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14TH ERU SYMPOSIUM, 2008: FACULTY OF ENGINEERING, UNIVERSITY OF MORATUWA
Design of an Ergonomically Efficient Chair
R.A.R.C. Gopura1
1Department of Advanced Systems Control Engineering, Saga University, Japan
K.G.A.H. Amarasena2
2Department of Textile and Clothing Technology, University of Moratuwa, Sri Lanka
E-mails: gopura@ieee.org, asiriamarase@yahoo.com
Abstract
Ergonomically designed chairs are important for long time seated workers to increase their productivity and also to
reduce low back injuries due to use of poorly designed chairs in ergonomic aspects. In addition, ergonomically
designed chairs increase the seating comfortability of the chair users. Most of the chairs designed for the long time
seated workers are not considered the full ergonomic aspects. In this paper, we discuss a design of an ergonomically
efficient chair for the long time seated workers to increase their productivity and also to reduce low back injuries.
1. Introduction
Ergonomics (Human factors) is the application of
scientific information concerning humans to the design of
objects, systems and environment for human use. Sitting
requires 20% less enery than that of standing to do the same
work, if the body is properly supported. Intradisc pressure
during reclined sitting is 50% less than that for erect
standing (Wilke et al., 1999). Many researchers (Ong et al.,
1989, Ignatius et al,. 1993) proved that low back pain risk
and musculoskeletal discomfort is related to mismatch
between chair and desk height and poor ergonomic
furniture, inclduing chair. Therefore ergonomically
designed chairs are important for seated workers to increase
their productivity and to reduce low back injuries due to use
of ergonomically poor designs of chairs. In addition,
ergonomically designed chairs increase the seating
comfortability of the chair users. Most of the existing chairs
designed (www.freepatentsonline.com) for the longtime
seated workers are not considered full ergonomic aspects
and a way of restrict the chair’s mobility on the floor. In this
paper, we discuss a design of an ergonomically efficient
chair for the long time seated workers. The chair is designed
considering overall ergonomics to increase their
productivity and also to reduce low back injuries. In the
design a floor grabber (which can be used to restrict the
chair mobility on the floor) is also proposed.
2. Design Specifications
In the design of ergonomically efficient chair, four main
parts and an aspect of the chair is considered to decide
design specification. Those four parts and the aspect and
specifications related to them are listed in TABLE 1. Here
mobility means ability to move the chair on the floor and
TABLE 1 Design Specifications
Part No. Part Name Specification
01 Back rest Height, posture, inclination, size, shape
02 Seat Height, posture, inclination, size, shape
03 Arm rest Height, inclination, posture, size, shape
04 Leg No. of pedestals
05 Mobility Swivel range, chair mobility
swivel ability (ability to turn) of the chair while seating. The
chair is designed for long time seated worker (sewing
machine operators, computer operators, type writer users,
clerical workers). In the design, anthropometric data of a Sri
Lankan male worker is used (Abeysekera et al., 1987).
3. Design Requirements-Anatomical and
Biological Point of View
A design outline incorporating the basic requirements for
a comfortable and protective seat based on knowledge of the
anatomical, physiological and pathological causes of low-
back discomfort and pain. The force diagram shows (Fig.1)
the forces applied in the body while seating. X, Y and Z
indicates the body forces and A, B, C and D indicates the
reaction force from chair. The chair should be designed to
obtain the indicated reaction forces. Main requirement of
chair is listed in below with explanatory figure (Fig. 3) and
notes (Goossens et al., 2005, Keegan et al.).
1. Placement of the primary back support over the lower
lumbar spine where most postural back symptoms are
located (Fig. 2: X-Back support)
2. The provision of a minimum angle of 105 degrees
between the trunk and the thigh to help preserve the
lumbar curve
3. Provision of an open or recessive space for the
posteriorly projecting sacrum and buttocks. This free
space permits constant contact with tine primary lower
lumbar back support.
4. The upper limit of the convex primary lower lumbar
back support in the short-backed “straight” chair
should be well below the lower angles of tine scapulae.
This permits unrestricted posterior placement of the
shoulders for relaxing change of position in the chair.
