Useful information regarding comfortable seating arrangement at workplace.

Fundamentals of seating

The advantages of a sedentary posture at work are:

• Taking the weight off the legs.
• Possibility of avoiding unnatural postures.
• Reduced energy expenditure.
• Lower demands on blood circulation.

These advantages must be set against certain drawbacks:

Prolonged sitting leads to a slackening of the abdominal muscles (‘sedentary tummy’), and to a curvature of the spine which, in turn, is unfavorable for the organs of digestion and breathing.

But the most serious problem involves the spine and the muscles of the back, which in many sitting positions are not only not relaxed but positively stressed in various ways.

  

The purpose of a seat is to provide stable bodily support in a posture that is: 

(i) Comfortable over a period of time;

(ii) Physiologically satisfactory;

(iii) Appropriate to the task or activity in question.

All seats are uncomfortable in the long run, but some seats become uncomfortable more rapidly than others, and in any particular seat, some people will be more uncomfortable than others. 

Comfort may also be influenced by the task or activity that the user is engaged in at the time. 

In other words, comfort (or more strictly the rate of onset of discomfort) will depend upon the interaction of seat characteristics, user characteristics, and task characteristics. 

In designing a seat therefore, the objective is to support the lumbar spine in its neutral position without the need for muscular effort, thus allowing the user to adopt a position that is both physiologically satisfactory and comfortably relaxed. In general this will be achieved by: 

• A semi-reclined sitting position (to the extent that this is permitted by the demands of the working task);
• A seat that is neither lower nor deeper than necessary;
• A backrest that makes an obtuse angle to the seat surface (thus minimizing the need for hip flexion) and is contoured to the form of the user’s lumbar spine.

General experience as well as a number of studies have yielded the following ‘golden rules’ for office chairs:

1. Office chairs must be adapted to both traditional office work and the modern equipment of information technology, especially to jobs at VDT workstations.

2. Office chairs must be conceived for a forward and reclined sitting posture.

3. The backrest should have an adjustable inclination, and it should be possible to lock the backrest at any desired inclination.

4. A backrest height of 48–52 cm vertically above the seat surface is an ergonomic necessity today. The upper part of the backrest should be slightly concave. A width of 32–36 cm for the backrest is advisable. It may alternatively be concave in all horizontal planes with a radius of 40–50 cm.

5. The backrest must have a well formed lumbar pad, which should offer good support to the lumbar spine between the third vertebra and the sacrum, e.g., at a height of 10–20 cm above the lowest point of the seat surface.

6. The seat should measure 40–45 cm across and 38–42 cm from back to front. A slight hollow in the seat, with the front edge turned upwards about 4–6 ° will prevent the buttocks from sliding forward. A light padding of foam rubber-2cm thick, covered with non-slip, permeable material is a great aid to comfort.

7.  Foot rests are important, so that small people can avoid sitting with hanging feet.

8.  An office chair must fulfil all requirements of a modern seat adjustable height (38–54 cm), swivel, rounded front edge of the seat surface, castors or glides, 5-arm base and user-friendly controls. The most important dimensions for a seat and working desk are shown in Figure 75.

Key features of chair design:

1. Seats should swivel and have heights adjustable between 38 and 54cm. Footrests should be provided for short users.

2. Free space for the legs must be provided both underneath the seat to allow the user to

flex the knees by 90 degrees or more and underneath the work surface to allow knee extension when reclining.

3. A 5-point base is recommended for stability if the chair has castors.

4. The function of the backrest is to stabilize the trunk. A backrest height of approximately 50cm above the seat is required to provide both lumbar and partial thoracic support.

5. If the backrest reclines, it should do so independently of the seat to provide trunk–thigh angle variation and consequent variation in the distribution of forces acting on the lumbar–pelvic region.

6. Lumbar support can be achieved either by using extra cushioning to form a lumbar pad, or by contouring the backrest. In either case, there must be open space between the lumbar support and the seat pan vertically below it to allow for posterior protrusion of the buttocks.

7. The seat pan must have a slight hollow in the buttock area to prevent the user’s pelvisfrom sliding forwards. This keeps the lower back in contact with the backrest when reclining.

The leading edge of the seat should curl downwards to reduce under thigh pressure.

8. Arm rests should be high enough to support the forearms when the user is sitting erect.

They should also end well short of the leading edge of the seat so as not to contact the front edge of the desk. If the armrests support the weight of the arms, less load is placed on the lumbar spine.

9. Modern chairs tend to have a thin layer of high-density padding. Layers of thick foam tend to destabilize the sitter. The foam can collapse after constant use.

10. Cloth upholstery provides friction to enhance the stability of the sitter.