12 January 2010
Duncan Abbott

Workers are the focus of ergonomics, where the intention is to prevent unsafe, inefficient and uncomfortable work practices by taking into account their physical and psychological capabilities and limitations.

Those workers come in all shapes and sizes, but most workplace equipment is designed for the mythical “average user”. Health and safety managers don’t have the luxury of assuming the people they have responsibility for will conform to the average; their risk assessments must cover short, tall, obese, disabled, old or young workers and make reasonable adjustments to help ensure they avoid injury or illness. In this article, we’ll concentrate on accommodating small, tall and overweight workers.

Designers rely on anthropometric data which gives them normative whole body and limb sizes to represent the human shape in designing the fittings and features of the workspace. The same anthropometric data will help health and safety practitioners tasked with accommodating workers to set up work areas that will allow the greatest number of people to work safely and make work systems usable. The data is valuable in determining appropriate dimensions for optimum fit between people and the work environment.

Measurements of different body parts, and the effect that these have on the way things are designed, are why anthropometrics is a very important aspect of ergonomics.

For a full list of anthropometric tables and an explanation of how to use them, the best reference is the book Bodyspace – Anthropometry, Ergonomics And The Design Of Work by Stephen Pheasant (available from www.crcpress.com for £43.78), a handbook widely used by ergonomists but easy to read for nonspecialists.

Tall story

Too often, tall workers are expected to work at the same workstations as average users. They are issued with the same chairs and a standard 72cm-high desk. In a display screen equipment (DSE) assessment, many assessors will  look only at the angle of the upper limbs and, providing these are at 90 degrees to the work surface, not bother with the lower limbs. This is wrong; the assessor must take the user’s seat away from the desk, so the chair can be set at the right height.

The user’s pelvis should be angled about 10 degrees higher than the knee joint, ensuring the pelvis is in the optimum position when the user is seated. If the knees are higher, the pelvis will rotate backwards and flatten out the curvature of the lower back, increasing discomfort and leading to potential injury.

Chairs are usually fitted with gas-lift stems that adjust the seat height and many of these will not be high enough for the tallest users; that may be another reason why they are sitting too low. If this is the case, see whether or not any other chairs in the office have higher gas stems. If not, check with the chair supplier to see if it is possible to get a higher gas stem, if the rest of the chair is suitable.

When the chair is at the right height and fully adjusted (with the backrest and lumbar support correctly placed), we need to check that the thigh is supported by the seat pan. Some chairs are fitted with a seat slide which is designed to decrease or increase the length of the seat pan for users with short or long thighs. If the chair doesn’t have a seat slide, and the thigh is not adequately supported – that is, the gap between the back of legs and the seat edge is greater than 50mm, you will need a chair with a longer seat pan.

Once we’ve made all the adjustments, we can take a measurement from the floor to the user’s elbow joint, which will determine optimum desk height. If the measurement is 78cm we need to subtract 4cm, to account for postural slump. Standard desk height is 72cm, which means we need to raise the desk to 74cm. To test this we can put the worker back at the desk, and raise the keyboard by 2cm  — use a book or telephone directory. This is a simulation which doubles up as a user trial; it will help determine if the user is happy to work at this height.

The golden rule of successful ergonomics is to simulate different options to gain feedback on whether proposed changes are suitable for the individual. In this instance the user may want the desk raised to 73cm; but above that height they would experience discomfort.
Incorrect seat height or seating that fails to support the user is one of the most common reasons tall users suffer back pain and discomfort. But all assessments must be thorough and cover all primary and secondary work areas that tall users work in.
Small problem
When setting up workstations for small users, we start off with the chair, just as for tall users. Lower the chair till you get the correct thigh angle, then adjust the chair so all its parts fully support the small user. We need to ensure that no part is too big or too wide for the occupant.

If the backrest has a fixed lumbar support, for example, this may be too high. The backrest may be too wide, or the seat pan too long. The latter will mean the user has to perch on the end of the chair and lean forward, which will increase the strain on their back, and result in the them resting their upper body weight on their wrists.

Therefore, if the seat doesn’t fit, the chair must be changed as it could lead eventually to musculoskeletal injury.
If the chair is adequate, we can push the chair back to the desk, and raise the seat to correctly position the user’s upper limbs – the forearms should be just below the 90-degree angle.

Don’t forget the other elements of the DSE. Keyboards with number pads will push the mouse out to the right, and for a person with narrow shoulders or short arms it will force them to overreach when using the mouse. Ideally, the keyboard should be changed for a smaller keyboard without a number pad. Also check the mouse is not too big for the user’s hands.

Volume issue

Some of the most obvious health problems associated with being overweight are a result of the extra burden placed on the structure of the body. Research has shown that low-back discomfort and arthritis is more prevalent in the overweight population.

Seriously overweight workers may find that their body mass can change the way the legs and arms are positioned when sitting at a desk. Arms can be forced out which can result in shoulder discomfort and ulnar deviation. This strain can be aggravated by unsuitable equipment such as small keyboards, small desks and small chairs.

As the average weight of the British worker increases, the needs of large, overweight and obese people are often overlooked by businesses when choosing appropriate seating for their staff.

The British Heart Foundation’s forecast a decade ago that weight trends could mean that a quarter of office workers would be too big for the average chair by this year may not have come true, but that is the case for a small minority of workers. The consequences of sitting in a chair which is too small can cause postural problems and even safety lapses (when a chair collapses under the user’s weight, which does happen).

Overweight and especially obese people need extra features that are not available in standard chairs. The back seat pan and back rest must be suitably proportioned, and the mechanisms must be designed for heavy-duty use. Obese workers often use their chairs differently to lighter users, pushing up from the chair from the armrests when rising, for example, and sitting into the chair more heavily.

The chair should have strengthened armrests that are width adjustable. The bases should be reinforced and six-star bases have been found to be more stable (the standard chair has a five-star base, where the star points carry the casters.)

Chairs come with a maximum guide weight which is around 21 stone, but there are chairs designed to support large people from 120 kilograms up to 203 kilograms. Some seating will go up to 255 kilos.

Heavy-duty chairs designed for the overweight or obese user will be more expensive than standard user chairs because of their construction, but the cost is minimal if it reduces sick leave that can result from postural problems or from costly litigation should the chair collapse.

When considering a heavy-duty chair you must take into account the physical changes that occur with increased body fat. The lumbar support needs to be height adjustable to at least 38cm above seat pan, as the fat in the buttocks and thighs can raise the worker up, and with a standard lumbar support the lower back will not be supported.

The lumbar support needs to be able to provide deep lumbar support, if not the person will try and flatten out their spine and this can result in a forward head posture.

If the office chair has a narrow seat, the user may have inadequate pelvic support, so their chairs should have a broader and longer seat pan. The width is generally required, but added depth can be a problem, especially for females with a short back of buttock to back of knee length.
To overcome this, some manufacturers will offer reduced chair functionality, limiting independent adjustment of seat back and seat pan tilt as a compromise to meet  the higher strength requirements of larger workers.

Women with very large breasts, people with large fronts (whether stomach or chest), and expectant mothers may not be able to sit right up to their desks, potentially causing them further problems with arm extension and posture. A bow-fronted desk can help here.

Large users’ body width means they often have a natural carrying angle with the shoulders in a flared posture. In order to use a standard keyboard, such workers need to deviate their wrists, arms and shoulders into a non-neutral posture, which can cause significant problems in the upper body. Split keyboards will help to accommodate overweight individuals.

Organisations must consider the changing shape of the working population when designing or fitting out workplaces or buying equipment, especially that subset of the population whose health and wellbeing they have day-to-day responsibility for maintaining.

Categories:
Ergonomics, Features

Related articles:




Share this page

Printer-friendly version