highly repetitive overhead sports activities such as baseball pitching and tennis serves 
 cause upper extremity injuries . these injuries result in a decline in performance and 
 longtime absence from sport participation , so prevention is highly important . shoulder 
 external rotation muscle weakness is a causative factor of shoulder injuries1,2,3 . therefore , many authors have emphasized the 
 need to enhance shoulder external rotation muscle strength and endurance during 
 rehabilitation and conditioning programs1 , 2 . we previously reported that shoulder 
 external rotation strength was maintained regardless of the shoulder elevation angle in the 
 scapular plane , and that scapular posterior tilting by the serratus anterior ( sa ) might 
 generate shoulder external rotation torque and maintain shoulder external rotation 
 strength4 . the scapular dysfunction called scapular dyskinesis5 involves alteration of the static scapular position or dynamic 
 scapular motion . it has been suggested to be another risk factor of upper extremity 
 injuries , and has been reported to be related to various shoulder pathologies6 . scapular dyskinesis seems to cause an 
 unstable basis of rotator cuff , and decline in the scapular posterior tilting force of the 
 sa . thus , scapular dyskinesis affects shoulder external rotation strength and muscle 
 activity , especially in the zero position . however , this expensive instrument can not often be used in the clinical practice because of 
 its cost . the visual assessment of scapular motion by mcclure et al.9 does not require the use of expensive instruments , and its 
 reliability and validity have been confirmed . scapular dyskinesis judged by visual 
 assessment shows alterations of scapular kinematics10 , 11 and muscle activity11 in shoulder elevation . therefore , this 
 assessment method could be utilized in clinical practice after the relationship between 
 scapular dyskinesis and shoulder function has been clarified . thus , the purpose of this 
 study was to clarify the relationship between scapular dyskinesis and shoulder external 
 rotation strength and muscle activity using visual scapular assessment . the subjects were 20 male volunteers ( mean age of 24.4  3.1  years , height of 173.2  
 5.2  cm , and weight of 66.4  7.3  kg ) who had no previous history of cervical or shoulder 
 injury or pathologies in the preceding 6 months , had negative neer impingement and hawkins 
 impingement test results , and had no rotator cuff tenderness . the pectoralis minor length12 and shoulder horizontal adduction angle , 
 abnormalities of which can cause scapular dyskinesis , were measured as soft tissue 
 flexibility assessments . ethical approval for this study was obtained from the ethics 
 committee of gunma university ( approval code 15 - 79 ) . this study was conducted in accordance 
 with the ethical principles of the declaration of helsinki . written informed consent the scapular dyskinesis test9 , a visual 
 scapular motion assessment , was used to classify scapular motion . the participants were 
 instructed to perform 5 consecutive repetitions of bilateral , active , and weighted shoulder 
 flexion and shoulder abduction using dumbbells while their scapular motion was videotaped 
 from the posterior using a digital camera ( ex - fc150 , casio computer co. , ltd , tokyo , japan ) . the weight of the dumbbell was set according to body weight , 1.4  kg ( 3 lb ) for those 
 weighing less than 68.1  kg ( 150 lb ) , and 2.3  kg ( 5 lb ) for those weighing 68.1  kg or more . the participants were asked to simultaneously elevate their arms overhead as far as possible 
 to a 3  s count , and then to lower them to a 3  s count paced by a metronome . the videotaped 
 scapular motion was classified into normal , subtly abnormal , or obviously abnormal shoulders 
 using the scale presented in table 1table 1.scapular dyskinesis test : rating scalerating scaleeach test movement ( flexion and abduction ) rated 
 asa ) normal motion : no evidence of abnormalityb ) subtle abnormality : mild or questionable evidence 
 of abnormality , not consistently presentc ) obvious abnormality : striking , clearly apparent 
 abnormality , evident on at least 3/5 trialsfinal rating is based on combined flexion and 
 abduction test movements.normal : both test motions are rated as normal or 1 
 motion is rated as normal and the other as having subtle abnormality.subtle abnormality : both flexion and abduction are 
 rated as having subtle abnormalities.obvious abnormality : either flexion or abduction is 
 rated as having obvious abnormality.this rating scale is reproduced from mcclure et al.9 . the subtly abnormal shoulders were excluded , and a comparative analysis of 
 the normal and obviously abnormal shoulders was performed . this rating scale is reproduced from mcclure et al.9 for measurement of the shoulder external rotation strength , a handheld dynamometer ( -tas 
 mf-01 , anima corp . , 1.the two measurement positions1st position : sitting in a chair with shoulder abduction and flexion at 0 , elbow 
 flexion at 90 , and forearm pronation / supination at neutral . zero position : lying 
 prone on the elbows with the shoulder elevated at the humeral shaft axis roughly in 
 alignment with the scapular spine . : 1 ) sitting in a chair with shoulder abduction and flexion at 0 , elbow flexion at 
 90 , and forearm pronation / supination in the neutral position ( 1st position ) ; 2 ) lying prone 
 on the elbows with the shoulder elevated at the humeral shaft axis roughly in alignment with 
 the scapular spine ( zero position ) . the measurements were performed for 5  s and repeated 
 three times . before the measurements , when the subjects performed compensatory 
 movements , the measurements were stopped and restarted after a rest period . the strength 
 ratio was calculated to quantify the change in shoulder external rotation strength between 
 the 1st position and the zero position ( strength in the zero position / 1st position ) . the two measurement positions 1st position : sitting in a chair with shoulder abduction and flexion at 0 , elbow 
 flexion at 90 , and forearm pronation / supination at neutral . zero position : lying 
 prone on the elbows with the shoulder elevated at the humeral shaft axis roughly in 
 alignment with the scapular spine . for electromyography ( emg ) , surface emg ( biolog dl-3100 , s&me , inc . , tokyo , japan ) was 
 used , with a sampling rate of 1,000  hz . the target muscles were the infraspinatus ( is ) , sa , 
 upper trapezius ( ut ) , middle trapezius ( mt ) , and lower trapezius ( lt ) . , tokyo , japan ) were applied parallel to each of the 
 muscle fibers after appropriate skin preparation . for the is , the 
 electrode was placed two fingerbreadths inferior to the center of the spina scapulae . for 
 the sa , the electrode was placed on the midaxillary line at the level of the inferior angle 
 of the scapula . for the ut , the electrode was placed midway between the spinous process of 
 the seventh cervical vertebra and the posterior tip of the acromion process along the line 
 of the trapezius . for the mt , the electrode was placed midway along the horizontal line 
 between the root of the spine of the scapula and the third thoracic spine . for the lt , the 
 electrode was placed midway between the spinous process of the seventh thoracic vertebrae 
 and the vertebral border of the scapula at the junction of the scapular spine . the raw emg data were processed using data analysis software ( m - scope , s&me , inc . raw emg data were filtered with a digital band - pass filter between 20 and 
 500  hz , converted to root mean square values using a 50-ms window , and the mean amplitude of 
 the middle 3  s , excluding the first and last second of each measurement , was calculated . each muscle activity was normalized using the muscle activity of maximum voluntary 
 contraction ( mvc ) . the statistical analyses were performed using spss version 21.0 for windows ( spss , chicago , 
 il , usa ) . shoulder external rotation strength , strength ratio , and each muscle activity in 
 the 1st and zero positions were compared between the normal and obviously abnormal 
 shoulders . the significance of differences was evaluated using the t - test , 
 and a significant level of 5% . the 40 shoulders included in the study were classified into normal ( 19 , 48% ) , subtly 
 abnormal ( 8 , 20% ) , and obviously abnormal shoulders ( 13 , 33% ) . the profiles of the normal and obviously abnormal 
 shoulder groups are presented in table 
 2table 2.the profiles of the subjectsnormal shouldergroup(n = 19)obviously abnormal shouldergroup ( n = 13)age ( years)25.4 ( 3.6)23.5 ( 2.7)height ( cm)173.7 ( 5.6)171.5 ( 4.3)weight ( kg)69.9 ( 8.1)62.2 ( 3.3 ) * arm dominance : dominant105non - dominant98soft tissue flexibilitypectoralis minor length ( cm)5.1 ( 0.9)4.5 ( 0.9)shoulder horizontal adduction angle (  ) 117.6 ( 5.3)119.5 ( 6.9)means ( standard deviation ) ; * p < 0.05 . the subjects in the normal shoulder group were significantly heavier ( p = 
 0.001 ) . no significant differences were found in any of the other profiles , including 
 shoulder soft tissue flexibility . means ( standard deviation ) ; * p < 0.05 the shoulder external rotation strength and muscle activities are presented in table 3table 3.shoulder external strength and muscle activitiesnormal shoulder group(n = 19)obviously abnormal shoulder group(n = 13)strength ( n)1st position80.6 ( 9.6)90.2 ( 13.3 ) * zero position74.8 ( 14.3)72.8 ( 12.5)strength ratio0.93 ( 0.16)0.81 ( 0.14 ) * muscle activity ( % mvc)1st positioninfraspinatus95.1 ( 20.2)108.8 ( 18.4)serratus anterior16.1 ( 12.3)15.6 ( 7.3)upper trapezius28.1 ( 17.1)41.2 ( 27.2)middle trapezius51.6 ( 26.3)73.1 ( 37.7)lower trapezius63.9 ( 28.5)74.2 ( 27.5)zero positioninfraspinatus77.5 ( 19.9)91.1 ( 21.3)serratus anterior74.7 ( 28.9)56.2 ( 15.5 ) * upper trapezius19.4 ( 14.4)21.5 ( 18.5)middle trapezius33.8 ( 20.6)35.9 ( 16.8)lower trapezius57.5 ( 18.4)57.2 ( 25.9)means ( standard deviation ) ; * p < 0.05 ; 1st position : sitting position , shoulder 
 0 abduction and 0 flexion position ; zero position : prone ( on elbows ) position with 
 the humeral shaft axis roughly in alignment with the scapular spine ; strength ratio : 
 the rate of muscle strength change = the strength in the zero position / 1st 
 position . the obviously abnormal shoulder group exhibited significantly greater 
 strength in the 1st position ( p = 0.024 ) than the normal shoulder group , but no significant 
 difference was observed in the zero position . the strength ratio of the obviously abnormal 
 shoulder group was significantly smaller ( p = 0.038 ) than that of the normal shoulder group . however , in the zero position , the sa of the obviously abnormal shoulder group was 
 significantly lower ( p = 0.044 ) than that of the normal shoulder group . means ( standard deviation ) ; * p < 0.05 ; 1st position : sitting position , shoulder 
 0 abduction and 0 flexion position ; zero position : prone ( on elbows ) position with 
 the humeral shaft axis roughly in alignment with the scapular spine ; strength ratio : 
 the rate of muscle strength change = the strength in the zero position / 1st 
 position for overhead sports athletes , shoulder external rotator muscle weakness is a risk factor of 
 shoulder injury1,2,3 . we previously reported that 
 shoulder external rotation strength was maintained regardless of shoulder elevation angle in 
 the scapular plane , and that scapular posterior tilting by the sa might generate shoulder 
 external rotation torque and maintain shoulder external rotation strength4 . in this study , we studied the relationship 
 between the scapular dysfunction called scapular dyskinesis and shoulder external rotation 
 strength and muscle activity . the shoulder external rotation strength of the obviously abnormal shoulder group was 
 stronger than that of the normal shoulder group in the 1st position , but no significant 
 difference was observed between the groups in the zero position . that is , shoulder external 
 rotation strength declined more in obviously abnormal shoulder group than in the normal 
 shoulder group when the position changed from the 1st position to the zero position . in the 
 comparison of the muscle activities , the obviously abnormal shoulder group showed 
 significantly lower sa activity moreover , the is showed higher activity in the obviously 
 abnormal shoulder group , but the difference was not significant . as mentioned above , we 
 previously reported that shoulder external rotation strength was maintained regardless of 
 shoulder elevation angle in the scapular plane , and that the sa is important in the shoulder 
 elevated position . in shoulder elevation motion , specifically , scapular dyskisnesis was 
 reported to show higher muscle activity of the ut11 , 
 16 and lower activity of the sa and 
 lt16 . considering the result of our 
 study , in shoulder external rotation in the zero position , in scapular dyskinesis , 
 recruitment of the sa is poor , weakening the shoulder external rotation strength . these 
 results reveal the negative effects of scapular dyskinesias on shoulder external rotation 
 function . a difference was not observed in the comparison of shoulder external rotation 
 strength in the zero position . this indicates that evaluation of shoulder external rotation 
 strength in various positions is important . various training methods have been reported to 
 improve the sa15 , 17,18,19 . another study reported improvement of the supraspinatus and is 
 strength by the rehabilitation of the scapular muscle20 . therefore , the effect of scapular muscle training on shoulder 
 injury prevention should be investigated in future studies . in baseball pitching and tennis serve motions , the greatest exertion of shoulder external 
 rotation strength is needed from ball release ( ball impact ) to the follow - through phase . pappas et al.21 reported that the maximum 
 shoulder internal rotation angular velocity in the acceleration phase of baseball pitching 
 is 6,180 /s , and recognized the importance of the shoulder external rotation muscles as 
 braking muscles . gandhi et al.22 reported 
 that the external rotation strength of baseball pitchers decreased after a game compared to 
 before the game because of is fatigue . in our study , is showed higher activity in the 
 obviously abnormal shoulder group than in the normal shoulder group , but the difference was 
 not significant . during shoulder external rotation in the zero position , the sa and is may 
 be considered to act as a co - muscle . in the obviously abnormal shoulder group , as the sa 
 activity was lower , the is activity may have been compensatorily higher . the obviously abnormal 
 shoulder group may be prone to fatigue due to the high activity of the is . therefore , their 
 shoulder external rotation strength will be weakened earlier , likely reducing their 
 performance and increasing their risk of shoulder injury . however , these are speculative 
 comments and the relationship between scapular dyskinesis and fatigue needs to be clarified 
 in the future . a limitation of this study was that it adopted an isometric task , although the shoulder 
 external rotator is often needed in eccentric contraction in sports activities . therefore , 
 it is important to consider actual sports activities and to clarify their relationships with 
 shoulder injuries . through this study , we wish to contribute to the development of the 
 preventive and rehabilitative strategies for the shoulder injuries of overhead sports 
 athletes .
[ purpose ] this study aimed to clarify the relationship between scapular dyskinesis and 
 shoulder external rotation strength and muscle activity . 
 [ subjects and methods ] both 
 shoulders of 20 healthy males were evaluated . 
 they were classified into 19 normal , 8 
 subtly abnormal , and 13 obviously abnormal shoulders using the scapular dyskinesis test . 
 subtly abnormal shoulders were subsequently excluded from the analysis . 
 shoulder external 
 rotation strength and muscle activity ( infraspinatus , serratus anterior , upper , middle , 
 and lower trapezius ) were measured in 2 positions using a handheld dynamometer and surface 
 electromyography while sitting in a chair with shoulder 0 abduction and flexion ( 1st 
 position ) , and while lying prone on the elbows with the shoulders elevated in the zero 
 position ( zero position ) . 
 the strength ratio was calculated to quantify the change in 
 strength between the positions ( zero position / 1st position ) . 
 [ results ] in the obviously 
 abnormal shoulder group , the strength in the 1st position was significantly stronger , the 
 strength ratio was significantly smaller , and the serratus anterior in the zero position 
 showed significantly lower activity than the normal shoulder group . [ conclusion ] in 
 shoulder external rotation in the zero position , in obviously abnormal shoulders , the 
 serratus anterior is poorly recruited , weakening the shoulder external rotation 
 strength .