the weight of the forearms alone eventually results in the operator’s wrists sagging onto the desk and angling up to reach the keys, throwing the carpal tunnel under stress.
The chair
My conclusion is that the ideal computer chair is something like a high-backed van seat, bucket shaped with a headrest and an adjustable reclining back typically set to about 20 degrees, mounted on four small castors.
My prototype is built from a scrap van passenger seat. It has one arm on the right (for a right handed user) which is fixed to the chair back and seat with a parallelogram motion. This carries a mouse mat and the keyboard is cantilevered from it in front of the user.
The keyboard is set above the mouse pad to resolve the conflict with the numeric keypad and angled so that it is based on a line parallel to the forarms.
The chair is intended to be positioned about 50cm from a wall-mounted monitor. As the chair is entered from the left side, there is no need to move it in normal use, so it is supported on small rollers rather than castors.
Future additions
- There is no constraint on the orientation of the chair. My prototype has the seat base inclned back about 10 degrees. This angle could be increased right up to the so called zero-g position with no change in design except that the arm has to be moved to keep it horizontal, and the seat height will need to be changed.
- The keyboard support can be extended upwards to make a document rest, like a music stand.
- A footrest could be usefully added. With shallow seat angles, there is a tendency to slide forward, with steep ones some leg support would be desirable.
- Loudspeakers can be fitted to the headrest for surround-sound.
- A small shock-absorbing pad under the keyboard would give a nice typing feel. The prototype's keyboard does not vibrate much but it is perceptible when typing fast. This is mainly due to resonance of the cantilever mount. An absorbent pad would significantly reduce the coupling of the key strokes into the arm and damp the resonance.
Left handed considerations
Left handed operators will require the mouse on the left. This basically means that the seat has to be constructed in mirror image, with the arm on the left side. As the standard keyboard right-handed, i.e. it has the numeric keypad on the right it will protrude beyond the open (right) side of the chair.
Design for Patent
Having used the prototype for seeral months I have now filed for a patent on this design. The key features are the parallelogram action of the chair arm and mouse pad, and the cantilever support for the keyboard. In order to make a manufacturable product which can be reassembled for left or right handed use I have added a short elbow rest on the left side, primarily to blank off the fixed bracket needed for the left-handed arm. This is curved to avoid the need for another parallelogram. I have put a horizontal joint in the cantilever so that the whole structure can be built from reversible components.
