forward head posture ( fhp ) is a commonly noted abnormal neck posture in students who carry 
 heavy loads or maintain a sitting posture for long periods1,2,3 .
fhp is defined as the hyperextension of the upper cervical and a 
 forward translation of the cervical vertebrae4 .
fhp increases the compressive loading on tissues in the cervical 
 spine , particularly the facet joints , and stress on the ligaments4 , 5 .
also , fhp can 
 induce neck pain and increase electromyographic activity in the neck muscles4 , 6 . 
 for these reasons
, fhp is known to lead not only to neck pain but also to changes in 
 cervical movement patterns7 , 8 .
alterations in cervical spine movement can provide clinicians with information that assists 
 assessment and treatment and in monitoring the efficacy of rehabilitation programs in fhp 
 populations1 , 9 , 10 .
in particular , the neck 
 rotation test is often performed for evaluation of cervical spine dysfunction7 . for the neck rotation test ,
the subject is 
 asked to maximally rotate the head by turning to one side , left or right , in a sitting 
 position11 .
comerford and mottram11 stated that generating lateral flexion 
 motion during neck rotation can cause dysfunction and pain in the cervical spine .
thus , to 
 prevent unwanted cervical lateral flexion during cervical rotation , many clinicians and 
 researchers emphasize cervical axial rotation so as not to complicate rotation with other 
 neck motions , such as flexion , extension , and lateral flexion12 , 13 .
cervical axial rotation 
 along the longitudinal axis is considered clinically important for painless neck motion14 . for precise cervical axial rotation ,
it is necessary to properly control the activation of 
 the neck muscles11 , 15 , 16 .
however , precise neck 
 movements in fhp populations are typically difficult because they have problems with 
 alterations in the length and activation of the neck muscles8 , 17 .
the primary problems in 
 fhp are shortening and hyperactivation of the sternocleidomastoid ( scm ) muscle7 , 8 , 18 .
the scm muscle acts in neck rotation and 
 lateral flexion as well as in neck flexion .
however , many previous studies of fhp and scm 
 muscles have focused largely on movements in the sagittal plane , such as flexion and 
 extension1 , 5 , 
 10 .
also , movements in the frontal 
 plane , which involve hyperactivation of the scm muscles , have been overlooked in patients 
 with fhp . although patients with fhp may be exposed to neck pain caused by hyperactivation of the scm 
 muscles , to our knowledge , kinematic analysis of lateral flexion movement patterns of 
 subjects with fhp during neck rotation has not been reported .
repeated cervical lateral 
 flexion and translation motions due to fhp posture that occur during cervical rotation in a 
 sitting position or in activities of daily life may cause tissue microtrauma , overuse 
 syndrome , mechanical dysfunction , and development of pain in the cervical spine12 , 19 .
for this reason , it is important to understand the kinematics of lateral flexion movement 
 patterns and scm muscle activation during neck rotation in subjects with fhp .
furthermore , 
 although the neck rotation test is commonly used to evaluate cervical motion in the sagittal 
 plane , there is potential for using this test for evaluating motion in the frontal plane as 
 part of a clinical evaluation for neck problems in subjects with fhp .
thus , the aims of this 
 study were to compare 1 ) the rotation - lateral flexion ratio , 2 ) the lateral flexion movement 
 onset time , 3 ) the rotation and lateral flexion angles , and 4 ) the muscle activation of scm 
 between individuals with fhp and controls during neck rotation .
we hypothesized that 1 ) the 
 rotation - lateral flexion ratios would be higher , 2 ) the lateral flexion movement onset time 
 would be earlier , and 3 ) the rotation and lateral flexion angle values would be higher 
 during neck rotation in the fhp group ; we also hypothesized that 4 ) the scm muscles would be 
 more highly activated during maintenance of end - range neck rotation in both directions in 
 the fhp group than in control subjects .
in total , 28 subjects were recruited from university students in korea ( 14 control 
 students , 14 fhp students ) .
subjects were recruited into the fhp group if they had a forward 
 translated neck alignment ; the fhp group was selected by evaluating the horizontal distance 
 from a vertical line through the lobe of the ear and the acromion process in the sagittal 
 plane ( fhp 5  cm)20 .
additionally , to 
 classify the groups , the cranio - vertebral angle ( cva ) was measured in all subjects .
the 
 intra - class correlation coefficient ( icc ) of the cva measurement method has been reported to 
 be high ( icc = 0.88)21 .
exclusion 
 criteria included past or present spinal canal stenosis , rheumatoid arthritis , and 
 vestibulobasilar insufficiency . before the experiment , we explained all procedures and the 
 purpose of the study to the subjects in detail .
all subjects signed an informed consent 
 form , which was approved by the inje university college of health science human studies 
 committee . in all sessions
, the kinematics were recorded using a three - dimensional ultrasound - based 
 motion analysis system and muscle activity was measured using electromyography .
data 
 collection using the two systems occurred simultaneously so that data were synchronized for 
 a more complete and valid analysis .
a three - dimensional ultrasound - based motion analysis 
 system ( zebris cms20 , zebris meditechnic gmbh , isny , germany ) was used to collect kinematic 
 data .
two components of ultrasound triple markers , one installed on the apex of the head and 
 another attached to the right lateral shoulder , were fixed to accept real time neck motion 
 data via a transducer sensor , which consisted of three microphones .
the transducer sensor 
 was placed perpendicular and to the right of the subject at a distance of 1  m. the preferred 
 neck position was set at zero based on the starting position before each test .
the kinematic 
 data - sampling rate was 20  hz . for measurement of neck movements , the icc for intra - session 
 test - retest reliability had been previously established as 0.730.90 , as well as 
 inter - session reliability of 0.430.6822 .
rotation and the lateral flexion angle of the neck were recorded in real time during neck 
 rotation to identify the ratio of rotation to lateral flexion movements to the ipsilateral 
 or contralateral side .
the collected kinematic raw data were converted to ascii files for 
 analysis . the movement onset time and the motion of the neck
the point exceeding a threshold angle of 1 was defined as the onset time of neck 
 movement23 .
the mean value of three trials was analyzed to determine 
 onset time and rotation - lateral flexion movement patterns during neck rotation . finally , the 
 rotation - lateral flexion ratio was calculated as the lateral flexion angle divided by the 
 rotation angle , multiplied by 100 .
electromyographic ( emg ) signals from the scm muscles were recorded and analyzed using a 
 surface electromyography system ( mp150 , biopac systems , inc .
, santa barbara , ca , usa ) . skin 
 preparation at the attachment sites consisted of shaving and cleaning the skin with an 
 alcohol swab .
emg data were recorded from the 
 scm muscle ( 1/3 of the distance from the sternal notch to the mastoid process , parallel and 
 over the muscle belly)24 .
( 20450  hz ) and a band - stop filter ( 60  hz ) were 
 used .
raw data were transformed into the root mean square with a window of 50  ms . for 
 normalization ,
muscle contraction reference data were collected while the subject performed 
 two maximal voluntary isometric contraction ( mvic ) trials over a 5-s period in a manual scm 
 muscle testing position4 .
the average value of the middle 3  s of the 5-s trial was used , 
 and the average of two trials was calculated for normalization . during the maintenance 
 phase ,
the average emg data from the middle 3  s of the 5-s measurement were used to compare 
 the fhp and control groups .
neck and trunk postures were not 
 adjusted , and the usual posture of each subject was maintained throughout the experiment . 

subjects were strapped to the chair to minimize compensation by trunk motion during the 
 test . before the test , each subject was instructed to perform neck rotation in the preferred 
 rotation range in two directions ( left and right ) randomly to identify end range of motion . 

neck rotation end range was defined as maximum range through which each subject could 
 actively rotate his / her cervical spine in each direction .
once end range of motion was 
 established , the neck rotation test was repeated three times in each direction .
the initial 
 direction of neck rotation test was randomized , and a 1-min rest was allowed between trials . 
 to minimize differences in vestibular function during the neck rotation , each subject 
 conducted the neck rotation for 6  s ( rotation phase ) at a speed of < 15/s25 and maintained the end - range position for 
 5  s ( maintenance phase ) , all timed using a metronome .
differences in the rotation - lateral flexion ratio , rotation angle , lateral flexion 
 angle , scm activation , and subject demographic in each direction between subjects with and 
 without fhp was evaluated using independent t - tests . the spss statistical package ( ver . 

18.0 , spss , chicago , il , usa ) was used for all statistical analyses and a p - value 0.05 was 
 considered to indicate statistical significance .
subjects in the two groups were of similar age , height , and weight ( p > 0.05 ) .
the 
 forward head distance was significantly increased in the fhp group compared with the control 
 group ( p < 0.05 ) .
the results of the kinematic analyses are presented in table 1table 1.rotation-lateral flexion movement patterns , mean ( sd ) , during neck rotation to 
 the left and right between two groupsdirectioncontrol group(n = 14)fhp groupn = 14)rotation - lateral flexion ratio ( % ) left10.01 ( 5.11)13.46 ( 10.92)right6.43 ( 3.58)6.81 ( 4.80)rotation angle ( )left52.15 ( 7.12)62.72 ( 6.94)right51.58 ( 7.90)59.45 ( 7.35)lateral flexion angle ( )left3.39 ( 1.95)6.22 ( 3.01)right3.46 ( 2.55)7.97 ( 6.21)lateral flexion onset time ( s)left1.68 ( 0.79)1.04 ( 0.45)right1.54 ( 0.75)0.88 ( 0.39)fhp : forward head posture ; sd : standard deviation . * p < 0.05 , all of which were significantly different between the fhp and control groups 
 ( p < 0.05 ) .
the rotation - lateral flexion ratio was higher in both directions in the fhp 
 group than that in the control group . at end range neck rotation ,
the fhp group showed 
 rotation angle was greater by approximately 810 degrees than the control group , 
 respectively . also , lateral flexion angle in fhp group was greater by 34 degrees than in 
 control group .
in addition , the onset of lateral flexion movement during neck rotation of 
 all directions in the fhp group occurred up to about one second earlier than in control 
 group ( p < 0.05 ) .
fhp : forward head posture ; sd : standard deviation . * p < 0.05 with regard to electromyography of the scm muscles during neck rotation in both directions , 
 the activity in the contralateral scm muscles was greater in subjects with than in those 
 without fhp ( p < 0.05 ) ( table 2table 2.muscle activation as a percentage of the maximum voluntary isometric contraction 
 ( % mvic ) during neck rotation to the left and right between the two groups ( n = 
 28)directionmusclemuscle activation ( % mvic)control group mean ( sd)fhp group mean ( sd)left rotationleft scm2.88 ( 1.35)3.52 ( 2.08)right scm16.90 ( 7.74)29.21 ( 14.69)right rotationleft scm18.29 ( 6.38)34.78 ( 21.48)right scm4.39 ( 2.17)3.32 ( 1.69)fhp : forward head posture ; sd : standard deviation . *
many researchers emphasize correct neck motion to minimize dysfunction in the adjacent 
 structures of the cervical spine17 , 26 .
although fhp is a common physical finding 
 in persons with malalignment of the neck , to our knowledge , there have not been any 
 publications investigating variations in neck kinematics in fhp during neck rotation3 , 27 , 28 . here , we investigated rotation - lateral 
 flexion movement patterns and scm activation during neck rotation in fhp and control 
 subjects .
our data suggest differences in the rotation - lateral flexion movement patterns and 
 activation of scm muscles between the fhp and control groups during neck rotation .
the data 
 showed relatively less axial neck rotation in the fhp group than in the control group .
this 
 may be because long - duration fhp maintenance induces alterations in cervical motor control 
 and muscle imbalance of the neck muscles , such as the scm .
faulty alignment , neck muscle imbalances , 
 and alteration of motor control of the cervical spine have been reported to contribute to 
 neck movements5 , 6 , 8 , 18 .
also , no other axial neck rotation could induce repetitive 
 translation motion in the transverse plane in the cervical spine as often as neck rotation , 
 and translation motion can induce cervical instability12 and spinal disorders31 .
the scm muscles of subjects with fhp showed greater imbalance between the left and right 
 muscles compared with the control group .
this may be because the scm muscles in the fhp 
 group are hyperactive during neck motion due to changes in the condition of the neck muscles 
 caused by maintaining a faulty posture such as fhp .
the muscle imbalance induced by fhp can 
 result in decreased muscular efficiency and increased activation of the additional muscles 
 needed to maintain neck and head posture29 . as a result , subjects with fhp used contralateral scm muscles to a 
 greater extent during neck rotation than did the control subjects .
these kinematics and emg data show that subjects with fhp have different pattern of neck 
 movement and scm activation than those without fhp .
these aspects may indicate that global 
 muscles , such as the scm , are being used instead of local muscles during neck rotation . 

global muscles are known to function as musculature for multi - segmental joints , and they 
 generate large movements rather than precise movements7 , 8 , 17 .
thus , excessive activation of scm - controlled lateral flexion 
 during neck rotation occurred significantly more in people with fhp than in control 
 subjects .
although statistical significance does not necessarily imply clinical 
 significance , clinicians may recognize the clinical implications of these findings for 
 preventing neck problems in fhp patients .
neck rotation 
 tests can be used to assess movement of the cervical spine7 , 11
. however , many clinicians 
 overlook motion in the frontal plane and examine only movement in the sagittal plane in 
 fhp1 , 32 , 
 33 .
thus , it should be recommended that clinicians 
 examine lateral flexion in the frontal plane when assessing and treating patients with 
 fhp . in this study ,
the angle of rotation in the fhp group was significantly higher than that in 
 the control group .
however , a previous study demonstrated that subjects with fhp had a 
 decreased range of motion of neck rotation compared with the control group30 ; this differs from our findings .
it is 
 possible that the subjects with fhp in this study may have compensated for the decreased 
 range of neck motion by lateral flexion the neck .
this may have resulted in our overlooking differences in the direction of lateral 
 flexion movements during preferred neck rotation . in future research ,
the direction of 
 lateral flexion movement generated during neck rotation should be identified and considered 
 within the interpretation of the results .
second , our results can not be generalized to other 
 age groups because all subjects were university students . finally , we did not measure emg 
 activity in the intrinsic neck rotator muscles .
our results demonstrate that neck rotation in subjects with fhp is characterized by lateral 
 flexion in addition to axial rotation compared with subjects without fhp .
the clinical 
 importance of this study lies in its confirmation of often - overlooked neck movement in the 
 frontal plane and the provision of baseline data for future clinical interventions 
 concerning lateral flexion in subjects with fhp .
additionally , significant differences in 
 activation patterns of the scm muscles were noted in individuals with fhp , specifically with 
 increased recruitment on the contralateral side during movement .
assessment of neck rotation 
 movement in patients with fhp should take into consideration lateral flexion in the frontal 
 plane and contralateral scm activation .
 [ purpose ] the present study investigated differences in the kinematics of the neck and 
 activation of the sternocleidomastoid ( scm ) muscle during neck rotation between subjects 
 with and without forward head posture ( fhp ) . [ subjects and methods ] twenty - eight subjects 
 participated in the study ( 14 with fhp , 14 without fhp ) . 
 subjects performed neck rotation 
 in two directions , left and right . 
 the kinematics of rotation - lateral flexion movement 
 patterns were recorded using motion analysis . 
 activity in the bilateral scm muscles was 
 measured using surface electromyography . 
 differences in neck kinematics and activation of 
 scm between the groups were analyzed by independent t - tests . 
 [ results ] maintaining fhp 
 increased the rotation - lateral flexion ratio significantly in both directions . 
 the fhp 
 group had significantly faster onset time for lateral flexion movement in both directions 
 during neck rotation . regarding the electromyography of the scm muscles during neck 
 rotation in both directions , the activity values of subjects with fhp were greater than 
 those of subjects without fhp for the contralateral scm muscles . 
 [ conclusion ] fhp can 
 induce changes in movement in the frontal plane and scm muscle activation during neck 
 rotation . 
 thus , clinicians should consider movement in the frontal plane as well as in the 
 sagittal plane when assessing and treating patients with forward head posture . 