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EŒects of sitting v ersus standing a nd scanner type on cashiers
K. R. L
EHMAN²
, J. P. P
SIHOGIOS²
* and R. G. J. M
EULENBROEK
³
²
NCR Corporation, Duluth, GA 30096, USA
³
Nijmegen Institute for Cognitio n and Information (NICI), University o f
Nijmegen, PO Box 9104, 6500 HE Nijmegen, The Netherlands
Keywords: Cashier; scanning; sitting; standing.
In the retail supermarket industry where cashiers perform repetitive, light manual
material-h and ling tasks when scanning and handling produ cts, reports of
musculosk eleta l disorders and discomfort are high. Ergonomics tradeoŒs exist
between sitting and stan ding postu res, which are further confounded by the
checkstan d design and point-of-sale techn olog y, such as the scanner. A
laborato ry experiment study was conducted to understand the eŒects of working
position (sitting versus standing) and scanner type (bi-optic versus single window)
on muscle activity, upper limb and spinal posture, and subjectiv e preference of
cashiers. Ten cashiers from a Dutch retailer participated in the study. Cashiers
exhibited lower muscle activity in the neck and shoulders when sta ndin g and
using a bi-optic scanner. Shoulder abduction was also less for standing
condition s. In addition, all cashiers preferred using the bi-optic scanner with
mixed preferences for sitting (n = 6) and standing (n = 4). Static loading of the
muscles was relatively high compa red with benchmarks, suggesting that during
the task of scanning, cashiers may not have adequate recovery time to prevent
fatigue. It is recommended that retailers integrate bi-optic scanners into s tand ing
checkstan ds to minimize postural stress, fatigue and discomfort in cashiers.
1. Introduction
Supermarket checkout work varies throughout the world. One major diŒerence in
such work is created by the workstation design and the average posture adopted
while working. Checkstands in North America, Asia and Australia are typically
designed to accommodate standing postures, whereas seated checkstands are the
norm in many European countries and in South America. Despite diŒerences in the
average working posture of cashiers, no geographical area or checkstand design is
exempt from reports of musculoskeletal disorders (MSD) or discomfort complaints.
The literature reports that MSD problems exist both in Europe among seated
cashiers (Sa
È
llstro
È
m and Schmidt 1984, Buckle 1987, Krueger et al. 1988, Hinnen et
al. 1992) and in North America among standing cashiers (Margolis and Kraus 1987,
Morgenstern and Kraus 1988, Ryan 1989, Wells et al. 1990, B aron et al. 1991,
Harber et al. 1992, 1993, Osorio et al. 1994). Although comprehensive information
*Author for correspondence. e-mail: Jennie.Psihogios@ncr.com
ERGONOMICS, 2001, VOL. 44, NO. 7, 719 ± 738
Ergonomics
ISSN 0014-0139 print/ISSN 1366-5847 online
Ó
2001 Taylor & Francis Ltd
http://www.tandf.co.uk/ journals
DOI: 10.1080/00140130110046180
on the severity a nd costs of MSD among cashiers globally does not exist, various
government agencies do provide statistics on the prevalence of MSD associated with
checkout work. The UK Health and Safety Executive (HSE 1996) reported that a
percentage of cashiers who experienced work loss in 1 year was due to problems
associated with the low back (32% ), wrist (28% ), neck (21% ) and shoulder (21% ).
In the USA, the grocery industry is ranked fourth highest in number of cases of
disorders associated with repeated trauma (BLS 1 996 ).
Researchers suggest tha t the following occupational risks may contribute to
MSD: shoulder load, static tension of the neck, shoulder, and arm muscles, highly
repetitive c ontractions in the shoulder muscles, work at or above shoulder level,
repetitive grasping, extreme deviations of the wrists, and repetitive lifting of loads
(Bjelle et al. 1979, 1981, Luopaja
È
rvi et al. 1979, Hagberg and Wegman 1987).
Grocery scanning is often described as a light and repetitive manual materials-
handling (MMH) task because it involves exerting low force to move products
repeatedly from one side of a checkout to the other. The largest component of a
cashier’s job, an average 45 ± 50% of customer transaction time, is spent scanning or
handling products (Lehman 1998).
For MMH tasks with light loads, the ergonomics literature discusses tradeoŒs
between standing and seated postures. Generally, the literature discourages a static
work posture (either sta nding or sitting) and states that changes in work posture are
important in reducing fatigue (Kroemer and Robinette 1969, Magora 1972). A
standing posture provides a more stable condition for the low back by preserving the
natural lordosis of the lumbar spine (Andersson 1979). Standing also allows for
dynamic use of the arms and trunk, which is better for handling loads, and enables
one to cover larger work areas because of the ability to move. On the other hand,
sitting has been shown to be less energy consuming than standing and less stressful
on the lower extremity joints (Grandjean 1988, Kroemer et al. 1994). However, the
literature cites increased risk of low back pain in seated jobs (Kroemer and
Robinette 1 969, Magora 1972, Kroemer et al. 1994) and greater disc pressure for a
seated than for a standing posture (Andersson et al. 1974). Work in a seated position
can also require greater shoulder abduction, which causes more stress on the
shoulder joints and shoulder/neck. Foot and leg swelling, reduced circulation,
varicose veins, and lower extremity discomfort have been shown to occur in both
standing and sitting occupations (Brand et al. 1988, Sadick 1992, Sisto et al. 1995)
although leg and foot activity reduce swelling (oedema) and increase circulation
(Winkel and Jorgensen 1986, Noddeland and Winkel 1988).
The literature oŒers numerous studies tha t compare ergonomic aspects of
diŒerent checkstand con® gurations. Marras et al. (1993, 1994, 1995), Grant et al.
(1993), Grant and Habes (1995) and Rodrigues (1989) assessed various checkstand
designs by use of instrumentation to quantify joint dynamics, expert evaluation,
posture estimation and heuristic analysis. These researchers recommended a front-
facing design (i.e. where the equipment is located directly in front of the cashier)
which promotes sharing of the load between right and left upper extremities and
minimizes twisting, lifting, forward bending and the moment arm between the load
and spine. Other researchers who have studied aspects of checkout work
recommend general ergonomics principles such as reducing the reach distance,
minimizing lifting, reducing the work surface thickness, and using footrests and
adjustable chairs (Wells et al. 1990, Orgel et al. 1991, Strausser et al. 1991, Wilson
and Grey 1994).
720
K. R. Lehman et al.
A few researchers have evaluated work posture in checkout work using a variety
of biomechanical and physiological measures. Lannerstern and Harms-Ringdahl
(1990) investigated diŒerences between sitting and standing in checkout work by
measuring the thoracic erector spinae, infraspinatus and trapezius muscle activity
and found higher levels in the seated posture. They recommended allowing cashiers
to alternate between sitting and standing positions. Unfortunately, the tasks in this
study were simulated using a non-functional scanner and therefore probably
underestimated the true EMG amplitude. Sandsjo
È
et al. (1996) reported high static
loading of the trapezius during seated cashier work and concluded that the muscles
did not relax during the scanning task.
Typical European supermarket retailers use single-window scanners mounted
vertically. Advances have been made in scanner technology that have provided
ergonomic bene® ts to cashiers. Bi-optic scanners consist of both horizontal and
vertical windows that can read barcodes on four or ® ve sides of a product, thereby
reducing the need to reorient the barcode toward the scanner. Bi-optic scanners have
been shown to reduce wrist accelerations, lifting and a wkward postures compared
with traditional single-window scanners (Lehman and Marras 1994, Lehman 19 96,
Madigan and Lehman 1996).
Guidelines for checkout ergonomics (INRS 1992, FMI 1992, 1996) and
ergonomics regulations of retail workstations (BSR 1991, SZW 1994, 1998,
Arbejdstilsynet 1996) are published in many countries. The guidelines and standards
are useful in terms of recommending workstation design parameters; however, many
standards are ba sed on vi deo display terminal (VDT) research. In virtually all
guidelines and standards reviewed on checkout work, an assumption about work
posture (standing or sitting) is made when making recommendations and
requirements for the checkout. Although these standards do mention that cashiers
should alternate between sitting and standing, they do not provide recommendations
as to how to design the checkstand to provide the option of using both postures.
Often, these publications do not address new technological advancements because
they are not regularly updated.
There has been insu cient research performed to understand the eŒect of posture
(both sitting and standing) on the entire body during checkout work. The literature
contains studies that consider only one part of the body and are restricted to one
work posture. Most researchers have chosen either to measure posture or muscle
activity but have not taken a comprehensive view to consider both for this work task.
While the bene® ts of bi-optic scanners to the hand and wrist have been shown,
measurement of eŒects to the neck and shoulders has not. Because tradeoŒs exist
between sitting and standing, and because the type of scanner confounds the
checkstand desig n for each posture, it is important to understand the implications of
the workstation design and its components. The objective of this study was to
provide a biomechanical and physi ological evaluation of the cashier while working
in sitting versus standing postures with diŒerent scanner types, in order to
understand MS D risk potential for cashiers.
