13 14 OBJECTIVE: This study explores the effect of different computer products’ usages with different tasks used to induce psychosocial stress on muscle activity.. Based on our literatur
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DOI:10.3233/WOR-162334
IOS Press
The effect of psychosocial stress on muscle
activity during computer work: Comparative study between desktop computer
and mobile computing products
1
2
3
4
Mohd Firdaus Mohd Taiba,b, Sangwoo Bahnc,∗and Myung Hwan Yuna
5
aDepartment of Industrial Engineering, Seoul National University, Seoul, South Korea
6
bDepartment of Material, Manufacturing and Industrial Engineering, Universiti Teknologi Malaysia,
Skudai, Johor, Malaysia
7
8
cDepartment of Industrial and Management Engineering, Myongji University, Yongin, South Korea
9
Received 13 December 2014
10
Accepted 28 May 2015
11
Abstract.
12
BACKGROUND: The popularity of mobile computing products is well known Thus, it is crucial to evaluate their contribution
to musculoskeletal disorders during computer usage under both comfortable and stressful environments
13
14
OBJECTIVE: This study explores the effect of different computer products’ usages with different tasks used to induce
psychosocial stress on muscle activity
15
16
METHODS: Fourteen male subjects performed computer tasks: sixteen combinations of four different computer products
with four different tasks used to induce stress Electromyography for four muscles on the forearm, shoulder and neck regions and task performances were recorded
17
18
19
RESULTS: The increment of trapezius muscle activity was dependent on the task used to induce the stress where a higher
level of stress made a greater increment However, this relationship was not found in the other three muscles Besides that, compared to desktop and laptop use, the lowest activity for all muscles was obtained during the use of a tablet or smart phone The best net performance was obtained in a comfortable environment However, during stressful conditions, the best performance can be obtained using the device that a user is most comfortable with or has the most experience with
20
21
22
23
24
CONCLUSIONS: Different computer products and different levels of stress play a big role in muscle activity during computer
work Both of these factors must be taken into account in order to reduce the occurrence of musculoskeletal disorders or problems
25
26
27
Keywords: Electromyography, performance, typing activity, trapezius, visual display unit
28
∗Address for correspondence: Sangwoo Bahn, Department of
Industrial & Management Engineering, Myongji University,
Yongin 449-728, Korea Tel: +82 31 330 6445; Fax: +82 31 330
6957; E-mail: panlot@gmail.com.
In the modern world, the use of computers and the 30
Internet are common things More and more people 31
feel the need to access them anytime and anywhere 32
1051-9815/16/$35.00 © 2016 – IOS Press and the authors All rights reserved
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The needs and use of this kind of technology
ubiq-33
uitously brought an explosion in the popularity of
34
mobile computing products The popularity of mobile
35
computing products raises several concerns including
36
well-known health problems such as
musculoskele-37
tal disorders (MSDs) MSDs not only affect workers’
38
health conditions but also reduce performance and
39
involve a very high cost [1, 2] MSDs were
responsi-40
ble for 34% of all workplace injuries and illnesses in
41
the year 2012 and reportedly cost between $45 to $54
42
billion to U.S economy [3, 4] Upper body pains such
43
as neck and shoulder pains are the most typical issues
44
among MSD patients as a result of static posture,
45
working technique and constant static muscle
activ-46
ity [1, 5–9, 18] These problems might be worse for
47
mobile computing product such as tablet and smart
48
phone users because of the inseparable screen and
49
keyboard This condition is worse for the laptop users
50
because it cannot be adjusted freely like general
dis-51
play terminals except for the angle [10] Even though
52
tablets can be used in various positions, a previous
53
study shows that head and neck flexion angles for
54
several typical positions during tablet usage are far
55
from recommended neutral angles for visual display
56
unit [11] Consequently, there might be more concern
57
for development of neck and shoulder discomfort
58
The same thing might happen with smart phone
59
usage because, like tablets, smart phones also have
60
the capability and flexibility to be used in various
61
positions Although flexibility provides a huge
bene-62
fit, it may also cause problems to users For example,
63
the size of smart phone allows users to use it in a small
64
and crowded space / area such as in a subway train In
65
this space, sometimes, it is inconvenient for the user
66
to place the phone higher (because of a privacy
rea-67
son or have difficulty to use it in that position) which
68
subsequently force users to bend or look down if they
69
want to use the phone
70
Many previous studies show that there is another
71
factor that might play a role in the MSD symptom
72
which is called psychosocial stress [12–15] The
def-73
inition of psychosocial stress was given as a stress
74
that associated with a non-physical factor It includes
75
time pressure, low social support, high job demands,
76
high mental workload, high memory demands, low
77
reward, surveillance of workers, and high efforts
78
[9, 12, 16–18] In order to study the effect of
psy-79
chosocial stress in a laboratory setting during
com-80
puter usage, previous studies use different kinds of
81
methods to induce stress including arithmetic tasks,
82
time pressure and color-word tasks The methods
83
used by previous studies are summarized in Table 1
Table 1 Summary of method used by previous studies to induce
psychosocial stress
Time pressure / speed task
The effect of psychosocial stress on muscle activity 84
might depend on the type of task used to induce the 85
stress Different tasks may produce different levels of 86
stress, hence lead to different effects on the muscle 87
activity Based on our literature review, stress induced 88
by a stressful environment (including noise, verbal 89
provocation, calling out any mistakes and evaluation 90
by a supervisor) has the largest effect on trapezius 91
muscle activity (but not on other muscle activity) 92
followed by skill and intelligence tasks, time pres- 93
sure tasks, arithmetic task and color-word tasks [1, 8, 94
12, 14, 18, 19, 28, 30] Therefore, we believed that 95
psychosocial stress would increase trapezius mus- 96
cle activity (First Hypothesis) The difference from 97
results obtained by previous researchers is because of 98
different levels of stress produced by each category 99
which consequently have a different level of effect on 100
trapezius muscle activity (Second Hypothesis) The 101
effect of these tasks on trapezius muscle activity is as 102
104
Stressfulenvironment> Skillandintelligencetask > 105
Timepressure>Arithmetictask >Color−wordtask 106
Previous studies regarding mobile computing 107
products show that they might cause worse MSDs 108
problems on the user compared to desktop computer 109
usage because of their physical factors For example, 110
detachable screens make the neck angles and head 111
tilt of mobile computing product users become larger 112
compared to desktop computer user [11, 36, 37] 113
A recent study by Kim et al [38] between four vir- 114
tual keyboards showed that muscle activity on the 115
shoulder muscle was slightly higher for smaller vir- 116
tual keyboards compared to other keyboards which 117
might be caused by the visual demand Meanwhile, a 118
study by Villanueva et al [39] on the effect of desktop 119
and four portable computer usages showed that mus- 120
cle activity in the neck extensor muscles for portable 121
computers were significantly higher than desktop 122
computers The discomfort survey also showed that
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subjects have most musculoskeletal complaints and
124
eye discomfort when they use the smallest portable
125
computer In addition, posture is also affected When
126
the size of visual display unit decreased, the degree of
127
backward tilting was increased Thus, it is expected
128
that muscle activity during mobile computing
prod-129
uct usage is higher compared to desktop computer
130
usage on the same task (Third Hypothesis)
131
Aside from that, many people are not aware that
132
these physical factors not only affect the physical
133
body but induce psychosocial stress conditions as
134
well For instance, a small screen might induce visual
135
demand that makes the user experience visual strain
136
and tiredness [36, 37, 39] and a small keypad might
137
require user’ concentration and precision [36, 39]
138
Furthermore, a smaller screen might produce larger
139
error rates and decrease satisfaction [40] In addition,
140
a small screen might limit the information that can
141
be obtained by the user, especially via -video or text
142
[41, 42] Consequently, it might increase
psychoso-143
cial stress Yet, in spite of the popularity of mobile
144
computing products, to our knowledge, there is no
145
study that has used any mobile computing product in
146
their psychosocial stress experiment It is expected
147
that the increment of muscle activity during a mobile
148
computing product’s usage under psychosocial stress
149
conditions is larger compared to personal computer
150
usage under the same conditions (Fourth Hypothesis)
151
Therefore, based on our first and second
hypothe-152
ses, the objective of this study is to see the effect of
153
different tasks (color-word, time pressure and
stress-154
