the spinal column and balance are closely related with each other , in order for the 
 adjustment of body posture to meet the stability and mobility requirements of the body 
 joints at the same time1 . the muscles of 
 the abdomen play an important role in stabilization of lumbar vertebra . especially , the 
 abdominal muscles , such as the transversus abdominis ( tra ) and the internus obliquus ( io ) , 
 enable functional movements by providing stability to each segment of the lumbo - pelvic 
 region when supporting the body 's weight2 , 3 . in short , the inner abdominal muscles , such 
 as tra , have impacts on balance by reacting to changes in body posture4 . in the case of stroke patients , the imbalance also it causes a disorder in posture adjustment by 
 reducing the ability to maintain the center of gravity of the trunk on the weight - bearing 
 surface and the reaction required to maintain a symmetric posture5 . compared to normal persons , back pain 
 patients have a difference in mobilization of motor units in the lumbar region , namely 
 asymmetric muscle activity , which works as a factor increasing damage to spinal 
 structures7 , 8 . changes caused by damage to spinal structures cause changes in 
 lumbar movement in back pain patients , reducing their balance ability1 . previous studies have investigated the reduction in balance ability caused by asymmetric 
 movement of trunk muscles , including the abdominal muscles , in stroke and back pain 
 patients . however , most of them were carried out by using electromyography , and there are 
 few studies which have been carried out using ultrasound imaging . also , the previous 
 ultrasound imaging studies of normal persons have focused on measurements of muscles9 , but few studies have focused on 
 correlations between the balance ( symmetry ) of muscles and body balance . accordingly , in 
 this study , thickness changes in the left and right side inner abdominal muscles , tra , io 
 and the externus obliquus ( eo ) , were analyzed using ultrasound imaging . we aimed to identify 
 the impact the thickness differences ( symmetry ) between the left and right abdominal muscles 
 have on balance ability . the study subjects were 41 healthy men and women in their 20s . according to the selection 
 criteria , the selected participants had no neurologic diseases or orthopedic problems , no 
 pains or dysfunction in their lumbar region in the last 6 months , and no disease that could 
 have affected the balance test used in this study . for the ultrasound measurement , images were captured using the b - mode of an ultrasound 
 diagnostic device ( logiq sonography system , -200 , samsung - ge medical systems inc . the thickness changes in abdominal muscles ( tra , io , eo ) on both on the 
 left and right sides were measured using a 7.5-mhz linear transducer , with subjects at rest 
 and performing the abdominal drawing in maneuver ( adim ) . the average value of three 
 measurements of each posture was used in the analysis . the measurements of abdominal muscle thicknesses in each 
 posture followed the general method used in previous studies10 , 11 . for the measurement of balance , the evaluation of static and dynamic balance was carried 
 out . a tetrax portable multiple system ( tetrax ltd , 56 miryam ramat gan , sunlight , israel ) 
 was used for the static balance evaluation . the force platforms of tetrax were positioned 
 for both feet , and the postural sway was evaluated by the changes in weight on the 4 points 
 ( 2 for each forefoot , and 2 for each heel ) . the stability test index ( sti ) the test was 
 carried out with the subjects ' eyes closed . a higher sti value a lower sti value means less postural sway13 . in order to measure the dynamic balance , the functional reach test ( frt ) was carried out . 
 for the frt , a tape measure parallel to the floor was attached to a wall at the height of 
 each subject 's acromion . after putting their feet parallel to each other , subjects rotated 
 both arms to 90 degrees to the horizontal position , and performed forward bending from a 
 position next to the starting point of tape measure with the wrists maintained in the 
 neutral position . pearson 's correlation 
 coefficients were calculated to analyze the impact of the thickness difference between the 
 leftside and rightside abdominal muscles on balance ability in the adim and resting 
 postures . the paired t - test was carried out to compare the thicknesses of tra , io and eo on 
 the left and right side when adopting taking each posture . * = p<0.01 the general characteristics of the participants were age of 20.81.0 years , height of 
 165.06.2  cm , weight of 57.47.5  kg , and a body mass index ( bmi ) of 
 21.02.0  kg / m . the 41 participants comprised 9 men and 32 women . according to the analysis of thickness changes between the resting and adim postures , the 
 muscle thicknesses of both left and right side tra and io increased significantly 
 ( p<0.01 ) ( table 1 ) , when adopting the adim 
 posture , compared to those of the resting posture . * = p<0.05 in contrast , in the case of eo , the muscle thickness was significantly greater in the 
 resting posture than in the adim posture ( p<0.01 ) ( table 1 ) . according to the correlation analysis of the balance ( tetrax , frt ) and thickness difference 
 between the left and right muscles of each posture , there were no significant correlations 
 ( table 2 ) . in the case of tra and io the 
 thickness difference reduced in the adim posture ( table 
 2 ) as in previous studies in which asymmetry of the inner abdominal muscles was measured on 
 using ultrasound images , we demonstrated that the thickness difference between the left and 
 right side abdominal muscles in the adim posture reduced , though asymmetry between the left 
 and right side muscles was not detected in this study . the reaction of the inner abdominal 
 muscles , tra and io , demonstrated in this study , may support those studies which have 
 suggested that these muscles provide the lumbar pelvis region with stability for functional 
 movement , and tra and io are closely related to the movement and balance of the spinal 
 column1,2,3 . in other studies , it has 
 been suggested that back pain patients show asymmetric muscle activities of the trunk 
 muscles , unlike healthy persons , which causes pain and affects balance7 , 8 . however , as seen in this 
 study , healthy persons showed symmetry of the abdominal both muscles , and they also showed 
 symmetric contraction , so we consider that thickness of the abdominal muscles does not have 
 a significant correlation with balance . though many studies have been carried out on the inner abdominal muscles , such as tra and 
 io , it is not easy to find studies about left - right imbalance of abdominal muscles . most studies have focused on the imbalance of trunk muscles in stroke patients and back 
 pain patients , and we could find no study which focused on correlation between muscle 
 imbalance and body balance . therefore , the results of this study of healthy persons may be 
 used as empirical data . according to the study of reeves and others , the thoracic region of back pain patients and the asymmetric muscle activity of left and right lumbar erector spinae are closely related 
 to pathological factors15 . renkawitz and 
 others have demonstrated a correlation between the neuromuscular imbalance of the erector 
 spinae and back pain16 . in general , 
 symmetric co - contraction between erector spinae and inner abdominal muscles enhances the 
 trunk stability , and motor control is used as a treatment method utilizing this 
 symmetry17 . namely , the symmetry of the 
 inner abdominal muscles , as well as that of the erector spinae is closely related to trunk 
 stability , and consequently , asymmetry of inner abdominal muscles may reduce balance 
 ability . it will be necessary to carry out studies in the future , recognizing the importance 
 of the inner abdominal muscles . as the number of participants was small , it 
 is difficult to generalize the data , and all the participants were young adults , so 
 generalization across ages is impossible . also , all the participants were healthy , so it 
 will be necessary to carry out comparative research with stoke patients and back pain 
 patients in the future . in conclusion , in this study , symmetric contraction of the abdominal muscles was induced by 
 adim , which is a re - education method focusing on inner abdominal muscles . this method should 
 be aggressively used to treat patients with central nervous system disorders , including 
 stroke patients who have asymmetric contraction , and patients with musculoskeletal diseases 
 such as back pain patients . also , the correlation analysis results of balances and the inner 
 abdominal muscles can be used empirical data .
this study aimed to identify what impact the thickness differences between the leftside 
 and rightside transversus abdominis ( tra ) , internal obliquus ( io ) and external obliquus 
 ( eo ) have on balance ability in the abdominal drawing - in maneuver ( adim ) and resting 
 postures . 
 [ subjects and methods ] in this study , 41 young adults were asked to adopt a 
 resting posture and to perform adim . 
 the thicknesses of the abdominal muscles ( tra , io , 
 eo ) were measured using ultrasound imaging , then balance ability was measured , so that a 
 comparative analysis could be carried out . 
 [ results ] according to the results , the 
 thicknesses of tra and io very significantly increased when adim was performed . 
 the 
 changes in thickness of the muscles on the left and right sides showed no significant 
 correlations with balance ability . 
 [ conclusion ] according to the study results , the 
 difference in thickness between the left and right side muscles in a normal person is 
 small ( symmetric ) , and the differences in the thickness of tra and io on the left and 
 right side reduced when the adim , which is a re - education method for abdominal muscles was 
 performed . 
 therefore , we consider that the adim should be used in future clinical trials 
 to induce symmetric contraction of the abdominal muscles . also , the correlation results of 
 muscle balance and body balance can be used as empirical data .