stability of the glenohumeral joint is provided by static elements ( articular surface and 
 ligament ) and dynamic elements ( muscles ) . the rotator cuff stabilizes the glenohumeral joint 
 in different shoulder positions and adjusts the translations of the humeral head1 . rotator cuff tear reduces 
 stability and triggers superior migration of the humeral head in abduction , which may lead 
 to acromial impingement2 . slight upward 
 translation occurs in the humeral head of normal shoulders , but severe rotator cuff tear 
 triggers major upward translation , which causes subacromial impingement . the cause of upward 
 displacement of the humeral head has not been clearly verified . laceration of the tendon 
 structure that maintains the space and lack of stability of the rotator cuff are considered 
 to increase traction of the deltoid muscle3 . scapular dyskinesis refers to an abnormal position or movement of the scapula during active 
 motions4 , the condition makes the 
 subacromial space smaller5 , and it 
 sometimes occurs together with subacromial impingement syndrome6,7,8 . coordinated movement of the scapula is necessary for the motions 
 and functions of the shoulders . desmeules and his co - researchers recommended ultrasonography 
 as a measurement tool for suspected when shoulder impingement syndrome , and reported that 
 there are changes in the subacromial space according to different positions of the 
 shoulders9 . it has been reported that 
 patients with rotator cuff disease have a small acromiohumeral distance compared with 
 healthy people10 , and that the 
 acromiohumeral distance is smaller in patients with impingement syndrome than in those with 
 healthy shoulders on magnetic resonance images ( mri)11 . ultrasound is less expensive and more practical than mri , and is a 
 valid radiographic examination method for the measurement of acromiohumeral distance 
 ( r=0.770.85)12 . clinically , diagnoses of shoulder impingement and rotator cuff tear are common . the 
 subacromial space is smaller in patients with symptoms of scapular dykinesia or impingement 
 syndrome . endo7 noted that impingement 
 occurs frequently at a humeral elevation of 90 and named it the impingement zone . the hypothesis of this study was that resistance exercise will have different effects on 
 the subacromial space according to the location of the arms and the degree of rotation of 
 the humerus . this study performed a comparative analysis of the effects of resistance 
 exercise on the subhumeral acromial space according to the location of the arms and the 
 rotation of the humerus . the criteria for selection were : no 
 history of orthopedic surgery , no fracture , pain , or neurological damage to the shoulders , 
 and no limitation in the movement of the arms . the acromiohumeral distance of the right side 
 of healthy subjects without shoulder pain was measured with a diagnostic ultrasound system 
 according to the locations of the arms and humeral rotation during resistance exercise . the 
 purpose of this study was sufficiently explained to the subjects and their consent to 
 voluntarily participate in this study was obtained . the ultrasound equipment used was a hdi 5000 of alt - philps machine , with a linear probe 
 transducer operated at a frequency at 7.5  mhz13 . to determine the acromiohumeral distance , the shortest distance 
 between the humeral head and the subacromial area was measured using the picture archiving 
 and communication system . the participants were seated upright on a chair , with their hips and knees flexed to 90 
 and their feet resting flat on the ground , looking forward ( straight ahead ) . to measure the 
 acromiohumeral distance at rest , three ultrasound measurements were obtained of the subjects 
 with their upper limbs placed in a position of 0 abduction , with a neutral shoulder 
 rotation , 90 elbow flexion , and their forearms in a mid - prone position . the subjects 
 adopted three shoulder positions : 90 flexion , scaption with 90 abduction , and 90 
 abduction in an upright sitting posture . ultrasound images were obtained of the three 
 different humeral rotations , neutral rotation , external rotation , and internal rotation , in 
 the following three shoulder positions , 90 flexion , scaption with 90 abduction , 90 
 abduction , in an upright sitting posture . the subjects were instructed to vertically push 
 the table to the maximum level with the humerus in internal , neutral , and external rotation . 
 in each position , internal rotation of the humerus was measured with the thumb downward , and 
 external rotation of the humerus was measured with the thumb outward to the maximum level , 
 with the shoulder not elevated . scaption was defined as 30 anterior to the frontal plane , 
 and the angle was verified with a goniometer . the ultrasound transducer was placed parallel to the flat surface at the most anterior 
 aspect of the acromion , with the long axis of the probe in the plane of the scapula to scan 
 the shoulder . the acromiohumeral distance was defined as the shortest distance in a straight 
 line connecting the most inferior aspect of the acromion and the humeral head . spss ver 12.0 statistical program was used for the analysis of the study data . repeated 
 measures analysis of variance was employed to examine the significance of differences in the 
 subacromial space distance according to the locations of the arms of the subjects and the 
 degree of humeral rotation . a posthoc test ( bonferroni test ) was conducted when of a 
 significant difference in the measured values of the different postures was found . to 
 analyze the intra - rater reliability of the ultrasonographic measurements , the intraclass 
 correlation coefficient for the measured values was calculated . a significance level of  
 = 0.05 the subacromial space was compared among the locations of the arms of the subjects and the 
 degree of rotation of the humerus . in the measurements of the distance of the subacromial 
 space , intra - rater reliability was high ( 0.881 minimum to 0.967 maximum ) ( table1table 1 . intrarater reliability of the acromiohumeral distance of each arm 
 positionresting90 flexionscaption90 abductionneutral0.947 ( 0.9090.971)0.966 ( 0.9410.982)0.954 ( 0.9200.975)0.967 ( 0.9420.982)ext - rot-0.946 ( 0.9070.971)0.864 ( 0.7750.924)0.922 ( 0.8670.957)int - rot-0.925 ( 0.8720.959)0.884 ( 0.8070.936)0.881 ( 0.8020.934)icc ( 95% ci ) : intraclass correlation coefficient ) . icc ( 95% ci ) : intraclass correlation coefficient the acromiohumeral distance was significantly different among the positions of humeral 
 rotation and the different locations of the arms . the acromiohumeral distance was largest 
 when the humerus was internally rotated in 90 abduction . when the acromiohumeral distance 
 was compared among the rotations of the humerus ( table 
 2table 2 . comparison of acromiohumeral distances of the arm postions and humerus 
 rotationflexion 90scaptionabduction 90rest0.8080.0380.9720.0380.9860.043*ext - rot0.9980.0410.9620.0430.9340.037int - rot1.1230.0431.2160.0441.2590.039*****significant difference ( p<0.05 ) . mean()sd ) , the difference in the distance between the neutral rotation and the internal 
 rotation of the humerus in the 90 abduction position was significant ( 0.2726 , p<0.05 ) . the difference in the distance between external rotation and internal rotation was also 
 significant ( 0.3244 , p<0.05 ) . mean()sd in the scaption position , the difference in the distance between neutral rotation and 
 internal rotation was significant ( 0.2435 , p<0.05 ) and the difference in the distance 
 between external rotation and internal rotation was significant ( 0.2542 , p<0.05 ) . in the 90 flexion position , the difference in the distance between neutral rotation and 
 internal rotation of the humerus was significant ( 0.1906 , p<0.05 ) , and the difference in 
 the distance between the neutral and the external rotation of the humerus was significant 
 ( 0.3153 , p<0.05 ) . the difference in the distances of the subacromial space in relation to the position of the 
 shoulder were compared , and the distance was greater in internal rotation than in neutral 
 and external rotation . the distance of the subacromial space was greatest in the neutral 
 position without rotation of the humerus . the variation , in the distance between the flexion 
 position and the scaption position was significant ( 0.164 ) , and the variation in the 
 distance between the flexion position and the abduction position was also significant 
 ( 0.178 , p<0.05 ) . there was a significant difference ( 0.1356 ) between the 90 flexion and 
 90 abduction with the internal rotation of the humerus ( p<0.05 ) . a recent study has shown that the acromiohumeral distance is related to functional disorder 
 resulting from shoulder disease and pain14 . karduna and his co - researchers reported that there was a 
 correlation between abnormalities in the acromiohumeral distance and abnormal scapular 
 movement15 . others have reported a 
 correlation between the location and the movement of the humerus and rupture of the rotator 
 cuff and impingement syndrome symptoms8 , 16,17,18 . ludewig and cook17 noted that subacromial impingement is associated with a 
 decrease in upward rotation of the scapula . dewhurst asserted that muscular function changes in patients with impingement syndrome 
 aggravat the condition , and that the subacromial space is affected by shortened pectoral and 
 levator scapulae muscles , thoracic kyphosis , and bad posture19 . that study further noted that strengthening of the supraspinatus 
 muscle , glenohumeral joint , infraspinatus muscle , teres minor muscle , and subscapularis 
 muscle , in other words , muscles affecting the stability and mobility of the rotator cuff , 
 was required in addition to interactions among the serratus anterior muscle , upper trapezius 
 muscle , and lower trapezius muscle , for effective scapular rotation . at 0 shoulder abduction , liu and his co - according to 
 hughes and an21 at 90 of shoulder 
 abduction , the deltoid plays a significant role in abduction . in general , muscle activities 
 of the anterior and middle part of the deltoid are high at scaptions of 60 and 90 , and the 
 activities of the supraspinatus , infraspinatus , and subscapularis are high between 30 and 
 6022 . jobe and pink23 observed that the 
 subacromial space became smaller during abduction and elevation of the arms , and that a 
 decreased subacromial space increased the load on the rotator cuff tendons , possibly 
 triggering impingement in tennis players . during the initial action of abduction , the 
 humeral cephalad moves toward the acromion process and the subscapularis forces the head of 
 the humerus downward24 . the infraspinatus 
 and teres minor comprise the posterior cuff and provide posteroinferior force to the humeral 
 head , and resistance against superior and anterior translation of the humeral head25 . azzoni and cabitza12 noted that the 
 ultrasonographic method was easy and useful for diagnosing patients with musculoskeletal 
 damages , but it had disadvantages in that results depend on the skill of the measurer who 
 requires in its use . however , in the present study , the intraclass correlation coefficient 
 of the ultrasonographic method used to measure the acromiohumeral distance was high ( 0.88 to 
 0.97 ) . this level of reliability is similar to that reported in other studies . in a study by 
 cotty and his coresearcher26 , the 
 acromiohumeral distance of a normal shoulder was 10.5  mm . in the present study , the average value for 
 females was 10.6  mm and that of males was 11.3  mm , and no significant difference between the 
 two groups was found by the independent t - test . this study also applied isometric resistance exercise in a vertical downward direction in 
 90 abduction , 90 flexion and scaption with 90 abduction , in which impingement commonly 
 occurs . the acromiohumeral distances of the postures of the shoulders with the humerus in a 
 neutral position were significantly different , with the distance largest in 90 abduction . the actions of the subscapularis , infraspinatus , and teres minor may have caused inferior 
 translation of the humeral head , and decreased the acromiohumeral distance . according to a 
 study by sharkey and marder25 , when 
 isometric resistance exercise was conducted with the shoulder in a 90 shoulder abduction 
 position , the scapularis and infraspinatus were activated simultaneously . according to 
 hughes and an21 to resist superior 
 translation of the humeral head , abductor moments should be made in an inferior direction , 
 because the teres minor below the scapula and the humerus create weak adductor torque . in addition , in each position , the acromiohumeral distance was significantly larger with 
 internal rotation of the humerus . as the subscapularis , an internal rotator , aids the 
 activities of the latissimus dorsi , pectoralis major , and the teres major this enhances , the activity of the anterior and posterior 
 fibers of the deltoid , the triceps long head , the teres major , and the pectoralis major and 
 triggers adduction , of the humerus , thereby increasing the acromiohumeral distance . the 
 increase in the acromiohumeral distance decreased pressure on the subhumeral acromial 
 space . the amount of change in the acromiohumeral distance necessary to influence a patient s 
 symptom and shoulder function remains unclear . a limitation of this study was that it did 
 not monitor scapular motion or muscle activity . this study used a diagnostic ultrasound system to comparatively analyze the effects of 
 resistance exercise on the subhumeral acromial space according to the locations of the arms 
 and the rotation of the humerus . the subhumeral acromial space was largest in 90 abduction 
 with the humerus internally rotated , and smallest in 90 flexion without humeral rotation . overall , the subhumeral acromial space was large when the humerus was internally rotated . therefore , the subhumeral acromial space appears to differ according to the amount of 
 humeral rotation and the locations of the arms . the larger size of the subhumeral acromial 
 space during internal rotation is due to the internally rotated muscles triggering inferior 
 translation of the humerus increasing the space .
[ purpose ] the purpose of this study was to compare the potential effects of resistance 
 exercise according to the positions of the arms and the rotation of the humerus on the 
 subhumeral acromial space . 
 [ subjects ] the study subjects were 34 subjects without shoulder 
 pain . [ methods ] ultrasonographic measurements of the acromiohumeral distance of the 
 subjects were made at three shoulder positions : 90 flexion , scaption at 90 abduction , 
 and 90 abduction in an upright sitting position . 
 the subjects were instructed to 
 vertically push against a table to the maximum level with the humerus in internal , 
 neutral , and external rotation . 
 the measurements were made three times in each position . 
 [ results ] there was a significant difference in acromiohuneral distance between neutral 
 and internal rotation of the humerus , and between external rotation and internal rotation 
 of the humerus . in the scaption position 
 , there was a significant difference between 
 neutral and internal rotation of the humerus , and between external rotation and internal 
 rotation of the humerus . in the 90 flexion position 
 , there was a significant difference 
 between neutral and internal rotation , and between neutral and external rotation . 
 there 
 was a significant difference between the flexion position and the abduction position , and 
 between the flexion position and the scaption position . in terms of the internal rotation 
 of the humerus 
 , there was a significant difference between 90 flexion and 90 abduction . 
 [ conclusion ] these findings can be applied in exercises prescribed to increase the 
 acromiohumeral distance and to aid the treatment and evaluation of shoulder 
 dysfunctions .