ful environment) used to induce the psychosocial
155
stress on muscle activity Meanwhile, based on our
156
third hypothesis, another objective of this study is to
157
see if there is any difference in effect for the usage of
158
different computer products (desktop computer,
lap-159
top, tablet and smart phone) while working on the
160
same task Finally, based on our fourth hypothesis,
161
the last objective of this study is to determine whether
162
psychosocial stress conditions will have a different
163
effect on muscle activity increment with different
164
products
165
2 Methods
166
2.1 Subjects
167
Fourteen healthy male students without
muscu-168
loskeletal symptoms in the neck, shoulder and arm
169
region were recruited from a university campus
Par-170
ticipants were experienced computer users They also
Table 2 Demographic data of the participants and their computer use
profile
used at least one of these four devices at least for 172
4 hours per day and have experience in using other 173
devices Each participant provided informed consent 174
before taking part in the experiment Most of the 175
participants have their own laptop and smart phone 176
Demographic data of the participants and their com- 177
puter use profile were summarized in Table 2 178
2.2 Workstation 179
Subjects sat at the same workstation (fixed table 180
height) for all device usage They can adjust their 181
position and chair height to the most comfortable 182
position before they start the experiment for each 183
device and psychosocial stress task There is no arm- 184
rest provided For the desktop computer, they can 185
adjust the height of the monitor and for other devices; 186
they can adjust the screen angle The monitor used 187
for desktop is Samsung CX1765 (445× 340 mm), 188
Lenovo Z480 (355.6× 230 mm) for laptop, iPad 189
Mini (200× 134.7 mm) for tablet and Galaxy Note 190
2.3 Procedure 192
In this experiment, if the participants are required 193
to perform all conditions using all devices, even with- 194
out any replication, they need to perform at least 16 195
trials It will take a lot of time to do the experiment 196
Besides, the participants need to rest for the same 197
amount of time in order to minimize the effect of 198
fatigue Thus, a large amount of trials and time needed 199
for each participant to perform all conditions for all 200
devices In order to reduce the influences of other 201
factors such as fatigue, or stress caused by a long 202
experiment, the participants were instructed to per- 203
form the four conditions by using only two devices 204
In order to avoid any bias, the devices that they used 205
were chosen randomly However, in order to allow 206
the participants to experience all the conditions used
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to induce stress, each of the participants used one of
208
these combinations: desktop and tablet, laptop and
209
tablet, desktop and smart phone, or laptop and smart
210
phone Besides that, each participant was needed to
211
do two extra tasks using a third device Subjects were
212
allowed to rest for five minutes after each task As
213
the hypothesis of this study is that a stressful
environ-214
ment has the largest effect on muscle activity followed
215
by color-word, time pressure and lastly plain
copy-216
ing; the experiment started with plain copying and
217
ended with stressful environment in order to avoid
218
any lasting effect from the last session
219
2.3.1 Plain copying
220
Plain copying was chosen as a reference because it
221
can be done using these entire products and there is
222
no need for the subjects to use any extra equipment
223
such as a mouse or stylus Many previous studies used
224
plain copying as a baseline for muscle activity [1, 14,
225
20, 27, 28] The participants need to copy some text
226
at their own comfortable pace and condition They
227
were instructed to make a correction if they see any
228
error as they hit the key, but not to try to find the error
229
by reading through all their works [1]
230
2.3.2 Color-word task
231
In this task, the name of a color was presented
232
in another color on a power point slide with black
233
background on a tablet screen while the researcher
234
pronounced the name of the third color using voice
235
[19, 28] The words appear in a random position
Sub-236
jects were needed to type in which color the words
237
were written on using four short keys: “D” = red,
238
“F” = green, J = “blue”, “K” = yellow Different sets
239
of color-word tasks were used for each different
240
device They were reminded not to miss any words
241
and they were told that if they make more than 10
242
mistakes, 10% out of their monetary compensation
243
would be deducted
244
2.3.3 Time pressure
245
This task is based on Hughes et al [20] study
246
In this task, participants were asked to type at 20%
247
faster than their comfortable pace In order to help
