when they look at the smartphone screen in this posture for a long time , they experience 
 shoulder and neck pain , and occasionally complain of headaches1 .
when continuous loads are applied in a static posture with the head 
 kept lowered , musculoskeletal diseases may develop2 .
this may lead to myofascial pain syndrome by causing stress and excessive tension in the 
 muscles around the neck and shoulders3 . 

myofascial pain syndrome may develop along with tissue damage , or abnormal conditions may 
 occur as a result of abrupt stress or excessive tension on the muscles4 .
jobs that require sitting at a desk for long hours , such as computer and office work , 
 negatively influence the neck and shoulder muscles5 .
in addition , changes in the strength of the deep cervical flexors , 
 endurance , and the location of the lower jaw have been shown in individuals with neck pain , 
 compared with individuals without neck pain6 .
kieine and schumman studied the activity of the upper trapezius associated with changes in 
 shoulder postures , and tepper studied the effects of computer work on the activity of the 
 upper trapezius7 , 8 .
these studies found that incorrect postures cause excessive tension 
 in the upper trapezius and the surrounding muscles .
such excessive tension is known to cause 
 pain by developing abnormal forces in the skeletal system9 , 10 .
the measurement of cervical - bent postures of women using computers showed increases in the 
 head tilting of women complaining of pain11 .
a large proportion of the existing studies have focused on the 
 visual display terminal ( vdt ) that is commonly seen with office computers .
in addition , some 
 studies have examined the fatigue and activity of cervical muscles under the postures 
 adopted while using a microscope in the workplace as well as the activity of cervical 
 muscles while individuals were watching a digital multimedia broadcasting ( dmb ) phone12,13,14 .
smartphones and tablets have recently 
 drawn a great amount of attention and have become almost daily necessities ; however , these 
 devices and other similar new devices may cause cervical deformities when used for extended 
 hours . therefore
, multiple studies on changes in the activity and fatigue of cervical 
 erectors in the postures of smartphone users should be conducted in various environments to 
 prevent skeletal diseases .
therefore , this study was conducted to investigate the changes in 
 the activity and fatigue of the splenius capitis and upper trapezius agonistic muscles , the 
 muscles that support the head , under the three postures adopted most frequently while using 
 a smartphone .
in this study , the three common postures while using a smartphone were considered as 
 independent variables , and the corresponding muscle activity and fatigue as dependent 
 variables .
the experiment was based on a within - group design in which tests were performed 
 on a single group under three different conditions .
the subjects were 15 college students in 
 their 20s who were given complete explanations about the experiment in advance .
the subjects 
 were required to be healthy individuals with no history of spinal damage or undergoing any 
 surgery , and no hand or wrist injuries or other surgeries during the preceding six months . 

all participants were instructed about potential risks and experimental design , and were 
 provided with an informed consent form to sign prior to participation , with the knowledge 
 that they could withdraw at any time .
general 
 characteristics of the subjects are shown in table 
 1table 1.general characteristics of the subjectsmeanectage ( yrs)23.6  2.4height ( cm)174.7  7.6weight ( kg)69.2  11.9gender ( men / women)8/7 .
the muscles targeted for measuring muscle activity were the splenius capitis and upper 
 trapezius , which are the agonistic muscles for cervical extension and are known to cause 
 neck pain11 , 15 .
the researcher identified the location of these muscles by 
 activating the areas around the subjects relevant muscles , and then attached electrodes , as 
 shown in table 2table 2.locations for attachment of electrodesmusclelocationsplenius capitismidpoint between the spinous process of c7 and the 
 peak of the shoulder bladeupper trapeziusthe point 2 cm away from the lateral side of t4 , by referring to previous studies15 .
the electrodes were attached after sterilization with alcohol 
 would be adequate , and the electrodes were attached after sterilization with alcohol using 
 cotton balls .
the markers of a three - dimensional motion analyzer were attached to 
 objectively determine the maximum cervical bending posture , middle cervical bending posture , 
 and neutral posture .
the markers were attached to the center of the forehead , the seventh 
 cervical vertebra ( c7 ) , and the seventh thoracic vertebra ( t7 ) .
a chair of the same height as that in a subway train was prepared , and each subject was 
 instructed to sit in the chair with his or her hips touching the back of the chair and the 
 pelvis fixed . in the maximum bending posture ,
the subject was instructed to place a 
 smartphone as high as the subject s elbow and grab it with both hands 5  cm away from the 
 trunk ( fig .
maximum bending posture in the middle bending posture , the subject s her upper arms were close to the trunk and the 
 neck was bent slightly in a comfortable manner ( fig . 
 2fig .
the edge of the upper part of the smartphone s screen and 
 the subject s eye level were kept horizontal ( fig . 

the average degree of neck - bending during each of the three postures was obtained 
 by measuring and averaging the degrees of the three motion markers for each posture during 
 preliminary tests on five subjects .
the average degree of neck - bending was 100 for the 
 maximum bending posture , 122 for the middle bending posture , and 131 for the neutral 
 posture .
a height - adjustable table was prepared to support the subjects arms during the 
 postures to avoid fatigue in the arms16 . 

surface electromyography ( semg ) was used to measure the 15 subjects while they remained in 
 the three postures , and measurements were performed randomly to reduce errors caused by 
 measuring in the sequence of the postures .
while each subject was adopting each posture for 
 five minutes , he or she was instructed to continuously type the sentences  the blue house is 
 not seen from the outside .  and
after five 
 minutes , the subject s muscle activity and fatigue were measured . after being measured under 
 one condition ,
each subject was provided with a 24-hour break to recover from fatigue , and 
 then was tested under the next condition17 . in this study , a chair having the same height as that of a seat in 
 a subway train and a height - adjustable vibrating table was used .
the smartphone used during 
 the experiment was one that is commonly used by koreans , and a three - dimensional motion 
 analyzer ( smart e , italy ) was employed to measure the degree of bending during each posture . 

a wireless electrode emg system ( free emg , bte , italy ) was used as an emg device to identify 
 the muscle activity and fatigue of the splenius capitis and upper trapezius .
emg data from 
 the maximum cervical bending postures , middle cervical bending postures , and neutral 
 postures were applied into root mean square ( rms ) for five minutes , and mean values were 
 also calculated for one  min and were normalized by % rvc18 .
middle bending posture the study data were analyzed statistically , using spss version 18.0 for windows .
the 
 kolmogorov - smirnov test was used to confirm that the characteristics of the data used in 
 this study represented normal distribution .
an analysis of variance ( anova ) test was 
 conducted to compare the activity and fatigue of the cervical erector during the three 
 different postures while using the smartphone .
scheffe s test was employed as a post - hoc 
 test on the statistically significant differences between the muscle groups , and the 
 statistical significance level for all data was set at =0.05 .
in terms of the differences in the muscle activity for each posture , the four muscles 
 measured ( right splenius capitis , right upper trapezius , left splenius capitis , left upper 
 trapezius ) did not show any statistically significant differences ( table 3table 3.comparison of the muscle activity under the posturesmuscleposturemeanstandard deviationright splenius capitisneutral posture14.27.4middle bending posture13.48.2maximum bending posture 18.825.9right upper trapeziusneutral posture8.95.8middle bending posture5.53.9maximum bending posture 7.15.7left splenius capitisneutral posture14.29.9middle bending posture13.111.0maximum bending posture 13.711.0left upper trapeziusneutral posture7.74.4middle bending posture5.63.9maximum bending posture7.91.2 ) .
the comparison of muscle fatigue among the postures showed statistically significant 
 differences for the right splenius capitis , left splenius capitis , and left upper trapezius 
 ( table 4table 4.comparison of the muscle activity under the posturesmuscleposturemeanstandard deviationright splenius capitisneutral posture0.20.0middle bending posture0.20.0maximum bending posture0.20.0right upper trapeziusneutral posture0.20.0middle bending posture0.20.0maximum bending posture0.20.0left splenius capitisneutral posture0.20.0middle bending posture0.20.0maximum bending posture0.20.0left upper trapeziusneutral posture0.20.0middle bending posture0.20.0maximum bending posture0.20.0values are shown as mean  sd , * p<0.05 . significant difference in 
 three postures . : significantly different compared with middle bending 
 posture . : significantly different compared with maximum bending 
 posture . ) .
values are shown as mean  sd , * p<0.05 . significant difference in 
 three postures .
the purpose of this study was to examine changes in muscle activity and fatigue under 
 different postures while using a smartphone , because of their rapidly increasing use . in 
 particular , polakowska reported that cervical degenerative changes appear due to working 
 postures , types of motions during work , and amplitude of vibration ; paccinni et al . reported 
 that improper postures are associated with abnormal cervical curvatures19 , 20 .
therefore , one 
 aim of this study was to provide basic data on correct postures while using a smartphone to 
 prevent neck pain and deformation that can occur due to their extended use in undesirable 
 postures . in terms of changes in muscle fatigue associated with postural changes during 
 smartphone use , this study showed that the maximum bending posture resulted in significantly 
 higher levels of muscle fatigue in the right splenius capitis , left splenius capitis , and 
 left upper trapezius , compared with middle bending posture .
there were no significant differences in muscle activity in the right splenius capitis , 
 left splenius capitis , and left upper trapezius among the three postures .
however , there 
 was no change in muscle activity because of the experimenter s tension by the awareness of 
 experiments was not controlled and the realization of real long - time smart phone use was 
 limited by measuring emg signal for short smart phone using time of five minutes21,22,23.this result is consistent with that of 
 jung s study , in which during the head - neck bending exercise of the cervical lordosis group , 
 kyphosis group , and hyperlordosis group , a comparative analysis was performed on the 
 activity of the deep flexor and the sternocleidomastoid using pressure and ultrasonic 
 images , and a higher level of pressure led to a corresponding increase in the activity of 
 the deep cervical flexor24 .
, these results may be because muscles exercise their forces in the 
 opposite direction to fight opposite movements25 .
however , the results of this study differ from those of some 
 previous studies .
for example , thuresson compared muscle activity during the cervical 
 neutral posture and cervical 20 forward bending posture on the axis of c7t1 , and found a 
 higher level of muscle activity during the 20 forward bending . in addition , yoo compared 
 muscle activity during the neutral posture and bending posture , and reported a statistically 
 significant higher level of muscle activity during the bending posture17 , 26 . according to 
 the results of villanueva s study , in which the trunk s movements affected the activity of 
 the muscles that surround it , continuous use of smartphone in the present study may have 
 caused contractions of muscles in the hands , wrists , and arms .
thus , muscle activation in 
 the cervical erector was not only confined to the cervical area but also spread into the 
 hands , wrists , and arms , thereby resulting in no statistically significant changes in muscle 
 activity27 . in this study ,
the muscle fatigue under the maximum bending posture and neutral posture 
 showed similar levels of increases , although this result was not statistically significant . 

