among the trunk muscles , the transversus abdominis ( tra ) reportedly acts as a feed - forward 
 control element prior to movement1 , and it 
 is also involved in the stabilization of the lumbar and pelvic regions . in healthy subjects 
 and those with low back pain , the activity and strength of the trunk muscles reportedly 
 differ2 , 3 . however , the response time and activity of the tra have been shown 
 to change4 . in recent years , some authors have reported that the change in muscle thickness of the tra 
 measured by ultrasound is a useful index for functional evaluation5,6,7 . functional evaluations of the tra during various movements have 
 been performed , but the operative mechanism of the tra during trunk rotation is not well 
 understood8 . trunk rotation is a key factor in 60% or more 
 of low back pain9 . therefore , to clarify 
 the anatomical and kinesiological characteristics of trunk rotation is important for 
 preventing and ameliorating low back pain . surface electromyography and needle 
 electromyography are the techniques used to evaluate the function of the abdominal muscles 
 during trunk rotation . studies concerning the relation between muscle activities of the external oblique ( eo ) , 
 internal oblique ( io ) , and tra using needle electromyography have reported that during trunk 
 rotation , the eo acts contralaterally whereas the io and tra act ipsilaterally10 . few studies of the thickness of the 
 abdominal muscles with trunk rotation have been performed using ultrasound , and there are no 
 studies in the literature which have investigated the thickness of abdominal muscles 
 horizontal trunk rotation . we previously examined the reliability of measuring the muscle 
 thicknesses of the eo , io , and tra by ultrasound in the neutral position and at 50% trunk 
 rotation , and reported that the mean value of three measurements showed a high coefficient 
 of reliability11 . to clarify the 
 characteristics of the abdominal muscles during trunk rotation using ultrasound thus , the aim of this study was to evaluate by ultrasound the characteristics of muscle 
 thickness changes in the eo , io , and tra in the neutral position and during trunk 
 rotation . the study subjects were 21 healthy young men with no history of orthopedic or neurological 
 disorders and no lumbar symptoms . in conformity with the principles of the declaration of 
 helsinki , each subject was informed about the study , privacy protection , and the absolute 
 right to withdraw at any time , and their consent to participation was obtained before the 
 start of the study . before measuring abdominal muscle thicknesses using ultrasound , the range of motion ( rom ) 
 at maximum trunk rotation and the maximum strength of trunk rotation were measured using a 
 hand - held dynamometer ( hhd ; the commander powertrack dynamometer ; j - tech medical , salt lake 
 city , ut , usa ) . the rom at maximum trunk rotation was measured once as the angle at the 
 intersection of the basic axis line linking the posterior superior iliac spines of both 
 sides and the movement axis line linking the acromions of both sides . maximum strength of 
 trunk rotation was measured with resistance applied to the greater tuberosity of the humerus 
 from the front . the maximum strength in the neutral position was measured three times and 
 the mean value was calculated . the subjects adopted a seated position with their feet flat on the ground and their arms 
 crossed on their chests . the hip and knee joints were flexed at 90 , and the thighs were 
 fixed with a belt . for all measurements , the pelvic position of the subjects was corrected 
 to the neutral position in the sagittal plane and the subjects were instructed to maintain 
 the neutral position . measurements were recorded with the trunk in the neutral position and 
 at 50% of the maximum trunk rotation ( 50% trunk rotation ) . measurements were also recorded 
 with the subjects at rest without voluntary contractions ( the resting state ) and with 
 resistance applied to isometric contractions to both sides of 50% of the maximum rotation 
 strength as measured by the hhd ( the resistive state ) . each of the two positions and states 
 was measured three times . moreover , one examiner operated the ultrasound equipment and the 
 other operated the hhd . the muscle thicknesses were measured on longitudinal images obtained using real - time b - mode 
 ultrasound equipment ( my lab25 ; hitachi medical corporation , japan ) with a 7.5  mhz 
 linear - array probe . the probe was first placed onto the right side of the abdomen in line 
 with the upper end of the umbilicus , after which it was fixed by hand when the muscle 
 thicknesses of eo , io , and tra were clearly observed on the ultrasound image . the muscle 
 thicknesses of the eo , io , and tra were recorded on video ( 5  s ) . a still image was extracted 
 of a position in which the three muscles were distinct , and the muscle thicknesses were 
 measured using imagej image analysis software ( national institutes of health , bethesda , md , 
 usa ) . muscle thicknesses were measured at 15  mm from the muscle tendon junction of the tra 
 towards the muscle belly ( fig . 1fig . 1.position at which muscle thickness was measured for external oblique ( eo ) , internal 
 oblique ( io ) , and transversus abdominis ( tra ) on ultrasound images ) . position at which muscle thickness was measured for external oblique ( eo ) , internal 
 oblique ( io ) , and transversus abdominis ( tra ) on ultrasound images for statistical analysis , one - way analysis of variance of the muscle thicknesses ( eo , io , 
 and tra ) for all conditions was performed . then , tukey s multiple comparison method was 
 performed to examine the significance of changes in muscle thickness with and without 
 muscular contraction . the muscle thicknesses of the eo , io , and tra in the resting state and 
 the resistive state for each position were compared . in addition , the t - test was performed 
 to examine the significance of the changes in muscle thickness at the different angles of 
 trunk rotation . the results of the muscle thickness measurements in the neutral position and 
 at 50% trunk rotation of both states were compared . we used spss , version 17.0 for 
 windows ( spss inc . , chicago , il , usa ) for the statistical analyses , and a significance level 
 of 5% . the subjects had a mean age of 23.8 ( 3.0 ) years , a mean height of 171.5 ( 7.0 ) cm , and a 
 mean weight of 64.1 ( 7.9 )  kg . the rom at maximum trunk rotation was 48.3 ( 7.8) in right 
 rotation and 49.3 ( 8.5) in left rotation . the maximum strength of right rotation was 147.4 
 ( 25.8 ) n , and 138.9 ( 22.7 ) n in left rotation . the rotation angle at 50% trunk rotation to 
 the right side was 24.2 ( 3.9) , and that at 50% trunk rotation to the left side was 24.6 
 ( 4.2). in the neutral position , the thickness of the eo was significantly reduced by left rotation 
 with resistance , and the thicknesses of the io and tra were significantly increased by right 
 rotation with resistance . at 50% trunk rotation to the right side , the thicknesses of the io 
 and tra were significantly increased . at 50% trunk rotation to the left side , the 
 thicknesses of the eo and io were significantly reduced . in the resting state and at 50% 
 trunk rotation to the right side , the thicknesses of the io and tra were significantly 
 increased . at 50% trunk rotation to the left side , there were no significant differences in 
 the thickness of the three muscles . in the resistive state and at 50% trunk rotation to the 
 right side , the thicknesses of the io and tra were significantly increased . at 50% trunk 
 rotation to the left side , the thicknesses of the io and tra were significantly reduced 
 ( table 1table 1.descriptive statistics for the muscle thicknesses of the external oblique , 
 internal oblique , and transversus abdominis with and without resistance and 50% trunk 
 rotation ( n=21)eoiotrarestingresistiverestingresistiverestingresistiverightneutral ( mm)7.4 ( 1.1)7.4 ( 1.9)7.2 ( 1.9)9.6 ( 2.9)3.6 ( 0.7)5.6 ( 1.8)50% ( mm)7.0 ( 1.2)7.7 ( 2.4)9.4 ( 2.7)12.0 ( 3.9)4.5 ( 0.9)6.6 ( 2.0)leftneutral ( mm)7.6 ( 1.1)6.6 ( 1.2)7.1 ( 1.9)6.7 ( 1.8)3.7 ( 0.8)4.2 ( 1.5)50% ( mm)8.0 ( 1.4)6.7 ( 1.3)6.9 ( 1.9)6.2 ( 2.3)3.6 ( 0.8)4.0 ( 1.6)mean ( sd ) , * significant difference ( p<0.01 ) . neutral , neutral position ; 50% , 50% 
 trunk rotation ; resting , resting state ; resistive , resistive state ; eo , external 
 oblique ; io , internal oblique ; tra , transversus abdominis ,   resting state 
 versus resistive state ,   neutral position versus 50% trunk rotation ) . neutral , neutral position ; 50% , 50% 
 trunk rotation ; resting , resting state ; resistive , resistive state ; eo , external 
 oblique ; io , internal oblique ; tra , transversus abdominis ,   resting state 
 versus resistive state ,   neutral position versus 50% trunk rotation in the neutral position , the thickness of the eo significantly reduced by left rotation 
 with resistance . in addition , the thicknesses of the io and tra significantly increased by 
 right rotation with resistance . first , the 
 muscle activity of the eo increased at trunk rotation angles of 20 or more12 , and second , the activity of the eo was 
 greater during trunk rotation with flexion in a horizontal rotation exercise in the neutral 
 position . in addition , the io and tra fixed the trunk by raising abdominal pressure to 
 resist . at 50% trunk rotation to the right side , the thicknesses of the io and tra were 
 significantly increased , because these muscles raised the abdominal pressure to resist 
 ipsilateral trunk rotation . at 50% trunk rotation to the left side , the thicknesses of the 
 eo and io were significantly reduced , because these muscles are stretched by thoracic 
 rotation and horizontal trunk rotation . furthermore , from an anatomical point of view , the 
 significant changes in the thickness of these muscles were caused by muscular stretch . there 
 were two reasons for this muscular stretch : the lowest fiber of the eo extends almost 
 vertically from the eighth lowest rib to the anterior of the iliac crest ; and the backward 
 ilium fiber of the io extends from the upper side of the inferior border to the lowest 34 
 ribs13 . with regard to the kinematic function of the abdominal muscles during trunk rotation , the 
 io and tra have been shown to have a functional role in ipsilateral trunk rotation , whereas 
 the eo has a role in contralateral trunk rotation10 . the thicknesses of the io and tra significantly increased in the 
 resting state at 50% trunk rotation to the right side , and there were no significant 
 differences in the thicknesses of the three muscles at 50% trunk rotation to the left side . the results indicate that the function of these muscles is to maintain a specific position 
 rather than to initiate joint movement . in the resistive state at 50% trunk rotation to the 
 right side , the io and tra showed the greatest thicknesses of all the conditions . at 50% 
 trunk rotation to the left side , the thicknesses of the io and tra were significantly 
 reduced , because the contralateral io and tra were extended to a greater degree through the 
 rectus sheath by strong contraction of the ipsilateral io and tra . however , with regard to 
 the eo , there was no significant difference in the resistive state during contralateral 
 trunk rotation , because the muscle fiber extends from the ribs to below the iliac crest and 
 is stretched by trunk rotation . hence , the significant change in the eo thickness was not 
 caused by muscle contraction . in the present study , there was no significant difference in the eo in the resistive state 
 during contralateral trunk rotation . first , 
 from an anatomical point of view , the muscle is easily extended from this position by trunk 
 rotation . second , the movement in this study was horizontal trunk rotation , and differed 
 from the trunk rotation with flexion that increased the muscle activity of eo and io . in left rotation , resistance 
 applied to the right shoulder strengthened the connection to the left abdomen and weakened 
 the activity of the right eo . however , it is difficult to measure the muscle thickness of 
 both sides at the same time using ultrasound , meaning this point could not be clarified . it 
 is important that the timing of the muscular contractions of the abdominal muscles coincides 
 with the activity and that the timing of the muscular contractions coincides with that of 
 the back muscles in trunk rotation . hence , future analysis is needed to evaluate trunk 
 function during trunk rotation using surface electromyography and three - dimensional motion 
 analysis , and to examine the activity of the abdominal and back muscles and their 
 association with trunk rotation angles . in addition , it is possible to measure muscle 
 thickness and quantitatively evaluate trunk function with various movements simply , easily , 
 and spontaneously by clarifying the association between muscle activity and muscle thickness 
 changes with different trunk rotation angles using ultrasound . in conclusion , the thicknesses of the io and tra during ipsilateral trunk rotation were 
 greater in the resistive state than in the resting state , and greater in the neutral 
 position than in trunk rotation . in addition , the thicknesses of the io and tra were 
 greatest in the resistive state during ipsilateral rotation . both the io and tra have 
 important functions in maintaining a specific position and initiating joint movement , and 
 these functions play a role in the mobility of the thoracolumbar region and pelvic 
 stability .
[ purpose ] the aim of this study was to investigate muscle thickness changes in the 
 external oblique ( eo ) , internal oblique ( io ) , and transversus abdominis ( tra ) muscles 
 between the neutral position and trunk rotation , under a state of rest without voluntary 
 contractions , and isometric contractions to both sides with resistance of 50% of the 
 maximum trunk rotation strength . [ subjects ] the subjects of this study were 21 healthy 
 young men . [ methods ] muscle thickness changes in the eo , io , and tra in each position and 
 state were evaluated by ultrasound . 
 the range of motion at maximum trunk rotation and the 
 maximum strength of trunk rotation were measured using a hand - held dynamometer . 
 [ results ] 
 in the neutral position and at 50% trunk rotation to the right side , the thicknesses of 
 the io and tra significantly increased with resistance . in both states , 
 the thicknesses of 
 the io and tra significantly increased at 50% trunk rotation to the right side . 
 [ conclusion ] the muscular contractions of the io and tra were stronger during ipsilateral 
 rotation than in the neutral position and with resistance than at rest . moreover , the 
 muscular contraction was strongest in the resistive state during ipsilateral rotation .