the flexion - relaxation phenomenon ( frp ) is the myoelectric silencing of a muscle during a 
 full - flexion maneuver1 .
the frp of the 
 erector spinae ( es ) muscles shows silencing during upright to full - flexion lumbar movement 
 in asymptomatic subjects .
the phenomenon involves the transfer of the role of the extension 
 moment producer to the deep back muscles to achieve spinal stability2 .
patients with low back pain ( lbp ) exhibit laxity of passive 
 structures and an altered neuromuscular activation pattern in the back muscles , in which the 
 frp of the es muscles is not present .
thus , the frp of es muscles has been used to evaluate 
 lbp and to monitor intervention - related factors after treatment3 .
previous studies have shown that frp occurs with full lumbar flexion in the sagittal plane . 

however , asymptomatic and symptomatic subjects can produce asymmetric lumbar flexion by 
 lumbar flexion combined with lateral bending and axial rotation .
although most healthy 
 people do not have lbp , they have the potential for asymmetry in the frp in the right and 
 left es muscles due to repetitive movement and poor posture in their work environments or 
 daily activities .
ning et al.4 reported 
 that asymmetric lumbar flexion elicits a loss of the frp in the ipsilateral muscle in 
 asymptomatic subjects .
although 90% of non - lbp subjects show frp5 , many of them were at risk of lbp , because of asymmetry in 
 the frp of the es muscles .
currently , lbp patients perform various deep muscle strengthening exercises for lumbar 
 tissue , with pilates , sling exercises , and lumbar stabilization exercises being the most 
 common6 . lumbar stabilization exercises , 
 especially the back bridge and hand - knee position , are the most effective at strengthening 
 the multifidus muscle7 .
marshall and 
 murphy et al.3 reported that a 12-week 
 exercise program ( side bridge , supine bridge , partial curl - up , bird - dog exercise , swiss ball 
 push - up , single leg hold , and rollouts ) decreased self - reported pain and es muscle activity 
 during the relaxation phase of frp tests in patients with chronic non - specific lbp .
lumbar 
 stabilization exercises elicit a reappearance of the frp through strengthening of the 
 multifidus muscle , and eventually this could result in symmetry of the frp in non - lbp 
 subjects with an asymmetric frp of the es muscles . the purpose of this study was to evaluate differences in the frp in the right and left es 
 muscles in asymptomatic subjects .
in addition , the effects of lumbar stabilization exercises 
 on differences in the frp of the right and left es muscles were investigated .
participants ( n=30 ; average age , 19.8  1.1  years ; height , 164.5  7.4  cm ; 
 body mass , 57.54  7.20  kg ) were recruited from among the healthy student population of 
 silla university .
participants were excluded from the study if they had a history of back 
 pain , vertebra surgery , or neuromuscular disorders .
all procedures were reviewed and approved by the 
 ethics committee of silla university . to analyze the frp of the es muscles , surface electromyography ( emg ) ( myotrace 400 ; noraxon 
 inc . ,
scottsdale , az , usa ) was used with two bipolar surface emg electrodes placed 
 bilaterally 2  cm apart over the right and left l3 area .
the data were collected at 1,000  hz and 
 processed using a 20450  hz band - pass filter and a 60-hz notch filter .
the root mean square 
 moving average of 300  ms duration of the processed emg data was calculated .
the relaxation 
 time was defined as when es activation decreased by two standard deviations ( sd ) of silence 
 activation in the fully flexed lumbar phase .
the absolute value of the difference in 
 relaxation time measured in the right and left es was calculated .
the experiment was done in 
 triplicate and the calculations were based on the averages of the three trials .
the position of the feet was 
 marked for consistency between trials . in all trials ,
the move from upright to full - forward 
 flexion ( forward flexion phase ) was performed over 3s , followed by 3s maintenance of full 
 flexion posture ( fully flexed phase ) , and another 3  s to move from full - forward flexion to 
 the upright posture ( re - extension phase)8 , 9 .
participants were assigned randomly to the control or exercise group using a table of 
 random numbers
the exercise group performed two 
 exercises that are commonly used in clinical practice and have been used in previous studies 
 ( back bridge and hand - knee exercise ) .
these exercises were performed 3 days per week with a 
 trainer and 2 days per week at home for 4 weeks .
the exercises were done in three sets with 
 three repetitions per set and a set break of 3  min .
instructors provided feedback to ensure that a consistent supine and lower limb posture was 
 maintained during the exercise .
for the back bridge exercise , the subjects lay supine on the 
 floor with their feet flat on the ground , their knees bent at 90 , their toes facing 
 forward , and their hands on the floor by their sides . with their palms facing down , the 
 subject raised the pelvis to achieve and maintain a neutral hip flexion angle .
the subjects 
 were requested to raise the back bridge from the floor ( stable condition ) and hold the 
 posture for 1  min . after the 1-min hold , the subject was given a 30-s break . for the 
 hand - knee exercise , the subjects started on their hands and knees with a neutral pelvis 
 position and breathing normally .
they lifted and held straight the right arm and the left 
 leg before doing the same with the opposite set of limbs . the subjects held each position 
 for 40  s with a 30-s break between positions .
18.0 ) was used to test the significance of differences between the 
 exercise and control groups and between pre- and post - exercise .
a subject factor analysis 
 was used to assess the statistical significance of the frp in the es muscles with a repeated 
 two - way analysis of variance .
the paired t - test was used to test the significance of 
 differences between the two groups characteristics .
a difference was seen in the frp between the right and left es muscles in asymptomatic 
 subjects ( control group 119.2  69.2  ms and exercise group 131.1  85.2  ms ; p>0.05 ) .
the exercise group showed a 
 significant decrease post - exercise ( 50.0  27.0  ms ) versus pre - exercise ( 112.3  41.5  ms ) in 
 the right and left frp of the es muscles ( p<0.05 ) .
in this study , the healthy participants showed a difference in the right and left frp in 
 the es muscles .
this lack of balance in the frp is thought to result from repetitive 
 asymmetric posture and activity .
previous research has indicated there is an increased risk 
 of back pain when working in asymmetric positions9 .
motor control deficits and motor dysfunction result in 
 increased passive system loading from repeated stress10 .
the students who participated in this experiment spent a great 
 deal of time at school in a sitting position .
this does not produce lbp , but they exhibited 
 an asymmetric frp in the right and left es muscles .
callaghan11 reported a difference in activation between the right and left es 
 in healthy subjects ( right 15 and left 17 of 20 participants ) .
this indicates that asymptomatic people can have 
 impairment and imbalance in the frp . when the students exhibited poor posture ( e.g. , sitting 
 cross - legged or one - leg weighted standing ) they experienced no pain , but they demonstrated 
 potential factors for lbp .
the bridge exercise is a popular lumbar stabilization exercise that activates weak 
 abdominal muscles and prevents unstable lumbar spine movement via co - activation of the 
 pelvic floor muscles , diaphragm , and deep abdominal muscles12 .
additionally , the swiss ball exercise activates deep and 
 superficial abdominal muscles by using the characteristics of an unstable environment13 .
marshall and murphy14 reported that swiss ball exercises increased the frp ( from 
 3.26  3.43 to 6.53  3.34 ) in lbp patients . in the present study , lumbar stabilization 
 exercises improved muscular balance by reducing asymmetry in frp onset .
howarth et al.15 reported that back muscle instability 
 caused by slumped sitting delayed the onset of the frp .
likewise , the intervention used in 
 the present study was meaningful for trunk asymmetry and instability .
at the onset of the 
 frp , the external moment generated around the lower back by the upper body s mass is 
 counteracted by the net internal moment produced from the tensile forces developed in 
 elongated passive tissue , instead of the forces generated by the active component of the 
 lower back musculature16 .
thus , lumbar 
 stabilization exercises can improve the motor control of lumbar spinal movement and lumbar 
 stabilization control . among the various exercises used for lumbar stabilization ,
the sling and swiss ball 
 exercises show the highest emg activation values17 ; however , this study used the back bridge and hand - knee exercises . 

these two exercises do not need any special equipment or space , so they are more suited for 
 home - based exercise than sling or ball exercises .
additional studies are needed to measure 
 changes in the frp in the es muscles elicited by other lumbar stabilization exercises .
while further study is needed , the results of this study suggest that lumbar stabilization 
 exercises can help to correct asymmetry of the frp in the right and left es muscles and this 
 would help prevent lbp in the general population .
 [ purpose ] this study evaluated the differences in the flexion - relaxation phenomenon ( frp ) 
 of the right and left erector spinae muscles in asymptomatic subjects and the effect of 
 lumbar stabilization exercises on these differences . [ subjects and methods ] twenty - six 
 participants ( 12 in the exercise group and 14 in the control group ) with a difference in 
 the frp in the right and left erector spinae muscles were recruited from among healthy 
 students attending silla university . 
 the exercise group performed two lumbar stabilization 
 exercises ( back bridge exercise and hand - knee exercise ) for 4 weeks . 
 the control group did 
 not exercise . 
 [ results ] no significant group - by - exercise interaction was found . 
 the right 
 and left erector spinae muscles did show a difference in frp between the control and 
 exercise groups ( 119.2  69.2 and 131.1  85.2  ms , respectively ) . 
 in addition , the 
 exercise group showed a significant decrease in post - exercise ( 50.0  27.0  ms ) compared to 
 pre - exercise ( 112.3  41.5  ms ) differences in the right and left frp . 
 [ conclusion ] these 
 results suggest that lumbar stabilization exercises may counter asymmetry of the frp in 
 the erector spinae muscles , possibly preventing low back pain in the general 
 population . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION