other than its primary purpose of telecommunication , smartphones have advantages as 
 portable devices allowing connection to the internet , games , and online communities1 .
however , because smartphone technology is 
 based on blue light , there is a high probability of damaging vision due to the shorter 
 wavelength of light when used for a long period of time2 . from the research on ocular fatigue induced by multimedia aimed 
 toward the general public
, it has been discovered that liquid crystal displays induce dry 
 eye symptoms more frequently than ordinary books and the displays increase the level of 
 visual fatigue from feeling dazzled3 .
if 
 the visual fatigue continues in everyday life , it can affect visual processing and can cause 
 various problems such as ocular pain , damaged corneal epithelial cells , conjunctival 
 hyperemia , and decreased visual acuity .
this continued visual fatigue also has a negative 
 effect on visual feedback processing that integrates the vestibular organ system with the 
 somatosensory network and can further cause an interruption in the body s postural control 
 system4,5,6 .
vision as part of the 
 postural control system may vary depending on ground stability and visual task .
therefore , 
 the purpose of this study was to determine the effects of visual fatigue from smartphones on 
 balance function depending on ground stability and visual task level and to provide 
 smartphone users basic vision care information for preventive measures .
this study included 22 young healthy adults ( 11 males and 11 females with an average height 
 of 169.9  7.8  cm and an average weight of 62.7  13.8  kg ) after they provided informed 
 consent to participate in this study .
the study was conducted in accordance with the ethical 
 principles of the declaration of helsinki and the local research ethics committee approved 
 the study protocol .
individuals with a history of ophthalmological injury or disease that may affect tear 
 secretion , who have had surgery or with a disorder that may affect postural control , have 
 had lasik surgery , were diagnosed with xerophthalmia , or who wear contact lenses were 
 excluded .
the size of the smartphones was limited to 4.75.7 inches and the equipment type 
 was limited to either the galaxy note 5 ( sm - n920s ) , the lg g5 ( f-700s ) , or the iphone 6s 
 ( a-1688 ) . after sufficient rest without visual fatigue was given , subjects were positioned upright 
 with their arms on a table and the smartphones were consistently held 40  cm away from the 
 eyes7 . viewing time to induce the visual 
 fatigue was limited to 40 minutes8 .
computerized posturography balance data were collected using the biodex balance system sd 
 ( bbs , biodex medical system , inc .
, new york , usa ) . the bbs was used to measure the postural 
 stability ( ps ) and the limit of stability ( los ) .
subjects were positioned on the apparatus 
 with their two hands together and their feet apart to adjust the center of pressure ( cop ) so 
 that it was at the center of the concentric circle .
measurements were calculated from the 
 process of the participants maintaining or moving their cop .
the subjects were instructed to maintain their cop at the center of the concentric circle 
 which was indicated on the monitor in order to measure static and dynamic ps before and 
 after the induction of visual fatigue .
three trials where two measurements were made during 
 a 20  s period at 10  s intervals were completed by the subjects and the measurements were 
 averaged over the three trials .
the dynamic ps was measured while the participant controlled 
 the unfixed footboard that provided 12 different levels of stability , levels 12 to 1 , 
 decreasing from 12 , which was the most stable , to 1 , the least stable .
the stability index scores were calculated from the 
 degree of tilt deviation from horizontal ; the lower postural stability index score indicated 
 superior balance function . for static los measurements before and after the induction of visual fatigue , the footboard 
 was fixed and eight randomly positioned markings flashed around the reference point in the 
 center of the monitor .
measurements were made as the participants changed the position of 
 the cop in response to the eight signals ; the return time was calculated .
the dynamic los 
 was measured while controlling the unfixed footboard at 12 different levels , level 12 to 1 , 
 with decreasing stability .
the apparatus was set at level 5 for assessment of static los , 
 and measurements were calculated using the same method .
the static and dynamic los running 
 time were recorded . the higher los score and shorten running time indicated superior balance 
 function .
the general characteristics of the participants were characterized as the average  
 standard deviation ( sd ) .
the kolmogorov - smirnov test was used to analyze the differences in 
 ps , los , and los running time before and after the induction of visual fatigue .
the paired 
 t - test was used for comparisons of normally distributed data sets .
patient characteristics are listed in table 
 1table 1.general patient characteristics ( n=22)characteristicsmean  sdrangegender ( m / f ) 11/11age ( years)25.3  3.82229height ( cm)169.9  7.8185153weight ( kg)62.7  13.89046values are expressed as mean  sd .
. the overall stability index ( osi ) assessing the changed cop from all 
 directions was used , with the lowest score referring to good postural balance control and 
 sufficient ps .
the osi of static ps before inducing visual fatigue ( 0.36  0.17 ) was not 
 significantly different than ps after inducing visual fatigue ( 0.38  0.2 ) .
however , the osi 
 of dynamic ps significantly increased from before ( 0.82  0.4 ) to after ( 1.02  0.59 ) the 
 induction of visual fatigue ( p<0.01 ) ( table 
 2table 2.comparison of postural control before and after inducing visual fatigue 
 ( n=22)valuebefore visual fatigueafter visual fatiguestatic ps0.36  0.20.38  0.2dynamic ps0.82  0.41.02  0.6**static los ( % ) 60.32  13.951.96  15.7**dynamic los ( % ) 35.68  11.529.55  12.2**static los running time ( sec)35.55  8.440.5  14.2*dynamic los running time ( sec)47.36  9.558.1  20.7*values are expressed as mean  sd .
* p<0.05 , * * p<0.01 the directional control score ( dcs ) assessed the degree of cop control with a higher dcs 
 indicating better performance during directional cop control .
the dcs of static los 
 significantly decreased from 60.32  13.88 before visual fatigue induction to 51.96  15.68 
 after induction ( p<0.01 ) .
the dcs of dynamic los changed significantly from 35.68  11.51 
 to 29.55  12.19 before and after the visual fatigue induction , respectively ( p<0.01 ) 
 ( table 2 ) .
the static los running time increased significantly after visual fatigue induction from 
 35.55  8.38  s before to 40.5  14.19  s after ( p<0.05 ) .
the dynamic los running time also 
 significantly increased from 47.36  9.5  s to 58.05  20.7  s before and after inducing 
 visual fatigue , respectively ( p<0.05 ) ( table 
 2 ) .
this study investigated the effects of visual fatigue from using smartphones on postural 
 control in 22 healthy adults .
visual fatigue was induced by placing smartphones 40  cm from 
 the eyes for 1 hr .
static ps , dynamic ps , static los , dynamic los , static los running time , 
 and dynamic los running time before and after the induction of visual fatigue were measured 
 using bbs .
postural control ability during dynamic ps ( p<0.01 ) , static los ( p<0.01 ) , 
 and dynamic los ( p<0.01 ) after induction of visual fatigue significantly decreased but 
 there were no significant changes in static ps .
this can indicate that the static ps results 
 in this study might have been affected by the measurement duration of 20  s , which may not be 
 long enough to effectively modulate visual fatigue .
shu et al.9 showed that the maximum time a person can hold their blinking eyes 
 with no visual fatigue was 23.12  s , but it was shortened to 17.09  s after 1 hr of computer 
 work , reducing the ocular protection index . for los
running time in this study , both static 
 los and dynamic los running time significantly increased ( p<0.05 ) , confirming that 
 postural control ability was diminished after the induction of visual fatigue .
therefore , 
 visual fatigue may also substantially affect other studies such as the visual perception 
 feedback study , where data is obtained regarding visual focusing ability , or in the mobility 
 induced setting or the visual searching study which measured the level of attention10 .
won et al.11 showed that changes in visual processing can affect the degree of 
 dependence on proprioception and postural sway can be increased if visual processing is 
 obstructed .
the limitation of this study was the limited number of participants who had no 
 balance deficits ( 22 young adults aged in the twenties ) . also , it was unable to control the 
 visual fatigue that may occur in the evaluation process bbs .
further studies should be 
 carried with a wide age range of subject through an objective indicator of the visual 
 fatigue index . in conclusion
, the results of this study demonstrated that visual fatigue 
 caused by smartphone use interferes with the visual ability to control posture and possibly 
 reduces the ability to balance .
thus , this study suggests that it is necessary for 
 smartphone users to reduce visual fatigue by taking appropriate rest .
also , proper 
 guidelines on the use of smartphones should be proposed on various symptoms caused by visual 
 fatigue from smartphones .
 [ purpose ] the purpose of this study was to determine the effects of visual fatigue caused 
 by smartphone use on balance function . 
 [ subjects and methods ] the participants consisted 
 of 22 healthy male and female adults . their postural stability , limit of stability , and 
 limit of stability running time were evaluated using a computerized posturography 
 apparatus before and after inducing visual fatigue . 
 postural stability and the limit of 
 stability were divided into static and dynamic conditions . 
 [ results ] there were 
 significant differences between the dynamic postural stability , the static and dynamic 
 limit of stability , and both the static and dynamic limit of stability running times after 
 the induction of visual fatigue . 
 [ conclusion ] the results showed that visual fatigue 
 caused by smartphone use has a negative effect on balance function . 
 therefore , reducing 
 visual fatigue through proper rest is necessary . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION