trunk muscle strength plays an important role in controlling posture , stability , and trunk 
 movement and is an essential element in the performance of movements of the extremities1 , 2 . the muscles used to achieve trunk stability are the trunk posterior muscles which are 
 composed of a superficial layer muscle group ( trapezius , lattisimus dorsi , rhomboids , etc . ) , 
 a middle layer muscle group ( serratus posterior superior , inferior , etc . ) , a deep layer 
 muscle group ( the erector spinae , transverse spinal , and short segmental groups ) , and 
 anterior and lateral trunk muscles composed of abdominal muscles ( rectus abdominis , internal 
 oblique abdominis , external oblique abdominis , and transverse abdominis ) . the active 
 muscular strength of these muscles plays a primary role in spinal stabilization3 , and weakening of these muscles leads to 
 instability of the trunk and low back pain4 . in addition , instability of the trunk could result in a decreased 
 ability to perform various exercise movements5 . j. gibson et al . stated that in order to have enough muscular 
 activity during upper limb movement , dynamic stability of the spine needs to be 
 established6 . dynamic trunk stability 
 especially involves the anterior and lateral trunk muscles , or abdominal muscles . 
 consequently , observational studies of these abdominal muscles are being actively 
 conducted7 . ultrasound imaging has been 
 widely used to examine the change in or ratio of abdominal muscle thicknesses in many 
 research studies8 . in addition , many 
 studies have investigated the application of exercise intervention to determine the optimal 
 degree of contraction9 , 10 . however , since humans perform many asymmetrical and 
 repetitive tasks , the tendency for a dominant hand occurs , and differences in muscular 
 strength and morphology appear in the left and right side muscles11 . accordingly , the abdominal muscles are expected to show 
 asymmetrical activation , but studies of the asymmetry of the left and right abdominal 
 muscles are lacking . rankin g et al . measured the changes in the thicknesses of the 
 abdominal muscles of general subjects in a stable state , while mannion et al . both of these 
 studies had limitations : the first because the measurements were made in a stable state , and 
 the second because hollowing occurred , which elicits the selective contraction of the 
 transverse abdominis . therefore , the aim of the present study was to compare contraction in 
 the left and right abdominals by applying bracing , which elicits overall contraction of the 
 abdomen . this study was conducted with 31 healthy male and female adults , who were attending d 
 university in busan . exclusion criteria included a bmi at the obesity level , any specific 
 pain in the lumbar area within the past 3 months , and any history of mental diseases , or 
 muscular skeletal diseases that would be provoked by participating in the experiment . the 
 subjects who met the above criteria participated in the experiment voluntarily , and provided 
 their informed consent . before participating in the experiment , subjects were informed of 
 the details of the experiment and the pain that might possibly result from the experiment . the general characteristics of the subjects are shown in table 1table 1.general characteristics of the subjectsgendernage ( years)height ( cm)weight ( kg)bmi*male1521.52.3175.35.566.56.821.72.2female1621.12.2132.95.654.47.420.52.3mean  sd . * bmi = weight ( kg)/height ( m)*height ( m ) a survey was conducted prior to the experiment to determine the dominant hand and only 
 those who answered that the right hand was their dominant hand were selected as subjects . the bracing exercise used in this study was performed by the subjects while they were in a 
 supine position with the pelvis and trunk aligned and bent legs . the subjects contracted the 
 lumbar - pelvic region imagining they were expanding the abdominal area in every direction , as 
 if reacting to the abdomen being hit , and maintained natural respiration12 . a prior training session was conducted 
 before starting the experiment , in order to reduce the error range of the experimental 
 results due to inadequate performance proficiency and to improve reliability . hixson , tn37343 , usa ) was used to measure the 
 maximum pressure five times for each subject , and the mean value was calculated . this mean 
 value was used as the value of 100% contraction , and it was also set as the base point for 
 the 50% contraction measurement . the contraction of the abdominal muscles was observed using 
 a logiq book xp ( ge healthcare product , milwaukee , wi , usa ) system . a standardized 
 ultrasound image was obtained by placing the probe at the midpoint of the costal cartilage 
 and the iliac crest10 . using b 
 ( brightness)-mode ultrasound , the transducer was positioned 2.5  cm anteromedially to the 
 mid - point between the iliac crest and the costal margin on the mid - axillary line , where the 
 fascial boundaries between tra , oi and oe and the superior edge of the tra fascia lie 
 parallel13 . the difference between the 
 muscle thickness on the left and right sides was determined by the paired t - test , and 
 significance was accepted for values of p<0.05 . this study was approved by the human 
 subject s research ethics committee of catholic university ( cupirb-2014 - 004 ) of pusan . the asymmetry in the left and right abdominal muscles was measured at maximum and 50% 
 contraction . the changes in the thicknesses before and after the contraction are shown in 
 table 2table 2.comparison of abdominal muscle thicknesses of each of the left and right side 
 ( cm)rightlefttraresting0.40.10.40.150% cont.0.50.20.50.2100% cont.0.60.40.60.3eoresting0.50.10.50.250% cont.0.60.10.50.1100% cont.*0.80.20.60.2ioresting0.80.30.80.350% cont.0.90.20.80.2100% cont.*1.00.30.90.2*p<0.05 . cont . : contraction , tra : transversus abdominis , eo : external oblique 
 abdominis , io : internal oblique abdominis . the anatomical characteristics of abdominal muscles may reduce the 
 reliability of thickness measurements made using ultrasound imaging . therefore , during the 
 experiment , each subject was measured at three different times of the day to obtain an 
 absolute value . the differences in the abdominal muscle thicknesses of each of the left and 
 right abdominal muscles at rest , at 50% contraction , and at maximum contraction are shown in 
 table 2 . as shown in table 2 , no significant differences were evident for the 
 transverse abdominis ( tra ) . the measured values of the external oblique abdominis ( eo ) and 
 the internal oblique abdominis ( io ) at rest and at 50% contraction also showed no 
 significant differences . however , the measurements of the eo and io at 100% contraction 
 showed significant differences ( p<0.05 ) . * cont . : contraction , tra : transversus abdominis , eo : external oblique 
 abdominis , io : internal oblique abdominis in this experiment , the measured thickness values of the right io ( 1.03 , 0.86 , and 0.17 ) 
 were larger and the thicknesses of the eo ( 0.76 , 0.65 , and 0.11 ) were also larger on the 
 right side . this might be because the subjects were all right - handed , resulting in the right 
 abdominal muscles being thicker . in contrast , the tra showed no differences between the left 
 and right sides when measured at rest , at 100% contraction , or at 50% contraction in the 
 bracing exercise . this exercise enhances the activity of global muscles so the contraction 
 level of the local muscles , like those located more to the central part , such as tra muscle , 
 etc . used a hollowing exercise to determine the level of 
 asymmetry between the left and right abdominal muscles , and reported a thickness difference 
 between the left and right tra muscles9 . therefore , the reason for the difference between the results of our study and those of 
 mannion s is assumed to be differences in methodology9 . apart from these studies on the abdominal muscle thickness , studies 
 related to low back pain are also being actively pursued14 . kang et al . chose the right side for abdominal muscle measurement , 
 and ha et al . applied an abdominal drawing - in exercise after a bridging exercise , but only 
 presented the increases in the abdominal internal oblique and abdominal external oblique , 
 which simply stabilize the trunk , without indicating on which side the abdominal muscle was 
 measured15 , 16 . . also did not state on which side the abdominal muscle 
 was measured with ultrasound imaging during a hollowing exercise , and only addressed the 
 reliability of ultrasound imaging for measuring the contraction of the abdominal 
 muscles17 . in the present study , thus , future 
 studies of the relationship of muscle thickness asymmetry with low back pain should consider 
 the level of asymmetry of contraction in the left and right muscles during an abdominal 
 bracing exercise . the fact that the experiment was conducted on adults in their 20s presents 
 limitations regarding the application of these results to subjects older than their 20s , or 
 those who have smaller muscles and lower muscle contraction . also , since the subjects were 
 healthy , the result may have limited applicability to left and right asymmetry in subjects 
 with impaired trunk stability , or those with low back pain .
[ purpose ] the purpose of this study was to investigate the effective strength levels of 
 abdominal muscle contraction using the bracing contraction method . [ subjects ] 
 the 
 experiment was conducted with 31 healthy male ( m=15 ) and female ( f=16 ) adults attending d 
 university in busan ; all participants had less than obesity level bmi ( bmi<30 ) . 
 [ methods ] bracing contraction was performed by the subjects in the hook - lying position at 
 maximum and minimum pressure levels , five times each , using a pressure biofeedback unit 
 ( pbu ) , and the mean measurement value was calculated . 
 the maximum pressure level was set 
 at 100% and the half maximum pressure level was set at 50% . 
 each subject s left and right 
 abdominal muscle thicknesses were then measured by ultrasound imaging in each state : at 
 rest , 100% contraction , and 50% contraction . 
 [ results ] no significant differences were 
 found between the left and right sides of the transversus abdominis ( tra ) at rest , 50% , or 
 100% contraction . 
 the external oblique abdominis ( eo ) and internal oblique abdominis ( io ) 
 showed no significant difference at rest or at the 50% contraction . 
 however , a significant 
 difference was noted at 100% contraction for the eo and io . 
 [ conclusion ] application of 
 abdominal contraction using bracing can achieve symmetry in the left and right abdominal 
 muscles at less than the maximum contractile strength . 
 the occurrence of asymmetry in the 
 left and right abdominal muscles at the maximum contractile strength suggests that the 
 most suitable contractile strength in this exercise is less than the maximum contractile 
 strength .