5. The shoulder support in high-backed chairs is
secondary to the lumbar support, placed at a minimum
angle of 105 degrees with the seat.
6. Increase of angle of back of seat is pivoted on a point
in line with the hip joint. This permits maintenance of
contact with the primary lower lumbar support.
7. The maximum length of the seat should be within suit-
2 14TH ERU SYMPOSIUM, 2008: FACULTY OF ENGINEERING, UNIVERSITY OF MORATUWA
Fig. 1 Force diagram Fig. 2 Lumbar support
able value, measured from the most prominent point of
the lower lumbar support. This allows free space back
of the knees for change of position of the legs.
8. The height of the seat should be within suitable value,
to permit comfortable placement of the feet on the
floor. If the seat is made higher for desk or table use, it
should be made shorter.
9. The front border of the seat is curved downward or
upholstered; this aids in molding the thighs over the
edge of the seat so that the feet, can reach the floor.
The joining support beneath the front border should be
at a 45-degree angle, so that it will not interfere with
placement of the feet beneath the seat.
10. Free space below the seat is provided to allow for
placement of the feet beneath the seat in rising and for
relaxation in sitting.
11. A tilt or upward inclination of about 5 degrees for the
seat is provided to aid in maintenance of proper
position against the lower lumbar back support.
Popliteal arch should not be in contact with the chair
seat pan.
4. The Design
In the design of ergonomically efficient chair four basic
parts of the chair are considered: Seat, back rest, hand rest
and legs. The design includes 06 degree of freedoms
(DOFs) for back rest (with back rest carrier), 10 DOFs for
both hand rests (with hand rest carriers) and 03 DOFs for
seat. All together the chair has 20 degrees of freedoms (with
the mobility of the chair on the floor). The height, posture
and inclination of the back rest, arm rest and seat, swivel
range of seat and chair mobility can be adjusted according
to the users necessity from the above 20 DOFs. Head rest
and foot rest are not considered for the design, since they
are out of the considered scope (not necessary for seated
workers). Three pedestals for the leg is included in the chair
as shown in Fig. 4 (05 pedestals can also be used). Caster
wheels are attached to end of each leg. Floor grabber (Fig.
4) can be used to stop the chair motion on the floor
according to the user’s necessity. Floor grabber is designed
such a way that once push it grabs the floor and for the
second push it releases the floor. The shape of the seat is
saddle and front of the seat pan is sloping for not to contact
with popliteal arch. The sloping is especially necessary for
TABLE 2 Adjustable Ranges of Design Specifications
Design Specification Adjustable Range
Seat height 15 -22 inches
Back rest angle 100
0
-120
0
Back rest height 6 -10 inches (above compressed seat)
Arm rest height 7.1- 10.6 inches (above compressed seat)
Fig. 3 Design requirements
proper blood flow of knee and thigh of long time seated
workers. The DOFs of each basic part is shown in Fig. 4.
The chair is design providing the ability to adjust the each
design specifications of main part of chair according to the
chair user. By providing adjustability, the designer expects
the user to adjust the specification according to his body
size, desired posture and positions. The long time seated
workers especially need to change their seated posture with
the time. The proposed chair design enhances the long time
seated users desire to change the posture of seating time to
time, since the adjustability is easy. The adjustable ranges of
few important design specifications are decided as per
TABLE 2.
Fig. 4 DOFs of basic parts of the chair
References
Abeysekera J. D. A and Shahnavaz, H. 1987. Body size data
of Sri Lankan workers and their variability with other
populations in the world: its impact on the use of
imported goods, Journal of human ergology,
16(2):193-208.
Goossens, R. Teeuw, R. and Snijders, C. 2005. Sensitivity
for pressure difference on the ischial tuberosity.
Ergonomics, 48 (7): 895-902,
http://www.freepatentsonline.com/result.html?p=1&query_t
xt=Ergonomic+chair+design
Ignatius et al. 1993. Journal of Human Ergology, 22:83-93.
Keegan J. J. Alterations of the Lumbar Curve Related to
Posture and Seating. Journal of Bone and Joint
Surgery, 35:589-603.
Ong et al. 1989. In Work with Computers, 330-337.
Wilke et al. 1999. Spine 24:755-62.
