This section describes postures that seek to minimize postural demands. However, it should be noted that any one posture becomes fatiguing after a while, and that changes in posture are important. Thus the posture described and illustrated in Figure 1, is a guideline as to general suitability of posture, and is not the only recommended posture.
As noted in the Canadian Standards Association (CSA) guideline movement is important to minimize postural fatigue and discomfort. Movement may include slightly adjusting the positioning of the head, shoulders, arms, back, hips, and legs. For example, hip angle changes as a person reclines in their chair. Leg and hip angles change as a person stretches their legs out in front. Shoulder and arm angles can be changed by moving the chair forward or back slightly.
Keyboard Position
When working at a keyboard, the operator should be sitting with the upper arms hanging naturally from the shoulders. The elbows should be bent at roughly a 90-degree angle when the fingers are in typing position on the home row of the keyboard. This posture allows the arms and wrists to be held in a natural and relaxed position that puts the least amount of physical stress on muscles and joints.
If work surfaces are too high, users must raise their arms and shoulders. This requires continuous muscular effort, called “static effort” or “static loading”. This static effort in the arms and shoulders may be fatiguing, and it may also hinder blood flow, adding to discomfort and even to the risk of injury. In addition, the wrist may be flexed (bent forward) to reach the keys, placing stress on forearm muscles and wrist tissues.
If the work surfaces are too low, the worker must lean forward, placing stresses on the arms and back. As well, the wrists will tend to be bent back, also stressing the muscles and tissues.
A desk height that is too high or too low for writing can result in the same kinds of problems.
Figure 1: Adjusting Your Computer Workstation
Note: This diagram is just an example. Workstation set ups will vary according to the particular desk style, monitor, tray mount or other accessories used.
- The monitor should be set at a height so that your neck will be straight.
- Your elbow joints should be at about 90 degrees, with the arms hanging naturally at the sides.
- Keep your hands in line with the forearms, so the wrists are straight, not bending up, down or to either side.
- Thighs should be roughly parallel to the floor, with your feet flat on the floor or footrest.
- If necessary, use a footrest to support your feet.
- Your chair should be fully adjustable (i.e. for seat height, backrest height and seat pan tilt, and, preferably, armrests). It should have a well-formed lumbar (lower back) support to help maintain the lumbar curve.
- There should be enough space to use the mouse. Use a wrist rest or armrest so that your wrist is straight and your arm muscles are not overworked (see Figure 2).
- Use an adjustable document holder to hold source documents at the same height, angle and distance as the monitor.
The Mouse and Other Input Devices
Input devices such as computer mouses, trackballs and digitizing tablets are used to perform a variety of types of computer work ranging from word processing to computer aided design (CAD). There are a number of types and styles of devices. For example, some mouses now have scroll buttons. Mouse settings can also be adjusted for left handed users and to change the speed and distance of mouse travel and clicking actions required. It is important that users, and purchasers of computers are aware of the range of devices and settings available, to determine which are most appropriate for their application and use.
Even with the appropriate device, poor positioning can lead to problems. Users may hold the arm they use to control the device in a fixed, raised or outstretched position. This results in static loading of the shoulder and in bent wrist postures that contribute to discomfort and risk of injury.
A mouse or a tablet should be placed as close to the worker’s side as possible at a height that allows the upper arm to hang relaxed from the shoulder with a “neutral” wrist position, with the hand in line with forearm. This position causes the least physical stress. The mouse should be also placed so the cord and items on the desk do not limit movement.
If a keyboard/mouse platform is used, take care that it allows the mouse to be placed as close to the keyboard as possible (at the same height and in the same plane), and that it provides a stable surface of sufficient size.
At CAD and other workstations where work is done with one arm for long periods, the forearm should be supported by a desk surface to the side of the operator or by adjustable armrests on the desk or the chair (see Figure 2). This support is necessary to reduce static loading.
The mouse or other hand-held input device should not contribute to cramped hand postures. This may require consideration of different-sized devices for different hand sizes. The device should be shaped so as to minimize bent wrist postures, or, failing that, the forearm should be supported on a raised smooth surface to allow a comfortable wrist posture.
The mouse buttons should be located so as to avoid awkward finger and hand postures. The activation force (the force needed to make a button click) should not be so great as to cause fatigue. But it should not be so little that buttons can be clicked inadvertently since users will then tend to hold their fingers up away from the buttons, causing static loading of the muscles.
Users should be encouraged to hold the mouse in a relaxed way, not to grip it tightly, and to move it from the shoulder rather than just the wrist. This better distributes muscular demands and reduces wrist movements and static loading.
As noted in the CSA guideline, keyboards are usually the main input device, and they should be properly designed and placed. Placement of the keyboard has already been discussed.
The keyboard selected should be suitable to the task and user. There are alternative keyboards available, which vary from the standard design. These should be tested prior to implementation to ensure that they are suitable for the individual and task. The CSA guideline provides more detail in this regard.
Figure 2: Surface beside keyboard for mouse use
Note: This diagram is just an example. Workstation set ups will vary according to the particular desk style, monitor, tray mount or other accessories used
Monitor Position
Monitors should be placed so that the top of the screen is at the operator’s eye level, though there are exceptions as noted in the next section on bifocals. The viewing distance between the operator’s eyes and the screen should be in the range of 40 to 74 centimetres. The size of the monitor often dictates viewing distance. If the monitor is large the workstation should be large enough to accommodate it. The increasing use of flat screen monitors is allowing for better space use and more flexibility in screen position.
If the screen is too low or too high, the muscles of the neck must work continuously to hold the head in a viewing position, which may result in fatigue and discomfort. If the screen is viewed continuously or frequently it should be directly in front of the worker to avoid having to keep the head turned to one side. Improper viewing distances or positions may result in fatiguing head positions and in visual fatigue caused by the effort needed to focus.
