however , because the structure of the spine is 
 more unstable than those of other bones , the roles of the deep muscles are particularly 
 important for spinal stabilization1 . representative deep muscles of lumbar stabilization include the transversus abdominis muscle 
 ( tra ) and the multifidus muscle2 . the fact 
 that tra greatly contributes to lumbar stabilization has been proven by many studies , and 
 the importance of this muscle has been established3 . in particular , through emg measurement , hodges and richardson 
 proved that tra contracts before any limb movement4 . diverse studies have been conducted investigating the functions and thickness of tra 
 affecting lumbar stabilization and balance . it has been shown that the thickness of tra 
 increases with lumbar stabilization exercises5 , and some studies have indicated that dynamic balance can also be 
 improved by lumbar stabilization exercises6 . tra has been studied using ultrasonic imaging to measure , changes 
 in the thickness of tra after pilates exercises , methods have been used to activate tra7 . it has been demonstrated that subjects with 
 low back pain have weaker tra contractile force using ultrasonic imaging8 , and changes in the thickness of tra in 
 various sitting positions have also been measured using ultrasonic imaging9 . some previous studies have examined the effects of the maximum contractile force of the 
 transverse abdominis muscle on the sacral angle , lumbar lordosis , lumbar rom , and dynamic 
 lumbar stability10 . these studies 
 compared the contractile force of the transverse abdominis muscle with dynamic lumbar 
 stability using a pressure biofeedback unit ( pbu ) , however the effects of the thicknesses of 
 tra and io on static and dynamic lumbar stability and balance have been insufficiently 
 studied . accordingly , the present study analyzed the correlations of the thicknesses of tra 
 and io with static and dynamic lumbar stability and balance , to provide basic data for use 
 in interventions for the improvement of lumbar stability and balance in clinics . measurement of abdominal muscle thickness the subjects of the present study were 40 female students in the department of physical 
 therapy of n university in chungcheong - do , korea . the subjects age ( mean  sd ) was 
 20.90.9  years , their height was 161.75.2  cm , and their weight was 54.57.2  kg . those who 
 had pain in the abdomen , those who were short of breath , and those who had difficulties with 
 balance due to musculoskeletal problems were excluded . all the subjects who participated in 
 the present study voluntarily agreed to participate after being sufficiently acquainted with 
 the purpose of the study and the experiment . the thicknesses of tra and the io were measured using an ultrasonic imaging diagnostic unit 
 ( mysono u5 , samsung medison , korea ) . the ultrasonograph was set to b - mode , and 12  mhz linear 
 probes were used . the measurer placed the probe transversely on the top of the iliac crest , moved it toward 
 the exact center of the abdomen in order to stop the screen at a position where all of the 
 external oblique abdominal muscle , the internal oblique abdominal muscle , and tra were 
 visible , and measured the thicknesses of the internal oblique abdominal muscle and tra at a 
 point 13  mm distal from the point at which the fascia of the internal oblique abdominal 
 muscle and the fascia of tra met ( fig . the 
 measurement was done by a measurer who had received instruction in the use of the ultrasonic 
 imaging unit and could use the unit proficiently . to assess static lumbar stability ( sls ) , the contractile force of tra was assessed using a 
 pbu ( chattanooga group , australia ) . a pbu is a simple device consisting of an inflatable 
 non - elastic bag connected to a pressure gauge that is used to record and monitor pressure 
 changes during the movement of the lumbar spinal / pelvic regions . the bag is 16.724.0  cm in 
 size and can measure pressures from 0 to 200 mmhg . excessive pressure changes during the 
 assessment indicate that the movements in the lumbar spinal / pelvic regions are not 
 controlled . to measure sls , when subjects were in the a prone position on a hard floor , the 
 pbu was placed at the point at which the two anterior superior iliac spines cross each other 
 below the navel . the subjects 
 pulled in their lower abdomens maximally , without moving the waist or the hip , in order to 
 reduce the pressure . when a constant pressure had been maintained for 10 seconds , the 
 pressure of change was recorded11 . to 
 measure dynamic lumbar stability ( dls ) , the subject performed the bent knee fall out ( bkfo ) , 
 as described by comerford and mottram12 . the bkfo was performed by placing the pbu vertically 2  cm below the posterior superior iliac 
 spines in the supine position , and to minimize trunk sway during movements , folded towels of 
 the same height were placed on both sides of the pbu . the measurement began with a baseline 
 pressure of 40 mmhg , and the pressure of change was recorded during abduction of the hip 
 joint in a posture in which the hip and knee of one leg were bent and the foot was in 
 contact with the floor , then returned to the original posture . a balance - measuring instrument ( tetrax , beammed ltd , israel ) was used to measure balance 
 ability . this equipment consists of four force plates : two 12  cm wide , 19  cm long 
 rectangular forefoot force plates , and two 12  cm wide , 12  cm long square rear foot force 
 plates . information on the pressure applied to the force plates when the subject was 
 standing on the force plates was amplified , filtered , input to a computer , and analyzed 
 using the tetrax software program . balance was assessed in a quiet room . the subjects stood 
 on both feet , without wearing shoes , in a comfortable posture . stability scores ( ss ) are an indicator of the stability of the center of 
 gravity . they measure the patterns of changes in the pressure applied to the individual 
 force plates , and larger values mean larger variations in the center of gravity . the weight 
 distribution index ( wdi ) shows the percentiles of the weight placed on the four force 
 plates , and the ideal is when 25% of the weight is placed on one force plate . when 
 deviations from the ideal 25% are small , the wdi value is close to 0 , and when deviations 
 are large , the wdi value increases . the measured data were analyzed using the spss 12.0 ko ( spss , chicago , il , usa ) statistical 
 program . to observe the correlations among 
 tra , io , sls , dls , wdi , and ss , pearson correlation coefficients were calculated . tra : transversus abdominis , io : internal obliquus abdominis , sls : static lumbar 
 stability , dls : dynamic lumbar stability , wdi : weight distribution index , ss : stability 
 score abbreviations are the same as in the table 
 1 . the means  sd of tra , io , sls , dls , wdi , and ss are presented in table 1 . regarding the correlations among tra , io , sls , dls , wdi , 
 and ss , tra and sls showed a weak positive correlation in which sls increased as tra 
 increased ( r=0.25 ) , dls showed a negative correlation with tra in which dls decreased as tra 
 increased ( r=0.34 ) , wdi showed a weak negative correlation ( r=0.27 ) , and ss showed a 
 strong negative correlation ( r=0.43 ) . sls and dls ( r=0.36 ) / wdi ( r=0.32 ) showed negative 
 correlations in which dls and wdi deceased as sls increased ( table 2 ) . previous studies have indicated that stabilization exercises strength tra , and that 
 stabilization exercises improve balance . in a study by endleman and critchley , 26 subjects 
 performed pilates exercises , which increased the thicknesses of tra and the io muscles7 . hides et al . measured that the thicknesses 
 of the tra and the io muscles while subjects with lower back pain ( n=20 ) and subjects 
 without lower back pain ( n=20 ) were performing a weight - bearing task , and their results 
 indicate that the subjects with lower back pain manifested thin tra contractions8 . studied changes in 
 the thickness of tra in diverse sitting positions using ultrasonography , and reported that , 
 the thickness of the tra increased more when subjects were sitting on a chair or gym ball 
 with one foot lifted from the ground than when both feet were placed on the ground9 . in a study by kim et al . , subjects dynamic balance improved after performing a lumbar 
 stabilization exercise program13 , and 
 carpes et al . reported that after applying a trunk strength and stability program for 
 subjects with lower back pain , lower back pain was relieved , and body balance was 
 improved6 . reported that 
 neutral lumbar postures can facilitate an increase in the thickness of the tra muscle while 
 performing a leg task14 . previous studies , as noted above , reported that stabilization exercises increase the 
 thickness of the tra and improve balance . however , there has been no previous study of 
 correlations among transversus abdominis thickness , lumbar stability , and balance . although 
 many researchers have stressed the importance of the transversus abdominis in lumbar 
 stabilization and reported that stability and balance improved through exercise 
 strengthening this muscle , there is no research in the literature on how the thicknesses of 
 the abdominal muscles affect lumbar stability and balance . therefore , this study examined 
 the correlations among transversus abdominis muscle thickness , lumbar stability , and 
 balance . the results of the present study indicate that as the thickness of the tra 
 increased , sls , dls , wdi , and ss improved . the results also show that as sls improved , dls 
 and wdi also improved . these results are similar to the results of studies reporting 
 increases in the thickness of the tra and improvement in balance in groups that performed a 
 trunk strength and stability program or pilates exercises . taken together , we consider that training to increase the muscle mass of the transversus 
 abdominis muscle is necessary to improve lumbar stability and balance . we hope that the 
 results of the present study will be utilized as basic data in the treatment of patients in 
 clinics .
[ purpose ] the purpose of the present study was to analyze the correlations of the 
 thicknesses of the transversus abdominis muscle ( tra ) and the internal obliquus abdominis 
 muscle ( io ) with static lumbar stability ( sls ) , dynamic lumbar stability ( dls ) , and 
 balance . 
 [ subjects ] the subjects of the present study were 40 female university students 
 who had no physical defects or pain . [ methods ] the thicknesses of tra and io muscles were 
 measured using an ultrasonic imaging diagnostic unit . 
 sls and dls were measured using a 
 pressure biofeedback unit ( pbu ) , and weight distribution indexes ( wdi ) and stability 
 scores ( ss ) were measured using a balance measuring unit . 
 [ results ] as the thickness of 
 the tra increased , sls , dls , wdi , and ss improved . as sls improved , dls and wdi were also 
 improved . 
 [ conclusion ] to improve lumbar stability and balance , training is needed in 
 order to increase the muscle mass of the transversus abdominis muscle .