2. Methods
2.1. Participants
Ten female cashiers at a Dutch supermarket volunteered to participate in the
laboratory study. The average (SD) age, height and weight of the participants
were, respectively, 21.1 (5.2) years, 176.8 (4.9) cm and 66.8 (9.0) kg. Participants
721
Posture and scanner effects on cashiers
had an average of 2.8 years of cashiering experience and no history of
musculoskeletal disorders. While all participants worked with vertically mounted
single-window scanners in seated checkouts for most of their career, they were
trained on proper scanning techniques and used the bi-optic scanner for 1 month
prior to testing. Training for the bi-optic scanner included techniques such as
sliding all items, no orienting of items, no ¯ ipping, twisting or rotating items, and
using two hands to share the load of heavier items (Lehman 1996). The procedure
was approved by the university and participants were allowed to withdra w at any
time.
2.2. Experimental design
2.2.1. Independent variables: Two diŒerent scanners, vertical and bi-optic, were
used for this study along with tw o diŒerent work postures, sit and stand. Because the
scanner type aŒected the checkstand con® guration due to its position, scanner and
posture were not viewed as independent of each other. Therefore, the independent
variable was called `scan posture’ and had four levels of combinations of scanner and
posture: sit bi-optic, stand bi-optic, sit vertical and stand vertical. Within each testing
session the four conditions were randomized.
2.2.2. Dependent variables: The dependent measures included muscle activity,
posture and subjective preferences. Muscle activity was obtained through surface
electromyography of the right and left anterior deltoid (RDELT, LDELT),
descending part of the trapezius (RTRAP, LTRAP), levator scapulae (RLEV,
LLEV), and the erector spinae at the level of L3 (RERES, LERES) (® g ure 1).
Posture was collected in three planes of motion for both the right and left arms
relative to the trunk (T1), the head relative to the trunk, the upper back (T1)
relative to the mid-back (L1) and the lower body (L5/S1) relative to the mid back
(L1) (® g ure 2). Roll, pitch and yaw movements were collected and interpreted as
rotation, lateral/abduction, and ¯ exion/extension movements. Subjective discom-
fort and preference s were obtained from each participant at the completion of
each condition. Comfort was rated on a seven-point scale anchored by very
comfortable (1) to very uncomfortable (7). In addition, participants were asked to
explain their discomfort and to comme nt on their preferences.
2.3. Task/equipment
Cashiers sca nned two similar sets of 15 grocery items that included a mix of product
sizes and shapes. Products included a variety of boxes, bags, cans, bottles and
¯ exibles, which were typical items in Dutch supermarket transactions (Lehman
1998). Product weights ranged from 100 to 1000 g with the exc eption of a 2- and 6-kg
product in each set. The checkstand was a front-facing design with incoming and
outgoing c onveyor belts. Additionally, the checkstand was designed to convert to
either a sitting or standing height by the removal of the ¯ oor from the sitting
con® guration (® gure 3). For standing, the distance from the ¯ oor to the top of the
checkstand was 98 cm, while for sitting it was 85 cm. These values are based on
standing and sitting elbow heights for Dutch women. T he chair height was adjusted
by the participant to a height that was the same as her typical seated working height,
which provided a 2 cm clearance between the thigh and underside of the counter.
The chair was obtained from a Dutch superma rket and met ergonomics standards
(padded, ® ve wheels, lumbar backrest) as required by the SZW (1998 ). A footrest
722
K. R. Lehman et al.
was also used and adjusted so that the participant’ s foot was properly supported.
Depth of the counter was the same for both seated conditions,
~
13 cm. The vertical
scanner was mounted slightly to the left of the participant’ s mid-sagitta l plane, which
is typical of European checkstands. The bi-optic scann er was centred directly in front
of the cashier.
The scanners used in the test included both a bi-optic and vertical window
scanner. The bi-optic scanner was the NC R 7875 scanner, which has the ability to
read bar codes from ® ve sides (® gure 4). The single wi ndow vertical scanner was
the NCR 7880 sca nner (® gure 5). Both scanners were fully operational for the
experiment. No other peripheral equipment (keyboard, cash box, printer, etc.) was
operating for the test. Each participant was tested in a single session that lasted
5 h.
2.4. Apparatus
Myoelectric activity was collected via 18 bipotential skin electrodes with a diameter
of 11 mm (SensorMedics, Yorba Linda, CA, USA). The RMS signal was collected
and then normalized to the maximal voluntary contraction of the participant. The
signal was ® ltered from both high- and low-pass frequencies between 20 and
1000 Hz. An A/D converter allow ed the data to be stored on a PC. The posture data
were collected through an Optotrak (Northern Digital, Canada) system that
quanti® ed movement in the three planes of motion. A sampling rate of 100 Hz with a
spatial accuracy of
<
0.2 mm in x, y and z dimensions was used. Infrared markers
were placed on the participant and tracked by three cameras from the Optotrak
system. The three cameras were mounted high on a wall behind the checkstand in
order to view all infrared markers.
Figure 1. Electrode placement.
723
Posture and scanner effects on cashiers
2.5. Procedure
The testing procedure for each participant occurred as follows: overview
information, anthropometric measurement, electrode preparation and placement,
maximum voluntary contractions, rigid body placement, testing conditions, and
subjective questioning. The participant’s skin was prepared for electrode placement
(Marras 1990), and surface electrodes were placed over the belly of the following
muscles bilaterally: anterior deltoid, 2 cm below and 1 cm medial to acromion
(Hagberg 1981); trapezius pars descendes, 2 cm lateral to half the distance between
Figure 2. Rigid body placement.
Figure 3. Front-fac ing checkstand design with interchangeable housing for vertical scanner
and bi-optic scanner. Item ¯ ow proceeds from right (incoming) to left (outgoing)
conveyor belts.
724
K. R. Lehman et al.
C7 and acromion (Ha
È
gg et al. 1987, Sommerich et al. 1998); levator scapulae, at base
of neck (Schu
È
ldt et al. 1986); erector spinae, 3 cm lateral to the spine at level of L3
(Mirka and Marras 1993) (® gure 1). Two ground electrodes were placed on the
participant’s clavicle and spinous process of the L1 vertebra.
After a brief warm-up and stretching exercizes, a series of six types of isometric
exertions were performed to elicit the maximum muscle activity from each muscle.
Participants were instructed to concentrate on using only the muscles of interest for
each exertion. The ® rst exertion, which generated a maximum force from the
trapezius, involved abducting the arm in a 90
8
posture with resistance from a strap
placed proximal to the elbow (with the elbow angle also at 90
8
) (Ha
È
gg et al. 1987).
This exercize was performed individually on both the right and left arms of the
participant. In order to generate a maximum exertion from each del toid muscle, the
arm was fully extended at 90
8
shoulder ¯ exion with resistance placed proximal to the
elbow. The participant was instructed to perform maximum anterior shoulder
¯ exion while keeping the arm straight (Christensen 1986). For the levator scapulae,
participants exerted maximal shoulder elevation by pulling up on two inextensible
straps that were secured to the platform on which the participant stood (Turville et
Figure 4. Bi-optic scanner.
Figure 5. Single-wind ow vertical scanner.
725
Posture and scanner effects on cashiers
al. 1998). For the erector spinae, the participant used an apparatus that allowed her
to hang her torso over a cushion while supporting her body weight through
resistance of the bent lower legs. The participant then performed a dynamic
contraction through her full range of motion, but was limited to a posture of
~
5
8
of
spinal extension by a strap placed around the shoulder blades which provided
resistance (McGill 1992).
After the voluntary maximal exertions, the rigid bodies for the posture a nalysis
were securely pla ced on the participant. A rigid body consisted of three infrared
markers that were previously calibrated to establish its location in three-dimensional
space, which were a xed to a thin piece of aluminium, so that no movement of
markers occurred relative to each other. Rigid bodies were placed on the left and
right arms by means of a lightweight cuŒ(® gure 6). For the head, a headpiece with
the rigid body was placed around the participant’ s head at the level of the temples.
The T1, L1 and L5/S1 rigid bodies were a xed directly on the skin w ith tape (® gures
2 and 6). Then the participant was taken to the checkstand and neutral trials were
collected with the participant standing (or sitting) still in an upright, neutral posture.
This neutral posture was assumed to be `0’ such that all subsequent postures were
reported as deviations from this posture. Participants were then asked to practise a
few transactions so that they would feel comfortable performing the scanning ta sks
with the additional equipment attached to their bodies. No restrictions were placed
on the participant in terms of her scanning speed: participants were simply asked to
scan at a normal, comfortable pace. However, participants were instructed not to
reach past a marked distan ce on the checkstand (30 cm) in order to keep all
participants’ reach envelopes within recommended ranges (SZW 1998).
During each scanning task, EMG and posture data collection began after the
cashier successfully scanned one item and continued until the next to the last product
had been scanned. One trial consisted of continuously scanning 13 items so the
sampling period was not time based, but instead based on number of products. F or
each condition 10 trials were performed by the participant. Two diŒerent product
sets were alternated through the trials. All products were introduced in a random
order on the conveyor belt. At the conclusion of 10 trials, the participants reported
their comfort level. Participants were instructed to consider both the scanner and the
posture when choosing a comfort level. At the end of the experiment participants
reported which condition they preferred overall.
2.6. Data analysis
To norma lize the RMS EMG for each participant, programmes were developed to
process the data as a percent of maximum voluntary contraction (% MVC). The
posture data were also evaluated in a similar manner, by `normalizing’ the posture to
the neutral values collected for each condition. Single-factor repeated measures
ANOVAs were run using SPSS to test for diŒerences between the four conditions.
Appropriate post-hoc tests were conducted using orthogonal contrasts in order to
compare between conditions. T enth and 50th percentile of the EMG data and 50th
percentile of the posture data were used in all analyses. Tenth percentile EMG data
were used to qua ntify the static load on the muscle in order to compare the data with
previous benchmarks (Bjo
È
rksten and Jonsson 1977). Two participants’ data w ere
dropped from the EMG analyses after careful evaluation (due to incomplete data
sets), resulting in EMG data reports from eight participants. Comfort results were
analysed using the Friedman test for non-parametric data.
726
K. R. Lehman et al.
3. Results
3.1. Electromyography
All muscle activity is reported by percent of maximum voluntary contraction
(% MVC). Tables 1 and 2 display the median and tenth percentile EMG data in
terms of the ANOVA analysis by condition. Statistically signi® cant diŒerences were
found for most muscles, especially those of the neck and shoulders. DiŒerences
between conditions for the median data (at p
<
0.05) were found for the left and right
deltoid, left and right levator scapulae, left trapezius, an d left erector spinae. For all
muscles, however, a general trend followed such that standing produced less muscle
Figure 6. A participant with rigid body placement and electrode placement.
727
Posture and scanner effects on cashiers
activity than sitting. In addition, bi-optic standing displayed less muscle a ctivity than
vertical standing, and the same trend was true for sitting. The post-hoc test results are
displayed in ® gure 7, which further illustrates the trends mentioned above. Although
there were no signi® cant diŒerences for the erector spinae muscles of the back, they
did follow the same trend as the upper extremity muscles.
3.2. Posture
Medians from the posture data are reported in table 3, while results from the post-
hoc tests are illustrated in ® gure 8. Postures of interest include left and right shoulder
abduction and ¯ exion and trunk and neck ¯ exion. Shoulder abduction for both arms
was signi® cantly less for the standing conditions (
~
20
8
) than for the sitting (27
8
),
because participants were able to scan with their arms closer to their torsos when
standing. Although there were no statistically signi® cant diŒerences for arm ¯ exion,
the trends are interesting to note. Because the vertical scanner was mounted slightly
to the left of the mid-sagittal plane, left arm ¯ exion is lower for the vertical scanner
while right arm ¯ exion is higher. For neck ¯ exion, the standing conditions produced
higher values than the sitting.
3.3. Performance
Cashier performance was measured as the time it took to scan 13 items of a typical
transaction. Data collecti on began after the ® rst item w as scanned a nd continued
until the ne xt-to-last item was released after scanning. The bi-optic scanner in both
Table 2. Tenth percentile EMGs by condition (% MVC).
10th F p Bioptic
stand
Vertical
stand
Bioptic
sit
Vertical
sit
Left delt 17.7 0.000 3.6 3.9 5.2 5.9
Right delt 7.9 0.001 3.9 4.3 5.5 5.7
Left lev 25.9 0.000 4.8 5.2 7.0 7.2
Right lev 4.1 0.020 6.2 7.1 7.6 8.0
Left trap 30.3 0.000 6.1 6.1 9.1 9.3
Right trap 2.4 0.098 6.0 6.6 6.9 8.0
Left eres 2.7 0.073 7.4 7.5 9.4 10.0
Right eres 1.7 0.205 6.8 7.3 8.4 8.5
*Muscles in bold indicate signi® cance at p
<
0.05.
Table 1. Median EMGs by condition ( % MVC).
Median F p Bioptic
stand
Vertical
stand
Bioptic
sit
Vertical
sit
Left delt 30.5 0.000 5.1 5.7 7.3 8.9
Right delt 11.1 0.000 6.4 6.8 8.7 9.5
Left lev 25.5 0.000 6.2 6.7 8.9 9.5
Right lev 4.5 0.014 8.1 9.3 9.4 10.4
Left trap 21.9 0.000 8.2 8.2 11.6 12.1
Right trap 2.9 0.055 7.8 8.8 9.1 10.6
Left eres 3.1 0.047 9.8 10.1 11.8 12.5
Right eres 1.6 0.215 8.5 9.3 10.0 10.4
*Muscles in bold indicate signi® cance at p
<
0.05.
728
K. R. Lehman et al.
postures had signi® cantly better performance than the vertical scanner in a seated
posture (® gure 9). On average, bi-optic trans actions (19 s) were
~
18% faster than
those using the vertical single-window scanner (23 s).
3.4. Preference
Subjective reports of discomfort and preference were provided by the cashiers after
completing 10 transactions of each condition and at the end of the experiment.
Comfort rankings are shown in ® gure 10 for each combination of scanner and
posture, along with statistical diŒerences among conditions. Sitting while using the
bi-optic scanner was rated 2.0 (on a scale of 1 ± 7), with one denoting very
comfortable. The next most comfortable condition for the cashiers was standing
while using the bi-optic sca nner, with an overall rating of 2.5. Overall, six of 10
Figure 7. Means used in the test for diŒerences between median EMGs (% MVC). Post-hoc
test results group diŒerences between conditions as A, B or C.
729
Posture and scanner effects on cashiers
cashiers preferred the sitting bi-optic condition, while four of 10 chose the standing
bi-optic condition.
No cashiers selected the vertical scanner in either posture as their preferred
condition. A few cashiers noted that they thought the bi-optic scanner was more
comfortable to use because they did not have to lift or turn items. For the cashiers
who preferred standing, some mentioned that they felt more comfortable in their
arms and could move around and reach for items more easi ly. For the cashiers who
preferred sitting to standing, most indicated that sitting w as less tiring.
4. Discussion
The results of this research indicate that for all muscle groups and for shoulder
posture the best condition was the standing posture and bi-optic scanner, while
the worst condition was the seated and vertically mounted single-window scanner.
Regardless of scanner type, the seated conditions resulted in greater muscle
activity in the shoulder and neck and more extreme postures for the shoulders
than the standing conditions. This may explain why cashiers in seated
workstations often experience MSD symptoms in the neck and shoulders
(Sa
È
llstro
È
m and Schmidt 1984, Buckle 1987, Krueger et al. 1988, Hinnen et al.
1992). Despite the physiological disadvantages for the shoulder and neck found in
the present study, the cashiers preferred sitting to standing. It is possible that
these cashiers were not aware of the possible long-term consequences of this stress
nor did they attribute any neck or shoulder symptoms they may experience to
their seated working conditions. In addition, since cashiers were asked about their
total body comfort, they may have been concerned about tiring their lower
extremities if required to work standing.
The median levels of trapezius muscle activity observed in this study (8 ±
12% MVC) were comparable with levels (14 ± 16% MVC) found in light load
Table 3. Med ian within-subject posture values averaged across condition (degrees from
neutral). A negative direction denotes the direction of body part movement for a negative
data value.
Posture-m edia n F p Bioptic
stand
Vertical
stand
Bioptic
sit
Vertical
sit
Direction
negative
(
Ð
)
Thoracic ¯ exion 2.0 0.139 3.0 5.8 2.0 2.7 back
Thoracic lateral 6.5 0.002
Ð
1.4
Ð
2.2 1.1 2.0 left
Thoracic rotation 3.1 0.045 1.4 2.0 1.6
Ð
1.3 ccw/left
Neck ¯ exion 5.8 0.005 17.7 17.7 11.1 10.2 back
Neck lateral 6.0 0.003
Ð
3.5
Ð
3.2
Ð
3.4
Ð
2.1 right
Neck rotation 2.5 0.082
Ð
13.4
Ð
12.6
Ð
14.8
Ð
13.2 right
Left arm abduction 9.8 0.000 18.2 19.9 25.0 28.8 in
Left arm ¯ exion 2.2 0.10 5 8.9 7.9 7.3 5.2 back
Left arm rotation 2.9 0.057 16.6 19.2 17.3 15.8 forward
Low back ¯ exion 0.3 0.824 3.1 2.1 1.8 0.6 back
Low back lateral 0.3 0.839 0.6
Ð
1.8
Ð
2.1
Ð
1.9 left
Low back rotation 2.3 0.103 3.2 0.7
Ð
0.3 1.1 ccw/left
Right arm abduction 4.7 0.010 20.2 20.3 27.1 27.3 in
Right arm ¯ exion 0.7 0.552 8.7 9.8 8.3 11.5 back
Right arm rotation 1.1 0.384
Ð
12.1
Ð
8.5
Ð
14.5
Ð
15.4 backward
*Posture s in bold indicate signi® canc e at p
<
0.05.
730
K. R. Lehman et al.
repetitive jobs (Christensen 1986, Jensen et al. 1993). In similar research on cashier
work posture, Lannerstern and Harms-Ringdahl (1990) reported lower levels of
muscle activity in the trapezius muscles (4 ± 9% MVC) and right levator scapulae (2 ±
3% MVC), but their ® ndings con® rmed that the standing posture produced lower
levels of EMG than sitting for the shoulder, neck, and back muscles tested. Lower
levels of muscle activity may have been measured in the previous work because the
Figure 8. Median postures (degrees from neutral). Post-hoc test results group diŒerences
between conditions as A, B or C.
731
Posture and scanner effects on cashiers
Figure 9. Performance data: time (s ) to complete a 13-item transaction. Post-ho c test results
group diŒerences between co ndit ions as A, B or C.
Figure 10. S ubjec tive median comfort rating of a scan ner and condition across participants.
Post-hoc test results group diŒerences between conditions as A, B or C.
732
K. R. Lehman et al.
scanner was not operational and because the data collection included other less
stressful tasks such as pa yment.
Levels of muscle activity were also compared with benchmarks for long duration
tasks recommended by Bjo
È
rksten and Jonsson (1977). The median muscle activity
for all muscles did not exceed their benchmark of 10 ± 14% MVC for any of the
conditions. However, the static lev els (p = 0.1) w ere well above their recommenda-
tion of 2 ± 5% MVC for most muscle s and conditions. High levels of sta tic loading
suggest that the muscles rarely return to re sting levels and therefore cannot fully
recover. Although high levels of static loading w ere observed during scanning, it
should be noted that the scanning task only represents 40 ± 50% of a customer
transaction (Lehman 1998). Additional tasks that ma y provide muscula r relief
include payment, waiting for the customer, wait time between customers and other
miscellaneous tasks.
Seated cashiers required shoulder postures that exceeded recommended joint
angles. Many researchers recommend that shoulder abduction a ngles should not
exceed 20
8
for continuous work (Tichauer 1968, Cha n and Andersson 1984,
Grandjean 1988). In the present study, the standing condition allowed the
participants to work with their shoulders abducted at 20
8
or less, but in the seated
condition, average shoulder abduction angles ranged between 25 and 29
8
. Aara
Ê
s
(1988) recommends shoulder ¯ exion angles of
<
15
8
for continuous tasks. Both
right and left shoulder ¯ exion were below this benchmark for all conditions. In the
bi-optic scanning conditions, the right and left shoulder joint angles were fairly
balanced, whereas a trend towards more right shoulder ¯ exion was observed for
the single window scanning condition. When the neutral trials were further
analysed, the average lumbar extension angle (lordosis) was 15.4
8
while sta nding
and 0.6
8
while sitting. These results indicate that participants’ normal spinal
curvature was ¯ attened when sitting, which results in higher disc pressure
(Andersson et al. 1974).
It is generally accepted that neck ¯ exion should be
<
20 ± 30
8
for a prolong ed
period and that 15
8
is acceptable for static jobs (Cha n and Andersson 1984,
Grandjean 1988). The standing conditions exhibited neck ¯ exions of 17 ± 18
8
and
only 10 ± 11
8
for seated conditions. However, in this study, the participants had no
display, ke yboard or customer with which to interact. Cashiers would probably be
less apt to focus on the products and scanner in actual w ork and more on the
customer or display, especially when using the bi-optic scanner which requires less
orienting of barcodes to a window.
Muscle activity was lowest in the bi-optic standing condition not only because
participants were able to scan without abducting their arms, but also because the
centred position allowed the participants to share the work more evenly between the
right and left upper extremities. Lifting, reaching and item manipulation were
reduced due to the scanner’s two-window technology. The ease of use of the bi-optic
scanner was validated by the productivity results, where cashiers scanned 18% faster
with the bi-optic. Furthermore, all participants preferred the bi-optic scanner and
rated it more comfortable. The post-hoc comparison gra phs show the trend that the
bi-optic produced less stress than the vertical scanner within each posture. Previous
research has already demonstrated the bene® ts of bi-optic scanning ove r single-
window scanners in reducing risk factors that may contribute to MSD of the hand/
wrist (Lehman 1996). It now appears that risk of MSD of the entire upper extremity
may be reduced by utilizing bi-optic rather than single-window scanners. Although
733
Posture and scanner effects on cashiers
more cashiers preferred sit ting to standing, these cashiers had never experienced a
standing worksta tion before this test. If standing workstations are to be
implemented, it is important that cashiers get accustomed to this diŒerent posture
before assessing their acceptance and whole body comfort.
Researchers have indicated that loads
<
1 kg may cause fatigue if handled
repetitively (AaraÊ s 1988, Wiker et al. 1989). In the bi-optic conditions, products
were usually pushed or slid across the scanner and therefore participants probably
did not exert as much force to counteract the entire product w eight. With further
analysis of EMG data within trials, average peak loads of 23.0, 24.8, 32.8 and
33.7% MVC were recorded for the right trapezius during the 6-kg box movement
for sitting bi-optic, standing bi-optic, standing vertical and sitting vertical
conditions, respectively. Less muscle activity was recorded for the bi-optic
conditions. Cashiers who used a smooth, two-handed sliding motion for this box
had lower muscle activity levels. The high peak loads observed for the heavy
product in each trial demonstrates that training cashi ers to minimize lifting is
important in reducing muscle load.
Ergonomics guidelines were followed by using a front-facing checkstand design,
reducing reach and lifting by moving conveyors inward, and providing an adjustable
chair and footrest. One cannot a ssume the same results for cashiers working at a
checkstand that does not meet these criteria. Checkstand design is of equal
importance as work posture and scanner type when designin g a solution to minimize
MSD in cashiering occupations. T ypically, European checkstands are 2 ± 3 cm
thinner in counter depth than the one used in this experiment, which may aŒect
shoulder abduction and muscle activity somewhat.
Methodological limitations of using EMG to record activity of muscles in free
dynamic tasks exist. It is di cult to determine whether the pick up area remains
constant as a muscle contracts and extends because the electrode on the skin may not
remain over the same muscle ® bres. In addition, as a muscle’s length changes, its
activation level will vary to produce a constant force lev el (Winter 1990). The MVCs
in this study were performed a t a single posture and therefore the muscle length ±
strength relationship was not quanti® ed. As a result, the % MV C might vary
somewhat depending on the posture. Finally, the velocity of muscle contraction has
also been shown to aŒect the EMG-muscle force relationship (Winter 1990).
Although the static loads reported were high, data were only collected for the
task of scanning. Most cashiers rece ive scheduled rest breaks and unscheduled
micro breaks when waiting between customer tra nsactions during non-peak times.
Before concluding that scanning leads to fatigue, an understanding of whether
tasks such as payment and microbreaks allow muscles to return to resting levels
with su cient frequency and duration to eliminate fatigue is needed. In addition,
further research is required to measure muscle loading and fatigue over a full
work shift to understand requirements for postural relief aids (e.g . lean bar, chair,
¯ oor mats), job rotation, rest break schedules, and other work organization
interventions.
5. Conclusions
It is recommended that retailers integrate bi-optic scanners centred with the ca shier’ s
mid-sagittal plane i nto a standing workstation that prov ides postural relief for
cashiers. Based on the results of this experiment, the researchers make the following
summary points:
734
K. R. Lehman et al.
·
Standing required signi® cantly lower muscle activity for shoulders and neck
than sitting.
·
Lower levels of muscle activity are required using the bi-optic versus the
single window vertically mounted scanner.
·
High sta tic levels of muscle loading were measured, indicating that muscles
may rarely return to resting levels during the activity of scanning.
·
Right and left shoulder abduction was signi® cantly lower for standing
conditions than seated conditions because participants could work below
elbow height.
·
For all muscle activity measures and for shoulder posture, the lowest values
were observed for the standing bi-optic condition whereas the highest w ere
seen for the seated vertical scanner condition.
·
Low back posture and muscle activity showed no signi® cant diŒerences
between the four posture/scanner conditions.
·
Six cashiers preferred the sitting condition compared with four who chose the
standing condition.
·
All cashiers preferred using the bi-optic scanner over the vertical scanner.
Because scanning is estimated to account for
<
50% of customer transaction tasks,
cashiers may have su cient time to rest their muscles during other tasks and rest
periods in order to minimize fatigue. Activities are underway to conduct further
research in live environments to assess standing checkstand design concepts, postural
relief aids a nd rest break recommendations to ensure adequate muscular rest for
cashiers.
Acknowledgements
The authors thank sincerely Myra van Esch-Bussemakers a nd Rasmus de Gruil who
supported every phase of this eŒort with interest, sensitivity and enthusia sm.
Without these team members’ invaluable assistance, it would not ha ve been possible
to overcome the challenges faced during the project.
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