adult mobile phone 
 usage was found to be one of the most important means of communication in an international 
 study in 20071 . the rapid increase in the 
 frequency of use of mobile phones among adults can adversely affect various aspects of human 
 health , and numerous studies2,3,4 have investigated 
 such adverse effects , particularly those focused on the neck and arms . kapandji5 reported that with a head 
 movement of 2.5  cm , an additional weight of 0.4  kg is imposed on the neck and upper body . greig et al.6 reported that muscle 
 activity is maintained at a maximum of 5% according to video screen - displayed 
 electromyography ( emg ) results from the sternocleidomastoid and upper trapezius muscles . straker et al.7 reported that the neck 
 bends more when an individual is using a tablet with a relatively small screen than when the 
 individual is using a general desktop computer and that muscle activity during tablet use 
 increases in the shoulders and surrounding muscles . this is because video devices with small 
 screens require more bending of the neck ( than devices with larger screens ) when the 
 extensor digitorum muscles are activated to balance the neck during the lowered gaze , 
 increasing the load on the sternocleidomastoid muscles6 , 8 . such fatigue is an important component of 
 human health and can lead to musculoskeletal injuries and psychological issues . they may 
 become a major cause of neck pain , which is closely correlated to the degrees of fatigue and 
 tension . these injuries can lead to chronic neck pain , resulting in serious problems in job 
 performance unless proper treatment is provided . ylinen et al.10 reported that stretching 
 exercises alleviate neck pain , thereby increasing the joint range of motion and contributing 
 to an increase in patient quality of life . however , morgan and proske11 reported that persistent muscle stretching can damage 
 muscle tissue , without much affecting joint range of motion . furthermore , muscular strength 
 can be decreased immediately after stretching12 , without much increase in muscular strength13 . previous studies mainly focused on subjects with neck pain rather than on mobile phone 
 users . thus , this study investigated the effects of stabilization exercises such as 
 stretching and neck exercises on the degree of muscle fatigue resulting from continuous use 
 of mobile phones . twenty - six volunteers were enrolled in this study ( 14 males and 12 females ) . individuals with congenital ( hereditary ) handicaps , patients with surgical and neurological 
 histories , and patients with severe arm injuries were excluded . the subjects mean age was 
 20.1  0.5  years ; mean height , 165.5  2.2  mm ; and mean weight , 62.1  4.2  kg . all test 
 protocols were approved by the ethics committee of the physical therapy faculty of kangwon 
 national university . they also used a telemyo 
 2400 direct transmission system for emg ( noraxon inc . , az , usa ) with a noraxon wi - fi 
 transmitter to receive and collect surface emg signals from the sternocleidomastoid and 
 upper trapezius muscles , which provide stability in the shoulder area during phone use14 . in this experiment , surface emg of the cervical erector spinae and upper trapezius , in 
 which muscle activity increases during cell phone use , was performed during the 
 participants mobile phone use8 . the 
 electrode for measuring the activity of the sternocleidomastoid muscles was attached on a 
 spot 2  cm from cervical vertebra 4 ( c4)15 , 16 , and the electrode for measuring the 
 activity of the upper trapezius muscles was attached on a spot slightly outside the midpoint 
 between cervical vertebra 7 ( c7 ) and the protruding peaks of the shoulder bones17 . to prevent unnecessary noise during the 
 emg measurement , the contact area selected the reference voluntary contraction ( rvc ) method was used to 
 quantify the emg values . the emg signals were collected while the subject was holding 1  kg 
 of dumbbells in both hands , with the subject s hands spread out 90 apart on the wing bone 
 plane , with the subject maintaining the position for 15 seconds . this study involved repeated , randomized , crossover experiments with the participants in 
 each group and a comparison of the resulting values . the experiments conducted were divided 
 into three different conditions and conducted on different days . on day 1 , the participants 
 used a mobile phone without any intervention to minimize the effects of fatigue from the 
 previous experiment ( condition 1 ) . on day 2 , the participants did some stretching exercises 
 before using a mobile phone and the measurements were taken while the participants were 
 using their phones ( condition 2 ) . on day 3 , the participants performed stabilization 
 exercises before phone use ( condition 3 ) . these experiments were conducted in random order 
 to avoid order effects . to equalize all the participants starting positions before the measurement , the 
 participants were told to sit comfortably with their hip and knee joints bent at 90 and 
 their feet flat on the floor . furthermore , they were told to raise their mobile phones up to 
 the level of their clavicle , with their elbow bent to avoid excessive bending of the waist . once phone use began , they were told to focus on their mobile phone use and ignore their 
 neck and waist positions . the experimental conditions were as follows : under condition 1 ( the reference group ) , emg 
 was performed for 10 minutes while the participants were using the phone without any 
 intervention . under condition 2 ( which included stretching exercises ) , the participants 
 engaged in a stretching exercise program that was modified from the training method used by 
 lee and yoo19 . the participants performed 
 passive stretching exercises in five motions for 10 seconds , each with an upright and fixed 
 posture , as follows : bending forward with the neck pulled forward while standing upright , 
 leaning backward , bending left and right , and turning the neck . breaks were provided for 10 seconds between the sessions and 5 minutes between 
 the sets . after the stretching exercises , the participants had a 1-minute break before emg 
 was performed for 10 minutes while they were using a mobile phone . under condition 3 ( which included stabilization exercises ) , the participants engaged in a 
 stabilization exercise program that was modified from the training method used by hur20 . these exercises enhance the muscular 
 strength of the long neck and long head muscles ( cardiac muscles ) , and involve the following 
 five motions : isometric contraction exercises for 10 seconds without moving the neck before 
 returning back to the starting position , neck bending exercises in the supine position , neck 
 stretching and greater pectoral muscle stretching exercises in the supine position , upper 
 back muscle - enhancing exercises while lying face down , placing both thumbs underneath the 
 chin while sitting , and leaning against the wall in a standing upright position with the 
 chin pulled in . five sessions were implemented for 10 seconds each for two sets in total , 
 and breaks were allowed for 10 seconds between the sessions and 5 minutes between the sets . after the stabilization exercises , the participants had a 1-minute break before emg was 
 performed for 10 minutes while they were using their mobile phones . testing of all variables using the 
 one - sample kolmogorov - smirnov test revealed that they were all normally distributed . a 
 repeated one - way analysis of variance determined the significance of temperature differences 
 between the upright sitting position and sitting with the legs crossed . scheffe s test was 
 conducted as a post hoc test to determine the significance of differences . statistical 
 analysis was performed by using spss 21.0 , and a significance level of p=0.05 was used . the three conditions were defined as follows : the day the participants used a mobile phone 
 without any intervention ( condition 1 ) , the day the participants performed some stretching 
 exercise before using a mobile phone and the measurements were taken while the participants 
 were using their phones ( condition 2 ) , and the day the participants performed stabilization 
 exercises before phone use ( condition 3 ) . the median frequency values for each condition are 
 shown in table 1table 1.changes in median frequency ( unit :  hz)condition 1condition 2condition 3right cervical erector spine36.1  6.736.6  11.137.4  10.1left cervical erector spine32.2  4.333.1  5.135.1  11.1right upper trapezius29.8  14.132.25  16.144.1  10.8*left upper trapezius27.3  12.331.1  14.437.2  15.5data are expressed as mean  sd . . significant difference 
 between conditions 1 and 3 .. the median frequency values increased in all the muscle groups in the 
 comparison between conditions 1 and 2 , showing a decrease in the degree of fatigue . the 
 decreased fatigue between conditions 1 and 3 was demonstrated by the increase in the median 
 frequency values in all the four muscle groups included in the comparison . in particular , 
 statistically significant differences were found between conditions 1 and 3 in the right 
 upper trapezius muscle group ( p<0.05 ) . however , researchers disagree about their effectiveness , and few studies have compared 
 and analyzed how such exercises when performed prior to using a mobile phone can affect neck 
 and shoulder fatigue . thus , this study compared and analyzed the effects of stretching and stabilization 
 exercises before mobile phone use on neck and shoulder muscle fatigue , and further 
 investigated the methods for preventing fatigue induced by mobile phone use . previous studies mentioned 
 three methods for measuring median frequency values as follows : the isometric contraction 
 method , the  comparison of reference activity  method , and joint analysis of emg amplitude 
 and spectrum14 . this study used the 
  comparison of reference activity  method to obtain the median frequency values . as a result , fatigue decreased in all the four muscle groups when the participants used 
 their mobile phones without any intervention ( condition 1 ) . furthermore , when the 
 participants performed stabilization exercises before phone use ( condition 3 ) , fatigue 
 decreased in all the four muscle groups . in particular , statistically significant 
 differences were found at the right upper trapezius muscle group between conditions 1 and 3 
 ( p<0.05 ) . the reasons for these results are as follows : stretching exercises increase the 
 inner pressure of the muscles through mild muscle contraction . the rhythmic contraction or 
 mild pressure of the muscles accelerates intravenous blood flow and increases blood 
 circulation , which can improve blood flow and remove fatigue - causing substances21 . neck stabilization exercises had positive 
 effects on the deep neck muscles and increased muscular strength and endurance22 . this study has the following limitations : first , the results can not be generalized to all 
 age groups , as the participants were limited to adult males and females in their twenties . second , the number of participants was too small to apply the results to the general 
 population . third , this study investigated only the short - term effects ( i.e. , one - time 
 stretching and stabilization exercises ) , excluding the long - term effects , of stretching and 
 stabilization exercises . lastly , all the study participants were right - handed . however , the 
 correlation between the dominant hand and decreased muscle fatigue was not investigated . in 
 the future , diverse age groups and a greater number of participants , as well as the 
 inclusion of adult males and females in their twenties , need to be considered . furthermore , 
 detailed studies are needed to compare the variations in changes in muscle fatigue that 
 occur over a longer intervention period and to investigate the correlation between the use 
 of the dominant hand and decreased muscle fatigue . as the potential effects of stretching 
 and stabilization exercises on muscle fatigue and other conditions are great , efforts are 
 needed in the future to secure an identical external environment and a sufficient number of 
 target subjects by using more quantitative and controlled methods that integrate with other 
 factors .
[ purpose ] the purpose of this study was to investigate the preventive effects of 
 stretching and stabilization exercises on muscle fatigue of the neck erector spine and 
 upper trapezius muscles . 
 [ subjects and methods ] the subjects of this study were 26 healthy 
 university students ( 14 males and 12 females ) . each subject was assigned randomly to each 
 of three study groups in order to prevent order effect . 
 the three groups included in this 
 study were the no - exercise , stretching exercise , and neck stabilization exercise groups . 
 the median electromyographic frequency was used as a gauge of muscle fatigue . 
 [ results ] 
 decreased muscle fatigue was demonstrated by an increase in the median electromyographic 
 frequency values in all the four muscle groups in the comparison between conditions 1 and 
 3 . in particular , statistically significant differences were found between the two 
 conditions in the right upper trapezius muscle group . 
 [ conclusion ] these findings suggest 
 that the effect of stretching and stabilization exercises can reduce muscle fatigue in 
 mobile phone users .