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Evaluation of a forward-sloping chair
Applied Ergonomics 1985, 16.1,41-47
Evaluation of a forward-sloping chair C.G. Drury and M. Francher Department of Industrial Engineering, State University of New York at Buffalo, Amherst, New York 14221, USA
A methodology of chair evaluation developed earlier was applied to a novel, forwardsloping chair. Two groups of six subjects each were trained to use the chair and then given an evaluation session of 21/2 h. The chair elicited mixed responses, with complaints of leg discomfort from terminal users and of entry and egress from typists. Overall, the chair was no better than conventional chairs and could be worse than well-designed conventional office chairs.
Keywords: Chairs, posture, product design Introduction Ergonomists are frequently asked for chair design recommendations and, until a few years ago, these were relatively easy to provide. Recommendations based on anthropometry and biomechanics were generally convergent in defining a 'good' chair. Recently the issue of chair design has become more controversial with the introduction, largely by Mandal (1981, 1982) of the concept of a forwardsloping seat instead of the conventional backward-sloping seat. Industrial concerns which must specify seating for their employees have frequently only been exposed to advertising or popular media presentations on one type of chair or the other. The marketplace is characterised by strongly held opinions and a relative absence of data. Because of an already existing methodology for chair evaluation (Drury and Coury, 1982), the authors were asked by an interested industrial chair user to evaluate an example of a chair with a forward-sloping seat. This paper is the result of that evaluation, lit should be emphasised that only a single example of a forward-sloping chair was tested and that no formal statistical comparison was made between the chair tested and the prototype chair tested earlier]. In addition to the obvious outcome of how well the chair worked, a major question of interest was whether the methodology developed earlier could be applied to such a novel chair.
school children and quoted the earlier workers in equating this with increased disc pressures in the lumbar region. In order to alleviate this condition, he proposed a chair with a forward tilt on the seat pan of 0 ° to 30 °, combined with a raised and rearward-tilting desk. Such a workplace would decrease the eye-to-workpoint distance as well as relieving the spinal stress. The system has apparently been tested with Danish school children (Mandal, 1982) although no results were presented. As evidence to support his tilted seat theory, Mandal measured the extension of back muscles in various sitting positions for 10 subjects. His data showed considerably less (back muscle) extension in a tilted chair when compared with a traditional chair. Mandal also measured seat pressure distribution by placing blood pressure cuffs in the front, middle and rear zones of the seat. His findings showed the tilted seat to have a more equal pressure distribution in comparison with other seated reading positions A critique of this chair was presented in Design magazine (Anon, 1976) in conjunction with descriptions of the Mandal chair and the Hunting and Grandjean chair. The abstract quoted Hunting as saying that the tilting seat solution was "not right". In the same article, J. Ward raised a number of objections. The chair appeared to her to violate three of the principles of good seating: 1. Low pressure under the thighs, 2. Adequate lumbar support,
The forward-sloping seat - pros and cons
3. Ability to change posture in long-term tasks.
Mandal's ideas on seat design stem from the fact that in sitting the pelvis rotates backwards by approximately 30 ° compared with a standing position. This rotation reduces the standing lumbar curve, described as the "normal anatomical posture, which corresponds closely to the military alert position" (Mandal, 1981). Drawings of the lumbar curve in sitting and standing (Keegan, 1953; Schoberth, 1962) were used to illustrate this reduction in lordosis while sitting. Mandal found a lack of lordosis in the seated posture of
Mandal (1981) states, and the Design writers confirm, that the forward-sloping seat may take some days to appear comfortable. He states 1 - 2 weeks (1981) while Design gives 3 - 4 days as a learning period. The chair evaluated here was not known to have been designed or even approved by Dr Mandal, although it did have a forward-sloping seat and did show diagrams of pelvic rotation similar to those in his paper in the advertising literature which accompanied the chair.
0003-6870/85/01 0041 07 $03.00 Q 1985 Butterworth & Co (Publishers) Ltd
Applied Ergonomics
March 1985
41
The chair evaluated Figs. 1 and 2 show two views of the chair used in the experiments reported here. The chair was supported on beechwood rockers so that different postures could be assumed and so that the chair could be 'swivelled' around on a hard floor to face different directions. Two other features were the lack of backrest and the presence of shin pads. These shin pads, covered in the same fabric as the seat, allowed the sitter to maintain posture by preventing the buttocks from sliding off the seat. In use, the seat had about a 15° forward slope.
Evaluation methodology
Measurements The Drury and Coury (1982) evaluation methodology could not be used in total because there were no anthropometric criteria for comparison and no adjustments to the chair tested. Hence the evaluation was done using the Shackel et al (1969) General Comfort Rating (GCR), the Corlett and Bishop (1976) Body Part Discomfort (BPD) scale and the Shackel et al Chair Feature Checklist (CFC). All were described in detail in the earlier paper. The Chair Feature Checklist contained specific questions about the backrest. As these obviously did not apply to the forwardsloping chair, they were replaced by questions concerning the shin pads.
Experimentaldesign In place of fitting trials, which seek to adjust the workplace to the user, a 'training period' was given for the user to adjust to the chair. Following the authors quoted earlier, five training periods were given on the five days prior to the
Fig. 2
Chair tested, side view
evaluation. Each training period consisted of half an hour of work, using the chair in place of the subject's usual chair. All subjects had the principles behind the novel chair explained to them at the start of the first training period, using the diagrams from the advertising literature accompanying the chair. GCR and BPD were taken at the end of each training period. After the five periods of training, subjects were given the chair for a 2-5 h evaluation session. Subjects performed their normal work at their normal workplace except that at the end of each half-hour they were interrupted to record their measures on the GCR and BPD scales provided, A new scale was used on each occasion to prevent ,carry over' effects from previous readings. After the 2-5 h evaluation period, the CFC was administered. Unsolicited comments of subjects at each measurement time were recorded, during both the training and evaluation periods. As in the earlier study, two groups of subjects were used, a typing group and a terminal,using group. Table 1 shows the composition of the two groups. The first group of six female secretaries used the novel chair in place of their usual typing chair during regular work periods. Since office duties required the user to leave the chair occasionally, subjects were allowed to move about during the ~ h trials and the 2.5 h working trial. The second group consisted of four males and two females using a Lear, Siegler computer terminal. All were students who frequently spent long periods interacting with one of these CRT terminals connected to the University computer. Subjects in this second group did not leave the chair during the 2.5 h trial so that their comfort ratings can be compared with the Shackel et al (1969) "long term sitting" group rather than the "typing" group. These two groups probably represent extremes of use by an office population and, as found in the earlier study, would be expected to have rather different :responses to a chaiL
~ts
Fig. 1
Chair tested, front view
42
AppliedErgonomics March 1985
The General Comfort Ratings as a function_of time during training and evaluation are shown in Fig. 3. The typing group showed a reduction in discomfort in training and a relatively level rating during evaluation, while the terminal-
Table 1: Composition of two subject groups
Group
Males
Females
Age (yrs)
Height (m)
Weight (kg)
Typing
0
6
35-52
1"57-1"65
54-70
Terminal-using
4
2
19-24
1.72-1 '80
68-84
using group showed relatively level rating during training and .~ an increase in discomfort during evaluation. Testing these results against the range of 10 chairs measured by Shackel et al (1969) gave the results in Table 2. The typing group found the novel chair better than the worst of Shackel et al's 10 chairs for the first 2 h. The terminal-using group found this chair better than the worst Shackel et al chair for the first hour and then worse than their best for the 1 h to 1½ h period.
Table 2: Significance of differences (p < 0-05) between
Body Part Discomfort results for both groups are given in Table 3 for each body part. Overall indices of discomfort were defined as:
chair tested and best, mid and worst of Shackel etal sample of 12 chairs using GCR.
Typing group
Body Part Discomfort Frequency (BPDF): the percentage of Body Part Discomfort responses which were non-zero at any measurement time.
Terminal-using group
Best Mid
Worst
Best
Mid
Worst
30 min
NS
NS
Better
NS
NS
Better
1h
NS
NS
Better
Worse
NS
Better
90 min
NS
NS
Better
Worse
NS
NS
2h
NS
NS
NS
NS
NS
NS
(NS = not significant)
Body Part Discomfort Severity (BPDS): the mean scale value of all non-zero responses at any measurement time. discomfort reported by the terminal-using group. The main regions of discomfort were the legs (including the response "knees" by the terminal-using group, even though this response was not on the Corlett and Bishop scale) and, to a lesser extent, the back and buttocks.
Both measures have been used in a series of studies of manual materials handling as well as for the paced inspection group in the previous chair evaluation paper to provide summary measures of workplace discomfort. Nots of BPDF and BPDS against time period are shown in Figs. 4 and 5. The terminal-using group had more Body Part Discomfort complaints than the typing group, although both had a similar severity of complaint. This impression is further emphasised by Table 3, which shows overall greater
Chair Feature Checklist results are shown for both groups in Figs. 6 and 7. Height, length arid width were adequate for both groups. The seat was found to slope too far forward but to be of adequate-to-good shape. Shin pads
j.f
3.5F
30 t Terminal-
T/ .E
usi.g
\/" \\
2.0
- - . - \,,
/
f
-~
0.... ~
iyp,ny
-o
o
t5
,,...0~ ~ . 0 /
~0.~
,~
!
t--
30 =BarelyComfortable 20 =QuiteComfortable 40 =Perfectly Comfortable 0.0 =CompletelyRelaxed
Q9
0.5
/
-
0LL_
..... l 2
[
1
1Itl
3
4
5
Training, Half-HourPeriods
I
0
1
l
I_
2
EvaluationPeriod, Hours
Fig. 3
General comfort ratings over time
Applied Ergonomics
March 1985
43
Table 3: Percentage of occasions on which each body part was reported as having pain/discomfort Typing group Training
Terminal-using
Evaluation
group
Training
Evaluation
Neck
-
20
17
Shoulder
--
10
13
-
13
7
-
3
-
17
13
3
20
-
13
27
-
20
27
-
27
20
Upper arms
-
Lower arms Hands
Upper back
20
10
Mid back Lower back
-
Buttocks
6
-
Thighs
-
-
33
33
(Knees)
-
_
40
47
Legs
50
60
43
43
./'----"
25 2O
/
•
"E
'= Q..
t5
EL
°'--.~o\
/o
\
Fig. 4
Body Part Discomfort Frequency over time
//0~
/
\~ing
I
I
I
I
t
2
3
4
~0------0 S
o/ I .,.,
5
I
.
0
J
I
1
i
I
2
EvaluationPeriod, Hours
Training, Half-Hour Periods
2.5
/P /
2.0 ram
/t°~
1.5
~
rj~ n 03
Tpi.g
I
I
I
I
I/!I
t
2
3
4
5
i 0
Training, Half- Hour Periods
44
Applied Ergonomics
•
Terrain01-
'V /
-
IO
March 1985
I
i
I
EvaluationPe'iod, Hours
I
2
~- ~ 0
Fig. 5
Body Part Discomfort SeVerity overtime
,
TOO HIGH SEAT HEIGHT ABOVE
CORRECT A
TOO LOW
CORRECT ~ | ~
TOO SHORT _j
rHE FLOOR
TOO LONG [
SEAT LENGTH
__
TOO NARROW
CORRECT
SEAT WIDTH
"
Z --"
SLOPES TOO FAR TOWARDS BACK I
SLOPE OF SEAT
CORRECT
ADEQUATE I =
POSITION OF SHINPADS
TOO HIGH L
CORRECT i ' t
SLOPE OF SHINPADS
POOR FIT l
Fig. 6
SEAT HEIGHT ABOVE
TOO HIGH I
THE FLOOR
SEAT LENGTH
SEAT WIDTH
SLOPE OF SEAT
-=
CORRECT
TOO LOW I
TOO LONG
CORRECT
L
I
TOO SHORT I
TOO NARROW I
CORRECT
SLOPES TOO FAR TOWARDS BACK L--
PITS 1 WELL . _ _ J
I
HINDERANCE
I
SLIGHTLY OBSTRUCTED
I
C L E A R A N C E FOR F E E T A N D C A L V E S UNDER CHAIR
TOO LOW / J
~
NEUTRAL
I t'
TOO LITTLE
Chair Feature Checklist results, typing group
I GOOD l
-=
ADEQUATE • l
J
USEFUL ROCKING ACTION
SLOPES TOO FAR = TOWARDS FRONT ' 1
a --
II
POOR I
SEAT SHAPE
I
I
i
TOO WIDE _j
j
ADEQUATE
•
i
TOO WIDE I
CORRECT
SLOPES TOO FAR TOWARDS FRONT 1
ADEQUATE SEAT SHAPE
POSITION OF SHINPADS
SLOPE OF SHINPADS
ROCKING
ACTION
POOR
I
TOO HIGH
~
GOOD
CORRECT ~
TOO LOW
t
j
POOR FIT t
~
USEFUL L 1 TOO LITTLE
C L E A R A N C E FOR F E E T A N D C A L V E S UNDER CHAIR
•
~
ADEQUATE l
NEUTRAL
FITS WELL J
HINDERANCE
t
F
J SLIGHTLY OBSTRUCTED
•
ADEQUATE
t
I
Fig. 7
Applied
Chair Feature Checklist results, terminal-using group
Ergonomics
March 1985
45
were judged rather low by the typing group and of rather poor fit by the terminal-using group. Both groups appreciated the rocking action but found leg clearance slightly obstructed. Table 4 shows the subjects' comments grouped under major headings. Generally the typing group found entry and egress problems, while the terminal-using group suffered from leg problems. This table also shows the wide variety of responses to the novel chair, all the way from preferring it to an ordinary chair to loudly voiced complaints of knee and shin pain. Discussion and conclusions
As in the previous chair evaluation study, considerable differences emerged between the two groups. The typing group, who spent a considerable proportion of their time getting in and out of the chair, had more concern with entry and egress but less overall discomfort on all scales. The terminal-using group, who spent a full 2½ h working period in the chair, said nothing about entry and egress but were rather uncomfortable, especially in the area of the legs. For this group, overall discomfort was quite high and generally became worse throughout the evaluation period. Table 5 examines the time trends shown in Figs. 3, 4 and 5, using a test of whether the slope of each line is significantly positive or negative. The terminal-using group became significantly less comfortable on all measures during evaluation, while the typing group showed an increase only in Body Part Discomfort Frequency. Also apparent from Table 5 is the low utility of the training period. Only the typing group benefited, and then only on one measure. Clearly, the novel chair, like other chairs, must be evaluated in terms of the task performed on the chair - different tasks give different results. Despite the training given in use of this chair, the overall comfort was not particularly good. Results were worse in overall magnitude than the earlier prototype conventional chair tested, and discomfort increased with time-on-task rather than remaining level. Responses to this chair were more widely spread, with fewer differences from Shackel et al's published results. The few significant differences found were "better than the worst" or "worse than the best" rather than "better than the mid point" or "better than the best" as was the case with the earlier prototype. Body parts affected by the novel chair were primarily the legs, particularly knees and shins. Pressure on the shins, particularly from female subjects wearing high boots or skirts which ended at shin-pad level, was perhaps the most vociferous complaint. Knee discomfort, presumably from the acute knee angle, was noticeable. For this increased leg discomfort there was little or no corresponding decrease in back discomfort. Although subjects had the theory of the chair explained to them during training and tried to sit with a lordotic spine, they often slumped forward to give a kyphotic curve instead. The lack of a backrest, combined with the very restricted posture of the legs, meant that a change of seating posture could only be achieved by alternating these 'good' and 'poor' back postures. Comments from the subjects, whether unsolicited or from the Chair Feature Checklist, confirmed the results of the other rating scales. The seat was found to slope too far forward and the shin pads did not fit all users (even in
46
Applied Ergonomics
March 1985
Table 4: Analysis of subject's comments
Number Number in in TerminalTyping using group group
Comment Genera/evaluation Chair is c o m f o r t a b l e
3
3
Not in chair much
4
-
Prefer t o ordinary chair
-
3
Chair should be adjusted
-
1
Rocking action useful for c o m f o r t
-
7
Entry and egress Problems getting in and out
5
-
Problems swivelling
8
--
Shin pads in w a y of desk
3
0
Lower limb pains Thighs ache
-
6
Knees ache
-
16
Shins hurt
8
11
Ankles hurt
-
3
Feet ache
--
6
Toes ache
-
1
Kept feet flat on f l o o r
4
--.
Need t o keep moving legs
-
5
Other body parts Back c o m f o r t a b l e
4
-
Back hurts
1
--
Pain in neck
-
t
Pain in shoulders
--
1
Table 5: Changes in rating during training and test
Function
Group
GCR
Typing
-
0
Terminal-using
0
+
Typing
0
0
Terminal-using
0
+
Typing
0
+
Terminal-using
0
+
BPDF
BPDS
Training
Evaluation
0 = no significant change
-
significant decrease (p < 0.05)
+ = significant increase (p < 0.05)
relatively homogeneous groups), although the rocking action was appreciated. There are chairs similar to the novel chair tested which do not have rocking action; one can only assume that the more restricted posture and lack of ability to swivel for entry and exit would make such chairs even less comfortable. In summary, the authors have distinct reservations about recommending the chair tested for either prolonged
industrial seating or for jobs requiring frequent chair entry and egress. The chair tested was no worse than the rather unselected sample of 15 years previously (Shackel et al, 1969) but does not compare well with a modern, conventional chair. Having said this, it should be noted that a minority of subjects preferred this chair to their own (rather indifferently designed) office chairs. The novel chair created considerable interest and comment and, with more attention to restricted leg posture and lack of available back support, could well have a place for certain subjects.
Corlett, E.N., and Bishop, R.P. 1976 Ergonomics, 19, 175-182. A technique tbr assessing
Finally, the simple methodology developed in the previous study worked well on this unusual chair. Many significant results were found, despite the relatively small sample size. Such a methodology can be used to give useful specific design feedback at modest cost in time and effort, without requiring either a large number of prototype chairs or a large number of comparison chairs.
Mandal, A. 1981 Applied Ergonomics, 12.1, 19-26. The seated man
postural discomfort. Drury, C.G., and Coury, B.G. 1982 Applied Ergonomics, 13.3, 195-202. A methodology for chair evaluation,.
Keegan, J.J. 1953 Journal ()/Bone and Joint Surgery, 35, 589-603. Alterations of the lumbar curve.
(Homo sedens). The seated work position, Theory and practice. Mandal, A. 1982 Human Factors, 24.3,257--269. The correct height of school furniture.
Shackel, B., Chidsey, K.D., and Shipley, P. 1969 Ergonomics, 12.2,269-306. The assessment of chair comfort.
Bibliography Anon
Schoberth, H.
1976 Design, 333, 32-35, Woik seat challenge.
1962 'Sitzhalten, Sitz~chaden, Sitzmobel'. Springer, Berlin.
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Applied Ergonomics March 1985
47
Evaluation of a forward-sloping chair C.G. Drury and M. Francher Department of Industrial Engineering, State University of New York at Buffalo, Amherst, New York 14221, USA
A methodology of chair evaluation developed earlier was applied to a novel, forwardsloping chair. Two groups of six subjects each were trained to use the chair and then given an evaluation session of 21/2 h. The chair elicited mixed responses, with complaints of leg discomfort from terminal users and of entry and egress from typists. Overall, the chair was no better than conventional chairs and could be worse than well-designed conventional office chairs.
Keywords: Chairs, posture, product design Introduction Ergonomists are frequently asked for chair design recommendations and, until a few years ago, these were relatively easy to provide. Recommendations based on anthropometry and biomechanics were generally convergent in defining a 'good' chair. Recently the issue of chair design has become more controversial with the introduction, largely by Mandal (1981, 1982) of the concept of a forwardsloping seat instead of the conventional backward-sloping seat. Industrial concerns which must specify seating for their employees have frequently only been exposed to advertising or popular media presentations on one type of chair or the other. The marketplace is characterised by strongly held opinions and a relative absence of data. Because of an already existing methodology for chair evaluation (Drury and Coury, 1982), the authors were asked by an interested industrial chair user to evaluate an example of a chair with a forward-sloping seat. This paper is the result of that evaluation, lit should be emphasised that only a single example of a forward-sloping chair was tested and that no formal statistical comparison was made between the chair tested and the prototype chair tested earlier]. In addition to the obvious outcome of how well the chair worked, a major question of interest was whether the methodology developed earlier could be applied to such a novel chair.
school children and quoted the earlier workers in equating this with increased disc pressures in the lumbar region. In order to alleviate this condition, he proposed a chair with a forward tilt on the seat pan of 0 ° to 30 °, combined with a raised and rearward-tilting desk. Such a workplace would decrease the eye-to-workpoint distance as well as relieving the spinal stress. The system has apparently been tested with Danish school children (Mandal, 1982) although no results were presented. As evidence to support his tilted seat theory, Mandal measured the extension of back muscles in various sitting positions for 10 subjects. His data showed considerably less (back muscle) extension in a tilted chair when compared with a traditional chair. Mandal also measured seat pressure distribution by placing blood pressure cuffs in the front, middle and rear zones of the seat. His findings showed the tilted seat to have a more equal pressure distribution in comparison with other seated reading positions A critique of this chair was presented in Design magazine (Anon, 1976) in conjunction with descriptions of the Mandal chair and the Hunting and Grandjean chair. The abstract quoted Hunting as saying that the tilting seat solution was "not right". In the same article, J. Ward raised a number of objections. The chair appeared to her to violate three of the principles of good seating: 1. Low pressure under the thighs, 2. Adequate lumbar support,
The forward-sloping seat - pros and cons
3. Ability to change posture in long-term tasks.
Mandal's ideas on seat design stem from the fact that in sitting the pelvis rotates backwards by approximately 30 ° compared with a standing position. This rotation reduces the standing lumbar curve, described as the "normal anatomical posture, which corresponds closely to the military alert position" (Mandal, 1981). Drawings of the lumbar curve in sitting and standing (Keegan, 1953; Schoberth, 1962) were used to illustrate this reduction in lordosis while sitting. Mandal found a lack of lordosis in the seated posture of
Mandal (1981) states, and the Design writers confirm, that the forward-sloping seat may take some days to appear comfortable. He states 1 - 2 weeks (1981) while Design gives 3 - 4 days as a learning period. The chair evaluated here was not known to have been designed or even approved by Dr Mandal, although it did have a forward-sloping seat and did show diagrams of pelvic rotation similar to those in his paper in the advertising literature which accompanied the chair.
0003-6870/85/01 0041 07 $03.00 Q 1985 Butterworth & Co (Publishers) Ltd
Applied Ergonomics
March 1985
41
The chair evaluated Figs. 1 and 2 show two views of the chair used in the experiments reported here. The chair was supported on beechwood rockers so that different postures could be assumed and so that the chair could be 'swivelled' around on a hard floor to face different directions. Two other features were the lack of backrest and the presence of shin pads. These shin pads, covered in the same fabric as the seat, allowed the sitter to maintain posture by preventing the buttocks from sliding off the seat. In use, the seat had about a 15° forward slope.
Evaluation methodology
Measurements The Drury and Coury (1982) evaluation methodology could not be used in total because there were no anthropometric criteria for comparison and no adjustments to the chair tested. Hence the evaluation was done using the Shackel et al (1969) General Comfort Rating (GCR), the Corlett and Bishop (1976) Body Part Discomfort (BPD) scale and the Shackel et al Chair Feature Checklist (CFC). All were described in detail in the earlier paper. The Chair Feature Checklist contained specific questions about the backrest. As these obviously did not apply to the forwardsloping chair, they were replaced by questions concerning the shin pads.
Experimentaldesign In place of fitting trials, which seek to adjust the workplace to the user, a 'training period' was given for the user to adjust to the chair. Following the authors quoted earlier, five training periods were given on the five days prior to the
Fig. 2
Chair tested, side view
evaluation. Each training period consisted of half an hour of work, using the chair in place of the subject's usual chair. All subjects had the principles behind the novel chair explained to them at the start of the first training period, using the diagrams from the advertising literature accompanying the chair. GCR and BPD were taken at the end of each training period. After the five periods of training, subjects were given the chair for a 2-5 h evaluation session. Subjects performed their normal work at their normal workplace except that at the end of each half-hour they were interrupted to record their measures on the GCR and BPD scales provided, A new scale was used on each occasion to prevent ,carry over' effects from previous readings. After the 2-5 h evaluation period, the CFC was administered. Unsolicited comments of subjects at each measurement time were recorded, during both the training and evaluation periods. As in the earlier study, two groups of subjects were used, a typing group and a terminal,using group. Table 1 shows the composition of the two groups. The first group of six female secretaries used the novel chair in place of their usual typing chair during regular work periods. Since office duties required the user to leave the chair occasionally, subjects were allowed to move about during the ~ h trials and the 2.5 h working trial. The second group consisted of four males and two females using a Lear, Siegler computer terminal. All were students who frequently spent long periods interacting with one of these CRT terminals connected to the University computer. Subjects in this second group did not leave the chair during the 2.5 h trial so that their comfort ratings can be compared with the Shackel et al (1969) "long term sitting" group rather than the "typing" group. These two groups probably represent extremes of use by an office population and, as found in the earlier study, would be expected to have rather different :responses to a chaiL
~ts
Fig. 1
Chair tested, front view
42
AppliedErgonomics March 1985
The General Comfort Ratings as a function_of time during training and evaluation are shown in Fig. 3. The typing group showed a reduction in discomfort in training and a relatively level rating during evaluation, while the terminal-
Table 1: Composition of two subject groups
Group
Males
Females
Age (yrs)
Height (m)
Weight (kg)
Typing
0
6
35-52
1"57-1"65
54-70
Terminal-using
4
2
19-24
1.72-1 '80
68-84
using group showed relatively level rating during training and .~ an increase in discomfort during evaluation. Testing these results against the range of 10 chairs measured by Shackel et al (1969) gave the results in Table 2. The typing group found the novel chair better than the worst of Shackel et al's 10 chairs for the first 2 h. The terminal-using group found this chair better than the worst Shackel et al chair for the first hour and then worse than their best for the 1 h to 1½ h period.
Table 2: Significance of differences (p < 0-05) between
Body Part Discomfort results for both groups are given in Table 3 for each body part. Overall indices of discomfort were defined as:
chair tested and best, mid and worst of Shackel etal sample of 12 chairs using GCR.
Typing group
Body Part Discomfort Frequency (BPDF): the percentage of Body Part Discomfort responses which were non-zero at any measurement time.
Terminal-using group
Best Mid
Worst
Best
Mid
Worst
30 min
NS
NS
Better
NS
NS
Better
1h
NS
NS
Better
Worse
NS
Better
90 min
NS
NS
Better
Worse
NS
NS
2h
NS
NS
NS
NS
NS
NS
(NS = not significant)
Body Part Discomfort Severity (BPDS): the mean scale value of all non-zero responses at any measurement time. discomfort reported by the terminal-using group. The main regions of discomfort were the legs (including the response "knees" by the terminal-using group, even though this response was not on the Corlett and Bishop scale) and, to a lesser extent, the back and buttocks.
Both measures have been used in a series of studies of manual materials handling as well as for the paced inspection group in the previous chair evaluation paper to provide summary measures of workplace discomfort. Nots of BPDF and BPDS against time period are shown in Figs. 4 and 5. The terminal-using group had more Body Part Discomfort complaints than the typing group, although both had a similar severity of complaint. This impression is further emphasised by Table 3, which shows overall greater
Chair Feature Checklist results are shown for both groups in Figs. 6 and 7. Height, length arid width were adequate for both groups. The seat was found to slope too far forward but to be of adequate-to-good shape. Shin pads
j.f
3.5F
30 t Terminal-
T/ .E
usi.g
\/" \\
2.0
- - . - \,,
/
f
-~
0.... ~
iyp,ny
-o
o
t5
,,...0~ ~ . 0 /
~0.~
,~
!
t--
30 =BarelyComfortable 20 =QuiteComfortable 40 =Perfectly Comfortable 0.0 =CompletelyRelaxed
Q9
0.5
/
-
0LL_
..... l 2
[
1
1Itl
3
4
5
Training, Half-HourPeriods
I
0
1
l
I_
2
EvaluationPeriod, Hours
Fig. 3
General comfort ratings over time
Applied Ergonomics
March 1985
43
Table 3: Percentage of occasions on which each body part was reported as having pain/discomfort Typing group Training
Terminal-using
Evaluation
group
Training
Evaluation
Neck
-
20
17
Shoulder
--
10
13
-
13
7
-
3
-
17
13
3
20
-
13
27
-
20
27
-
27
20
Upper arms
-
Lower arms Hands
Upper back
20
10
Mid back Lower back
-
Buttocks
6
-
Thighs
-
-
33
33
(Knees)
-
_
40
47
Legs
50
60
43
43
./'----"
25 2O
/
•
"E
'= Q..
t5
EL
°'--.~o\
/o
\
Fig. 4
Body Part Discomfort Frequency over time
//0~
/
\~ing
I
I
I
I
t
2
3
4
~0------0 S
o/ I .,.,
5
I
.
0
J
I
1
i
I
2
EvaluationPeriod, Hours
Training, Half-Hour Periods
2.5
/P /
2.0 ram
/t°~
1.5
~
rj~ n 03
Tpi.g
I
I
I
I
I/!I
t
2
3
4
5
i 0
Training, Half- Hour Periods
44
Applied Ergonomics
•
Terrain01-
'V /
-
IO
March 1985
I
i
I
EvaluationPe'iod, Hours
I
2
~- ~ 0
Fig. 5
Body Part Discomfort SeVerity overtime
,
TOO HIGH SEAT HEIGHT ABOVE
CORRECT A
TOO LOW
CORRECT ~ | ~
TOO SHORT _j
rHE FLOOR
TOO LONG [
SEAT LENGTH
__
TOO NARROW
CORRECT
SEAT WIDTH
"
Z --"
SLOPES TOO FAR TOWARDS BACK I
SLOPE OF SEAT
CORRECT
ADEQUATE I =
POSITION OF SHINPADS
TOO HIGH L
CORRECT i ' t
SLOPE OF SHINPADS
POOR FIT l
Fig. 6
SEAT HEIGHT ABOVE
TOO HIGH I
THE FLOOR
SEAT LENGTH
SEAT WIDTH
SLOPE OF SEAT
-=
CORRECT
TOO LOW I
TOO LONG
CORRECT
L
I
TOO SHORT I
TOO NARROW I
CORRECT
SLOPES TOO FAR TOWARDS BACK L--
PITS 1 WELL . _ _ J
I
HINDERANCE
I
SLIGHTLY OBSTRUCTED
I
C L E A R A N C E FOR F E E T A N D C A L V E S UNDER CHAIR
TOO LOW / J
~
NEUTRAL
I t'
TOO LITTLE
Chair Feature Checklist results, typing group
I GOOD l
-=
ADEQUATE • l
J
USEFUL ROCKING ACTION
SLOPES TOO FAR = TOWARDS FRONT ' 1
a --
II
POOR I
SEAT SHAPE
I
I
i
TOO WIDE _j
j
ADEQUATE
•
i
TOO WIDE I
CORRECT
SLOPES TOO FAR TOWARDS FRONT 1
ADEQUATE SEAT SHAPE
POSITION OF SHINPADS
SLOPE OF SHINPADS
ROCKING
ACTION
POOR
I
TOO HIGH
~
GOOD
CORRECT ~
TOO LOW
t
j
POOR FIT t
~
USEFUL L 1 TOO LITTLE
C L E A R A N C E FOR F E E T A N D C A L V E S UNDER CHAIR
•
~
ADEQUATE l
NEUTRAL
FITS WELL J
HINDERANCE
t
F
J SLIGHTLY OBSTRUCTED
•
ADEQUATE
t
I
Fig. 7
Applied
Chair Feature Checklist results, terminal-using group
Ergonomics
March 1985
45
were judged rather low by the typing group and of rather poor fit by the terminal-using group. Both groups appreciated the rocking action but found leg clearance slightly obstructed. Table 4 shows the subjects' comments grouped under major headings. Generally the typing group found entry and egress problems, while the terminal-using group suffered from leg problems. This table also shows the wide variety of responses to the novel chair, all the way from preferring it to an ordinary chair to loudly voiced complaints of knee and shin pain. Discussion and conclusions
As in the previous chair evaluation study, considerable differences emerged between the two groups. The typing group, who spent a considerable proportion of their time getting in and out of the chair, had more concern with entry and egress but less overall discomfort on all scales. The terminal-using group, who spent a full 2½ h working period in the chair, said nothing about entry and egress but were rather uncomfortable, especially in the area of the legs. For this group, overall discomfort was quite high and generally became worse throughout the evaluation period. Table 5 examines the time trends shown in Figs. 3, 4 and 5, using a test of whether the slope of each line is significantly positive or negative. The terminal-using group became significantly less comfortable on all measures during evaluation, while the typing group showed an increase only in Body Part Discomfort Frequency. Also apparent from Table 5 is the low utility of the training period. Only the typing group benefited, and then only on one measure. Clearly, the novel chair, like other chairs, must be evaluated in terms of the task performed on the chair - different tasks give different results. Despite the training given in use of this chair, the overall comfort was not particularly good. Results were worse in overall magnitude than the earlier prototype conventional chair tested, and discomfort increased with time-on-task rather than remaining level. Responses to this chair were more widely spread, with fewer differences from Shackel et al's published results. The few significant differences found were "better than the worst" or "worse than the best" rather than "better than the mid point" or "better than the best" as was the case with the earlier prototype. Body parts affected by the novel chair were primarily the legs, particularly knees and shins. Pressure on the shins, particularly from female subjects wearing high boots or skirts which ended at shin-pad level, was perhaps the most vociferous complaint. Knee discomfort, presumably from the acute knee angle, was noticeable. For this increased leg discomfort there was little or no corresponding decrease in back discomfort. Although subjects had the theory of the chair explained to them during training and tried to sit with a lordotic spine, they often slumped forward to give a kyphotic curve instead. The lack of a backrest, combined with the very restricted posture of the legs, meant that a change of seating posture could only be achieved by alternating these 'good' and 'poor' back postures. Comments from the subjects, whether unsolicited or from the Chair Feature Checklist, confirmed the results of the other rating scales. The seat was found to slope too far forward and the shin pads did not fit all users (even in
46
Applied Ergonomics
March 1985
Table 4: Analysis of subject's comments
Number Number in in TerminalTyping using group group
Comment Genera/evaluation Chair is c o m f o r t a b l e
3
3
Not in chair much
4
-
Prefer t o ordinary chair
-
3
Chair should be adjusted
-
1
Rocking action useful for c o m f o r t
-
7
Entry and egress Problems getting in and out
5
-
Problems swivelling
8
--
Shin pads in w a y of desk
3
0
Lower limb pains Thighs ache
-
6
Knees ache
-
16
Shins hurt
8
11
Ankles hurt
-
3
Feet ache
--
6
Toes ache
-
1
Kept feet flat on f l o o r
4
--.
Need t o keep moving legs
-
5
Other body parts Back c o m f o r t a b l e
4
-
Back hurts
1
--
Pain in neck
-
t
Pain in shoulders
--
1
Table 5: Changes in rating during training and test
Function
Group
GCR
Typing
-
0
Terminal-using
0
+
Typing
0
0
Terminal-using
0
+
Typing
0
+
Terminal-using
0
+
BPDF
BPDS
Training
Evaluation
0 = no significant change
-
significant decrease (p < 0.05)
+ = significant increase (p < 0.05)
relatively homogeneous groups), although the rocking action was appreciated. There are chairs similar to the novel chair tested which do not have rocking action; one can only assume that the more restricted posture and lack of ability to swivel for entry and exit would make such chairs even less comfortable. In summary, the authors have distinct reservations about recommending the chair tested for either prolonged
industrial seating or for jobs requiring frequent chair entry and egress. The chair tested was no worse than the rather unselected sample of 15 years previously (Shackel et al, 1969) but does not compare well with a modern, conventional chair. Having said this, it should be noted that a minority of subjects preferred this chair to their own (rather indifferently designed) office chairs. The novel chair created considerable interest and comment and, with more attention to restricted leg posture and lack of available back support, could well have a place for certain subjects.
Corlett, E.N., and Bishop, R.P. 1976 Ergonomics, 19, 175-182. A technique tbr assessing
Finally, the simple methodology developed in the previous study worked well on this unusual chair. Many significant results were found, despite the relatively small sample size. Such a methodology can be used to give useful specific design feedback at modest cost in time and effort, without requiring either a large number of prototype chairs or a large number of comparison chairs.
Mandal, A. 1981 Applied Ergonomics, 12.1, 19-26. The seated man
postural discomfort. Drury, C.G., and Coury, B.G. 1982 Applied Ergonomics, 13.3, 195-202. A methodology for chair evaluation,.
Keegan, J.J. 1953 Journal ()/Bone and Joint Surgery, 35, 589-603. Alterations of the lumbar curve.
(Homo sedens). The seated work position, Theory and practice. Mandal, A. 1982 Human Factors, 24.3,257--269. The correct height of school furniture.
Shackel, B., Chidsey, K.D., and Shipley, P. 1969 Ergonomics, 12.2,269-306. The assessment of chair comfort.
Bibliography Anon
Schoberth, H.
1976 Design, 333, 32-35, Woik seat challenge.
1962 'Sitzhalten, Sitz~chaden, Sitzmobel'. Springer, Berlin.
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Applied Ergonomics March 1985
47