lower limb asymmetry is highly related to damage of the lower limbs that causes changes in 
 lower limb function during daily or sports activities1 , 2 .
lower limb asymmetry causes 
 changes in the movement of the ankle , knee , and hip joints and in the combined 
 proprioceptive sensation and neuromuscular control required for change of direction3 .
therefore , the different movement pattern 
 of the non - affected or affected side could change the mechanism of alternating movements 
 such as walking , running , jumping , and landing and damage the lower limb during such 
 movements . asymmetry after this damage exacerbates the condition , resulting in even greater 
 damage4 , 5 .
diagnostic tests can help 
 identify healthy people who are at risk of lower limb damage2 .
moreover , diagnostic tests serve as a criterion for rehabilitation 
 of damage and are helpful in determining the appropriate time for a person to return to the 
 community6 .
a study by fitzgerald et 
 al.7 suggested that the functional 
 performance test could predict the possibility of knee damage and serve as a tool to assess 
 postoperative improvement in patients .
the functional performance test includes a single leg 
 hop for distance , single leg triple hop for distance , single leg internal / external jump , and 
 single leg vertical jump ( slvj ) , all of which have been widely used both clinically and 
 academically8 .
the slvj , in particular , 
 is one of the most common tests of functional performance and has been used to evaluate 
 lower limb performance by applying similar stimulations as those experienced during daily or 
 sporting activities ; moreover , the slvj has a reported test - retest reliability of 
 0.880.979 . as an officially certified 
 assessment tool
, the slvj comprehensively measures flexibility , balance , power , and 
 neuromuscular control and provides objective measurements based on subject performance10 .
the slvj can be used to assess the functional performance ability of both lower limbs , and 
 the results can be represented using the limb symmetry index ( lsi ) .
the lsi is the ratio of 
 the jump height of the measured lower limb to the jump height of the contralateral lower 
 limb . for classification of normal and abnormal limb symmetry ,
odonnell et al.11 suggested that an lsi  90% should be 
 considered in the normal range .
therefore , functional ability as measured by the slvj is 
 abnormal , and asymmetry exists , if there is > 10% difference between the two lower 
 limbs . to restore symmetry , a significant goal can be to improve the functional ability of the 
 weaker side of the asymmetric lower limbs .
elements related to functional ability include 
 muscle strength , power , lower extremity joint stability , endurance , muscle flexibility , 
 balance , proprioception , speed , and agility .
diverse methods including muscular strengthening exercise , endurance training , balance 
 training , perturbation training , and agility training have been used to improve the 
 functional ability of a weaker lower limb . however
, these exercises and training methods 
 only improve specific elements of lower limb functional ability at a time . in this sense ,
a 
 number of elements need to be trained to achieve sufficient capacity of the lower limb . over the most recent decade , wbv has emerged as a new training method that involves diverse 
 elements by improving bone density , blood circulation , muscular strength , endurance , 
 proprioception , and balancing ability12,13,14 . 

wbv applies fast repeated , alternating concentric  eccentric stimulations while the subject 
 is in a static standing posture ; the external vibration affects the muscles and nervous 
 system15 .
other existing training 
 methods with stronger stimulation are more likely to harm the subjects , but wbv is 
 associated with only a low risk of such harm16 .
several studies have investigated the potential of whole body vibration ( wbv ) to improve 
 subsequent performance .
traditionally , bilateral vibratory stimulation transmitted through 
 both lower limbs has been provided to enhance muscular activity and functional performance . 

however , training using bilateral vibratory stimulation may not be proper in the case of 
 subjects showing a severe difference between both lower limbs .
the amount of vibratory 
 stimulation accepted by each lower limb may differ while supporting body weight against 
 gravity and the magnitude of the effect depends on the condition of each limb .
therefore , 
 training using unilateral vibratory stimulation can be a way to improve the functional 
 ability of a weak lower limb and the symmetry of functional ability between both lower 
 limbs .
but it is not well understood whether unilateral vibratory stimulation will cause 
 improvement in the performance of a trained lower limb and the other limb , and no studies 
 have examined the effect of unilateral vibratory stimulation compared with bilateral 
 vibratory stimulation in the same position .
thus , in recognition of this , the present study aimed to differentiate the weaker and 
 strong lower limbs and identify the effect of unilateral wbv training on the slvj height as 
 well as limb symmetry .
all subjects were classified as 
 recreationally active , which was operationally defined as participation in regular 
 structured exercise training or competition .
none of the subjects were exposed to wbv 
 training before participation in this study17 .
the subjects were excluded if they had pain , recent or possible 
 thrombosis , severe headache , vestibular disorder , advanced arthritis , lower limb implant , 
 lumbar disc disorder , medication that could interfere with the study , a recent sprain , a 
 recent fracture , a gall bladder or kidney stone , or a malignancy18 .
the subjects were fully informed about the possible risk and discomfort that might result 
 from the investigations .
the protocol was approved by the institutional review board and 
 ethics committee of sahmyook university , south korea .
five subjects were excluded because of 
 knee joint pain ( n = 3 ) , recent sprain ( n = 1 ) , or lumbar disc disorder ( n = 1 ) .
after 
 interviews for exclusion , thirty subjects were randomly assigned into one of three groups , 
 namely the unilateral vibratory stimulation group ( uvsg , n=10 ) , bilateral vibratory 
 stimulation group ( bvsg , n=10 ) , and no vibratory stimulation group ( nvsg , n = 10 ) .
random 
 allocation software ( version 1.0 ) was used to minimize selection bias19 ( table 
 1table 1.subject characteristics ( n=30)uvsgbvsgnvs(n=10)(n=10)(n=10)age ( years)21.1  2.420.9  1.521.2  2.3height ( cm)175.9  5.8175.3  4.6174.7  4.4weight ( kg)65.6  7.667.8  6.265.4
 7.6values are expressed as the mean  standard deviation.uvsg : unilateral vibratory stimulation group ; bvsg : bilateral vibratory stimulation 
 group ; nvsg : no vibratory stimulation group ) .
uvsg : unilateral vibratory stimulation group ; bvsg : bilateral vibratory stimulation 
 group ; nvsg : no vibratory stimulation group during the course of the study , participants were not permitted to undertake any power or 
 strength training and were also instructed to avoid any vigorous activity 24 hours prior to 
 the intervention20 .
based on the findings of cochrane and 
 stannard21 , each participant was 
 familiarized with the vibration platform . in the study by cochrane and stannard ,
vibratory 
 stimulation was provided for 5  min ; however , in the present study , the subjects could not 
 maintain the squat posture for more than 3 minutes because of calf muscle spasm .
the galileo 2000 ( novotec , pforzheim , germany ) was used for bilateral wbv training , and a 
 researcher provided a full explanation of the study purpose and methods , as well as the 
 proper exercise posture , to the subjects .
based on the methods of cochrane and stannard21 , the exercise posture was corrected , and 
 the subject stood on a foothold while barefoot and maintained a specific distance from the 
 rotation axis .
the subject crossed his arms , and a researcher supported the subject s waist 
 to prevent falling .
the vibratory stimulation was administered once at a vibration frequency of 26  hz 
 for 3  min .
the proper exercise 
 posture included maintenance of the knee and hip joint angles at 140 degrees and 90 degrees , 
 respectively .
both feet were placed on the vibratory plate to transfer vibratory stimulation 
 through both the lower limbs . for unilateral wbv training of the lower limb on the weak 
 side ,
all the conditions were the same as those for the isometric squat exercise with 
 bilateral vibratory stimulation .
the foot of the strong side was placed on a foothold at the 
 same height as the vibration plate , and the foot of the weak side was placed on the 
 vibration plate so that the vibratory stimulation was transferred only through the weak 
 lower limb .
training without wbv was performed by the same method as bilateral wbv training 
 except for vibratory stimulation .
both feet were placed on the vibratory plate ; however , 
 vibratory stimulation was not provided .
no attempt was made to randomize the order in which the lower limbs 
 were tested .
prior to the experimental procedure , assessors were trained regarding their 
 behavior and to be aware of the problems that could occur .
the slvj test was used to assess 
 the lower - limb performance capacity . for the slvj
, it was recommended that the subjects 
 stand on one leg and be unsupported at take off .
the subjects then jumped as quickly and as 
 high as possible and landed on the same extremity .
the subjects kept their hands on the 
 pelvis and were informed that they could swing their arms freely or use a selected 
 countermovement without stepping prior to jumping .
the tests were performed with an optogait 
 system ( optogait , microgate srl , bolzano , italy ) , which measured the flying time .
an 
 optogait with high - density photoelectric cells that allows for quantification of 
 spatiotemporal gait parameters on basically all flat surfaces has recently been introduced . 

the optogait system is composed of transmitting and receiving bars that are placed parallel 
 to each other .
when a subject passes between the transmitting bar and receiving bar , the 
 system detects any interruption in light signal due to the presence of feet within the 
 recording area and automatically calculates spatiotemporal parameters .
it has been shown to 
 have high concurrent validity and test - retest reliability22 .
ground contact and ight time during a jump or series of jumps 
 were measured .
the flight time was used to estimate the height of the body s center of 
 gravity during the vertical jump .
the average value of three measurements was used as the 
 test record . the ight time ( tf ) and acceleration due to gravity ( g )
were used to 
 calculate the vertical rise ( h ) of the center of gravity of the 
 body .
performance 
 in the weak limb ( h1 ) and performance in the strong limb ( h2 ) were 
 used to calculate for lsi .
thus , physical performance in the weak limb was expressed as a 
 percentage of the physical performance in the strong limb .
all statistical analyses were performed using the ibm spss statistics version 19.0 
 statistical software ( ibm corp . ,
, the shapiro - wilk test was used to check if the 
 distribution of all parameters was normal .
when data were normally distributed , one - way 
 analysis of variance ( anova ) was used to assess differences in continuous variables among 
 groups .
the paired t - test was used to assess differences in continuous variables within 
 groups before and after interventions for each group .
the slvj heights of the strong lower limb and weak lower limb were significantly different 
 in the uvsg , bvsg , and nvsg ( p<0.05 ) ( table 
 2table 2.differences between the strong and weak lower limbs ( n=30)uvsgbvsgnvsg(n=10)(n=10)(n=10)slvj height ( cm)wll17.8  1.717.9  1.517.7  1.5sll21.7  1.721.4  1.721.7  1.5sll - wll3.9  0.2 * 3.6  0.8 * 4.0  0.2*values were expressed as the mean  standard deviation .
slvj : symmetry of single leg vertical jump ; uvs : unilateral vibratory stimulation 
 group ; bvsg : bilateral vibratory stimulation group ; nvsg : no vibratory stimulation 
 group ; wll : weak lower limb ; sll : strong lower limb ) .
slvj : symmetry of single leg vertical jump ; uvs : unilateral vibratory stimulation 
 group ; bvsg : bilateral vibratory stimulation group ; nvsg : no vibratory stimulation 
 group ; wll : weak lower limb ; sll : strong lower limb the slvj height of the weak lower limb showed significant improvement in the nvsg , bvsg , 
 and nvsg ( p<0.05 ) .
the slvj height of the weak lower limb showed significant improvement 
 in both the uvsg and bvsg compared with the nvsg ( p<0.05 ) .
in addition , significant 
 improvement was observed in the uvsg compared with the bvsg ( p<0.05 ) .
the slvj height of 
 the strong lower limb showed significant improvement in the nvsg , bvsg , and nvsg 
 ( p<0.05 ) .
the slvj height of the strong lower limb showed a significant improvement for 
 the bvsg compared with the uvsg and nvsg ( table 
 3table 3.changes in single leg vertical jump height ( n=30)uvsg ( a)bvsg ( b)nvsg ( c)post hoc(n=10)(n=10)(n=10)wll ( cm)pre17.8  1.717.8  1.517.7  1.5post21.5  1.720.2  1.718.03  1.5post - pre 3.7  0.5 2.3  0.7 0.35  0.2a  b  csll ( cm)pre21.7  1.721.42  1.721.68  1.5post23.0  1.823.70  1.922.11  1.6post - pre 1.2  0.6 2.27  0.5 0.42  0.3b  a , cvalues were expressed as mean  standard deviation .
* there is a significant 
 difference between the pre- and post - intervention values .
uvsg : unilateral vibratory stimulation group ; bvsg : bilateral vibratory stimulation 
 group ; nvsg : no vibratory stimulation group ; wll : weak lower limb ; sll : strong lower 
 limb ) .
* there is a significant 
 difference between the pre- and post - intervention values .
uvsg : unilateral vibratory stimulation group ; bvsg : bilateral vibratory stimulation 
 group ; nvsg : no vibratory stimulation group ; wll : weak lower limb ; sll : strong lower 
 limb slvj symmetry showed significant improvement in the uvsg and bvsg ( p<0.05 ) , but not in 
 the nvsg .
slvj symmetry showed significant improvement in both the uvsg and bvsg compared 
 with the nvsg ( p<0.05 ) .
in addition , significant improvement was observed in the uvsg 
 compared with the bvsg ( p<0.05 ) ( table 
 4table 4.changes in symmetry ( n=30)uvsg ( a)bvsg ( b)nvsg ( c)post hoc(n=10)(n=10)(n=10)lsi ( % ) pre81.91.683.43.581.51.5post93.81.285.12.781.51.7post - pre11.91.9 1.72.50.01.3a  b  cvalues were expressed as the mean  standard deviation .
* there is a significant 
 difference between the pre- and post - intervention values .
lsi : limb symmetry index ; uvsg : unilateral vibratory stimulation group ; bvsg : 
 bilateral vibratory stimulation group ; nvsg : no vibratory stimulation group ) .
* there is a significant 
 difference between the pre- and post - intervention values .
lsi : limb symmetry index ; uvsg : unilateral vibratory stimulation group ; bvsg : 
 bilateral vibratory stimulation group ; nvsg : no vibratory stimulation group
one session of 
 an isometric squat exercise and unilateral vibratory stimulation applied only to the weak 
 side improved slvj height and symmetry .
a variety of functional performance tests have been described , including single leg hop 
 tests for distance or time , hop and stop tests , and vertical jump tests7 . in particular , the slvj is typically used to assess 
 elements of lower limb function and the ability of the lower limb to perform challenging 
 tasks .
previous studies have described the relationships between hop test measurements and 
 physical impairments , such as muscle weakness , passive joint laxity , and knee joint 
 proprioception deficits23 .
the slvj height in both the uvsg and bvsg showed a considerably larger increase than that 
 in the nvsg . with an increase of 21% in slvj height in the weak lower limb ,
the uvsg showed 
 a considerably larger increase than the bvsg , which had an increase of 12% .
although the 
 present study resulted in only small improvements in the average slvj height after the wbv 
 training , the improvement was obvious .
torvinen et al.16 demonstrated an improvement in isometric knee joint extension 
 torque after one 4-min wbv session .
this 
 improved lower limb extension torque is likely because wbv constantly expands muscular 
 capacity25 .
the wbv stimulates the ia afferent tendency of muscle spindles , and continued stimulation 
 of the stretch reflex mechanism activates motor neurons , increasing the sensitivity of 
 primary ending .
small type i muscles are 
 first recruited under the muscle recruitment principle , followed by large type ii muscles 
 thereafter , and the more large type ii muscles are recruited , the better the improvements in 
 movements and muscle strength10 .
these 
 all imply that brief wbv has a significant effect on muscle recruitment required for the 
 slvj . as mentioned above
, the neuron work and muscle enhancement elicited by both concentric and 
 eccentric contractions with the wbv also follows the process of the slvj .
each joint of the 
 lower limbs is flexed for instant extension for a jump , and the stretch - shortening cycle of 
 extended muscles activates the spinal reflex for a burst of concentric contraction , in which 
 the stretch receptors are activated in the eccentric loading phase21 .
this mechanism increases the slvj height such that it is 
 higher than that of the squat jump .
therefore , the concentric and eccentric muscle contractions induced by wbv activates the 
 stretch reflex mechanism and increases the muscular recruitment required for jumping , and 
 the effect of the stretch - shortening cycle elicited by the eccentric contraction during the 
 slvj further increases the jump height .
of course , the nvsg also showed the effect of the 
 stretch - shortening cycle , but it did not improve , seemingly because there was no stretch 
 reflex mechanism , as this group did not receive wbv .
in contrast , based on the results of the present study , vibratory stimulation in the bvsg 
 was transmitted though both lower limbs , not only the weak lower limb .
therefore , a certain 
 amount of vibration was divided between the legs , and the vibratory stimulation transmitted 
 to the weak lower limb in the bvsg might have been less than that in the uvsg .
this meant 
 that the stretch reflex mechanism and muscular recruitment were not sufficiently experienced 
 in the weak lower limb in the bvsg compared with the uvsg . with an slvj height increase of 5% in the strong lower limb ,
the uvsg showed a considerably 
 larger increase than the nvsg but not a considerably larger increase than the bvsg , which 
 showed an increase of 10% . with unilateral vibratory stimulation ,
the strong side failed to 
 experience the stretch - reflex mechanism , as in the case of no vibratory stimulation . 

the aforementioned effects of improved lower limb extension torque of the non - stimulated 
 lower limb could be caused by supporting the body weight against external sway without 
 losing balance .
in addition , the cross - training effect documented in a previous study also 
 enhances muscle strength in the untrained lower limb compared with unilateral resistance 
 training27 . the gain in muscle strength 
 through the cross transfer of resistance training could be caused by modifications in neural 
 control .
neural modifications could be elicited from the cortical areas responsible for 
 excitatory responses of the appropriate cortex area during voluntary contraction28 .
the characteristics of the cross - training 
 effect indicated that it was likely to occur even though the untrained lower limb did not 
 participate directly in the interventions29 .
therefore , improved lower limb extension torque as a result of supporting body weight 
 against external sway in addition to the cross - training effect during unilateral wbv 
 training led to enhanced function of the non - stimulated lower limb during performance of the 
 slvj . with an increase in symmetry of 14% ,
unilateral vibratory stimulation increases the stability of the 
 lumbosacral area and activates the muscles of the lower limb , improving slvj performance 
 compared with the pre - intervention performance .
accordingly , the ability of the weak lower 
 limb improves more than that of the strong lower limb , which did not receive vibratory 
 stimulation .
therefore , this increases the symmetry of the functional performance of both 
 lower limbs .
bilateral vibratory stimulation improves the capacity of not only the weak 
 lower limb but also the strong lower limb at the same time .
therefore , the symmetry of the 
 bvsg did not improve compared with that of the uvsg . in this sense , for enhanced symmetry between the lower limbs , unilateral vibratory 
 stimulation applied to only the weak lower limb could be a good approach .
 [ purpose ] the aim of the present study was to investigate the acute effects of unilateral 
 whole body vibration training on height and symmetry of the single leg vertical jump in 
 healthy men . [ subjects ] thirty males with no history of lower limb dysfunction 
 participated in this study . 
 [ methods ] the participants were randomly allocated to one of 
 three groups : the unilateral vibratory stimulation group ( n=10 ) , bilateral vibratory 
 stimulation group ( n=10 ) , and , no vibratory stimulation group ( n=10 ) . 
 the subjects in the 
 unilateral and bilateral stimulation groups participated in one session of whole body 
 vibration training at 26  hz for 3  min . 
 the no vibratory stimulation group subjects 
 underwent the same training for 3  min without whole body vibration . 
 all participants 
 performed the single leg vertical jump for each lower limb , to account for the strong and 
 weak sides . 
 the single leg vertical jump height and symmetry were measured before and 
 after the intervention . 
 [ results ] the single leg vertical jump height of the weak lower 
 limb significantly improved in the unilateral vibratory stimulation group , but not in the 
 other groups . 
 the single leg vertical jump height of the strong lower limb significantly 
 improved in the bilateral vibratory stimulation group , but not in the other groups . 
 the 
 single leg vertical jump symmetry significantly improved in the unilateral vibratory 
 stimulation group , but not in the other groups . 
 [ conclusion ] therefore , the present study 
 found that the effects of whole body vibration training were different depending on the 
 type of application . to improve the single leg vertical jump height in the weak lower 
 limbs as well as limb symmetry , 
 unilateral vibratory stimulation might be more 
 desirable . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION