the use of smartphones to send or receive messages and to access the internet is on the 
 rise1 . although no epidemiological 
 studies have been reported , case reports and laboratory studies indicate that the use of a 
 mobile device may damage musculoskeletal health2 . furthermore , because most mobile device tasks require users to look 
 downwards or to hold their arms in front of them to view the screen , this could cause 
 fatigue and pain in the neck and shoulders3 . villanueva et al . identified the effects of screen height on the 
 electromyographic activities of the neck and shoulder muscles in 10 healthy subjects4 . when the height was low , the neck was more 
 flexed , and it produced significantly higher neck extensor muscle activities . floyd and 
 silver first defined the flexion - relaxation phenomenon ( frp ) , and it refers to a reduced or 
 sudden onset of myoelectric silence in the erector spinae muscles during full trunk 
 flexion5 . a transfer of the extension 
 movement from the active muscular structures to passive structures of the spinal column 
 possibly explains this phenomenon6 . some 
 studies have reported the absence or delay of frp during complete trunk flexion , which could 
 be used to differentiate between healthy subjects and subjects with low back pain6 . the cervical frp has been observed to be 
 similar to that of the lumbar region7 . recent studies have suggested that the flexion - relaxation ratio ( frr ) may be an important 
 marker of neuromuscular impairment6 , 8 . murphy et al . found that the cervical frr 
 is significantly lower in neck pain patients than in healthy subjects9 . another study suggested that frr could be used to identify 
 the potential risks of neck discomfort in computer workers10 . the purpose of this study was to measure the cervical frr and neck pain 
 intensity in healthy subjects , and to identify the differences according to the postures 
 employed while using a smartphone . subjects were excluded if , in 
 the past year , they had experienced neck pain , spinal trauma , or cervical surgery . all 
 subjects provided their informed consent after receiving a detailed explanation of the 
 study . the average age of the subjects was 26.075.73 ( meansd ) years , and their average height and 
 weight were 173.210.05  cm and 69.3313.01  kg , respectively . the muscle activity of the 
 cervical erector spinae ( ces ) muscle was recorded with an mp150wsw ( biopac system inc . , 
 santa barbara , ca , usa ) , the analog signal was converted to a digital signal , and the data 
 was processed using the acqknowledge 4.1 software . a sampling rate of 1,000  hz was used , and 
 the data was band - pass filtered between 10 and 500  hz . the electrode pairs were placed 
 approximately 2  cm from the spinous process over the belly of the muscle at the level of c4 . subjects were required to sit erect in a 
 straight - backed chair with their hips and knees at 90 , feet positioned shoulder width 
 apart , and with their arms relaxed by their sides , while viewing a point at eye level . to 
 measure the frr , subjects were asked to perform a standardized cervical flexion - extension 
 movement in three phases : phase i , full cervical flexion to last for 5  s ; phase ii , 
 sustained cervical full flexion for 5  s ; and phase iii , cervical extension with return to 
 the starting position for 5 s. while measuring the frr , to prevent the effects of speed on 
 frp and to provide standardized test times , the speed was regulated by the beat of a digital 
 metronome . the frr was calculated by dividing the maximum 
 muscle activity during the 5  s of phase iii by the average activation during phase ii . after the initial 
 measurements for frr ( baseline i ) , a smartphone , galaxy s3 ( shv - e210s , samsung electronics 
 co. , ltd . ) smartphone , size 136.670.69.0  mm ( lengthbreadththickness ) , weight of 138.5  g , 
 was given to the participants , and they used the applications of their choice for 15 
 minutes . the postures adopted while using the smartphone were distinguished between desk 
 posture and lap posture , and the testing sequence of the settings was randomized . in the 
 desk posture , participants viewed a point on a smartphone screen with the neck in a neutral 
 position . then , subjects were asked to use both hands to hold the smartphone , keeping their 
 elbows on the desk . in the lap posture , subjects were instructed to use both hands to hold 
 the smartphone in the lap while performing neck flexion . while performing the task in the 
 lap posture , the cervical range of motion ( rom ) was measured using a cervical rom instrument 
 ( performance attainment associates , st . after using the smartphone for 15 
 minutes in the desk posture or lap posture , measurements for frr were performed in the same 
 manner as the baseline measurement , and a break time of 30  min was provided . then , 
 measurements for the frr of the second baseline ( baseline ii ) were conducted , and the 
 subjects used the smartphone for 15 minutes in the next posture and the measurements for frr 
 were repeated . before making emg measurements , neck pain intensity was recorded using a 
 visual analog scale ( vas ) with values between 0 ( no pain ) and 10 ( the worst imaginable 
 pain ) . spss 21.0 ( ibm corporation , armonk , ny , usa ) was used for statistical analysis . the 
 frr was compared using a one - factor repeated measures analysis of variance ( anova ) . post hoc 
 analysis was performed using the bonferroni correction , and statistical significance was 
 accepted for values of <0.05 . no significant differences were found in the frrs among desk posture , lap posture , and 
 baseline . the frr for the desk posture was 1.890.61 on the right and 2.150.92 on the left , 
 2.140.80 and 2.421.18 for the lap posture , 2.160.64 and 2.360.80 for baseline i , and 
 2.070.71 and 2.301.13 for baseline ii , respectively . the cervical rom of neck flexion in 
 the lap posture was 444.31. before measuring baseline i , the results of the vas were 0 . however , after using the smartphone in a desk posture and a lap posture , the mean values of 
 vas were 1.7 and 5.2 , respectively . this study measured cervical frrs and the intensity of neck pain to identify differences 
 elicited by the various postures employed while using the smartphone . there was a difference 
 in neck pain intensity , according to the different postures , but no significant differences 
 in cervical frrs were found . however , chronic low - back pain patients demonstrate the absence 
 of or delayed lumbar frr during forward flexion compared to healthy adults6 , 8 , 11 . murphy et al . found that the frr of neck 
 pain patients was significantly lower than that of asymptomatic subjects in a control 
 group9 . in these studies , chronic neck 
 pain or low back pain patients had higher muscle activities in forward flexion than subjects 
 in the control group , so frr was low . the reduced frr could be the result of altered neural 
 reflexes that make the ces muscles more active during sustained full cervical flexion ( phase 
 ii ) to protect the spine from secondary injury . in addition , the absence of frr in low - back 
 pain patients is due to an imbalance between neural discharges to the muscles from 
 pathologic structures , and this dysfunction in the reflex arc is expressed as continuous 
 paraspinal activity to protect the spinal structures12 . however , the participants in this study were healthy adults with 
 no neck pain and no pathological conditions surrounding the neck muscles and structures . 
 also , after using smartphones in a lap posture , muscle activity did not change in phase ii . 
 thus , the frr may be a significant criterion of neuromuscular impairment and function in 
 chronic neck pain or lumbar pain patients9 ; however , it may not be possible to distinguish neck pain that is 
 caused by performing tasks for a short time . the frrs observed in the present study were 
 lower than those in the low back musculature ( 1215 ) noted in the study by watson et al13 . the reason for this is that the ces 
 muscle has a lower anatomic cross - sectional area than the lumbar extensors . the vas values after using a smartphone in the lap posture were higher than those of the 
 desk posture . greig et al . reported 
 that ces and upper trapezius muscle activities were higher with a laptop set - up than with a 
 desktop set - up14 . found that 
 the activities of the trapezius and erector spinae muscles increased when viewing a lower 
 screen height15 . this would cause 
 fatigue , as the load of the muscles around the neck is increased . thus , persistent neck 
 flexion while using a smartphone or laptop computer may cause structural damage in the 
 tissues around the cervical vertebra which may elicit a risk of neck pain3 , 16 . therefore , using a smartphone in a desk posture with a neutral neck position might reduce 
 the incidence of neck pain . in addition , using smartphones for a long time in one posture 
 should be avoided as it often results in stretching of the neck and shoulders . first , because measurements for the calculation of frr 
 were taken after using smartphones for 15 minutes , we could not identify the effects of 
 prolonged smartphone use in specific postures . second , in the lap posture , the angles of 
 neck flexion were not identical , among the subject , whose movements were not controlled . therefore , future studies , will be required to compare frrs among postures with prolonged 
 use of smartphones .
[ purpose ] the purpose of this study was to measure the cervical flexion - relaxation ratio 
 ( frr ) and intensity of neck pain and identify the differences according to postures 
 adopted while using smartphones . [ subjects ] fifteen healthy adults with no neck pain , 
 spinal trauma , or history cervical surgery participated in this study . 
 [ methods ] the 
 activity of the cervical erector spinae muscle was recorded while performing a 
 standardized cervical flexion - extension movement in three phases ( flexion , sustained full 
 flexion , extension ) . and neck pain intensity was recorded using a visual analog scale 
 ( vas ) with values between 0 and 10 . 
 postures held while using a smartphone are 
 distinguished between desk postures and lap postures . 
 the frr was calculated by dividing 
 the maximal muscle activation during the extension phase by average activation during the 
 complete flexion phase . 
 [ results ] no significant differences were found in the frr between 
 desk posture , lap posture , and baseline , though the intensity of the neck pain increased 
 in the lap posture . 
 [ conclusion ] the frr could be a significant criterion of neuromuscular 
 impairment in chronic neck pain or lumbar pain patients , but it is impossible to 
 distinguish neck pain that is caused by performing task for a short time . 
 prolonged lap 
 posture might cause neck pain , so the use of smartphones for a long time in this posture 
 should be avoided .