the sedentary life style easily leads to a forward head posture ( fhp ) , and the clinical 
 expenses related to fhp has increased1 . 

fhp is a poor habitual neck posture that induces biomechanical changes in the head and 
 spinal columns2 , 3 .
fhp is strongly associated with various symptoms such as neck 
 muscle pain and fatigue , headaches , and less motilities of the neck1 , 2 , 4,5,6 .
many researchers have studied fhp and its relationships with the neck muscles because of 
 its prevalence in musculoskeletal disorders of the head and neck1 , 2 , 4,5,6,7 . however , to our knowledge , 
 no research has reported on the neck and scapular stabilizer muscle activations according to 
 various neck flexion and extension angles in subjects with fhps .
the previous study8 have only been studied in healthy 
 individuals or have focused on either the neck muscles or scapular stabilizer muscles in a 
 specific neck position7 , 9,10,11 .
therefore , in this study , we investigated upper trunk and neck muscle activities according 
 to the angle of neck flexion and extension in subjects with fhps .
lateral views of each 
 subject were photographed to measure the craniovertebral angle , which was defined as the 
 angle between the horizontal line running through the c7 spinous process and the line 
 connecting the tragus of the external auditory meatus and the c7 spinous process .
subjects 
 with angles less than 53 were considered to have fhps10 . in this study ,
a telemyo 2400 ( noraxon , usa ) was used to measure 
 the activities of the muscles surrounding the neck .
the muscle activity was measured three 
 times in each of three positions ( the neutral position , an angle of 30 , and a full rom ) in 
 both neck flexion and extension in accordance with the protocols of the previous study8 .
the electrodes were attached over the 
 sternocleidomastoid muscle , splenius capitis and splenius cervicis muscles , upper trapezius 
 muscle , and middle trapezius muscle8 , 10 .
the purpose and procedures of this study 
 were explained to all subjects , and they provided written informed consent prior to 
 participation .
a repeated measure anova and a post hoc lsd test were conducted to determine 
 muscle activity according to neck movement angles .
in the present study , we observed changes in neck muscle activity according to neck 
 position .
there were significant differences in the muscle activities of the 
 sternocleidomastoid muscle and middle trapezius muscle ( p<0.05 ) ( table 1table 1.comparison of muscle activities according to the neck positionmuscleactionneutral30-degree anglefull romsternocleidomastoid muscle ( % mvic)flexion4.71  4.095.36  4.587.28 
6.15 extension6.72  4.77 9.45  8.82splenius capitis & splenius cervicis 
 ( % mvic)flexion13.37  16.6314.98  18.6714.65  13.25extension12.16  14.5713.14  13.26upper trapezius ( % mvic)flexion5.31  5.005.42  5.625.36  5.29extension5.32  5.295.23  4.93middle trapezius ( % mvic)flexion11.75  8.1112.61  9.1512.51  8.55extension11.18  7.89 10.69  7.76 values are expressed as mean  sd .
the activity of the sternocleidomastoid muscles showed significant 
 differences between the 30 flexed position and the full rom position , and between the 
 neutral position and the full rom position ( p<0.05 ) .
for the activity of the middle 
 trapezius muscle , it showed a significant reduction in the 30 extended position and the 
 full rom position compared to the neutral position ( p<0.05 ) .
this study investigated upper trunk and neck muscle activities according to the angle of 
 neck flexion and extension .
we compared three neck flexion positions : ( 1 ) neutral neck 
 position , ( 2 ) 30 degree neck flexion ( 3 ) , and maximum neck flexion .
we also compared three 
 neck extension positions : ( 1 ) neutral neck position ( 2 ) 30 neck extension , and ( 3 ) maximum 
 neck extension .
the results showed that the sternocleidomastoid muscle and middle trapezius 
 muscle activities are different according to neck position .
the first finding of this study was that the activation of the sternocleidomastoid muscle 
 was greatest in the full flexed position , but there were no significant differences between 
 the neutral and middle ranged neck flexion .
lee et al.8 reported that sternocleidomastoid muscle activities between the 
 neutral and middle range neck flexion also have significant differences in healthy adults . 

they suggested that the mean data of activations of the sternocleidomastoid muscle in the 
 neutral position demonstrated 3.34  2.2 , which was lower than we found in our study . 

therefore , we assumed that higher activations of the sternocleidomastoid muscle in the 
 neutral position in subjects with fhp would not be able to cause significant differences 
 between the neutral and middle range neck flexed position .
we also presumed that the muscle 
 activities of the sternocleidomastoid muscle were significantly activated in the full flexed 
 position in subjects with fhps .
cheng et al.7 reported that sternocleidomastoid muscle activities are 
 significantly increased with the increasing flexion and extension angles , which is similar 
 to the trends found in our study .
moreover , we found that greater neck extension angles showed less muscle activation in the 
 middle trapezius .
a previous study9 
 reported that the activation of the middle trapezius increased significantly in the slouched 
 sitting posture compared to the erect one .
furthermore , gaffney et al.11 reported that scapular adduction was coupled with cervical 
 extension .
therefore , we estimate that the middle trapezius is reduced in cervical extension 
 because it may lead to scapular adduction
. the limitations of this study were that we did not measure the neck and scapular motion . 

thus , we were not able to determine the relationships between scapular movements and neck 
 motions , which could affect the upper trunk and neck muscle activities
. however , the results 
 of this study could provide fundamental data for understanding neck and scapular muscle 
 activation in head flexed and extended positions in subjects with fhps , which are commonly 
 performed in daily life .
 [ purpose ] this study investigated changes in neck and upper trunk muscle activities 
 according to the angle of movement of the neck in subjects with forward head posture . 
 [ subjects and methods ] twenty subjects with forward head postures were recruited . 
 the 
 activities of the sternocleidomastoid muscle , splenius capitis and splenius cervicis 
 muscles , upper trapezius muscle , and middle trapezius muscle during flexion and extension 
 were assessed . 
 [ results ] the activity of the sternocleidomastoid muscle showed significant 
 differences between the 30 flexed position and the full range of motion position , and 
 between the neutral position and the full rom position . 
 the activity of the middle 
 trapezius muscle showed a significant reduction in the 30 extended position and the full 
 rom position as compared to the neutral position . 
 [ conclusion ] in the full flexed 
 position , sternocleidomastoid muscle activity increased significantly , and during 
 extension position , the middle trapezius muscle reduced its activities . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION