use of computers by students has reportedly increased due the current information society 
 and rapid economic growth1 .
students 
 lifestyle choices including computer overuse , along with a lack of health education and 
 exercise lead to changes in physical posture and increases in pain2 .
trunk stability is related to the body s ability to move and 
 involves increased activity of the core muscles and erector spinae3 , 4 .
muscles that comprise trunk stability stabilize the body and the spine irrespective of limb 
 movements since they act like a corset .
studies have reported that once trunk 
 stabilization is secured , activities of the abdominal muscles , pelvis , waist , and hips 
 harmonize and movements of the limbs occur more smoothly6 .
in addition , trunk stabilization exercises are important to 
 preventing functional movement impairment of the abdominal muscles7 .
researchers have also reported that trunk stabilization 
 exercises increase weight bearing toward the immobile side and that they are effective for 
 postural control and activities of daily living8 , 
 9 .
trunk stabilization exercises maximize spinal movement and stability through repeated 
 reinforcement10 . a sling , which is used 
 to enhance stability and mobility during trunk exercises , has the following advantages : the 
 participant s own weight serves as the resistance and the exercises can be performed in 
 small areas , are simple , and include a variety of motions .
thus , the use of a sling is 
 popular11 . a previous study reported on 
 how one can maintain balance in the prone position as well as advantages of trunk 
 stabilization exercises12 .
when trunk 
 movement is parallel to the provided external resistor , transversus abdominis ( tra ) 
 contractions are activated to maintain a neutral trunk position13 .
when the shoulder joints are bent to a 90 horizontal 
 alignment of the pelvis posture , abdominal internal oblique ( io ) and tra activities are 
 high11 .
oh et al . compared abdominal 
 external oblique ( eo ) and abdominal io by controlling the elbow joint angle during push - ups 
 at 0 , 45 , and 90. as a result , activity was higher when the angle during sling exercises 
 was 0. those authors concluded that trunk stabilization exercises effectively promote 
 stability of the trunk as well as the muscles around the proximal joint14 .
as shown above , although trunk stabilization exercises have clinical therapeutic effects , 
 there have been few formal studies on the effect of different shoulder joint angles on trunk 
 stabilization exercises .
the purpose of this study was to identify whether bending the 
 shoulder joints with a sling strapped at an angle in the prone position effectively thickens 
 the core muscles and increases static and dynamic balance abilities in a standing 
 position .
in this study , 91 students of both genders , all in their 20s , and who were enrolled at sun 
 moon university located in asan city were randomly selected as subjects .
the study was 
 approved by the sun moon university institutional bioethics committee ( smu-14 - 07 - 03 ) .
all 
 participants provided written informed consent and were informed of the study s purpose and 
 procedures .
the inclusion criteria involved 1 ) a full understanding of the researchers 
 instructions and purpose of study ; 2 ) good health ; and 3 ) written informed consent . 

exclusion criteria consisted of 1 ) structural abnormalities of the spine during the three 
 months prior to the study ; 2 ) pain in the spine , such as back pain ; 3 ) drug use ; 4 ) 
 regularly performing weight - training activities that may affect the mechanical structure of 
 the abdominal muscles .
subjects were randomized into 
 three group : the shoulder joints 60 flexed trunk stabilization exercises group ( group 1 ) , 
 shoulder joints 90 flexed trunk stabilization exercises group ( group 2 ) , and shoulder 
 joints 120 flexed trunk stabilization exercises group ( group 3 ) .
the experiment was 
 conducted one by one in a separate time in order to blind subjects to the information about 
 the angle . in group 1 ,
the exercises were performed with a sling strapped on both ankles at 
 the shoulder joints bending 60 in the push - up position . in group 2 ,
the same condition at 
 the shoulder joints bending 90 was used , while in group 3 , the same condition at the 
 shoulder joints bending 120 was used .
the intervention period lasted a total of 8 weeks , 
 after which time the thickness , static balance , and dynamic balance of the core muscles were 
 measured to identify the pre- versus post - intervention changes .
when the measurer says 
  start  in a state when a sling has been strapped , the participant must move both knees away 
 from the floor11 .
before both knees fall 
 back to the floor , the shoulder joints bending angles ( 60 , 90 , and 120 ) are set using an 
 electronic goniometer .
after both knees are away from the ground , the hip joint angle must be maintained at 0. 
