- For most office tasks, especially when working with a computer, the desk surface should be slightly below elbow level, when your arms are by your side and bent at a 90 degree angle.
- Raise or lower the seat as needed to achieve the correct position. If your feet cannot reach the floor when the chair is correctly adjusted in relation to the desk, the use of a foot rest or height adjustable desk would be ideal. A foot rest should be made available to anyone who requires one.
- The appropriate height for a chair in practical terms means that when the feet are planted firmly on the floor, the thighs are approximately parallel to the floor. This provides even support to the underside of the thighs without localised pressure, particularly to the backs of the knees. This also helps ensure that the torso remains upright and the spine retains its neutral s-shaped curve (the lumbar curve). A correctly adjusted chair also prevents the user from crouching forward and trying to support their body weight elsewhere, such as on the arm rests or the desks work surface.
- Seat depth is also a factor. A correct seat pan depth will allow the back rest to provide good supporting contact for the sitters back and still maintaining clearance between the front edge of the seat and the back of the sitter’s legs (approximately 2 fingers width).
- Most chairs offer some form of recline function. In most situations the chair should be adjusted to recline about 10 to 25 degrees, similar to the correct driving position. The other benefits of recline are mainly in the area of dynamic variation of posture – a chance to rest some muscle groups, whilst bringing others into action, reducing the amount of work being done by the spine and spinal muscles and transferring some of the torsos weight on to the chair back rest. This function also opens up the angle between the torso and the thighs, reducing pelvic rotation and spinal deformation. Extreme angles of recline should not been seen as alternative working postures.
- Arm rests can help support the weight of the arms and shoulders, reducing the load on the spinal structure and also provide additional support to the body when sitting down or standing up, when the body can be subject to extremely high transitional forces. The armrest should be set slightly below elbow height. In this way it can provide support when resting between periods of keying without interfering with arm movement when working.
- For prolonged tasks involving intensive use of the mouse, some ergonomists suggest that support for the forearm whilst still permitting the appropriate movement of the hand and wrist. However, it is important to note that arm rests can sometimes clash with the edge of the desk work surface. If this prevents the user from adopting the correct position relative to the desk, then this problem could outweigh the benefits of armrest use.
The main requirements for an office seat are that:
The seat shall be stable and allow user easy freedom of movement and a comfortable position. Shall be adjustable in height. Seat back shall be adjustable in height and tilt. Footrest shall be made available to any user who wishes one.
- Blood circulation to the lower limbs is not restricted
- It is easy to change and maintain a variety of postures with little muscular effort
- Loading or stress on the spine is minimised
- The seat surface has sufficient friction to avoid slipping off
- For thermal comfort, fabrics are permeable
- Compatibility with the work surface – especially height and the layout of equipment.
‘Independent height and tilt adjustment of backrest is not a requirement’. The primary requirement is that is that user should be able to achieve a comfortable position. Other requirements are:
- seat should adjust in height
- backrest should adjust in height and tilt
- 5 star base with castors
Standards applicable to task chairs
- BS EN 1335 Part 1 Dimensions
- BS 5459 Part 2 Strength and Stability for 24-hour use for people weighing up to 150kg or
- BS EN 1335 Parts 2 and 3 – Strength and stability for 8-hour use by people weighing up to 110kg
- BS EN ISO 9241 Ergonomic requirements for office work with visual display terminals (VDT’s) Part 5 – work station layout and postural requirements
- The appropriate seat height for a seated user – popliteal height plus footwear thickness
- Backrest to give support to the back in all sitting positions. Particular support to the lumbar region.
- Lower edge of the backrest should start above the major protuberances of the buttocks and conclude below the level of the shoulder blades.
- Arm support should not restrict the user’s preferred working posture. Should not restrict access to the workplace, e.g. should not prevent chair from being slid under work surface.
DesignEye 