poor posture in students in the growth period causes chronic pain in the neck and shoulder 
 joint and is related to trunk postures1 . biomechanical alterations in the scapula affect muscle activity2 , causing an abnormal activity pattern in the shoulder 
 complex followed by imbalance of surrounding muscles3 , 
 4 . for treatment of imbalance of the 
 surrounding muscles , the scapula should returned to its ideal position by exercising the 
 muscles that contribute to scapular stability5 . the serratus anterior , upper trapezius , lower trapezius , and rotator cuff , as scapular 
 stabilizers , provide stability through upward rotation of the scapula in flexion and 
 abduction of the upper extremities7 . patients 
 with scapula joint pain showed decreased serratus anterior muscle activity and increased 
 upper trapezius muscle activity8 . this 
 causes a compensatory response in the form of abnormal upward rotation of the scapula , 
 inducing pain and subacromial impingement3 , 4 , 9 . therefore , scapular lateral movement while raising the upper extremity is important for 
 maintenance of the ideal scapulohumeral rhythm10 . wickham et al . reported the angle with the highest muscle activity 
 among different angles of shoulder abduction11 . reported the rotator cuff and deltoid activation 
 angles in accordance with different angles of shoulder joint12 . therefore , shoulder muscle activity is affected by the angles of 
 flexion and abduction . it is important to understand the relationship between muscle strength and joint angle to 
 conduct exercise for enhancement of muscle activity effectively . a change in joint angle 
 alters the length of the moment arm and upper limb muscles , therefore causing a change in 
 muscle contraction13 . each joint of the 
 body has a specific joint angle with optimal mechanical benefit14 , and each muscle has a length that can trigger the 
 greatest strength at that angle15 . there are two types of shoulder joint stability exercise : open and closed kinetic chain 
 exercises . in open kinetic chain exercises in particular the distal portion of the limb 
 moves while the proximal portion is fixed . these exercises are suitable for patients with a 
 limited range of motion for muscle strengthening , as they increase acceleration and 
 facilitate functional activities16 . recently , there have been studies on a combination of vibration stimulus and open kinetic 
 chain exercise17 , 18 . the flexi - bar , a 152-cm - long elastic bar , is used as a training 
 tool that provides a vibration stimulus . shaking the bar actively delivers a vibration 
 stimulus of 5  hz throughout the body and facilitates muscle activity of the limbs and 
 trunk18 . in addition , it has been 
 reported that the vibration stimulus from the flexi - bar improves muscle strength , 
 coordination , and balance ability19 . proper control ( location , posture , and amplitude ) of a vibration stimulus of flexi - bar by a 
 distal limb segment ( hand ) affects lumbar stability20 . the purpose of this study was to analyze the effect of the vibration stimulus provided by 
 the flexi - bar during open kinetic chain exercise according to different shoulder joint 
 angles and the most effective joint angle during the shoulder joint exercise . a sample of 30 healthy male students who were 19 to 24  years of age and attending s 
 university in namwon city , jeollabuk - do province , republic of korea , was were randomly 
 selected . the subjects understood the purpose and procedure of this study and agreed to 
 participate . all subjects provided written informed consent prior to participation according 
 to the ethical standards of the declaration of helsinki . the exercise groups , each consisting of 10 subjects , were as follows : group i ( shoulder 
 joint angle of 90 ) , group ii ( shoulder joint angle of 130 ) , and group iii ( shoulder joint 
 angle of 180 ) . those who had congenital anomalies or severe surgical or neurological 
 disease of the upper limb , injury in upper limbs , or neck , back , or shoulder pain or who 
 participated in a regular upper limb exercise program in the last six months were excluded . all subjects were right handed . the average age , height , and weight of group i were 21.52.1  years , 173.85.2  cm , and 
 67.98.0  kg , respectively . those of group ii were 20.81.6  years , 174.53.7  cm , and 
 68.78.4  kg , respectively . those of group iii were 21.81.6  years , 174.74.9  cm , and 
 68.49.4  kg , respectively . the subjects had adjustment period for 34 days to learn how to use the tool . they 
 conducted warm - up stretching and walking for 5 minutes before the test . the flexi - bar ( flexi - sports , bisley , stroud , uk ) , an elastic bar made of glass fiber that 
 is 1,520  mm long and weighs 719  g , was used as an exercise tool . it has a rubber grip 
 ( 17.9  cm long , located in the middle of the bar , with rubber end weights ) . holding the grip 
 and shaking the bar delivers 5  hz vibration throughout the hands , arms , and trunk . before the test , the subjects maintained a standing position facing forward with the feet 
 10  cm apart and arms in a neutral position . adjustable bars were placed beside the subjects 
 to control the height at which the flexi - bar was held such that it was at shoulder joint 
 height . guide bars were placed at 90 , 130 , and 180 in accordance with the shoulder joint 
 angles for the flexi - bar as a guide to help subjects maintain the fixed angles . four sets of 3 minutes of exercise and then 5 
 minutes of rest were conducted . the subjects were in a 
 standing position with the feet shoulder - width apart , and a ruler was placed horizontally at 
 shoulder height . the subjects raised their arms straight out in front of them with their 
 hands formed into fists and conducted elbow extension and shoulder joint flexion of 90. 
 while maintaining this position for 5 seconds , the location of the 3rd metacarpal head wes 
 marked and recorded . measurementa are presented as the mean values of three assessments21 . a y - balance test kit ( move2perform , evansville , in , usa ) was used to test the stability of 
 the shoulder joint . the y - balance test is a dynamic balance test that evaluates muscle 
 strength , flexibility , and proprioception related to shoulder joint instability . it is 
 designed to be a standardized test and to increase the repeatability of measurement22 . the y - balance test kit contains has a 
 platform to which three pvc pipe rulers are attached in the anterior , posteromedial , and 
 posterolateral directions , with reach indicators attached to each of them . each pipe is marked in 5  mm increments , and the 
 anterior pipe is positioned 135 from the posterior pipes and the posterior pipes at 90 
 angles to each other . the length between the 7th cervical vertebra and index finger of each 
 subject was measured for upper limb length measurement for normalization of reach distance 
 values . the starting position was the push - up position with the feet shoulder - width apart . the subject had to start the test again if he missed the reach indicator , touched the floor , 
 or failed to push the reach indicator with hands or to return to the start position during 
 the movement . ancova was conducted to 
 investigate the differences in each group between before and after the intervention . analysis of covariance was conducted with values before the intervention as covariates to 
 analyze the differences among groups i , ii , and iii in the frt and y - balance test . in the 
 frt group i showed 
 significantly greater improvement than group iii according to the bonferroni post hoc 
 comparison ( p<0.05 ) ( table 1table 1.changes in the results of the frt and y - balance test after flexi - bar 
 exercisegroup igroup iigroup iiiprepostprepostprepostfrt ( cm)89.45.092.14.692.24.894.04.192.96.691.55.5y - balance test ( cm)45.87.150.15.946.67.156.57.443.26.147.65.0meansd . group i : shoulder joint angle of 90 ; group ii : shoulder joint angle of 130 ; group 
 iii : shoulder joint angle of 180. frt : functional reach test . group ii showed significantly greater improvement than groups i and iii 
 according to the bonferroni post hoc comparison ( p<0.05 ) . group i : shoulder joint angle of 90 ; group ii : shoulder joint angle of 130 ; group 
 iii : shoulder joint angle of 180. frt : functional reach test . the aim of this study was to introduce an effective shoulder joint exercise program using 
 the flexi - bar , which generates a vibration stimulus , through comparison of functional 
 stretching ability , proprioception , and stability according to different shoulder joint 
 flexion angles between three groups and analyzing the differences among the groups . the efficiency of 
 movements varies according to the angles between the bones and origin and insertion23 . absolute muscle force , joint angle during 
 measurement , and the length of force applied from the joint axis affects the expressed 
 muscle strength24 . introduced the scapulohumeral rhythm as the balance between the movements of the shoulder 
 and glenohumeral joints25 . this showed 
 that coordination between the scapula and shoulder joint , which is important for range of 
 motion of the shoulder complex , is necessary to move the upper limb efficiently . myers et 
 al . presented that the shoulder joint has both mobility and stability26 . this result is in accordance with that of a study by an 
 et al.13 who showed that muscles 
 contracted most efficiently at an angle of 90 between the force and object surface . in 
 addition , this result is considered to be the effect of low amplitude of 5  hz from the 
 flexi - bar , which was delivered to upper limb , shoulder , and then the whole body18 . the balance between the movements of the shoulder and glenohumeral joints is called the 
 scapulohumeral rhythm , the scapulohumeral rhythm is important for the range of motion of the 
 shoulder complex25 . constant 
 length - tension relationships among the surrounding muscles during glenohumeral joint 
 movement should be maintainedto provide stability of the scapula27 . the stability of the shoulder joint is closely related to 
 improvement of proprioceptive function . the y - balance test is a tool for dynamic stability 
 evaluation of joints . proprioception can be measured by maintaining the center of gravity of 
 the upper limb and shoulder during this test22 . this study measured the stability of the shoulder joint using the y - balance test . shoulder 
 stability improved in group ii after the flexi - bar exercise more than in groups i and iii . tucker et al . reported that lower trapezius muscle activation increases with abduction of 
 more than 90 , for example 125 and 14528 . the moment arm shrunk when the upper and middle trapezius muscles 
 were abducted to more than 90 , but it became longer when the lower trapezius muscle was 
 abducted to more than 9029 . reported that a continuous vibration stimulus stimulated muscle spindles and improved 
 proprioception , strengthening muscles involved in posture stability30 . a vibration stimulus strongly stimulates -motor neuron 
 and induces tonic vibration reflex because mechanical vibration applied to the muscle belly 
 or tendon reflectively contracts muscles . therefore , a vibration stimulus stimulates 
 proprioception , acting as a greater external loads during exercise and increasing muscle 
 activity of the upper limb and trunk according to the angle at which it is applied . the 
 combination of existing shoulder rehabilitation therapies and an active vibration stimulus 
 such as that generated by a flexi - bar can be a more effective tool for muscle activation and 
 joint stability . more objective verification methods with various types of subject groups 
 and more functional programs should be analyzed in further studies .
[ purpose ] the purpose of this study was to analyze the effect of an active vibration 
 stimulus exercise according to shoulder joint angles on functional reach and stability of 
 the shoulder joint . 
 [ subjects and methods ] thirty healthy male students participated in 
 this study . upper limb length of each subject was measured to obtain normalized 
 measurement values . 
 the exercise groups were as follows : group i ( n=10 , shoulder joint 
 angle of 90 ) , group ii ( n=10 , shoulder joint angle of 130 ) , and group iii ( n=10 , 
 shoulder joint angle of 180 ) . 
 after warm - up , an active vibration stimulus was applied to 
 the subjects with a flexi - bar . 
 the functional reach test and y - balance test were conducted 
 for measurement of shoulder stability . 
 [ results ] analysis of covariance was conducted with 
 values before the intervention as covariates to analyze the differences among the groups 
 in the two tests . there were significant differences among the groups . according to 
 bonferroni post hoc comparison , group i showed greater improvement than group iii in the 
 functional reach test , and group ii showed greater improvement than group i and group iii 
 in the y - balance test . [ conclusion ] the effect of the exercise with different shoulder 
 joint angles revealed that the shoulder joint has a certain effective joint angle for its 
 functionality and stability . 
 in addition , application of an active vibration stimulus with 
 a flexi - bar can be a very effective tool for improvement of functionality and stability of 
 the shoulder joint .