 the measurer should always observe whether the participant maintains the posture and angle 
 in the prone state . according to each shoulder joint angles , trunk stabilization exercises 
 were performed for 30 minutes three times a week for a total of 8 weeks ( warm - up , 5 minutes ; 
 exercises , 20 minutes ; cool - down , 5 minutes ) .
after the exercises were conducted for 40 
 seconds , each subjected rested for 20 seconds .
a 3.5-mhz convex array transducer with an ultrasonic measurement device was used to measure 
 trunk muscle thickness . on the ultrasound images ,
one physical therapist skillful at measuring ultrasound images applied 
 ultrasound gel between the transducer and the skin and placed the b - mode scanner so that the 
 center of the transducer contacted the 2.5  cm site frontward at the midpoint between the 
 12th rib and the iliac spine ridge centered around the armpit15 , 16 . to measure muscle 
 thickness ,
a vertical line was then 
 drawn at the left and right ends of the image and the muscle thickness was measured in the 
 order of tra , abdominal io , and abdominal eo17 . for a measurement posture
the participant was asked to comfortably bend the hip and knee joints in 
 the supine position to minimize lumbar lordosis and pull the belly upward .
when the 
 participant performed an abdominal throw - operation , the therapist gave visual feedback for 
 the abdominal muscle to contract while looking at the monitor .
a dynamic balance functional reaching test was used to measure dynamic stability18 . during this inspection ,
the subject is 
 asked to extend his / her legs as far as possible with heels not falling . at this time
, arm 
 movement is measured from end to end using a ruler pasted on the wall . in other words , this 
 procedure aims to test a subject s ability to extend their legs as far as possible while 
 stepping forward without losing their balance .
the ability to stretch farther indicates a 
 wider stability limit , suggesting better dynamic balance16 .
balance measuring equipment was used to evaluate static balance . a 
 stability index ( st ) and a weight distribution index ( wdi )
an st base on each of 
 the 4 force - plates for the toes and heels of the right and left feet showed the balance 
 stability by measuring the posture fluctuation based on changes in weight , while the wdi 
 showed a weight - loaded value using percentages19 .
the subject was asked to place one bare heel on the force place 
 where the sole - shaped heel portion was drawn .
measured postures were divided into eyes open , eyes closed , standing on a soft 
 supportive surface , and eyes closed while standing on a soft supportive surface . for each 
 posture , the subject was instructed to maintain it for 32 seconds . for the eyes
after the operation , the st and the wdi 
 were comprehensively calculated and displayed on a computer screen connected to the 
 equipment . in this study ,
a comprehensive st value and a wdi value were used , for which 
 lower values indicate good balance19 .
we 
 performed the shapiro - wilk test to determine the distribution of the subjects .
to compare 
 pre- versus post - experimental data in each group , the paired samples t - test was used .
to 
 compare pre- versus post - experimental data of the three groups , one - way analysis of variance 
 ( anova ) was performed .
when a significant difference was found , post hoc comparisons were 
 performed using a bonferroni correction .
the measurement data of each item are presented as 
 means and standard deviations . the significance level for statistical verification
the mean age height , weight , and bmi were not significantly different among the groups 
 ( table 1table 1.general characteristicsgroup 1(n=14)group 2(n=15)group 3(n=14)age ( years)20.6  0.920.5  1.120.2  0.4height ( cm)16,930  8.1167.3  7.8171.8  7.0weight ( kg)60.7  12.163.0  8.367.6  11.7bmi ( kg / m)22.1  2.119.6 
1.221.2  2.2 meansd , group 1 : the shoulder joints 60 bending trunk stabilization 
 exercises group , group 2 : shoulder joints 90 bending trunk stabilization exercises 
 group , group 3 : shoulder joints 120 bending trunk stabilization exercises group , bmi : 
 body mass index ) .  
meansd , group 1 : the shoulder joints 60 bending trunk stabilization 
 exercises group , group 2 : shoulder joints 90 bending trunk stabilization exercises 
 group , group 3 : shoulder joints 120 bending trunk stabilization exercises group , bmi : 
 body mass index group 1 and group 2 showed significantly increased tra thickness after the experiment ( p 
 < 0.05 ) .
group 3 showed a significant post - experiment increase in eo thickness ( p < 
 0.05 ) ( table 2table 2.comparison of core muscle thickness pre- vs. post - interventions(mm)group 1(n=14)group 2(n=15)group 3(n=14)eopre0.99  0.301.00  0.170.96  0.18post0.87  0.180.86  0.160.94  0.13iopre1.03  0.120.96  0.141.06  0.22post1.05  0.101.08  0.201.03 
0.23tra pre1.20  0.161.21  0.131.28  0.24post1.43  0.261.58  0.311.49  0.46meansd , group 1 : the shoulder joints 60 bending trunk stabilization 
 exercises group , group 2 : shoulder joints 90 bending trunk stabilization exercises 
 group , group 3 : shoulder joints 120 bending trunk stabilization exercises group , eo : 
 external oblique , io : internal oblique , tra : transverse abdominal , comparison of the 
 three groups pre- vs. post - interventions using one - way anova , comparison of 
 differences before and after interventions in each group using a paired t - test , 
 * p<0.05 , * * * p<0.001 ) .
meansd , group 1 : the shoulder joints 60 bending trunk stabilization 
 exercises group , group 2 : shoulder joints 90 bending trunk stabilization exercises 
 group , group 3 : shoulder joints 120 bending trunk stabilization exercises group , eo : 
 external oblique , io : internal oblique , tra : transverse abdominal , comparison of the 
 three groups pre- vs. post - interventions using one - way anova , comparison of 
 differences before and after interventions in each group using a paired t - test , 
 * p<0.05 , * * * p<0.001 changes in dynamic balance increased from 18.00  4.51 to 21.98  3.86 in group 2 .
the post - test results 
 revealed no statistical significance in the differences before and after dynamic balance at 
 60 , 90 , and 120 of bending .
changes in static balance showed statistical differences in eyes closed , pillow with eyes 
 closed , and stability index values in group 2 , in which the shoulder joints were bent at 90 
 using a sling ( p < 0.05 ) .
the post - test results found no statistical significance in each 
 difference in st , wdi and in 60 , 90 , and 120.
this study found that groups in which the shoulder joints were bent to 60 and 90 showed 
 improved tra strength .
this finding suggests that the tra muscle activity was increased . in 
 previous studies , when the shoulder joints are bent at 90 or less , the closer the trunk , 
 the more significant the activation of the tra11 , 
 19 .
tra fibers run parallel around the abdominal wall , playing a role in 
 allowing a force the same as the rim following contraction .
in addition , tra stabilizes the 
 lumbar spine and is first activated by the weight of the torso due to the limb movement18 .
this study s experimental method requires 
 a force to align the trunk against gravity with the shoulder bent at various joints .
, it is reported that 
 shoulder joints can securely move since their bending angle is related to the activation of 
 the follicle through the significant difference from 30 to 110 in the shoulder joint 
 bending angle19 .
reported 
 that the movement of the far limbs may affect muscle activity and that the force of 
 contracting muscles can be transferred to other connected muscles and tendons through the 
 site of origin .
in addition , myers et al . reported that the deltoid was involved when the 
 shoulder joint connected to the trapezius is bent20 .
based on previous research , this study also found that trunk 
 stabilization exercises depending on the shoulder joint angle impact the contraction of the 
 muscle belly through the myofascial meridians of the dorsal arm line , where the arm muscles 
 deeply contract and the chest waist fascia to increasing the activity21 . in the group
in which the shoulder joint was bent to 120 , the eo strength was 
 significantly improved compared to that in the other groups . for stevens et al . , the muscle 
 activity of the eo in the leg lifted in a posture of lifting the arms and legs was higher 
 because the eo on the lifted side of the torso turning direction in maintaining the spine 
 against gravity contracts more by adjusting the body turning occurring in the lifting 
 process22
. additionally , it may be 
 because the eccentric contraction of the eo resulted in increasing muscle activity to 
 maintain trunk stability as the trunk and arms go farther .
for static balance , group 2 showed a statistically significant difference in ec , pc , and st 
 values .
for st , a significant difference was seen only in the eyes - closed posture .
visual 
 and proprioceptive senses perform an important function in reflective postures and movement 
 control23 .
static balance on a soft 
 supportive surface is significantly reduced when vision is blocked24 .
reported that trunk 
 stability is divided into three sub - systems : a passive system in charge of the stability 
 provided to the non - contractile tissue in a passive tense form ; the transmission of power , 
 an active sub - system made by shrinking organizations , reducing the stress exerted on the 
 vertebral body and spinal joint , adjusting pain , and reinforcing the joint with active 
 stabilization ; and an adjustable sub - system consisting of proprioceptive senses and the 
 central nervous system25 . ec and pc 
 better demonstrate the effects of balance , and an improved balance ability helps with 
 neurological stabilization , which consists of an active sub - system of trunk muscles , 
 proprioceptive senses , and the central nervous system . however , for wdi , no significant 
 change was seen in eo , ec , or po .
periyasamy et al . reported that foot pressure distribution 
 is affected by a number of factors such as foot anatomy , body mass , gender , and joint range 
 of motion26 . in this study , since 
 measurements were performed three times in each standing posture and foot position , no 
 statistically significant difference was created in wdi dynamic balance was improved overall 
 in all three groups , but statistically significant differences were found only in group 2 . 
 muscle stability and strength increase as the motor control for harmonious muscle 
 mobilization between the large trunk muscles and the small intrinsic muscles is 
 emphasized27 .
in addition , the 
 automatic activity of the tra is thought to be the protective mechanism of the lumbar 
 spine13 and has been reported as part 
 of the deep muscle that provides the lumbar stability during functional movements .
the tra 
 is reportedly involved in proactive attitude adjustments regardless of upper and lower limb 
 movement direction28 . from these results , 

trunk stabilization exercises of the shoulder joint bent at 90 were the most effective 
 since they provide trunk stability , which then improves one s dynamic balance sense .
such 
 increases are considered to contribute to the dynamic stability required by functional 
 movements .
first , subjects were limited to healthy adults in their 
 20s . as such , the subject cohort was small , making it difficult to generalize the results 
 obtained .
second , compared to the 8-week intervention period , the pre- and post - intervention 
 comparison was made and no subsequent evaluation was conducted .
thus , it was impossible to 
 determine the long - term effect of the intervention .
third , the subjects private lives , sex , 
 habits , and personal athletic career were not considered .
therefore , further studies of a 
 larger number of subjects with follow - up studies are required to evaluate the retention of 
 the improved effects . the program applied in this study
thus , it is necessary to normalize the program as a more specific and structured 
 intervention program through design improvements and modifications .
therefore , trunk stabilization exercises according to shoulder joint angle using a sling 
 have positive effects on core muscle strength and balance .
the entire muscle was 
 strengthened under trunk stabilization exercises applied at the shoulder joints at angles of 
 120 or higher . in trunk stabilization exercises using a sling according to various shoulder 
 joints angles , the shorter distance between the arm and the trunk allows the tra to act , 
 whereas a longer distance contributes to trunk stability due to the contraction of the 
 eccentricity of abdominal eo .
to improve one s balance capacity for functional activities , 
 the shoulder joints should be bent at 90 before training .
this study is significant in that 
 it has presented the orientation for muscle strengthening training involved in trunk 
 stabilization for use in future clinical experiments .
related studies to demonstrate and 
 systemize the effects on the core muscle are required .
 [ purpose ] to date , core muscle activity detected using ultrasonography during prone 
 bridge exercises has not been reported . 
 here we investigated the effects of core muscle 
 thickness and balance on sling exercise efficacy by shoulder joint angle in healthy 
 individuals . 
 [ subjects and methods ] forty - three healthy university students were enrolled 
 in this study . 
 ultrasonography thickness of external oblique , internal oblique , and 
 transversus abdominis during sling workouts was investigated . 
 muscle thickness was 
 measured on ultrasonography imaging before and after the experiment . 
 dynamic balance was 
 tested using a functional reaching test . 
 static balance was tested using a tetrax 
 interactive balance system . 
 [ results ] different muscle thicknesses were observed during 
 the prone bridge exercise with the shoulder flexed at 60 , 90 or 120. shoulder flexion 
 at 60 and 90 in the prone bridge exercise with a sling generated the greatest thickness 
 of most transversus abdominis muscles . 
 shoulder flexion at 120 in the prone bridge 
 exercise with a sling generated the greatest thickness of most external oblique muscles . 
 
 [ conclusion ] the results suggest that the prone bridge exercise with shoulder joint angle 
 is an effective method of increasing global and local muscle strength . 