core stabilization exercises may prevent or improve 
 symptoms of low back pain because it provides the basis for force production by the 
 limbs2 . broad benefits of core 
 stabilization have been reported , including improvement of athletic performance and 
 prevention of injuries3 . the core muscles include the abdominal muscles in the front , paraspinal and gluteal muscles 
 in the back , the diaphragm as the roof , and the pelvic floor and hip girdle musculature at 
 the bottom . this core is composed of 29 muscle pairs that help to stabilize the spine , 
 pelvis , and kinetic chain during functional movements . without these muscles , traditional core 
 exercises , core stability exercises , ball and device exercises , free - weight exercises , and 
 noncore free - weight training have been studied as methods for core stability1 . methods of increasing core stability also 
 involve the use of an unstable support surface ( uss ) , such as a ball or balance 
 platform4 , 5 . the use of uss that increases trunk muscle activity may also be 
 effective during core stability training6,7,8,9 . in addition , movements of the limbs and 
 body weight during core stabilization exercises can be used to provide resistance to trunk 
 muscles10 , 11 . moreover , shoulder resistance exercises increase the endurance and 
 strength of core stability muscles12 . several studies have been performed using uss during core stability training13,14,15,16 , 
 but no study has addressed the effects of a uss on core muscle activities during a shoulder 
 exercise . therefore , the purpose of this study was to determine the effects of uss on trunk 
 and lower extremity muscle activities during pulley - based shoulder exercises . subjects were volunteers recruited 
 from a university in seoul and were accepted for this study if they were in good health , had 
 no current musculoskeletal , neuromuscular , or cardiovascular problems , and had a normal 
 range of shoulder joint motion . subjects were excluded if they had trauma or pain in the 
 trunk or a shoulder joint or a history of surgery . in addition , those who had experienced 
 core stabilization training or therapy within the last 3 months were excluded . subjects 
 signed an informed consent form prior to participation , and the study protocol was approved 
 by the institutional review board of the university of sahmyook . forty - eight hours before starting the study , all subjects abstained from excessive 
 exercise . a 5-min warm - up stretching exercise was conducted prior to the performing of the 
 exercises . prior to electrode placement , sites were prepared by abrading the skin with fine 
 sandpaper and cleaning it with 70% isopropyl alcohol . muscle 
 activations of the trunk were measured during pulley - based shoulder exercise ( pbse ) using 
 14-kg resistance on the uss . pbse comprised shoulder abduction , adduction , flexion , 
 extension , internal rotation , and external rotation . the orders of the six pbse exercises 
 were randomized , and each exercise was repeated five times . a metronome , set at 80 counts 
 per minute , was used during the exercises to control speed . all exercises were conducted with the feet apart ( within shoulder 
 width ) and parallel to the shoulders . abduction and flexion exercises were conducted from 0 
 to 90 of the shoulder range of motion with the elbow joint in extension . adduction exercise 
 involved 90 to 0 of the shoulder range of motion with the elbow joint in extension . extension exercise involved 180 to 90 of the shoulder range of motion with the elbow joint 
 in extension . external rotation exercise involved internal rotation 45 to external rotation 
 45 with the elbow joint in 90 of flexion . internal rotation exercise was conducted from 
 external rotation 45 to internal rotation 45 with the elbow joint in 90 of flexion17 . a pulley machine ( pulley ex , sanimed , 
 ibbenbren , germany ) was used for all shoulder exercises . the flat floor of the laboratory 
 was used as the stable support surface , and a pedalo - vestimed # 50 
 ( diameter 50  cm , height 19  cm ) was used for the uss during shoulder exercises . the top and 
 bottom of the pedalo device were connected by four fixed springs , and the top of the pedalo 
 device was able to move up and down in all directions . to measure trunk muscle activities , emg data were collected from the rectus abdominis ( ra ) , 
 external oblique ( eo ) , and erector spinae ( es ) ; to measure lower extremity muscle 
 activities , emg data were collected from the rectus femoris ( rf ) , tibialis anterior ( ta ) , 
 and gastrocnemius ( ga ) . for the ra , electrodes were placed 2  cm lateral , 1  cm superior , and 
 1  cm inferior to the umbilicus . for the eo , the electrode was placed midway between the 
 anterior superior iliac spine and the most inferior aspect of the rib cage . for the es , the 
 electrodes were placed 2  cm lateral to the 3rd lumbar vertebra and parallel to the 
 lateral - most high iliac crest . for the rf , the electrode was placed midway between the knee 
 and iliac crest on the front of the thigh . for the ta , the electrode was placed on the 
 lateral side of the tibia three - fourths of the distance from the knee to the ankle . for the the locations of 
 all electrodes were determined using the method described by cram and kasman18 , and all electrodes were placed on the 
 dominant sides . a datalog p3x8 data acquisition unit ( biometrics ltd , gwent , uk ) surface emg signals were extracted with the biometrics analysis 
 software ( v7.50 ) in ascii and were processed using a root mean square ( rms ) algorithm in 
 myoresearch xp master edition 1.06 ( noraxon u.s.a . the sampling 
 rate was set at 1,000  hz per channel , emg signals were band - pass filtered from 20 to 450  hz , 
 and a 60-hz notch filter was used to reduce noise . descriptive statistics were used to analyze general subject characteristics , and a one - way 
 repeated - measures analysis of variance was used for each muscle . twenty healthy adult male subjects ( height , 176  5  cm ; weight , 70  9  kg ; age , 22  
 3  years ; values are presented as means  sd ) participated in this study ( table 1table 1.subject characteristicscharacteristicsage ( years)22.4  2.7height ( cm)176.4  5.1weight ( kg)69.8  8.6dominant siterightvalues are expressed as mean  sd ) . values are expressed as mean  sd the ra and es showed significantly less activation on the uss ( p < 0.05 ) during shoulder 
 external rotation , but they showed no significant difference on the uss during shoulder 
 abduction , adduction , extension , flexion , and internal rotation . the eo and rf showed no 
 significant difference on the uss during any shoulder exercises ( table 2table 2.mean ( sd ) of the emg activity of trunk muscles on ussshoulder exercisestable support surfaceunstable support surfacerectus abdominis1 . however , the ta showed significantly greater activation ( p < 0.05 ) on the 
 uss during shoulder abduction , adduction , external rotation , and internal rotation . on the 
 other hand , the ta showed no significant difference during shoulder extension and flexion on 
 the uss , whereas the ga showed significantly greater activation ( p < 0.05 ) on the uss 
 during shoulder abduction , extension , and internal rotation . however , during shoulder 
 adduction , flexion , and external rotation , the ga showed no significant difference on either 
 a stable surface or the uss ( table 3table 3.mean ( sd ) of the emg activity of lower extremity muscles on ussshoulder exercisestable support surfaceunstable support surfacerectus femoris1 . . internal rotation23.4  10.930.5  8.2**statistical significance ( p > 0.05 ) ) . * statistical significance ( p < 0.05 ) * statistical significance ( p > 0.05 ) this study indicates that the use of uss during pbse did not increase trunk muscle 
 activity . on the contrary , this is probably because of the compensation strategy of the ankle rather than 
 because of that the trunk muscles . this ankle compensation is a postural control strategy 
 that first appears when there is a slight instability in the support surface . balance in the 
 upright posture can be restored through muscle contraction near the ankle joint , and a close 
 relationship exists between this ankle strategy and the activities of the ta and ga . in the 
 case of postural sway under unstable conditions , balance is recovered by an ankle strategy , 
 a hip strategy , or both simultaneously19 . 
 in the case of rapid shaking of the support surface , balance recovery is achieved via an 
 integrated ankle / hip joint strategy , and in the case of swaying of the support surface in an 
 upright posture , balance can be maintained by ankle movement without hip joint 
 extension20 . in addition , to maintain 
 postural control , normal adults mainly use an ankle strategy , whereas elderly individuals 
 and children mainly use a hip strategy21 . 
 in this study , we compared muscle activities in young male subjects on stable and unstable 
 support surfaces . based on the results , it would appear that compensation for the uss 
 involved an ankle strategy rather than trunk muscle activity . shoulder abduction and 
 adduction exercises involve movements in the frontal plane17 , and postural sway from the uss occurs in the medial / lateral 
 direction . the results showed significantly increased ta and ga activities ( p < 0.05 ) 
 during shoulder abduction when uss was used but only showed significantly increased ta 
 activity during shoulder adduction . although ga activity did not significantly increase 
 during shoulder adduction , muscle activity increased slightly . in this study , the use of uss 
 significantly increased ga muscle activity during shoulder extension ( p < 0.05 ) . the 
 extension and flexion exercises involve movements in the sagittal plane19 ; thus , postural sway occurs as the center of gravity moves 
 forward and backward on the support base . the maintenance of balance against such 
 forward / backward sway is dependent on the alternating activities of the ta and ga . ga 
 activity begins before the body collapses in the forward direction , whereas ta activity 
 appears before collapse in the backward direction23 . in the present study , shoulder flexion resulted in no significant 
 differences in lower extremity or trunk muscle activity on the unstable surface . however , 
 the most activity was seen in the ta compared with the other muscles . in the normal upright 
 posture , the center of gravity is on the front side22 , and ankle dorsiflexor activity is low ; on the other hand , ankle 
 plantar flexor activity is high21 , 23 , 24 . the ankle plantar flexor plays an important role in torque 
 adjustment at the ankle joint and in the maintenance of upright posture25 . therefore , back sway occurs during shoulder flexion due 
 to use of uss , but because of the influence of the center of gravity at the ankle joint , it 
 appears to have no significant impact on ta activity due to the ankle dorsiflexors . shoulder 
 external and internal rotation exercises change the center of gravity with a complex form in 
 the forward / backward direction as well as in the medial / lateral direction . during shoulder 
 external rotation on uss , the body sways in the lateral backward direction , and an ankle 
 strategy can be used to maintain balance . in this study , the ta activity increased 
 significantly on the uss ( p < 0.05 ) , showing that the ta compensated for lateral backward 
 sway via eccentric contraction . shoulder internal rotation causes the body to sway in a 
 forward medial direction , and the ankle strategy is used to maintain balance in a manner 
 similar to that during shoulder external rotation ( p < 0.05 ) . in this study , pbse may have been able to increase core stability , but using a uss to 
 enhance core stability may not be effective . further confirmation of these results is necessary in larger , more diverse populations , 
 including females and older individuals , and there is a need to measure the deep muscles 
 such as the transverse abdominis and internal oblique .