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Paper text:
smartphones provide various conveniences , such as sending and receiving e - mail , accessing 
 the internet , and engaging in entertainment1 .
the number of smartphone users has increased dramatically in recent 
 years because of these conveniences .
however , the use of smartphones is reportedly 
 associated with physical symptoms in some users2 .
some researchers have suggested that frequent smartphone use can lead to the use of a 
 non - neutral neck posture or the development of musculoskeletal disorders3 , 4 . 

berolo , wells , and amick4 surveyed a 
 canadian university population and reported that the duration and frequency of use of mobile 
 handheld devices were related to the prevalence of neck pain .
a cause of neck disorders 
 among smartphone users may be the ability to reposition the display of the mobile device , 
 such as on a desk or below the shoulder5 . 

many people may use smartphones with the head shifted forward and the smartphone placed near 
 the waist or lap while in a sitting position6 .
this flexed neck posture can increase the moment of the cervical 
 spine and induce muscle strain in adjacent portions of the cervical spine7 .
although smartphone use is associated with 
 causal factors of neck pain or musculoskeletal disease , fewer experimental studies have been 
 performed on the effects of smartphone use than on the effects of visual display terminal 
 ( vdt ) work .
maintenance of a non - neutral neck posture , such as a flexed posture , is a well - known cause 
 of neck pain8 . in a previous study ,
small 
 forward shifts of the head in the sagittal plane increased the load on the supporting 
 structures and activated the neck muscles7 .
harrison et al.9 
 found that the compressive load on the cervical discs in the neck - forward flexed position 
 was 10  kg greater than that in the upright neck position .
these biomechanical variations or 
 the presence of neck pain can induce proprioceptive deficits in the cervical region .
szeto , 
 straker , and osullivan10 reported that 
 more symptomatic than asymptomatic individuals used a flexed neck position while using a 
 vdt .
therefore , we postulated that smartphone users with neck pain might more frequently 
 utilize a non - neutral neck posture than asymptomatic users .
many previous studies have investigated the alterations in cervical movement patterns 
 during computer use11 , 12 .
however , few experimental studies have addressed how 
 smartphone use changes cervical movement patterns .
additionally , no study has reported the 
 effect of mild neck pain ( mnp ) on smartphone use .
thus , this study compared alterations in 
 the cervical spine posture of young adults with and without mnp during smartphone use .
we 
 hypothesized that the cervical spine of young adults with mnp would be more flexed than that 
 of asymptomatic young adults during smartphone use .
for this study , 27 young adults ( 12 male , 15 female ) who had used a smartphone for at least 
 1  year were recruited from the university of gimhae , south korea , by print media 
 advertisement .
all subjects who were included in the study had experienced cervical symptoms 
 during smartphone use within the last year .
individuals with a history of neck trauma or 
 surgery or with a medical diagnosis of fibromyalgia , cervical radiculopathy , a systemic 
 illness , or a connective tissue disorder were excluded from this study .
all subjects were 
 grouped into either the mnp group or the control group based on their neck disability index 
 ( ndi ) scores ( > 8 and  8) .
the ndi involves a 10-item questionnaire regarding the effects 
 of neck pain and symptoms during a range of functional activities .
each subject was 
 instructed to select one of six options concerning the severity of each item ( 05 ) .
the ndi 
 score was calculated as the total score multiplied by two ( score range = 0100 ) .
the intraclass correlation coefficient of the 
 ndi scoring method in the korean language version is reported to be excellent , at 0.906 .
the inje university faculty of health science human ethics committee granted approval for 
 this study , and all subjects provided their written informed consent prior to 
 participation .
kinematic data of the upper cervical ( uc ) and lower cervical ( lc ) angles in the sagittal 
 plane during smartphone use were collected using an ultrasound - based motion analysis system 
 ( cms20 ; zebris medical gmbh , isny , germany ) . for measurement of the uc and lc flexion 
 angles ,
four active single markers were attached , one each over the zygomatic bone , tragus , 
 first thoracic spinous process , and sternum13 . each subject was seated on an adjustable - height chair without a backrest , with the knee and 
 hip joints at 90 angles and the feet on the floor .
the subject initially adopted a neutral 
 cervical posture and held the smartphone in both hands while sitting .
the neutral position 
 was defined as a cervical posture without rotation , lateral bending , or excessive cervical 
 lordosis in the sitting posture , but with slight lumbar lordosis and a relaxed thorax . 

straker , jones , and miller7 reported that 
 the discomfort score was higher among workers performing vdt work in a flexed neck posture . 
 in the present study ,
all subjects were instructed to maintain their preferred shoulder and 
 arm postures with the exception of placing the arm and hand on the thigh during the 
 experiment .
the subjects freely used the messenger application or the internet on a galaxy 
 sii phone ( shw - m250s ; samsung electronics co. ltd . ,
each subject 
 performed three test trials with a 3-min rest period between trials . to analyze the cervical angle ,
the raw kinematic data was collected at a 10-hz sampling 
 rate and converted to ascii files .
then , the flexion angles were analyzed using microsoft 
 excel 2010 ( microsoft corp . ,
a negative cervical angle was defined as 
 cervical flexion . in each of the three trials , the flexion angle data points of 100 , 200 , 
 and 300  s of smartphone use
the mean of the three trial values 
 was analyzed to determine the uc and lc flexion angles during smartphone use .
the cervical flexion angles were analyzed using 2 ( groups )  3 ( time ) analysis of variance 
 with repeated measures of both factors .
a greenhouse - geisser correction was used to adjust 
 the degrees of freedom whenever an inequality of sphericity was rejected by mauchly s test . 

differences in the ndi score and anthropometric characteristics between the two groups were 
 analyzed using the independent t - test .
statistical analyses were performed using spss 
 version 20.0 for windows ( ibm corp . , armonk
twenty - seven subjects were recruited from among young adults who used a smartphone in 
 korea .
the ndi score of the mnp group was significantly higher than that of the control 
 group ( table 1table 1.summary of the anthropometric characteristics and neck disability indices of the 
 study participantscharacteristiccontrol group(n = 13)mnp group(n = 14)sex ( n , male)66age ( years ) , mean  sd20.6  1.620.6  1.5height ( cm ) , mean  sd167.3  6.9168.0  8.0weight ( kg ) , mean  sd60.7  10.761.0  12.4neck disability index ( % ) , mean  sd3.3  2.616.9  7.1*mnp , mild neck pain ; sd , standard deviation .
no significant differences in age , height , or weight were observed between 
 the mnp and control groups .
table 1 shows the mean uc and lc flexion angles 
 of subjects with and without mnp at each measured time point during smartphone use .
the cervical spine 
 flexion angles were significantly higher in the mnp group than in the control group ( p < 
 0.05 ) .
the uc of subjects with mnp exhibited greater flexion than that of subjects without 
 mnd ( p < 0.05 ) .
the variations in the degrees of lc flexion in the mnd group were 
 significantly greater than those in the control group ( p < 0.05 ) .
there were no 
 significant differences in the uc and lc segments over time between the two groups ( table 2table 2.cervical spine angle of subjects with and without mnp according to timesegmentcontrol group ( n = 13)mnp group ( n = 14)100 s200 s300 s100 s200 s300 supper cervical spine2.68  1.462.82  2.213.20  2.474.95  3.015.85  3.025.87  3.63lower cervical spine3.99  2.575.14  2.555.84  2.938.19  5.059.20  3.869.95  4.22mnp , mild neck pain , mean  sd .
in the present study , the mnp group exhibited greater uc and lc flexion angles than the 
 control group during smartphone use .
we found differences in the position of the cervical 
 spine during smartphone use between individuals with and without mnp .
young adults with mnp 
 exhibited greater flexion of their cervical spines when using a smartphone than users 
 without mnp .
these findings indicate that young adults with mnp experience difficulty in 
 maintaining a neutral neck posture during smartphone use .
possible explanations for the 
 increased neck flexion angles in subjects with mnp include neck pain altering the motor 
 control of the neck muscles10 .
based on 
 these findings , it appears that induction factors of neck pain occurred in control subjects 
 during consistent use of the smartphone .
thus , we believe that cognition of posture and 
 maintenance of a neutral neck posture are important for subjects both with and without neck 
 pain .
our findings demonstrated differences in the cervical flexion angles with time during 
 smartphone use .
the cervical flexion angles were influenced in the uc and lc regions by the 
 passage of time during smartphone use .
possible explanations for the effect of time on the 
 cervical angle include the subjects maintaining a static sitting posture during smartphone 
 use .
they reported that cervical angles showed gradual increase of flexion 
 with time in asymptomatic subjects during vdt work .
however , some 
 studies have suggested that subtle but significant differences in these angles might correct 
 postural malalignment associated with musculoskeletal disorders15 , 16 . considering 
 the findings of these previous studies , we suggest that the significant changes in the 
 cervical flexion angles observed in the present study might reduce mechanical stress on the 
 cervical spine , even though the differences in the angles were subtle .
a strength of this study was that the characteristics of the cervical posture of subjects 
 with mnp during smartphone use were considered , and that this study demonstrated that 
 subjects with mnp can not maintain a neutral cervical posture as well as control subjects 
 can .
first , we did not measure the flexion 
 angle of the lumbar spine during smartphone use .
we speculate that lumbar flexion might 
 induce cervical flexion because the lumbar spine is connected in a chain - like manner to the 
 cervical spine17 . however , this study 
 focused on the posture of the cervical spine in subjects with mnp during smartphone use . 

changes in lumbar flexion combined with cervical flexion when using a smartphone in a seated 
 position should be assessed in future studies .
electromyography should be performed to examine the muscle activity of the 
 cervical erector spinae in relation to the duration of smartphone use .
further research is needed to identify alterations 
 in cervical flexion angles depending on the grade of neck pain during smartphone use . in conclusion ,
our findings indicate that individuals with mnp adopt a posture of greater 
 neck flexion than individuals without mnp when using a smartphone .
our findings suggest that 
 young adults with mnp must be aware of their posture and modify their non - neutral cervical 
 alignment when using a smartphone .
to reduce the risk of developing severe neck pain , 
 clinicians should instruct smartphone users to maintain a correct neck posture .
[ purpose ] smartphone use reportedly changes posture . 
 however , how neck posture is altered 
 in smartphone users with neck pain is unknown . 
 this study examined changes in the posture 
 of young adults with and without mild neck pain ( mnp ) when using a smartphone . 
 [ subjects ] 
 thirteen control subjects and 14 subjects with mnp who used smartphones were recruited . 
 
 [ methods ] the upper cervical ( uc ) and lower cervical ( lc ) angles in the sagittal plane 
 were measured using an ultrasound - based motion analysis system while the seated subjects 
 used a smartphone for 5  min . 
 [ results ] during smartphone use , the mnp group exhibited 
 greater uc and lc flexion angles than the control group . 
 [ conclusion ] these findings 
 suggest that young adults with mnp are more careful and more frequently utilize a neutral 
 neck posture than young adults without mnp when using a smartphone while sitting .