abdominal muscle strengthening exercises are frequently used for the promotion of health 
 and prevention of musculoskeletal disease , as well as for rehabilitation of lower back 
 pain1 , 2 .
strengthening the abdominal muscles helps to stabilize the trunk 
 and lessen the stress on the lumbar spine3 .
the curl - up exercise has been shown to produce reasonable levels of 
 activity in the rectus abdominis ( ra ) muscle while minimizing the resultant load on the 
 spine4 .
however , the curl - up exercise may activate the superficial cervical flexors , such as the 
 sternocleidomastoid ( scm ) , because the head and shoulders are raised off the floor during 
 the neck flexion .
superficial cervical flexors are the dominant muscle group during neck 
 flexion movement5 . a neck flexion position 
 that causes neck pain could be a contributing factor to increased back pain , because the 
 vertebral regions are connected to each other6 .
according to lee et al.7 the activity of the scm muscle increases as the neck flexion angle 
 increases .
muscle activity of the superficial neck flexor is significantly increased in 
 cervical pain during the neck stabilizing task ; increased activity of the scm could be 
 compensatory for poor segmental stability of the deep muscles .
therefore , an intervention to 
 decrease scm activity with neck flexion restriction during the curl - up exercise is 
 required .
research has focused primarily on increased abdominal activity during curl - up exercises 
 using an unstable surface8 and different 
 positions of the upper and lower limbs9 to 
 identify the most effective position to increase abdominal strength . the question of whether 
 neck flexion restriction can alter the muscle activity of the ra , external oblique ( eo ) , and 
 scm muscles during a curl - up has not been investigated .
thus , the purpose of this study was 
 to assess the effect of neck flexion restriction on scm and abdominal muscle activity during 
 the traditional curl - up exercise and the curl - up exercise with neck flexion restriction .
in total , 13 healthy male subjects were recruited from inje university , gyeongsangnam - do , 
 republic of korea .
the mean age of the subjects was 23.7  0.4  years , and their mean height 
 and weight were 173.2  5.25  cm and 66.5  5.7  kg , respectively .
subjects were excluded if 
 they had a history of abdominal or low - back pain within 6 weeks prior to the study and an 
 inability to correctly perform all the exercises in a pain - free manner .
prior to 
 participation , all subjects read and signed an informed consent form that was approved by 
 the institutional research review committee of inje university .
surface electromyographic ( emg ) signals were collected with a trigno wireless emg system 
 ( delsys , inc . ,
emg data were collected from the scm , ra , and eo muscles on 
 the dominant side .
the electrode placements were as follows : scm , half the distance between 
 the mastoid process and the sternal notch ; ra , 2  cm lateral to the umbilicus ; and eo , the 
 inferior edge of the eighth rib superolateral to the costal margin10 .
root mean square values were calculated for all raw data . 
 to normalize the data , the maximum voluntary isometric contraction ( mvic ) of each muscle
the subject was instructed to lay supine with both arms down 
 by their side , knees flexed to approximately 90 , hips flexed to approximately 45 , and feet 
 placed flat on the floor .
subjects elevated their trunk by lifting their head and shoulders 
 until the scapulae were lifted above the floor .
a target bar was placed at the chest to 
 provide feedback when the scapulae were off the floor .
second , subjects performed a 
 traditional curl - up exercise with neck flexion restriction .
the subject began in a supine 
 hook - lying position and was asked to move to the chin tuck position .
lightly touch your right hands on your mandible  to provide a tactile cue for 
 neck flexion restriction during curl - up .
the subject was asked to touch the target bar 
 during each curl - up exercise and to hold the position for 5 s. all test trials were repeated 
 three times , with a 1-min rest between trials to avoid fatigue .
the mean value of these 
 measurements was used for data analysis . to compare the differences in normalized emg muscle activity of the scm , ra , and eo 
 muscles , between the traditional curl - up and curl - up with neck flexion restriction 
 exercises ,
there was significantly lower emg activity of the scm during the curl - up exercise with neck 
 flexion restriction than the traditional curl - up exercise ( p < 0.05 ) .
conversely , the 
 activity of the ra and eo muscles was significantly higher during the curl - up exercise with 
 neck flexion restriction compared to the traditional curl - up exercise ( p < 0.05 ; table 1table 1.electromyography activity ( % maximum voluntary isometric contraction ) in the 
 sternocleidomastoid , rectus abdominis , and external oblique muscles during the 
 traditional curl - up and curl - up with neck flexion restriction exercisesmuscletraditional curl - upcurl - up with neck flexion restrictionsternocleidomastoid32.2  4.5 * 22.1  6.1rectus abdominis46.5  12.356.2  11.1*external oblique38.8  15.550.4  17.8**p<0.05 ) .
the results of this study showed that the curl - up exercise with neck flexion restriction 
 decreased the activity of the scm muscle and increased the activity of the abdominal muscles 
 significantly .
this may have important clinical implications for abdominal muscle 
 strengthening in patients with neck pain . in this study
, we showed that during the curl - up exercise in which neck flexion was 
 restricted by the subject s hand , the muscle activity of the scm was decreased compared to 
 the traditional curl - up exercise .
recently , a self - monitored method using tactile 
 stimulation through the subject s own finger touching has been shown to be a useful clinical 
 method to enhance abdominal muscle training and to prevent compensatory movement12 .
superficial cervical flexors ( scm and 
 anterior scalenes ) contribute to forward translator motion in the cervical region5 .
considering the significantly decreased scm 
 activity in this study , having subjects touch their hands on the mandible could be a good 
 strategy to prevent forward translation in the cervical region .
previous studies focused on decreased scm and increased abdominal activity during the 
 curl - up exercise13 , 14 .
yoon et al.14 
 reported that the muscle activity of scm during curl - up with craniocervical flexion ( ccf ) 
 was significantly lower than during the traditional curl - up exercise , but the abdominal 
 muscle activity did not differ between the two tasks .
however , we found that decreased scm 
 activity could influence increased abdominal muscle activity during curl - up with neck 
 flexion restriction .
it is possible that neck flexion restriction by the subject s hand 
 ( scm : 22.1%mvic ) could be used to greatly reduce cervical curve flexion relative to ccf 
 ( right scm : 39.5 , left scm : 38.2%mvic ) due to decreased scm activity .
thus , reduced cervical 
 flexion during the curl - up exercise with neck flexion restriction could increase abdominal 
 muscle activity because of the lengthened lever arm of the trunk .
further studies are needed to 
 assess not only the activity of the abdominal muscle , but also kinematic data in the 
 cervical region .
this study provides empirical evidence that the curl - up exercise in which neck flexion is 
 restricted by the hand is more useful for enhancing abdominal muscle activity compared to 
 the traditional curl - up exercise .
this suggests that it may be used as an alternative 
 abdominal strengthening exercise for the management of patients with neck disability . 

therefore , the curl - up exercise with neck flexion restriction is recommended to prevent 
 excessive activation of the superficial cervical flexors during the curl - up exercise .
 [ purpose ] the purpose of this study was to investigate the effect of neck flexion 
 restriction on sternocleidomastoid ( scm ) , rectus abdominis ( ra ) , and external oblique ( eo ) 
 muscle activity during a traditional curl - up exercise and a curl - up with neck flexion 
 restriction . 
 [ subjects ] in total , 13 healthy male subjects volunteered for this study . 
 
 [ methods ] all subjects performed a traditional curl - up exercise and a curl - up exercise in 
 which neck flexion was restricted by the subject s hand 
 . surface electromyography ( emg ) 
 signals were recorded from the scm , ra , and eo during the curl - up . 
 [ results ] there was 
 significantly lower emg activity of the scm during the curl - up exercise with neck flexion 
 restriction compared to the traditional curl - up exercise . 
 conversely , the activity of the 
 ra and eo muscles was significantly higher in the curl - up exercise with neck flexion 
 restriction than in the traditional curl - up exercise . 
 [ conclusion ] neck flexion 
 restriction is recommended to prevent excessive activation of superficial cervical flexors 
 during the curl - up exercise . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION