the 
 shoulder joint is a joint that relies on interactions and balance among multiple joints1 , 2 .
the shoulder joint relies more on muscles than on bones 
 or ligaments to support the shoulder and maintain the stability of the shoulder3 .
stabilization relates to the 
 human ability to consciously or subconsciously control large or small movements occurring in 
 the joints4 .
the movements of the shoulder 
 bone and the humeral bone are controlled by the coupling force of two muscles : the 
 rotator - cuff muscle and the elevator muscle of the scapular .
if this coupling force 
 collapses , the muscle - coordination activity does not occur and the rotator cuff is 
 compressed5 . in this case , the humeral 
 head only partially maintains contact with the articular fovea , leading to many pathologic 
 conditions , such as shoulder instability3 , 6 .
the activity of the scapular stabilizer 
 muscles is important in counteracting the instability that occurs due to the aforementioned 
 structural problems .
the scapular stabilizer muscles include the serratus anterior , the 
 upper trapezius , and the lower trapezius7 . 
 in particular
, the activity of the serratus anterior muscle is very important for scapular 
 movements and control8 , 9 .
extensive electromyography ( emg ) studies of the stabilizer muscles 
 that contribute to shoulder stabilization have been carried out .
the muscle activity of the 
 serratus anterior and that of the upper trapezius have been shown to be imbalanced in 
 subacromial - impingement syndrome , subacromial bursitis , rotator - cuff laceration , and 
 rotator - cuff and biceps - brachii tendinitis10 .
closed - chain exercises are frequently used for shoulder - joint dysfunction prevention and in 
 rehabilitation programs11 .
closed - chain 
 exercises are frequently used in therapeutic exercise programs to generate dynamic stability 
 in the joints and to maintain posture , since these exercises not only improve strength and 
 endurance , but also induce the co - contraction of many muscles through mechanical compression 
 of articular surfaces eliciting larger proprioceptive responses by stimulating the afferent 
 receptors around the joints12 .
along with 
 closed - chain exercises , sling exercises , which require performing exercises while hanging on 
 swinging ropes , are one of the treatment methods that can be used to prevent injuries , since 
 these exercises stimulate the proprioceptive senses to strengthen weakened muscles and give 
 the necessary stimuli to maintain joint stability .
kirkesola13 reported that if vibrations of the rope were given during sling 
 exercises , instability would be increased , further stimulating the proprioceptive sensory 
 organs in the joints , and incorrect neuromuscular control could be corrected , reducing 
 muscle imbalance .
therefore , the present study compared the muscle activity of the upper trapezius with those 
 of the serratus anterior , and the lower trapezius when slings , unstable surfaces , were 
 laterally vibrated , to examine the effects of vibration during sling exercises on the 
 shoulder stabilization muscles .
the subjects of the present study were 15 adult men without any history of damage to the 
 shoulder girdle or shoulder pain .
the subjects average age was 26.13  5.13  years , their 
 average height was 174.00  5.44  cm , and their average weight was 73.47  13.46  kg .
all subjects read and signed a 
 written consent form . the subjects performed push - up exercises with knee flexion .
al.1 advised that push - up exercises with 
 knee flexion produce emgs similar to those produced during push - up exercises with knee 
 extension , while eliciting less force than push - up exercises with knee extension . before 
 emgs
were measured , the subjects performed push - up exercises with their hands placed on the 
 floor at shoulder width , their knees placed on the floor , and their elbow joints extended . 

the 
 subject performed push - up exercises on an unstable surface ( sling ) and maintained isometric 
 contraction for five seconds in a state of scapular protraction and elbow extension , in the 
 final stage of the motion , and emg signals from the serratus anterior , the upper trapezius 
 muscle , and the lower trapezius muscle were collected during state of isometric contraction . 
 in the second exercise ,
the subject performed push - up exercises on a sling and maintained 
 isometric contraction in the last stage , and the rope of the sling was manually vibrated 
 during the isometric contraction .
vibrations was given within a range of 10  cm for five 
 seconds at a frequency of 1  hz , in time with a metronome . in the third and fourth exercises , 
 vibrations of 3  hz and 3.5  hz , were respectively applied .
usa ) was used to measure muscle activities during 
 the exercises . for the measurement
, electrodes were attached to the upper trapezius , the 
 lower trapezius , and the serratus anterior .
18.0 ) , and one - way anova was used to compare muscle activities among the various vibration 
 frequencies on the unstable surface .
the upper trapezius and the lower trapezius did not show any significant differences among 
 isometric contraction without vibration , isometric contraction with vibration of 1  hz , and 
 isometric contraction with vibration of 3  hz , but showed significant differences at 
 isometric contraction with vibrations of 3.5  hz . the serratus anterior showed a significant 
 differences between isometric contraction with vibration of 3  hz and isometric contraction 
 with vibration of 3.5  hz ( table 1table 1 .
comparison of muscle activation among the various vibration frequencies ( unit : 
 % rvc)ic1  hz3  hz3.5  hzut*0.034  0.0130.034  0.0130.035  0.0130.057  0.010lt*0.025  0.0080.027  0.0070.027  0.0070.053  0.009sa*0.051  0.0120.050  0.0110.083  0.0140.083  0.014*p<0.05 , mean  sd , ic : isometric contraction , ut : upper trapezius , lt : lower 
 trapezius , sa : serratus anterior ) .
* p<0.05 , mean  sd , ic : isometric contraction , ut : upper trapezius , lt : lower 
 trapezius , sa : serratus anterior
the coupling force of the upper trapezius , the serratus anterior , and the lower trapezius 
 is the core of scapular movements14 .
if 
 this muscle recruitment is changed , or the motions of these muscles are limited by an 
 antagonist , the movement pattern will collapse .
it has been revealed that in shoulder 
 impingement syndrome , patients shoulders are elevated , the muscle activity of the upper 
 trapezius increases , and the muscle activity of the serratus anterior decreases .
in 
 particular , the serratus anterior , which is an important muscle scapular movement imbalance , 
 has been studied by many researchers .
fatigue of the serratus anterior decreases scapular 
 rotation and protraction , and displaces the humeral head in a forward and upward 
 direction1 , 10 , 11 .
therefore , it may cause 
 secondary impingement syndrome and rotator - cuff tear , decrease the range of scapular motion , 
 or disturb the contraction timing of the serratus anterior , thereby inducing stress in the 
 glenohumeral joint15 . if abnormal 
 scapular movements appear , due to problems in the serratus anterior
, the position of the 
 fossa would be affected , and the force and alignment involved in movements around the 
 glenohumeral joint would eventually be affected16 .
kirkesola14 
 reported that when the rope used for sling exercises was vibrated , instability increased 
 further stimulating the proprioceptive sensory organs in the joints , and incorrect 
 neuromuscular control could be corrected , reducing muscle imbalance .
as the load - bearing surface becomes more unstable , the muscle activities of the serratus 
 anterior and the lower trapezius would increase , selectively strengthening the 
 scapular - stabilizer muscles , reducing muscle imbalance .
exercises on a sling elicit 
 significantly higher muscle activities than exercises on a fixed surface .
therefore , 
 exercises on a sling should simultaneously strengthen many muscles that contribute to 
 shoulder joint stability , increasing the stability of the shoulder complex . in the present 
 study , to examine the effects of diverse vibration frequencies on the upper trapezius , the 
 lower trapezius , and the serratus anterior during exercises on a sling , an unstable surface , 
 vibration frequencies of 1  hz , 3  hz , and 3.5  hz were used . in the present study ,
all three 
 muscles showed gradual activity increases as the frequency of vibration increased from 1  hz 
 to 3  hz and 3.5  hz . the upper trapezius and the lower trapezius showed significant changes 
 in muscle activity at 3.5  hz , and the serratus anterior showed prominent changes in muscle 
 activity at 3  hz and 3.5  hz .
therefore , as the vibration frequency increased , making the 
 bearing surface more unstable , the recruitment of the upper trapezius , the lower trapezius , 
 and the serratus anterior increased . to perform exercises that will selectively strengthen 
 the serratus anterior ,
   [ purpose ] the present study compared the muscle activity of the upper trapezius with 
 those of the serratus anterior and the lower trapezius when slings , unstable surfaces , 
 were laterally vibrated , to examine the effects of vibration during sling exercises on 
 shoulder stabilization muscles . [ methods ] the subjects performed push - up exercises on a 
 sling and maintained isometric contraction in the final stage , while vibration was 
 manually administered to the rope of the sling during the isometric - contraction stage . 
 
 vibration within a range of 10  cm was delivered for five seconds at a frequency of 1  hz in 
 time with a metronome . 
 vibrations were applied for five seconds at 3  hz and 3.5  hz , 
 respectively . [ results ] 
 the serratus anterior showed a significant differences between 
 isometric contraction with vibration of 3  hz and isometric contraction with vibration of 
 3.5  hz . [ conclusion ] the upper trapezius and the lower trapezius showed prominent changes 
 in muscle activity at 3.5  hz , and the serratus anterior showed prominent changes in muscle 
 activity at 3  hz and 3.5  hz . 
 therefore , as vibration frequency increased , making the 
 load - bearing surface more unstable , the recruitment of the upper trapezius , the lower 
 trapezius , and the serratus anterior increased . to perform exercises 
 that selectively 
 strengthen the serratus anterior , the exercises should be performed at a vibration 
 frequency of 3  hz . 