the local muscle system consists of muscles with insertion in or origin from ( or both ) the 
 lumbar vertebrae and includes the transversus abdominis ( tra ) and multifidus muscles . the 
 global muscle system comprises muscles with origin from the pelvis and insertion in the 
 thoracic cage , and includes the rectus abdominis , internal oblique ( io ) and external oblique 
 ( eo ) muscles1 . these anatomical 
 differences in muscle attachment influences the function of each muscle on the lumbar spine . 
 in short , local muscles are believed to contribute to the stability of the lumbar spine , 
 whereas global muscles contribute to gross movement of the lumbar spine1 , 2 . the tra muscle has been considered to have a protective role during activity that 
 challenges the integrity of the lumbar spine2,3,4 . many reports have suggested a preferential ability for the tra muscle to increase spinal 
 stability over other muscles2 , 4,5,6,7 . in particular , it is 
 suggested that the tra muscle should be recruited while suppressing activity of the io and 
 eo muscles . richardson et al . called this the ideal contraction pattern for abdominal muscle 
 activation in terms of spinal stability7 . contraction of the tra without increasing activation of other abdominal muscles is 
 described as an abdominal hollowing maneuver ( ahm)7 , 
 8 . the ahm is usually conducted in the 
 supine position because a tester can easily observe and palpate the muscle activation . however , this position is not ideal , as it is not functional and the effect of an unstable 
 surface such as a balance ball on muscle activation can not be determined . by contrast , the 
 sitting position is more functional than the supine position and provides the opportunity to 
 introduce greater challenge to muscle activation such as that in unstable surfaces . validity studies have compared ultrasonographic measurements with those obtained by using 
 magnetic resonance imaging and electromyographic activity to determine muscle activation in 
 low - force contractions9,10,11 . these findings 
 support the use of ultrasonographic imaging as a reliable and noninvasive technique to 
 measure abdominal muscle thickness12 . studies have measured tra and io muscle activation by using ultrasonography during hip 
 flexion while sitting on a balance ball . changes in muscle thickness were significantly 
 greater in the sitting position than in the supine position12 , 13 . evidence suggest that the ahm is effective for muscle activation and hence re - education of 
 the tra muscle2 , 8 , 14 , 15 . the tra and io muscles are also known to contract while sitting on 
 a balance ball and increase in thickness with increasing levels of instability12 . given the above - mentioned findings , the 
 purpose of this study was to investigate the changes in the thicknesses of the tra and io 
 muscles during sitting on a stable surface or a balance disc during 4 different levels of 
 trunk stability tasks accomplished by sitting , sitting with single hip flexion , an ahm , and 
 an ahm with single hip flexion . the study hypothesis was that the thickness of the tra 
 muscle would increase with an ahm during sitting on an unstable surface . twenty healthy men ( mean  sd : age , 22.2  2.7  years ; height , 170.4  5.6  cm ; weight , 63.4 
  6.3  kg ) volunteered to participate in this study . this study was conducted in accordance 
 with the declaration of helsinki and was approved by the ethics committee at the saitama 
 medical university , saitama , japan ( m-37 ) . ultrasonographic imaging of the abdominal muscles was performed under 8 conditions as 
 follows : ( a ) sitting , ( b ) sitting with left hip flexion , ( c ) sitting with an ahm , and ( d ) 
 sitting with an ahm and left hip flexion , either with or without a balance disc ( 39209 , 
 regent far east , co. ltd . ) on a stable chair ( fig . 1.the 8 testing positions ( 2 stability levels of sitting position and 4 tasksthe upper row shows tasks sitting on a chair , and the lower row shows tasks sitting 
 on an unstable surface . from left to right : ( a ) sitting , ( b ) sitting with left hip 
 flexion , ( c ) sitting with an abdominal hollowing maneuver ( ahm ) , and ( d ) sitting with 
 an ahm and left hip flexion ) ) . during ultrasonographic measurements , the subjects sat with their arms folded across their chest and spine in 
 neutral position while breathing normally . during hip flexion , the subjects held their left 
 foot 10  cm above the floor for tasks ( b ) and ( d ) . to establish intrarater reliability , the 
 ultrasonographic measurements in all the positions were repeated with an interval of 1 to 2 
 weeks in the first 11 of the 20 subjects . the 8 testing positions ( 2 stability levels of sitting position and 4 tasks the upper row shows tasks sitting on a chair , and the lower row shows tasks sitting 
 on an unstable surface . from left to right : ( a ) sitting , ( b ) sitting with left hip 
 flexion , ( c ) sitting with an abdominal hollowing maneuver ( ahm ) , and ( d ) sitting with 
 an ahm and left hip flexion ) a diagnostic ultrasonographic imaging unit set in b - mode ( prosound 6 , aloka ) with a 7.5-mhz 
 linear head transducer was used to measure the thickness of the tra and io in all of the 
 testing positions . the ultrasound transducer was transversely placed across the abdominal wall over the 
 anterior axillary line , midway between the 12th rib and the iliac crest , in order to obtain 
 a clear image of the deep abdominal layers12 . the image was frozen at the end of expiration , and a vertical 
 straight line through the center of the ultrasonographic image was used to ensure 
 standardized placement of the measurement line . by using the cursor points , muscle thickness 
 was carefully measured between the inside edge of the fascial bands in millimeters ( mm ) . the change in the 
 thickness of each abdominal muscle under different sitting positions was normalized to 
 actual muscle thickness at a resting position in the supine position . the visual analogue 
 scale ( vas ) was used to identify the perceived difficulty of the task in each testing 
 position . the kolmogorov - smirnov 
 test was used to assess the normality of the distribution of the tested variables . the 
 intraclass correlation coefficient ( icc ) ( 1,1 ) was used to assess the intrarater reliability 
 of the measurements of muscle thickness on two occasions . the icc reflects the percentage of 
 agreement between the values from two repetitions of the measurements . a 2-way mixed - design 
 analysis of variance ( anova ; 2 stability levels of sitting position  4 tasks ) for each 
 muscle was used to test the main effects of stability of the sitting position , tasks , and 
 their interactions on the thickness of the tested muscles . significant main effects and interactions were further analyzed by using the 
 post hoc ( bonferroni ) multiple range test . icc(1,1 ) was greater than 0.76 for the repeated measurements of the thicknesses of the tra 
 (  = 0.760.90 ) and io (  = 0.840.91 ) during all the test positions ( table 1table 1.intrarater reliability of the measurements of muscle thickness ( a)*(b)*(c)*(d)*(a)(b)(c)(d)tra0.820.780.870.900.900.880.900.84io0.850.900.840.870.860.900.910.88tra : transverse abdominis , io : internal oblique . * one - way intraclass correlation coefficient ( icc ) values : ( a ) sitting , 
 ( b ) sitting with left hip flexion , ( c ) sitting with an abdominal hollowing maneuver 
 ( ahm ) , and ( d ) sitting with an ahm and left hip flexion ) . one - way intraclass correlation coefficient ( icc ) values : ( a ) sitting , 
 ( b ) sitting with left hip flexion , ( c ) sitting with an abdominal hollowing maneuver 
 ( ahm ) , and ( d ) sitting with an ahm and left hip flexion the thicknesses of the tra and io in the supine position were 3.1  0.7 and 9.3  1.7  mm , 
 respectively . the tra thickness in sitting with an ahm on a balance disc significantly 
 increased about twofold greater than that in sitting with an ahm . that in left hip flexion 
 on a balance disc also significantly increased about 2.4 times greater than that in the 
 supine position . furthermore , the tra thickness with an ahm and left hip flexion on a 
 balance disc also increased significantly 2.6 times greater than that in the supine position 
 ( table 2table 2.relative changes in abdominal muscle thickness during 2 levels of sitting 
 stability challenge and 4 tasks in comparison with those in the supine 
 positionmusclestabilitysupineabcdtrastable*1.01.20.21.60.22.10.52.40.4unstable1.30.21.70.22.10.52.60.4iostable*1.01.00.11.40.21.40.41.70.3unstable1.10.11.60.21.40.41.90.2tra : transverse abdominis , io : internal oblique . * sitting on a chair ; $ sitting on a 
 balance disc . ( a ) sitting , ( b ) sitting with left hip flexion , ( c ) sitting with an 
 abdominal hollowing maneuver ( ahm ) , and ( d ) sitting with an ahm and left hip 
 flexion ) . ( a ) sitting , ( b ) sitting with left hip flexion , ( c ) sitting with an 
 abdominal hollowing maneuver ( ahm ) , and ( d ) sitting with an ahm and left hip 
 flexion the results of the 2-way anova revealed a significant difference in the main effects of the 
 task ( f = 93.3 , p < 0.01 ) but no significant difference in those of the stability levels 
 in the sitting position ( f = 2.82 , p > 0.05 ) on the relative changes in the thickness of 
 the tra muscle . the pairwise comparisons revealed that the relative thickness of the tra 
 significantly differed between the 4 tasks such that in task ( a ) , the thickness was less 
 than that in task ( b ) , which was less than that in task ( c ) , which was less than that in 
 task ( d ) ( table 3table 3.pairwise comparison of the changes in the thicknesses of the tested abdominal 
 muscles ( in millimeter ) between the positions in all the subjectsmuscleposition and muscle thickness ( meanse)position and muscle thickness ( meanse)mean differencep value traa ( 3.770.26)b ( 5.010.32)1.33<0.01 a ( 3.770.26)c ( 6.570.39)2.8<0.01 a ( 3.770.26)d ( 7.860.43)4.09<0.01 b ( 5.010.32)c ( 6.570.39)1.47<0.01 b ( 5.010.32)d ( 7.860.43)2.76<0.01 c ( 6.570.39)d ( 7.860.43)1.29<0.01   ioa ( 9.580.36)b ( 13.740.55)4.16<0.01 a ( 9.580.36)c ( 13.140.73)3.56<0.01 a ( 9.580.36)d ( 16.840.68)7.26<0.01 b ( 13.740.55)c ( 13.140.73)0.60.8b ( 13.740.55)d ( 16.840.68)3.11<0.01 c ( 13.140.73)d ( 16.840.68)3.71<0.01 tra : transverse abdominis , io : internal oblique ; ( a ) sitting , ( b ) sitting with left 
 hip flexion , ( c ) sitting with an abdominal hollowing maneuver ( ahm ) , and ( d ) sitting 
 with an ahm and left hip flexion ) . tra : transverse abdominis , io : internal oblique ; ( a ) sitting , ( b ) sitting with left 
 hip flexion , ( c ) sitting with an abdominal hollowing maneuver ( ahm ) , and ( d ) sitting 
 with an ahm and left hip flexion in regard to the io , the results of the 2-way anova revealed a significant difference in 
 the main effects of task ( f = 97.2 , p < 0.01 ) and a significant difference in those of 
 stability levels ( f = 6.89 , p < 0.01 ) on the relative changes in muscle thickness . the 
 pairwise comparisons revealed that the relative thickness of the io significantly differed 
 between the 2 stability levels . in short , that in sitting on a balance disc was greater than 
 that in sitting on a chair . furthermore , muscle thickness significantly differed among the 4 
 tasks such that muscle thickness in task ( a ) was less than that in task ( b ) and that in task 
 ( c ) , which was less than that in task ( d ) . additionally , we found no significant difference 
 in the thickness of the io between task ( b ) and task ( c ) ( table 3 ) . as for task difficulty scored by using the vas , the results of the 2-way anova revealed a 
 significant difference in the interaction between task and stability levels in the sitting 
 position ( f = 3.27 , p < 0.05 ) according to the relative change in muscle thickness . with 
 respect to the effects of task and stability levels in the sitting positions , significant 
 changes were observed for each task ( f = 43.5 , p < 0.01 ; f = 76.7 , p < 0.01 ) . the vas 
 score during sitting on an unstable surface was greater than that during sitting on a stable 
 surface ( p < 0.01 ) . as for tasks , the multiple comparisons of the vas scores between the 
 4 tasks in sitting on a chair showed significant differences in that the vas score in ( a ) 
 was less than that in ( b ) , which was less than that in ( c ) , which was less than that in ( d ) . 
 however , the vas scores in the 4 tasks in sitting on an unstable surface showed that the 
 score was less in ( a ) , which was less than that in ( b ) and that in ( c ) , which was less than 
 that in ( d ) . the ultrasonographic measurement of the thicknesses of the tra and io muscles during single 
 hip flexion on or off an unstable surface showed excellent intrarater reliability15 . reliability 
 may have been enhanced by factors such as marking for the measurement point , picture capture 
 at the same end point of expiration , and rigorously defined motor tasks . no significant difference in tra thickness was observed between the tasks performed in 
 sitting on a stable or unstable surface . by contrast , io thickness was greater when sitting 
 on an unstable surface than when sitting on a more stable surface . these results indicate 
 that stability levels did not affect the muscle activation of the local muscle ( tra ) . global muscles such as the eo and io are thought to be 
 easily recruited and to often substitute for local muscles such as the tra and multifidus . 
 based on the results of this study , it was postulated that tra contraction was not enough to 
 stabilize the trunk when sitting on an unstable surface and io might therefore have been 
 recruited . while the tra muscle inserts on the costal process of the lumbar spine via the 
 thoracolumbar fascia and acts to stabilize the spine , the middle and lower part of the tra 
 are known to be anatomically only 0.37 to 0.39 times thicker than the io muscle17 . the study results taken together with 
 these anatomical observations suggest that io activation may reinforce tra muscle activation 
 under increasing levels of lumbar stability challenge . finally , the results suggest that 
 exercise performed while sitting on a chair leads to a more isolated tra activation without 
 io activation than sitting on a balance disc , which might be preferential from the 
 perspective of load on the lumbar spine . one study showed that tra muscle thickness with an ahm was almost equal to that 
 during a 25% maximum voluntary contraction ( mvc)8 . in addition , a study using wire electromyography showed that active 
 straight - leg raising induced 5% of the mvc for the tra muscle18 . furthermore , a study that performed ultrasonographic measurement 
 showed that tra thickness with an ahm was twofold greater and that during active 
 straight - leg raising was 1.25 times greater than that at rest19 . based on the above - mentioned findings , ahm can be considered to 
 have a much stronger effect on tra activation than active straight - leg raising . when 
 comparing between the abdominal muscles , the tra was recruited more readily than the eo , io , 
 and rectus abdominis20 . the present 
 results were obtained in a similar fashion , and tra was found to be activated by the ahm 
 more selectively than hip flexion in the sitting position . two reasons can explain why the thicknesses of the tra and io muscle with the ahm combined 
 with hip flexion increased greater than those with either the ahm or hip flexion alone . first , psoas major activation during hip flexion is likely to stress the lumbar spine , which 
 is countered by the tra . the thoracolumbar fascia and tra have a synergic role in 
 stabilizing the lumbar spine and sacroiliac joint21,22,23,24,25 . the eo and io muscles differ from the tra muscle with respect to 
 lumbar spine stability . even though the eo and io muscles continue to the middle portion of 
 the thoracolumbar fascia23 , 26 , these two anatomical structures are almost perpendicular 
 and their force is considered relatively weak26 . a second factor is the function of the contralateral gluteus 
 maximus muscle during the ahm and hip flexion . because the deep fibers of 
 the multifidus muscle stabilizes the lumbar spine and is synchronously activated with the 
 tra muscle28 , it is considered that the 
 ahm together with hip flexion increases tension of the gluteus maximus , thoracolumbar 
 fascia , and tra . in addition , based on the present results and in accordance with the 
 results reported by richardson7 , to obtain 
 an ideal contraction pattern for the lumbar spine - stabilizing muscles , the ahm in the 
 sitting position should be chosen because of the increased tra muscle thickness and 
 suppressed change in io muscle thickness . in addition , strong exercise demands such as a 
 combination of the ahm and hip flexion might be chosen if both the tra and io are required 
 to be highly activated . the study results indicate that stability of the sitting position alters the perceived 
 difficulty of the exercise task . this difference in perceived difficulty is related to io 
 muscle thickness , which potentially indicates greater recruitment of that muscle in an 
 attempt to stabilize the lumbar spine . the perceived difficulty of the task is usually an 
 important factor that influences the choice of an exercise . according to the present 
 results , the ahm in sitting on a stable surface might be chosen as an exercise position for 
 people of all levels of ability and with low back pain who want to improve their tra 
 activation . on the other hand , the ahm with hip flexion should be chosen for those who want 
 to increase activation of both the tra and io muscles despite its perceived difficulty . one limitation of this study was that all of the subjects were young and healthy and 
 without pain . the study results may not be applicable to other groups such as older people 
 or those with low back pain . the motivation and learning ability of the subjects might have 
 also influenced the results . further studies are needed to measure the thickness of all 
 parts of the tra and its relationship to other muscles surrounding the hip and lumbar 
 spine . tra muscle thickness was influenced by the ahm rather than by hip flexion regardless of 
 stability of the sitting position . sitting on a stable surface should be chosen as the ideal 
 position for the optimal activation pattern for abdominal muscles that induce tra muscle 
 activation while suppressing the io muscle . the ahm with hip flexion might be chosen for 
 maximizing the activation of both the tra and io muscles even though its perceived 
 difficulty is increased .
[ purpose ] this study was conducted to investigate ultrasonically determined changes in 
 the thickness of the transversus abdominis and internal oblique muscles during different 
 sitting conditions . 
 [ subjects and methods ] twenty healthy men volunteered to participate 
 in this study . 
 four different sitting conditions including ( a ) sitting , ( b ) sitting with 
 left hip flexion , ( c ) sitting with an abdominal hollowing maneuver ( ahm ) , and ( d ) sitting 
 with an ahm and left hip flexion , were used . 
 subjective exercise difficulty was evaluated . 
 [ results ] transversus abdominis and internal oblique muscle thicknesses significantly 
 differed between conditions , with significantly greater thickness between positions from 
 ( a ) to ( d ) . 
 stability of the surface when sitting had no effect on the muscle thickness of 
 the transversus abdominis . 
 by contrast , sitting on an unstable surface caused an increase 
 in muscle thickness of the internal oblique in each condition . 
 the subjects reported 
 progressively increasing difficulty in performing each exercise in a stable position from 
 ( a ) to ( d ) , while the difficulty in an unstable position was significantly different 
 between ( a ) and ( b ) , and between ( c ) and ( d ) . 
 [ conclusion ] our findings suggest that task 
 ( b ) on a stable surface should be chosen for maximal activation of transversus abdominis 
 without inducing overactivation of the internal oblique muscle .