for jumping , the lower limbs explosive and organic motions are required to act on the 
 ground against a player s weight and gravity with strong force , by effectively delivering 
 muscle strength generated from the hip and knee joints to the ankles and feet . the physical 
 stress occurring at this time acts on the ankle joints directly
; therefore , they are the 
 body parts experiencing the greatest load1 . many studies of volleyball players have reported chronic ankle 
 injuries . as a volleyball game progresses ,
motions placing burden on the ankle joints , such 
 as jumping and landing , which are known to place excessive load on the ankle joints are 
 numerously repeated2 , 3 . recently , as interest in systematic and scientific training methods has increased in many 
 sports , and diverse training methods have been developed and applied . in volleyball , high 
 jumping ability greatly affects a player s ability and , therefore , diverse methods for 
 exerting explosive power in the lower limbs have been introduced . among them , plyometric 
 training enables the exertion of explosive power using the extension reflex by the action of 
 actin - myosin within the muscles4 . 

therefore , it is widely applied as a training method to develop greater power , through the 
 improvement of muscle strength and speed , for to producing superior exercise performance in 
 ordinary people , as well as in athletes whose sports rely heavily on jumping , such as 
 basketball , and track and field , as well as volleyball5,6,7,8 .
the application of whole body vibration exercise , another training method , is being used 
 for elite athletes of diverse sports since research results have shown that whole body 
 vibration exercises are safe , convenient , and equally influence the circulation and 
 endocrine systems , in addition to the musculoskeletal system .
reported that 
 whole body vibration training increased knee extension strength and women s speed .
torvinen 
 et al . reported the effects of vibration training on body composition and the muscles of 
 women who did not do exercise9 , 10 .
in addition , choi et al . through the improvement of after 
 conducting whole body vibration training with patients with hemiplegia resulting from 
 stroke .
bosco et al . reported that after 10 minutes of vibration training of constant 
 intensity , there was an improvement in the average power of the thighs of female volleyball 
 players who were national team members11 , 12 .
a study of elderly subjects reported significant increases in balance ability , and another 
 study of concluded that vibration exercise positively affected their lower limb power and 
 jumping ability male college students in their twenties13 , 14 .
there is a considerable 
 body of previous research demonstrating that whole body vibration exercise positively 
 affects the body15 , 16 .
accordingly , this study investigated differences in jumping ability and lower limb balance 
 ability elicited by plyometric training and vibration exercise in male professional 
 volleyball players with and without ankle injuries , which occur frequently in korean 
 professional volleyball players .
the subjects of this study were 28 male athletes registered as professional volleyball 
 players in the korean volleyball federation .
none of them had any orthopedic diseases that 
 would have made it difficult or impossible to participate in training or regular league 
 games .
the subjects were divided into a total of four groups based 
 on whether or not they had ankle injury and the types of training they underwent ( plyometric 
 or vibration exercises ) ( table 1table 1.general characteristics of the subjectsitempai ( n=8)pan ( n=6)vai ( n=7)van ( n=7)height ( cm)190.3  9.0190.8  8.2193.7  1.6188.5  6.9weight ( kg)82.8  7.984.1  6.984.8  5.4582.0  7.4body fat percentage ( % ) 11.7  3.79.7  2.78.9  2.79.4  3.0values are shown as the mean  sd , p<0.05 , pai : plyometric with ankle injury ; pan : 
 plyometric with no ankle injury ; vai : vibrator with ankle injury ; van : vibrator with 
 no ankle injury ) .
values are shown as the mean  sd , p<0.05 , pai : plyometric with ankle injury ; pan : 
 plyometric with no ankle injury ; vai : vibrator with ankle injury ; van : vibrator with 
 no ankle injury to measure vertical jumping ability , the subjects stood on the flank of a wall , where a 
 digital meter system had been attached , with one leg contacting the wall .
then , the subjects 
 raised one arm and stretched it and the height to the end of the fingers was marked . at a 
 spot 20  cm distant from the wall ,
the subject put both feet together and jumped as high as 
 possible using rebound .
the vertical distance between the height of the fingers at rest and 
 their maximum jump height was measured in millimeters .
the subjects performed this action 
 twice and the best result was used . for side step measurement ,
a central line was marked and 
 120  cm lines were marked to the left and right sides .
the subjects spread their legs from 
 the central line to the left and right sides and moved quickly as follows : central line to 
 the right line , back to the central line , to the left line , back to the central line .
this 
 pattern was repeated for 20 seconds and the number of movements was recorded .
the subjects 
 closed their eyes , put both arms on the waist , flexed the hip joint and the knee joint of 
 one leg to 90 degrees , and stood with the other limb stationary .
then , a stopwatch was used 
 to measure balance maintenance time twice and the average value was calculated .
this equipment measures subjects postural 
 control ability by quantifying parameters of body sway from the center of the foot plate 
 based on changes in the ground reaction force with gradual changes in the support plate s 
 stability .
when the subjects stood on the support plate on one leg with both eyes open , 
 measurements were taken after inputting the location of the foot according to the direction 
 of the program , and postural control ability was evaluated according to the degree of 
 stability in the forward , backward , internal , external , and all directions , with stability 
 of the support plate adjusted from one to eight .
the 
 test results were converted to indexes with lower values indicating higher dynamic postural 
 control ability .
the plyometric exercise was composed of boxes and huddles based on prior research17 , 18 .
the exercise time of one session was a total of 50 minutes , 
 including warm - up and cool - down exercises .
the subjects conducted this exercise three times 
 per week for eight weeks . the program of plyometric exercise was performed for 10  min 
 warm - up , and double - leg hop / dept jump , single - leg hop ( both)/depth jump , side - hop and two 
 step double - leg hop was also performed for 30  min and then cool - down was 10  min .
the whole body vibration exercise was conducted for eight weeks using the natural wave , 
 sound wave vibration training system .
the warm up exercise , main exercise , and cool down exercise were 
 conducted for 10 min , 30 min , and 10 min , respectively .
the first and second weeks were used 
 for adaptation and three sets of exercises were performed . from the third to the sixth weeks 

the exercise 
 intensity was adjusted with the frequency of vibration , the number of sets , and motion 
 composition .
the vibration frequency was 8  hz from the first week to the second week and 
 26  hz from the third week to the sixth week19 .
the program of vibration exercise was performed for 10  min warm - up 
 in the standing position followed by squat / plank , calf raise , and lunge for 30  min , and 
 cool - down 10  min .
the data was analyzed using spss ver . 15.0 . in order to examine differences in data between 
 the pre - test and post - test after the whole body vibration exercise , 2-way repeated analysis 
 of variance ( anova ) was conducted , and for simple effect verification , the paired t - test was 
 carried out .
the measurements of vertical jumping ability and side - step ability in consideration of 
 whether or not there was an ankle injury , and according to the types of training , are 
 presented in table 2table 2.the subjects jumping ability and balance ability resultsitemgroupprepostjump abilityside step test ( sec)pai68.1  2.367.0  4.6pan68.2  6.265.3  4.9vai63.2  1.166.7  2.5van62.0  4.262.57  4.2vertical jump ( cm)pai56.2  3.560.8  4.1pan56.0  3.761.0  4.4vai63.0  9.365.2  8.2van60.3  3.265.2  2.1balance ability(index)frontpai62.6  5.159.3  3.7pan54.0  6.258.2  8.5vai56.8  3.059.4  4.2van54.8  8.861.4  5.6backpai56.6  12.558.3  2.9pan53.0  8.357.0  9.9vai51.4  8.355.1  5.0van53.5  12.362.8  7.4leftpai62.6  8.761.3  8.0pan61.5  10.060.2  5.3vai57.7  6.763.5  7.3van65.1  11.272.1  12.0right pai72.0  7.871.0  13.6pan64.0  8.862.5  6.5vai64.5  5.268.5  6.7van69.7  11.878.8  11.left/frontpai79.0  15.172.3  13.7pan73.2  12.872.0  11.6vai70.8  11.276.0  9.5van79.4  13.286.4  11.7right / frontpai79.3  16.879.3  6.5pan74.0  10.275.0  9.9vai72.5  11.177.2  6.6van80.8  11.985.1  7.4left / backpai78.6  19.284.0  16.5pan74.0  7.978.5  8.9vai69.1  12.275.5  7.6van78.7  9.189.8  6.4right / backpai78.6  19.380.6  16.3pan75.0  7.781.5  14.0vai69.7  10.577.1  9.0van78.8  10.994.0  4.0romberg test ( left)pai41.3  19.936.1  19.2pan27.6  17.924.6  18.8vai34.5  19.656.43  14.2van10.7  9.634.71  14.8romberg test ( right)pai36.6 
 8.9values are shown as the mean  sd . * significant difference between ( p<0.05 ) the 
 pre- and post - tests within each group .
pai : plyometric with ankle injury ; pan : plyometric with no 
 ankle injury ; vai : vibrator with ankle injury ; van : vibrator with no ankle injury . 

: significantly different from pai , : significantly 
 different from pan , : significantly different from vai , : 
 significantly different from van .. in the side - step test , the effect of interaction between the groups and 
 timing was significant and according to the results of the paired t - test , the vibration 
 exercise group with an ankle injury showed significant differences between pre- and 
 post - intervention .
there were significant improvements in both groups with ankle injury and 
 the group without it in the vertical jump .
values are shown as the mean  sd . * significant difference between ( p<0.05 ) the 
 pre- and post - tests within each group .
pai : plyometric with ankle injury ; pan : plyometric with no 
 ankle injury ; vai : vibrator with ankle injury ; van : vibrator with no ankle injury . 