some studies have reported that a smaller degree of joint flexion led to a statistically 
 significant higher level of muscle fatigue , which may also support the present study 
 result28 , 29 .
in addition , a smaller degree of cervical flexion increases the 
 level of head - lowering , and the head is then influenced by gravity to a larger extent30 . here , a larger load is applied to the 
 cervical erector to support the relevant position , which may cause increased levels of 
 fatigue .
first , the subjects were measured while they were 
 using a smartphone for a short time of five minutes ; therefore , the study did not fully 
 reproduce the situation of individuals using their smartphones in the subway , which is 
 typically for longer periods .
in addition , the subjects were 15 college students in their 
 20s , and thus , the results derived can not be generalized for various age groups .
moreover , 
 it was difficult to control the subjects daily activities during the 24-hour break provided 
 after each measurement . consequently , a follow - up study should fully reproduce the 
 conditions in which individuals use their smartphones on public transportation , and test 
 subjects from various age groups for longer durations of smartphone use .
 [ purpose ] the purpose of this study was to identify changes in the activity and fatigue 
 of the splenius capitis and upper trapezius muscles , which are agonists to the muscles 
 supporting the head , under the three postures most frequently adopted while using a 
 smartphone . [ subjects and methods ] the subjects were 15 college students in their 20s . 
 
 they formed a single group and had to adopt three different postures ( maximum bending , 
 middle bending , and neutral ) . while the 15 subjects maintained the postures , muscle 
 activity and fatigue 
 were measured using surface electromyography . 
 [ results ] comparison of 
 the muscle fatigue caused by each posture showed statistically significant differences for 
 the right splenius capitis , left splenius capitis , and left upper trapezius muscles . in 
 addition , maintaining the maximum bending posture while using a smartphone resulted in 
 higher levels of fatigue in the right splenius capitis , left splenius capitis , and left 
 upper trapezius muscles compared with those for the middle bending posture . 
 [ conclusion ] 
 therefore , this study suggests that individuals should bend their neck slightly when using 
 a smartphone , rather than bending it too much , or keep their neck straight to reduce 
 fatigue of the cervical erector muscles . 