248
the participants to work in suitable pace, the new
typ-249
ing speeds were calculated and the target end word
250
for each 1 minute interval were underlined Subjects
251
were told the time for every 1 minute interval to help
252
them identify their performance during the task
Par-253
ticipants were advised that the main objective was to
254
achieve the target, even if it meant committing more
255
typing errors It was not only that, the participants
were also reminded if they could not achieve the target 257
after five minutes, 10% of their compensation would 258
2.3.4 Stressful environment 260
For this task, subjects needed to do the same plain 261
copying task and they were asked to type as fast and 262
as accurate as possible Aside from working under 263
supervision, participants were not allowed to do any 264
correction and every time subjects made a mistake, 265
the experimenter said it out loud In addition, par- 266
ticipants were encouraged to work faster every 30 267
seconds Furthermore, an alarm clock with sound 268
was placed in the same room [1] The participants 269
were told if they could not achieve the target after 270
5 minutes or made more than 10 mistakes, another 271
10% of their compensation would be deducted, 272
2.4 Questionnaire 274
Participants needed to fill out another question- 275
naire after they had finished the experiment There 276
were three parts of this questionnaire, namely: 277
They need to compare the perceived stress between 279
three tasks (color-word, time pressure and stress- 280
ful environment) with the plain-copying task The 281
scale are from “much more relax”, “quite relax”, 282
“slightly relax”, “no different”, “slightly stressful”, 283
“quite stressful” and “much more stressful” 284
All other tasks (color-word, time pressure and 285
stressful environment) were compared to the plain- 286
copying task in order to see the effect of different 287
task on perceived stress clearly by letting the plain- 288
copying task to act as a reference point This is 289
because plain-copying task was done under stress- 290
free condition while all other tasks were done under 291
some stressors which allow the comparison of per- 292
ceived stress between stress and no stress tasks Since 293
all other tasks were anchored on one reference point, 294
which is plain-copying task, this can increase the 295
They need to compare the perceived stress between 298
at least two devices for the same task The same scales 299
Some tasks have three or more stressors imposed 302
on the participants simultaneously For instance, the 303
stressful environment task contains noise, time pres- 304
sure, monetary reduction, verbal provocation and
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negative feedback Thus, this part was used to find
306
the effect of every stressor towards the participants
307
The participants were asked to rate from “0” for not
308
stressful at all up to “5” for very stressful
309
2.5 Electromyography (EMG) and Maximum
310
Voluntary Contractions (MVC)
311
Muscle activity was recorded from the
domi-312
nant upper trapezius, deltoid, extensor digitorum and
313
extensor carpi ulnaris muscles using bipolar Ag-Cl
314
surface electrodes The distance used between
record-315
ing areas was 20 millimeters [31, 32] The skin was
316
prepared by cleaning the located area The EMG
sig-317
nals were sampled at 1024 Hz The precise locations
318
of EMG were based and adopted from previous
stud-319
ies [8, 43] The EMG signals were measured using an
320
EMG LAXTHA device and the signals were analyzed
321
using TeleScan software version 3.09 (LAXTHA
322
Inc., Korea) Isometric maximum voluntary
contrac-323
tions were performed for each muscle At least three
324
MVC were made for each muscle, and each MVC
325
lasted at least three seconds
326
2.6 Data analysis
327
The data were band filtered using 5 Hz and 500 Hz
328
and then root mean square was calculated for three
329
5 s epochs at 60 s, 180 s and 300 s after the task was
330
started The value was then normalized with
maxi-331
mum EMG obtained from MVC The average value
332
was calculated from these three epochs
333
Significant differences of perceived task stress,
334
perceived device stress, perceived condition stress
335
and EMG increment or decrement between
differ-336
ent tasks and different devices were evaluated using
337
repeated measures in Analysis of Variance (ANOVA)
338
Statistical Package for the Social Sciences (SPSS)
339
version 20.0 was used for data analysis with
sig-340
nificance level set at p < 0.05 with 95% confidence
341
interval
342
3 Result
343
3.1 Perceived task stress
344
The difference of mean stress between
plain-345
copying (0.000) and color-word (0.1667) was very
346
low and not significant (p = 0.638) Thus,
color-347
word can be considered as a “no stress” task The
348
time pressure task (mean = 1.4167) is significantly