 : significantly different from pai , : significantly 
 different from pan , : significantly different from vai , : 
 significantly different from van . in the balance ability test ,
the main effect of timing was signficant according to the 
 results of the posterior test , and there was a significant difference in timing between the 
 plyometric training groups with and without ankle injury .
in addition , according to the 
 result of test of the right side , there were a significant differences in timing in the 
 left / posterior test and right / posterior test , and group , timing , and interaction effects 
 were significant in the one - leg standing test with the eyes closed ( table 2 ) .
in volleyball , dominating command of the air based on the players height and jumping 
 ability is a very important factor in determining victory or defeat .
height is considerably 
 affected by innate factors , but jumping ability can be improved by training . to this end , 
 improving the muscle strength of the lower limbs and anaerobic exercise ability may increase 
 jumping ability20 . in particular , the 
 thigh muscle group plays a key role in performing jumping motions and its ability can be 
 evaluated through a test such as the vertical jump21 .
plyometric training enables an individual to exert great force instanteously using the 
 extension reflex through the actin - myosin action in the muscles .
therefore , it is in the 
 spotlight as a training method in many sports , including volleyball , where jumping ability 
 is emphasized .
however , diverse physical strength , including muscle strength of a certain 
 level or higher , is required as a precondition for plyometric training .
plyometric training 
 exerts a considerable physical burden on the muscle joints , and , although it carries the 
 risk of injury , it has nevertheless been proposed as an effective training method for the 
 enhancement of jumping ability22 . in the 
 present study , according to the vertical jump results , in the plyometric group , the group 
 with ankle injury and the group without it demonstrated improvements in jump height of 3.63% 
 and 3.72% , respectively , compared to their pre - test results .
there was a statistically 
 significant improvement after training compared to the pre - test values , consistent with 
 previous research20 , 23 which reported improvements in jumping ability after plyometric 
 training .
on the other hand , the vibration exercise group exhibited improvements in jump 
 height compared to the pre - test but they were not statistically significant ; therefore 
 plyometric training was more effective than vibration exercises at improving the jumping 
 ability of the volleyball players .
in the balance ability test , conducted in order to examine balance ability , there were no 
 differences between the ankle injury groups of the training methods used , but in the romberg 
 test ( standing on one leg with the eyes closed ) on the right and left sides , the ankle 
 injury group which performed vibration exercise showed the greatest improvements ; 
 therefore , , the vibration exercise may be recommended as a training for improving volleyball 
 players balance ability .
one hundred adults with ankle injuries performed whole body 
 vibration exercises and there was an effect on their ankle muscle strength . according to by 
 kiers et al .
a positive change in ankle muscle strength improves balance ability , suggesting 
 whole body vibration exercises are effective for the improvement of balance24 .
whole body vibration exercise delivers stimulus to the tissues , including the muscles 
 around the ankle joints , which play an important role in balance ability , thereby increasing 
 muscle strength , enhancing proprioceptive sense , and mobility ; and vibration exercise has 
 consistently been reported to be very effective at improving balance ability25 , 26 .
however , such effects are different according to the direction of 
 vibration stimulation and the number of vibrations27 , and in order to clarify the effect of one - leg standing with the 
 eyes closed , research which measures other factors known to affect equilibrium should be 
 studied together .
in addition , in the balance ability test ( where another measurement method 
 was conducted to test balance ability ) no significant differences were found , suggesting the 
 measurement of the effects of vibration exercise using diverse variables of balance ability 
 should be attempted . to summarize the results of this study ,
plyometric training was effective at improving the 
 jumping ability of volleyball players , and whole body vibration exercise was effective for 
 the improvement of their balance ability . in order to clarify the results of this study , 
 additional research that with larger numbers of subjects and a wider range of measurement 
 variables is considered necessary .
 [ purpose ] the purpose of this study was to investigate differences in jumping ability and 
 lower limb balance ability elicited by plyometric training and vibration exercise , of 
 volleyball players with and without ankle injuries , which frequently occur among korean 
 professional volleyball players . 
 [ subjects and methods ] twenty - eight volleyball players 
 were divided into the following groups : plyometric with ankle injury ( pai ) group ; 
 plyometric with non - ankle injury ( pan ) group ; vibrator with ankle injury ( vai ) group ; and 
 vibrator with non - ankle injury ( van ) group . 
 after exercise and whole body vibration 
 stimulation , their vertical jumping abilities , side step , and static equilibrium ability 
 were measured . 
 [ results ] the vibration exercise group which had experienced ankle injuries 
 showed significant improvements in the sidestep test after the intervention compared to 
 before the intervention . in vertical jumping as well , significant improvements were 
 observed in the vai group and the van group following vibration exercise . in the balance 
 
 ability test , significant improvesments in the pan group and the pai group were observed 
 after the intervention . according to the results of the right side 
 , there was significant 
 change in the left / back side test and the right / back side test before and after the 
 intervention ; and in the test of one - leg standing with eyes closed , there were significant 
 group , timing , and interaction effects . 
 [ conclusions ] the training method which 
 effectively improved the jumping ability of volleyball players was plyometric training , 
 and for balance ability improvement , whole body vibration exercise was effective . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION