proprioception provides sensory feedback from the body to the nervous system ; therefore , it 
 contributes to the maintenance of optimal body alignment1,2,3,4 . it includes several 
 submodalities such as joint position sense , kinesthesia , and sense of tension . joint 
 position sense is used for not only recognizing the location of joints , but also for 
 measuring proprioception . sense of tension is also used to measure proprioception , and 
 kinesthesia is considered to perceive active and passive movements2 , 5 . various receptors 
 such as ruffini receptors and parcinian corpuscles are involved in the relay of information 
 to the central nervous system ; however , the receptors in the muscle called muscle spindles , 
 particularly play a major role in proprioception2 , 
 6 . changes in the muscle length caused 
 by poor posture for a sustained period of time result in musculoskeletal problems such as 
 forward head posture ( fhp)7 , 8 . fhp is defined when that the head is anterior to a vertical line through the center of 
 gravity . it is considered a common postural disorder related to abnormalities in 
 musculoskeletal balance9 , 10 . many studies have reported that fhp is correlated to 
 headache , temporomandibular disorders , myofacial pain syndrome , and abnormal scapular 
 movement . neck pain resulting from a reduction not only in the length of muscle fibers , but 
 also in the capacity to generate tension in muscles is also attributed to fhp11,12,13 . since there are many mechanoreceptors in 
 the cervical muscle , the cervical region is considered to play a crucial role in the 
 transmission of information . multifactorial problems in this region are mainly caused by 
 decreased joint sense7 , 14 , 15 . thus far , many studies have been conducted investigating the correlation between fhp and 
 pain8 , 9 , 
 16 , 17 . however , investigations concerning whether fhp can affect 
 proprioception are few . in the current study , we investigated the effect of fhp on 
 proprioception via the measurement of cervical position - reposition error . thirty - nine subjects with no history of neuromuscular disorder , fracture , or moderate or 
 severe scoliosis were recruited for this study . they were divided into two groups in 
 accordance with their craniovertebral angles : the fhp group ( n=19 ) and the control group 
 ( n=20 ) ( table 1table 1.general characteristics of the subjectsfhp groupcontrol groupgender ( m / f)7/1212/8age ( years)22.21.922.72.1height ( cm)166.07.4169.77.3weight ( kg)63.812.364.012.6values are expressed as the meansd . ) . the craniovertebral angle of all subjects was measured , and when the angle 
 was less than 53 , subjects were included in the fhp group10 , 11 , 18 . all subjects were informed of the purpose of this study and 
 provided their written informed consent prior to their participation . a very common method for the assessment of fhp is taking a picture of the lateral view of 
 the subject18 , 19 . the base of the camera was set at the 
 height of the subjects shoulder . the tragus of the ear was marked , and a plastic pointer 
 was taped to the skin overlying the spinous process of the c7 vertebra . we measured the 
 craniovertebral angle , which is defined as the angle between a horizontal line passing 
 through c7 and a line extending from the tragus of the ear to c7 . the error value of cervical position sense was evaluated using a head repositioning 
 accuracy ( hra ) test20,21,22,23 . the hra test was used to measure the difference between the start 
 ( 0 position ) and return positions . a laser pointer attached to a cycling helmet was firmly 
 placed on the subjects heads . with their head in a natural resting position , the subjects 
 were requested to focus on a target that was positioned at eye level . all subjects were then 
 instructed to close their eyes , and the target was moved so that the laser pointer s beam 
 projected onto the target . the subjects were told to memorize this position because this was 
 the reference position . then they performed a cervical full flexion at their preferred speed 
 and held this position for 5 seconds . following this , the subjects , with their eyes still 
 closed , were instructed to return to the reference position at their preferred speed . the absolute error value was 
 measured as the distance between the two marked points . the same procedure was followed to 
 assess extension , right rotation , and left rotation , which were performed at random . data satisfying the normal distribution were examined with a parametric test . to assess the 
 differences in the error values of the joint position sense ( cervical flexion , extension , 
 and rotation ) between the fhp and control groups , the independent t - test was performed . in 
 addition , pearson correlation coefficients were used to assess the degree of correlation 
 between the craniovertebral angle and the error value of each joint position sense . there were significant differences between the error values of the joint position sense 
 ( cervical flexion , extension , and rotations ) of the fhp and control groups ( p<0.05 ) . in 
 addition , there was an inverse correlation between the craniovertebral angle and the error 
 values of position sense ( p<0.05 ) . the statistical values are shown in tables 2 and 
 3table 2.comparison of the cervical position sense errors of the fhp and control groups 
 ( unit : cm)fhp groupcontrol groupflexion6.231.91 * 4.141.74extension6.232.20 * 4.561.62right rotation7.082.27 * 5.131.51left rotation5.602.03 * 4.231.62values are expressed as the meansd . an asterisk ( * ) indicates a significant 
 difference ( p<0.05).table 3.the correlations between the craniovertebral angle and the error values of 
 position senseflexionextensionright rotationleft rotationpearson s correlation ( r)0.597*0.421*0.428*0.389*an asterisk ( * ) indicates a significant correlation ( p<0.05 ) .. values are expressed as the meansd . an asterisk ( * ) indicates a significant 
 difference ( p<0.05 ) . an asterisk ( * ) nowadays , the use of visual display terminals ( vdt ) of computers and smart phones in almost 
 all homes and organizations is very common24 . excessive use of vdts results in musculoskeletal disorders . among 
 these disorders , fhp is one of the most common conditions10 , 16 , 25 . these 
 changes in the cervical region may lead to musculoskeletal dysfunction such as an upper 
 crossed syndrome  resulting from maintaining poor head position for a long duration of 
 time24 , 26 , 
 27 . these pains cause a reduction of joint sense which influences 
 abnormal proprioception and poor postural balance7 . the present study examined the position - reposition error of the cervical region in order to 
 investigate whether fhp affects joint position sense . higher error rates were shown by the 
 group with fhp compared to the group without fhp . in all movements ( flexion , extension , and 
 rotation ) , there were significant differences in repositioning errors between the two 
 groups . the present study also 
 demonstrated that there is a correlation between the degree of fhp and joint position sense , 
 additionally suggesting that as fhp becomes more severe , joint position sense becomes 
 worse . it is the 
 ability to recognize the joint location , and influences body alignment and joint 
 stability2,3,4,5 . muscle spindles have a primary ending which responds to changes in 
 the length and speed of muscle stretch as well as a secondary ending that responds only to 
 changes in the muscle length2 , 28,29,30 . in general , fhp leads to changes in the lengths of the anterior and posterior neck muscles , 
 and prolonged fhp causes sustained loading on the cervical spine8 , 26 . in this regard , 
 the results of the present study imply that changes in the lengths of muscles , as a result 
 of fhp , have a bad influence on the activity of the muscle spindles , and this is the reason 
 why poor joint position sense is induced by fhp . further studies of the effect of 
 therapeutic exercise for improving position sense of subjects with fhp should be 
 encouraged . in conclusion , this result implies that a 
 change in the muscle length caused by fhp affects decreases joint position sense .