neck pain and its disability result in a significant burden on individuals and family 
 members , communities , health - care systems , and businesses1,2,3 .
recently , studies have been conducted in an attempt to identify the 
 disability of deep cervical muscles contributing to stabilization of the cervical spine .
a 
 comparison study reported that people with neck pain have reduced activity of deep neck 
 flexor ( dcf ) muscles with excessive activity of long - lever superficial muscles ( e.g. , 
 sternocleidomastoid and anterior scalene muscles ) , resulting in unstable segmental motion in 
 the intervertebral joint during repetitive upper limbs tasks4 .
disability of the dcf muscles in people with neck pain is regarded as a general 
 consideration that a health professional might consider in planning a treatment strategy . in 
 a control trial ,
jull et al.5 reported 
 that targeted training of the dcf muscles in people with neck pain demonstrated a greater 
 reduction in score for the neck disability index ( ndi ) and intensity of neck pain .
however , 
 for health professionals , the only feasible assessment method known to detect the activity 
 of the dcf muscles in a clinical setting , due to their location , is electromyographic 
 testing by applying needle or tube insertions , which is not a simplistic approach clinically 
 and legally .
therefore , in a clinical setting , an easy to apply approach may benefit rehabilitation 
 practices for the imbalance of muscle recruitment in neck flexor muscles due to lack of dcf 
 muscle activity .
ultrasonography is currently receiving attention as a tool for examination 
 of deep layer muscles , but few studies have successfully reported muscle recruitment and 
 neuromuscular responses of dcf muscles using ultrasonography6 .
the purpose of this study was to identify the imbalance of muscle 
 recruitment in cervical flexor muscles during the craniocervical flexion test ( ccft ) using 
 ultrasonography in people with neck pain and to use the findings to further guide explicit 
 exercise management for neck flexor disabilities .
the study subject were 18 students ( 9 males ) from d university gyeongbuk province with a 
 mean age , weight , and height of 21.03  2.67  years , 57.62  12.9  kg , and 168.01  7.61  cm , 
 respectively .
participants were invited to participate in the study if they reported a 
 current neck pain score of 5 points or greater for the ndi ( total score is 50 points)7 .
participants were excluded if they reported 
 signs of neurological disorders , visited a health professional , or had experienced whiplash . 
 in accordance with the ethical standards of the declaration of helsinki , information on the 
 study
was provided to all the subjects , and informed consent was received from them prior to 
 participation in the study .
real - time ultrasonography images were obtained from the longitudinal cross section of the 
 right deep cervical flexor unit ( longus colli and capitis ) and right sternocleidomastoid 
 during the ccft in accordance with the previously established protocol6 .
a 7.5  mhz transducer of a z.one ultra convertible 
 ultrasound system ( zonare medical systems , inc .
, mountain view , ca , usa ) was positioned 
 diagonally against the front of the neck area running along the trachea s line and 5  cm away 
 from the midline of the cervical spine .
subjects were positioned in 
 the supine position with the head and neck in a mid position , and the task required subjects 
 to perform five gradually increasing movements of craniocervical flexion motion adjusted by 
 visual feedback from an air - filled pressure sensor right underneath their neck ; the applied 
 pressures were 22 , 24 , 26 , 28 , and 30 mmhg the images obtained from the trial were transmitted to the nih imagej software ( version 
 1.44 for windows ) and subjected to image analysis process to measure the muscle thicknesses . 

a four - line grid with a 0.5  cm interval between lines was drawn on each image , and the first 
 line was centered in the middle of each image .
the average value for distance estimated from 
 the second , third , and fourth line was designated as the thickness of each muscle .
the 
 amount of change in muscle thickness at each pressure level from the reference pressure ( 20 
 mmhg ) was calculated by the following formula : ( muscle thickness at the target pressure  
 muscle thickness at the reference pressure ) / muscle thickness at the reference 
 pressure8 .
the imbalance of muscle 
 recruitment at each pressure level was represented as the ratio of the change in muscle 
 thickness of the sternocleidomastoid muscle ( scm ) to the change in muscle thickness in the 
 dcf muscles ( imbalance ratio : thickness of scm / thickness of dcf ) .
this study used ibm spss statistics for windows ( version 20.0 ) to analyze the data . 

repeated measures analysis of variance ( anova ) was used to examine the ratio of the amount 
 of muscle thickness changes in two comparison muscles according to incremental cervical 
 flexion movements .
the muscle thickness ratio of the scm / dcf according to the incremental pressure showed 
 significant differences between 22 mmhg and 24 mmhg , between 24 mmhg and 28 mmhg , between 24 
 mmhg and 30 mmhg , and between 26 mmhg and 28 mmhg ( p<0.05 ) .
the muscle thickness ratio of 
 the scm / dcf was smallest at 24 mmhg ( table 
 1table 1.comparison of the ratio of dcf thickness to scm thickness according to pressure 
 increasepressure22 mmhg24 mmhg26 mmhg28 mmhg30 mmhg22 mmhg2.281.271.330.42 * 1.470.471.940.602.040.8124 mmhg1.470.471.940.60 * 2.040.81 * 26 mmhg1.940.60 * 2.040.8128 mmhg2.040.81*p<0.05 ( meansd ) .
in this study , ultrasonographic assessment of the muscle recruitment in cervical flexor 
 muscles was conducted in people with neck pain .
the secondary aim of this study was to 
 determine the clinical applicability of using ultrasonography in evaluating the disability 
 of cervical flexor muscles .
multiple studies have successfully visualized the recruitment 
 pattern of deep cervical muscles in noninvasive and low - cost manners6 , 9,10,11 , and the present study 
 highly suggested that ultrasonography could be used for this purpose .
the real values for the thicknesses of the dcf and scm gradually increased as the pressure 
 increased .
heightened recruitment of the scm during the ccft , was observed in the present 
 study , which is consistent with the results of previous studies5 , 12 , 13 .
nonetheless , the aim of this study , unlike the previous 
 studies , was to compare the muscle recruitment ratios of the superficial and deep cervical 
 muscles and to suggest the optimal level of pressure for balanced muscle recruitment during 
 the ccft .
as depicted in table 1 , the greatest imbalance 
 in muscle recruitment between the deep and superficial muscles during the ccft was revealed 
 at a pressure of 22 mmhg due to considerable thickness changes in the scm .
the second and 
 third greatest imbalances , also due to increased changes in recruitment of the scm , were 30 
 mmhg and 28 mmhg , respectively , while the dcf muscles maintained steady amounts of 
 recruitment changes at both of these pressure levels .
this finding suggests that health 
 professionals need to take into consideration differences in applied pressures during 
 interventions for patients with a disability of the dcf . as the activation of the scm during 

the ccft is smallest at the pressures of 24 mmhg and 26 mmhg , a recommendation for an 
 adequate exercise intervention for dcf training might be established at a pressure level 
 between 24 mmhg and 26 mmhg .
although this study found great imbalances at pressure levels of 22 mmhg , 28 mmhg , and 30 
 mmhg , the standard muscle recruitment ratio between deep and superficial cervical muscles 
 was not evaluated in a healthy population . for a comprehensive assessment of the muscle 
 recruitment between symptomatic and non - symptomatic populations ,
 [ purpose ] the purpose of this study was to identify the imbalance of muscle recruitment 
 in cervical flexor muscles during the craniocervical flexion test by using ultrasonography 
 and to propose the optimal level of pressure in clinical craniocervical flexion exercise 
 for people with neck pain . 
 [ subjects and methods ] a total of 18 students ( 9 males and 9 
 females ) with neck pain at d university in gyeongsangbuk - do , south korea , participated in 
 this study . 
 the change in muscle thickness in superficial and deep cervical flexor muscles 
 during the craniocervical flexion test was measured using ultrasonography . 
 the ratio of 
 muscle thickness changes between superficial and deep muscles during the test were 
 obtained to interpret the imbalance of muscle recruitment in cervical flexor muscles . 
 
 [ results ] the muscle thickness ratio of the sternocleidomastoid muscle / deep cervical 
 flexor muscles according to the incremental pressure showed significant differences 
 between 22 mmhg and 24 mmhg , between 24 mmhg and 28 mmhg , between 24 mmhg and 30 mmhg , and 
 between 26 mmhg and 28 mmhg . 
 [ conclusion ] ultrasonography can be applied for examination 
 of cervical flexor muscles in clinical environment , and practical suggestion for 
 intervention exercise of craniocervical flexors can be expected on the pressure level 
 between 24 mmhg and 26 mmhg enabling the smallest activation of the sternocleidomastoid 
 muscle . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION