8 FURNITURE
8.3 Design Specifications
8.3.3 SEATING .1 Height
A seated 5th percentile female will be accommodated at the 56 cm height and 0 degree tilt angle. A standing 95th percentile male will be accommodated at a 96 cm height with a surface that is tilted 45 degrees below horizontal; see Figure 8-4. Tilt angles greater than 45 degrees below horizontal are not acceptable.
The location of the input device on its support surface is a complex issue that integrates the recommended positions of the shoulder, upper arm, elbow, forearm, wrist, and hand with provision of clearance space for the legs underneath. Height and/or tilt adjustments are necessary to accommodate different sizes of workers and different working postures. Moreover, height and tilt requirements interact. These issues can be jointly addressed by an input-device support surface that supports placement of the input device or devices within a three-dimensional space or volume. Such an input- device support surface benefits from adjustability in both height and tilt. (See Section 5.2.4 for specification of the location of input devices relative to the user.)
Research suggests that input devices are most comfortable to use, and place less demand on the shoulder, when they are located so as to minimize shoulder abduction (Harvey & Peper, 1997;
Karlqvist et al., 1996; Paul & Nair, 1996). This is achieved when the devices are located approximately within the shoulder span of the user and within forearm reach. Similarly, users report equal comfort and productivity for a wide range of elbow angles (Cushman, 1984: Grandjean et al., 1983; Miller & Suther, 1981; Weberet al., 1984). Tilting support surfaces for input devices facilitates neutral wrist postures (Berqvist, Wolgast, Nilsson, & Voss, 1995; Hedge, Morimoto, & McCrobie, 1999; Hedge & Powers, 1995).
Research supports the benefits of alternating between seating and standing postures (Dainoff, Paasche, Simons, & Terlaga, 1999; Nerhood & Thomson, 1994; Paul & Helander, 1995). Accordingly, the standard specifies minimum height and tilt adjustment ranges for sit/stand input-device support surfaces. Finally, research indicates that placement of input devices at multiple levels represents a viable solution to the location of multiple input devices within the recommended space (Paul & Nair, 1996). However, care must be taken that the postural design criteria of Section 5.2.1.1 are followed when multi-tiered input-device supports are used.
• Be adjustable by the user over a minimum range of 11.4 cm (4.5 in.) within the recommended range of 38 to 56 cm (15 to 22 in.)
The manufacturer shall
• Provide information to show which of the three seated postures the chair will accommodate Seat height is determined by a combination of chair dimensions (seat pan depth, seat pan angle) and anthropometric dimensions (popliteal height, buttock-knee length, buttock-popliteal length). Appendix A contains the anthropometric models and detailed rationale for these values. It should be pointed out that, for reasons relating to mechanical constraints on existing adjustment mechanisms, the required 11.4-cm seat height adjustment range does not accommodate the entire user population specified in Section 8.1.
8.3.3.2 Depth and Front Edge of the Seat Pan
The seat depth shall
• If nonadjustable, be no greater than 43 cm (16.9 in.)
• Include 43 cm (16.9 in.) if adjustable The front edge of the seat pan should
• Be rounded
An appropriate seat depth allows the user’s back to be in full contact with the backrest while preventing contact between the back of the knee and the front of the seat pan. For nonadjustable seat depths, this is achieved by setting the maximum depth to the length of the small female (5th percentile) buttock-popliteal length, less a 1-cm margin. Rounding the front edge of the seat pan helps avoid undue pressure on the underside of the thigh (Chaffin & Andersson, 1991).
8.3.3.3 Seat Pan Width
The seat pan shall
• Be at least 45 cm wide (17.7 in.)
The hip breadth of a 95th percentile female defines the minimum width of 45 cm to accommodate the user’s thighs, including clothing (Gordon et al., 1989).
8.3.3.4 Seat Pan Angle
The seat pan shall
• Have a user-adjustable range of at least 4 degrees, which includes a reclined position of 3 degrees
NOTE: User adjustment is defined as any method of activating the movement of the seat pan/backrest.
This can be either through the use of manual devices (levers, knobs, adjustments, etc.) or by movement of the body.
Studies point out the advantages of adjustability of seat pan angles in either reclined backrest, declined seat pan, or upright postures (Grandjean et al., 1983; Mark, Dainoff, Moritz, & Vogele, 1991).
However, these studies do not specify exact limits of seat pan angle adjustability. The values selected are based on industry practice.
8.3.3.5 Seat Pan–Backrest Angle
The backrest shall
• Be able to achieve a position that is vertical or to the rear of vertical.
If the backrest is adjustable, it shall
• Have an adjustment range of 15 degrees or more within the range of 90 degrees and 120 degrees relative to horizontal
If the backrest recline angle exceeds 120 degrees from the horizontal, the backrest should
• Have a headrest, preferably user adjustable
An angle of less than 90 degrees between trunk and thighs may lead to fatigue or discomfort. Studies have shown that a recline-angle of 30 degrees from vertical reduces fatigue (Andersson, 1987;
Andersson, Murphy, Ortengren, & Nachemson, 1979). If the backrest is reclined more than 30 degrees from vertical and there is no headrest, the user is forced to maintain the head in a static position. This may result in fatigue.
8.3.3.6 Backrest Height and Width
The top of the backrest should
• Be at least 45 cm (17.7 in.) above the compressed seat height If fixed, the lumbar support area of the backrest should
• Be located between 15 and 25 cm (5.9 and 9.8 in.) above the compressed seat height The position of the center of the lumbar support should
• Be user adjustable between 15 and 25 cm (5.9 and 9.8 in.) above the compressed seat height The width of the backrest should
• Be at least 36 cm (14.2 in.)
A high backrest provides support to the thoracic spine in addition to supporting the lumbar spine. This allows the user to lean back in the chair to reduce compressive loading of the spine. For this purpose, the backrest must be sufficiently tall and wide (Andersson, 1987; Andersson et al., 1979; Andersson &
Ortengren, 1974; Dowell, 1995; W. F. Floyd & Roberts, 1958; W. S. Floyd & Ward, 1969; Grandjean et al., 1983; Knutsson, Lindh, & Telhag, 1966).