Ergonomics is the science of designing jobs, equipment and workplaces to fit workers, or in the words of renowned ergonomist, Etienne Grandjean, 'Fitting the task to the man'.
Ergonomics in the workplace has to do largely with designing the work environment, the equipment and tasks to be performed in a way that takes cognisance of human physiology to minimise the amount of stress and stain on the person. In its simplest form, the work environment or area needs to have good lighting and ventilation, it should be clean, non toxic and safe, while the actual task should be designed so that lifting, reaching, pulling, stretching, bending and static seating is minimised. Finally, the equipment needs to be designed and setup in a manner that will simplify the task and reduce the risk of personal injury, both long and short-term.
In any work environment, the most important ergonomic considerations are:
Once the environmental factors have been addressed, the office chair is arguably the most important piece of equipment in any office, and one that has a huge impact on the long-term well-being of the user.
Figure 1. Traditional 'upright' sitting posture, with elbows, hips and knees all at 90 degree angles
When sitting, the body weight is transferred to the seat, floor, and to a lesser extent, the backrest and armrests. Traditionally, we have always been taught to sit in an upright position, much like the old Egyptian Pharaohs, with elbows, hips and knees at 90 degree angles. However, we seldom see people actually sitting in this position, simply because our body is not designed to allow us to sit in this position for any extended period of time. Sitting in this traditional upright position increases the pressure in the intervertebral discs in the lumbar region of the back (see figure 2), which causes lower back pain, so people will automatically adjust their sitting position to alleviate any associated discomfort. As can be seen from figure 2, the pressure in the intervertebral discs in the lumbar region (L3, L4 and L5), is steadily reduced when the angle between the back and thigh increases from the 'traditional' 90 degrees, and reaches a minimum when this angle is approximately 130 degrees; the so-called horse-riding or foetal position (figure 3).
Figure 2. Effect of seat angle on disc pressure in the Lumbar Vertebrae
Figure 3. When sitting on a horse or lying in the foetal position, the trunk-thigh angle is approximately 130 degrees, which results in the in the lowest pressure distribution in the lumbar intervertebral discs
When we sit in the traditional 90 degree upright position, our hip joint only rotates by approximately 60 degrees. To make up the 90 degree angle between our thigh and back, our pelvis must rotate through the remaining 30 degrees. This rotation flattens the natural elongated 'S' shape of the spine, predominantly in the lumbar region of the back (figure 4).
Figure 4. The rotation of the pelvis and the consequent flattening of the lumbar curve, increases the pressure on the intervertebral discs, predominantly between the 3rd, 4th and 5th lumbar vertebrae. This directly results in lower back pain.
One solution to this problem can be found in the kneeling chair. The Wellback kneeling chair developed by Karo in 1986, opens the body angle by lowering the level of the knees to well below the level of the hips to create a 110 degree trunk-thigh angle. The open position dramatically reduces the pressure in the lumbar discs (see figure 2), while the 110 degree angle ensures that the pressure on knees/shins does not exceed 20% of the total body weight. It should be noted that the primary function of the kneepad is not to support the body weight, but to prevent the user 'sliding' off the chair.
Figure 5. The Wellback kneeling chair with the 'open' trunk-thigh angle of 110 degrees
Reducing the Incidence of Lower Back Pain during sitting
A good ergonomically designed Task Chair is able to significantly reduce the incidence of lower back pain, provided the following important features are incorporated in its design:
- Good quality seat foam that provides adequate support and will not collapse after a short period of use. Typically, polyurethane injection moulded foam is used in the seats of well designed ergonomic Task Chairs
- The seat should incorporate a waterfall front edge to reduce the possibility of pressure points behind the knees.
- The seat depth should be adjustable to accommodate a broader spectrum of users.
- The chair should be height adjustable. You should be able to adjust the height of the seat so that the level of your knees is slightly below the level of your hips and your feet are firmly on the floor.
- The height adjustment mechanism should be easy to reach and operate when you are seated. In some cases, a free-standing footrest provides additional comfort.
- The contoured backrest should incorporate an adjustable lumbar support.
- Movement of the backrest while seated helps to maintain a healthy spine, so the backrest should be able to recline relative to the seat, thereby allowing the angle between the trunk and thigh to change from the traditional 90 degree upright seated position, to approximately 110 degrees when fully reclined. During this reclining movement, the backrest should still provide adequate spinal support.
- The position of the backrest should be lockable.
- All Task Chairs should be fitted with armrests that are height, and ideally also width adjustable. Muscular pain across the shoulders is often caused by armrests that are either too high or too low, forcing the user to "raise" or "droop" the shoulders. By using Height Adjustable Armrests that support the forearms and/or elbows, the incidences of such muscular pain is significantly reduced.
- The 5-star base should have castors that freely glide over the floor surface. For most floor surfaces, 60mm diameter nylon or soft PU castors are the preferred options.
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