Bifocals, Trifocals and Single-Focus Glasses
A computer operator who wears bifocals may tilt the head back to view the monitor through the bottom, close-vision, part of the glasses. If bifocals cause discomfort or awkward head positions, several approaches can be taken. The screen should be lowered such that the head is in a neutral position when viewing the top line of text or other material.
Alternatively, one could wear single-focus glasses designed specifically for computer work, with the focal distance chosen for the viewing distance between the worker and the screen. In this case, it is important that a document holder is also used, to position documents at the same viewing distance.
Other options are graduated bifocals, which have no sharp line between the two parts of the lens, trifocals, or the use of reverse bifocal lenses, where the computer screen prescription is in the upper part of the lens.
Computer users should have their eyes checked regularly and discuss their computer use with their optometrists.
Seating
A height-adjustable chair can help in placing the operator at a proper height for typing and viewing the monitor, especially when height-adjustable tables are not available. The height of the chair should allow the feet to rest flat on the floor with the thighs roughly parallel to the floor. To place some shorter workers at a comfortable typing height, the chair must be raised. If a worker’s feet then cannot reach the floor, the front edge of the chair may press into the underside of the worker’s thighs, which may impair circulation and cause discomfort. These problems can be avoided by using a footrest.
The size of the worker is an important consideration in buying a chair. Many newer models of chairs come in different sizes to accommodate the variation in user sizes
An office chair should have:
- a five-prong base, with its feet set in a circle at least as big as the seat itself,
- seat height adjustable from 42 cm to 51 cm (standard), 38 cm to 45 cm (low height),
- a covering made with a breathable material and a dense foam that gives way no more than 2.5 centimetres,
- a seat pan that:
- is large enough to provide support for thighs and buttocks but not so long that the front edge presses into the backs of the operator’s lower legs,
- small: 38-42 cm
- medium: 42-46 cm
- large: > 46 cm
- has a “waterfall” front edge (i.e. rounded, downward-curving) to reduce pressure on the underside of the thighs,
- is adjustable, so that it can tilt from 3° forward (i.e., with the front edge 3° down from the horizontal) to 4° back, and
- has a width of ≥ 45 cm
- is large enough to provide support for thighs and buttocks but not so long that the front edge presses into the backs of the operator’s lower legs,
- a backrest that:
- with lumbar support adjustable to 15-25 cm. above the seat. If the backrest is fixed (a less desirable option) the lumbar support should be within this range.
- with the upper edge at a height of 45-55 cm above the seat pan for a standard back. A high back should be ≥ 7.5 cm higher than a standard back.
- adjustable angle within a range of 93°-13°. A fixed backrest should be in the range of 93°-103°.
- armrests:
- that are height adjustable to 19-24 cm or fixed within same range,
- of length ≥ 18 cm but set back at least 15 cm from the edge of the seat,
- should not impede computer work, or positioning of chair, and
- with a distance between armrests of at least 45 cm (or adjustable).
Footrests, where they are necessary, should have a stable surface and be large enough to accommodate both feet easily. The footrest angle could be adjustable, though a fixed footrest is suitable if it allows for comfortable ankle angles (roughly 90° between foot and leg). Generally fixed footrest angles are in the range of 0 – 30°.
Make sure that workers are aware of the importance of adjusting their chairs correctly and know how to make adjustments themselves. Chairs should be readjusted when workers change the height they will be working at for any length of time: for example, from keyboarding to writing.
Desks
The best way to provide the proper screen and keyboard heights for all operators is to use split-level tables or desks that allow each height to be adjusted independently. This allows for proper work postures for a range of user sizes. However, a fixed desk of suitable height, the correct use of an adjustable chair, and a footrest and/ or monitor stand where necessary, will also allow for suitable postures.
The CSA guideline promotes adjustable worksurfaces but also gives a recommendation for a fixed desk height of 73 cm +/- 2.5 cm.
Any table, desk or stand used for computer work must be deep enough for both the keyboard and the monitor to be in front of the worker. The CSA guideline recommends a minimum of 76 centimetres. In cases where space is limited, the use of flat screens is one option for freeing up space.
There should be sufficient leg-room. The CSA guideline calls for 43 centimetres of horizontal knee space and 60 centimetres of “toe space”, the total horizontal space for leg and foot. The vertical clearance at the front edge of the work surface should be at least 68 cm. The width of the leg space should be at least 50 cm.
Document Holders
Computer work often involves entering information from source documents. These should be located beside the screen and in the same plane. This reduces the size and amount of head and eye movements between the document and the screen and decreases the likelihood of muscular and visual fatigue. The best way to position documents correctly is to use an adjustable document holder. These are usually mounted on a flexible arm that is fixed to a base or clamps to the edge of the desk. The clamping type is preferable if desk space is limited. Before purchasing a holder, consider the size and thickness of the documents to be used and choose a holder that will accommodate them. A slant board is one option for supporting larger document.
Telephone Work
Increasingly, workers are required to use a keyboard while on the telephone. This often results in awkward head, neck and back postures with the receiver cradled between the shoulder and head to leave both hands free. Workers required to use a computer while on the telephone for long periods tend to experience discomfort, particularly in the head and back. In such cases, headsets should be used. Hands-free phones are also an option, where the office space and task are appropriate. A spacer or cradle that mounts to the handset is not a preferred option. Although it improves the head position, a static effort is still needed to hold the handset in place.
Taken from http://www.labour.gov.on.ca/english/hs/pubs/comp_erg/gl_comp_erg_2.php
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