considered as in the middle of “slightly stressful” 350
and “quite stressful” compared to the plain-copying 351
task (p < 0.000) while the stressful environment task 352
(mean = 2.1667) is considered as slightly more than 353
“quite stressful” compared to plain-copying task 354
(p < 0.000) The perceived stress between the time 355
pressure and stressful environment task is also sig- 356
3.2 Perceived device stress 358
Unexpectedly, there were no significant differ- 359
ences in stress between devices for any task 360
3.3 Perceived stress for each of the stressor 361
In this section, the highest stress was induced by 362
“the needs to change the screen between alphabets 363
and symbols”, followed by noise and typing accu- 364
racy Other significant stressors were time pressure, 365
verbal provocation, negative feedback, and small key- 366
pad There are other stressors that can be considered 367
as insignificant to the participants which are differ- 368
ent color-word for both on screen and using voice, 369
random positioning of appearance during color-word 370
task, supervision by the researcher, small screen and 371
3.4 Muscle activity 373
EMG values for each muscle were analyzed into 374
two categories which are by the effect of different 375
tasks and by the effect of different devices Fig- 376
ure 1 (a – d) shows a box and whisker plot with 377
medians and 25–75 percentiles of electromyogra- 378
phy activity (%EMGmax) for the upper trapezius, 379
extensor digitorum, extensor carpi ulnaris and ante- 380
rior deltoid respectively There are 16 combinations 381
of devices and tasks The first two or three alphabets 382
were devices (DC = desktop computer, LAP = laptop, 383
TAB = tablet and SP = smart phone) and the last two 384
alphabets were tasks (CW = color word, PC = plain 385
copying, TP = time pressure and SE = stressful envi- 386
color-word task was done using desktop computer 388
Some data need to be excluded because of certain 389
technical errors Thus, on average, there were eight 390
participants for each combination of device and task 391
There is a clear effect of the level of stress on the 392
upper trapezius muscle activity Generally, trapezius 393
muscle activity increased during time pressure task 394
compared to plain-copying or color-word tasks and
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Fig 1 Box and whisker plot with medians and 25–75 percentiles of electromyography activity (%EMG max ) under 16 combinations of devices and tasks for a) upper trapezius, b) extensor digitorum, c) extensor carpi ulnaris and d) anterior deltoid.
had a greater increment when they did stressful
envi-396
ronment task The statistically significant results for
397
trapezius muscle activity were obtained for several
398
task comparisons for each of the devices (Table 3)
399
Aside from the trapezius muscle, other muscles also
400
show significant increment However, there is no clear
401
relationship of the increment pattern
402
Unexpectedly, the only increment that happened
403
between a desktop computer and a mobile computing
404
product was between a desktop computer and laptop
405
However, it is not statistically significant At the same
406
time, the comparison between a desktop computer
407
and tablet / smart phone or between laptop and tablet /
408
smart phone shows a significant decrement (Table 4)
409
Muscle activity during the usage of a tablet and smart
410
phone for all muscles and tasks is much lower
com-411
pared to the muscle activity during the usage of a
412
desktop computer and laptop
3.4 Performances 414
The performance of participants was different 415
depending on the task and the devices The fastest typ- 416
ing speed was during the stressful environment task 417
Generally, the result shows that there are significant 418
increments of typing speed from the plain-copying 419
task to the time pressure and stressful environment 420
tasks However, there is no significant difference of 421
typing speed between different devices for the same 422
task which means most of the participants can type on 423
smart phones or tablets as fast they type on desktops 424
or laptops Nevertheless, it should be noted that this 425
might be true only for slow typists Meanwhile, num- 426
bers of errors increased significantly for each task and 427
the same thing happened for all devices The lowest 428
number of errors was obtained during the usage of a 429
smart phone The mean of typing speed in terms of
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Table 3 The significant increment of muscle activity between two tasks
Task (i) Task (j)
1 For devices: DC = desktop computer, Lap = laptop, Tab = Tablet computer, SP = smart phone 2 For tasks:
CW = color word, PC = plain copying, TP = time pressure, SE = stressful environment 3 Repeated measures ANOVA
(bold indicates significant effect p < 0.05).
words per minute (WPM) and number of error were
431
summarized in Table 5
432
Net values for performance were calculated to see
433
the actual performance by the participants
Interest-434
ingly, the highest performance was obtained during
435
the plain copying task using a desktop In addition,
436
the net performances of the participants were better
437
during comfortable conditions compared to during
438
stressful conditions Besides that, during the time
439
pressure and stressful environment tasks, the best
per-440
formance was obtained during the usage of a laptop
441
4 Discussion
442
Previous studies showed a different result in
443
regards to the effect of psychosocial stress on muscle
444
activity Even for the most common muscle studied
which is the trapezius muscle, the effect of stress on 446
muscle activity can be divided into two main groups 447
One group found that when a certain psychoso- 448
cial stress exists, the trapezius muscle will become 449
affected and the muscle activity will be increased sig- 450
nificantly [8, 12, 18, 21, 29–31, 35] On the other 451
hand, another group of researchers found that the 452
existence of a certain psychosocial stress did not have 453
any significant effect on trapezius muscle activity [9, 454
14, 18, 19, 23, 24, 27, 28] Meanwhile, there are sev- 455
eral studies that can be categorized in a third group 456
where this group obtained a combination of the two 457
It is predicted that there is a relationship between 459
the task used to induce the stress and the effect 460
on trapezius muscle activity We believe that this 461
relationship is the main reason that leads to the 462
result obtained by these previous researchers This 463
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Table 4 The significant decrement of muscle activity between two devices
Device (i) Device (j)
1 For tasks: CW = color word, PC = plain copying, TP = time pressure, SE = stressful environment.2For devices:
DC = desktop computer, Lap = laptop, Tab = tablet computer, SP = smart phone 3 Repeated measures ANOVA (bold
indicates significant effect p < 0.05).
Table 5 Mean of the participants’ performance in terms of WPM and number of error
Plain copying Time pressure Stressful environment Plain copying Time pressure Stressful environment
is because each of the tasks used to induce stress
pro-464
duces a different level of stress and consequently has
465
a different effect on muscle activity
466
There is a wide range of stressors used in this study
467
since normally, in the real work environment,
multi-468
ple stressors or factors are present simultaneously
469
The effect of each one of them might be small but the
470
combination of them might create a big effect
Pre-471
vious researchers who obtained a significant activity
472
increment in trapezius muscle activity used different
473
kinds of stressors In order to differentiate the effect
474
of each stressor, part 3 of the questionnaire was used
475
so that participants could rate the effect of each
stres-476
sor separately As a result, there were seven stressors
477
that can be considered as significant in inducing the
478
stress in this study
479
This result was supported by part 1 of the
question-480
naire that shows the color-word task was considered
as a ‘no stress’ task This is because four insignificant 482
stressors were used in color-word task At the same 483
time, time pressure task was in the position between 484
‘slightly’ and ‘quite stressful’ tasks This is expected 485
as the time pressure task contained several stressors 486
that can cause significant stress to the participants 487
Finally, the stressful environment task was consid- 488
ered ‘quite stressful’ because not only did it have 489
the same stressor as the time pressure task, but also 490
some additional stressors that can cause significant 491
As we were interested in finding the relation of 493
muscle activity to the level of stress, the core part of 494
this study is to prove that there is a different level 495
of perceived stress between each of the tasks Then, 496
the result was validated by an EMG result in order to 497
determine the effect of psychosocial stress on muscle 498
activity, especially on the trapezius muscle
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4.1 The effect of different tasks used to induce
500
psychosocial stress on muscle activity
501
The results obtained from the EMG measurements
502
have proven the first and second hypotheses The
503
first hypothesis is accepted as true because the result
504
showed a clear increment of trapezius muscle activity
505
for the time pressure and stressful environment tasks
506
compared to the plain-copying or color-word tasks
507
Meanwhile, the second hypothesis stated that the
508
increment is dependent on the level of stress The
509
higher the level of stress is, the higher the increment
510
of trapezius muscle activity This hypothesis stated
511
that the plain-copying task has the lowest effect on
512
trapezius muscle activity while the stressful
environ-513
ment has the highest effect It was expected that the
514
highest trapezius muscle activity in this study would
515
result from the stressful environment task, followed
516
by the time pressure task, and finally the color-word
517
task The result obtained has proven this hypothesis
518
The EMG result was matched with part 1 of the
ques-519
tionnaire’s result This is because in most conditions,
520
there was a significant increment of trapezius
mus-521
cle activity between the plain-copying or color-word
522
task with the stressful environment task even though
523
there was no significant increment between the
plain-524
copying or color-word task with time pressure task
525
In addition, in the event there is a significant
incre-526
ment, the increment in trapezius muscle activity for
527
the first comparisons is higher and more significant
528
than the latter Furthermore, there is no significant
529
difference of trapezius muscle activity between the
530
plain-copying and time pressure task even though
531
at the same time there is significant difference in
532
typing speed for both tasks The significant
incre-533
ment that happened between the color-word task
534
and time pressure task is most probably due to the
535
increment of typing activity itself since the
color-536
word task did not involve much typing Besides that,
537
the different increment of trapezius muscle activity
538
can be seen clearly where the lowest activity was
539
obtained during the plain copying task and the
high-540
est was obtained during the stressful environment
541
task for all devices This result matches Mclean and
542
Urquhart [1] where they found that the increment
543
in typing speed during a data entry task does not
544
affect the muscle activation amplitude in the trapezius
545
muscle
546
This result is concurrent with previous research
547
regarding the effect of psychosocial stress on
trapez-548
ius muscle activity These researches also showed
549
that there were no significant differences for trapezius
muscle activity during the color-word task [19, 28], 551
greater effect during the time pressure task [8, 14] 552
and the highest effect happened during the stressful 553
Aside from the trapezius muscle, other muscles did 555
not show this kind of relationship Besides that, as 556
mentioned before, the significant increment that only 557
happened between the color-word task and the other 558
three tasks were most probably due to the increment 559
of typing activity itself since the color-word task did 560
4.2 The effect of different devices used under 562
psychosocial stress on muscle activity 563
In third hypothesis, it is believed that mobile 564
computing products can produce psychosocial stress 565
because of its characteristics However, this hypoth- 566
esis cannot be accepted as true This is because in 567
comparison to the desktop computer, only a laptop 568
showed a greater trapezius muscle activity However, 569
none of them were statistically significant Not only 570
that, all significant results show that muscle activity 571
during the usage of tablet and smart phone is lower 572
than during the usage of desktop computers and lap- 573
tops There is a contradiction between this study and 574
Kim et al [38] which showed that muscle activity in 575
the shoulder muscle was slightly higher for a smaller 576
virtual keyboard However, this inconsistency might 577
happen because of the differences in study design 578
and equipment Thus, direct comparison might not 579
There are several reasons why this result was 581
obtained First, in many conditions, laptops and desk- 582
tops are quite similar The main difference is that 583
laptops do not have a detachable monitor which 584
makes the angle of viewing quite limited compared to 585
desktops Thus, the result obtained was as expected 586
However, it is not significant enough Secondly, most 587
of the participants were not people who could do 588
the typing process without needing to take a look 589
at the keyboard Thus, when they do the copying 590
on a desktop computer or laptop, their heads move 591
from monitor to keyboard and to monitor again in 592
the process This movement is minimized during the 593
usage of tablets and smart phones, and consequently 594
might lower their muscle activity Meanwhile, the 595
significantly lower activation in other muscles can 596
be explained by the difference of keyboard size and 597
touch screen capability Since the keyboard / keypad 598
size is very small for tablets and smart phones, the par- 599
ticipants do not move or use their muscles as much
Trang 10Uncorrected Author Proof
as when they use desktop computers and laptops In
601
addition, the force they need to press a touch screen
602
button is lower than the actual keyboard Finally, in
603
a study to determine the effect of precision demand
604
and mental pressure on the load of the upper
extrem-605
ity, the authors found that unlike mental pressure,
606
the precision had a small effect on trapezius
mus-607
cle activity [29] However, they also argued that in
608
a case where the performance is essential, precision
609
might has a hidden effect This is because precision
610
plays an important role in performance during
com-611
puter work, and consequently on mental pressure
612
This relationship is also found in another study [22]
613
Thus, based on this argument, we also want to argue
614
that since the participants can do the typing task with
615
significantly less number of errors during the usage
616
of tablet and smart phone, this might be another
rea-617
son why trapezius muscle activity is significantly less
618
compared during the usage of a laptop or desktop
619
computer
620
The fourth hypothesis cannot be taken as true as
621
the third one has been rejected Furthermore, there is
622
no fixed patent of muscle activity increment found in
623
any muscles for any devices
624
4.3 Performance
625
In the earlier stage of this study, it was believed that,
626
for word processing tasks, the performance during
627
the usage of desktop computers would be
signifi-628
cantly better in terms of typing speed and rate of error
629
However, the result of this study is quite the opposite
630
since not only is the laptop better in terms of
typ-631
ing speed, but also the rate of error during the usage
632
of tablet and smart phone is significantly lower than
633
during the usage of desktop computer during stressful
634
condition
635
Nevertheless, it should be noted that this result
636
might be true only for slow typists and this certain
637
posture This might lead to an argument that this
638
result was obtained because most of the participants
639
use smart phones much longer than desktop
comput-640
ers or laptops and are much more comfortable using
641
smart phones than other devices Yet, the same result
642
was obtained for tablet even though most of the
par-643
ticipants used tablet very seldom compared to other
644
devices In addition, the fact that the hour of usage
645
for tablet is much lower compared to other devices
646
did not affect their performance much both in terms
647
of WPM and rate of error They still can do the typing
648
process using a tablet as fast as using other devices
649
with a significant lower rate of error compared to
desktop computer and laptop during stressful envi- 651
Yet, it should be noted that for net performance, 653
the best performance was obtained during the usage 654
of desktop computer and plain copying task This 655
is interesting because the best result was obtained 656
during the comfortable condition without any stress 657
Even though the participant can increase their typing 658
speed, the stress made them make more mistakes 659
This result indicates that the best environment for 660
the workers is the comfortable environment Besides 661
that, during the time pressure and stressful environ- 662
ment tasks, the best performance was obtained during 663
the usage of laptop This result might indicate that 664
under stressful conditions, the best performance can 665
be obtained using the device that they are most com- 666
4.4 Stressors 668
There are many stressors used in this study to 669
induce psychosocial stress Even though this study 670
cannot clearly differentiate the effect of each stressor, 671
the result from questionnaire (part 3) found that there 672
were six stressors which considered as not stressful 673
enough to increase the trapezius muscle activity This 674
result is matched with some previous studies which 675
used some of these stressors and no significant dif- 676
ferences in trapezius muscle activity were found For 677
instance, the effect of different color-word either on 678
screen or using voice [19, 28], supervision by the 679
experimenter [9] and adding to or deducting the com- 680
pensation [26] However, the comparison between 681
this study and the previous ones for significant stres- 682
sors are difficult to be made as these previous studies 683
also combine more than one stressor in their experi- 684
4.5 Limitations 686
There are several limitations in this study First 687
of all, there is no female participant involved Many 688
previous studies described that gender plays a big 689
role in MSDs symptoms However, none of the pre- 690
vious studies regarding the effect of psychosocial 691
stress on trapezius muscle activity reported any differ- 692
ence in effect between men and women [20, 28, 29] 693
Thus, it is believed that it will not affect the result of 694
this study that much Another limitation is the limited 695
choice of posture and small number of participants 696
As the design of the experiment for this particular 697
study is quite big, only one posture was used for each