poor postural control during ambulation or activities of daily living ( adl ) is a common 
 disability after stroke1 . asymmetric 
 weight bearing in a standing position has been shown to have a negative relationship with 
 independence in performance of adl2 . a 
 previous study demonstrated that humans required three elements in order to control 
 functional standing balance : maintenance of the standing position in static circumstances , 
 maintenance of standing while experiencing internally produced perturbations associated with 
 movements of their extremities , and maintenance of standing while experiencing externally 
 produced perturbations3 . postural control 
 is essential for independence in performance of ambulation and adl in poststroke patients ; 
 therefore , accurate evaluation of the postural control system is important4 . in general , traditional balance measures , such as the berg balance scale5 , timed up and go test6 , and functional reaching test7 , are most commonly used as assessment tools for evaluation of 
 postural control ability in stroke patients . however , these tools can not be regarded as 
 quantitative and objective methods of measurement8 . recently , in the clinic or laboratory , computerized measurement and 
 feedback systems have been used for evaluation of both static and dynamic balance control 
 abilities9 , 10 . among the various types of equipment used for measurement of 
 postural control , force platform technology can provide a method for quantifying an 
 individual s postural control ability by measuring the center of pressure11 . in particular , the good balance 
 system consists of an equilateral triangular force platform 
 connected to a computer through a three - channel amplifier with an analog to digital ( a / d ) 
 converter . the following variables are calculated in the good balance 
 system : the extent of mediolateral ( ml ) movement of the center 
 of pressure ( cop ) ( x movement ) , the extent of anteroposterior ( ap ) movement of the cop ( y 
 movement ) , and the mean values for all of the measurement points in relation to the midline 
 of the platform ( lateral displacement ) . despite use of the good balance system in many studies as 
 equipment for measurement of postural control ability10 , 12 , 13 , there is no evidence regarding the reliability of the good balance 
 system in stroke patients . the reliability of measurement using 
 the good balance system need to be determined before it can be used 
 as a potential predictor of functional performance . thus , the purpose of this study was to 
 examine the test - retest reliability of the good balance system for 
 measurement of postural sway in poststroke patients . we hypothesized that postural sway 
 while maintaining postural control in a standing posture has adequate reliability for 
 research and clinical use in stroke patients . this single - group repeated - measures design involved a baseline measurement session and a 
 follow - up session one week later . subjects were included if they had ( 1 ) hemiparesis 
 resulting from stroke for more than six months ; ( 2 ) were able to understand and follow 
 simple verbal instructions ( korean version of the mini - mental state examination score > 
 21 ) ; ( 3 ) had no known musculoskeletal conditions that would affect the ability to stand 
 safely ; and ( 4 ) had no serious visual impairment or hearing disorder . potential subjects 
 were excluded if they had cardiovascular or other conditions affecting their balance and 
 were unable to provide informed consent . all subjects underwent conventional rehabilitation 
 programs consisting of physical and occupational therapy during the duration of 
 hospitalization . six of the 66 potential subjects were excluded because they met the exclusion 
 criteria . we explained the objective and 
 requirements of our study to all participants , and they voluntarily signed informed consent 
 forms . ethical approval for the study was granted by the sahmyook university institutional 
 review board . the good balance system ( good balance system , metitur oy , 
 jyvaskyla , finland ) was used for measurement of postural sway velocity and velocity moment 
 of subjects in the standing posture . on the basis of these coordinate values for x and y , the following parameters were calculated : ( 1 ) mean speed of the movement of the cop in the 
 ap direction ( mm / s ) ; ( 2 ) mean speed of the movement of the cop in the ml direction ( mm / s ) , 
 and ( 3 ) mean velocity moment ( mm / s ) . postural sway velocity moment is defined as 
 the average horizontal area covered by movement of the center ( ap and ml direction ) of force 
 per second . the force platform , which was an equilateral triangle ( 800 mm ) , was connected to 
 a three - channel dc amplifier . signals from the amplifier were converted into digital form 
 using a 12-byte converter ( sampling frequency=50 hz ) and stored on the hard disk of a 
 personal computer . to measure postural sway , a subject stood on the force plate with their legs spread at 
 shoulder width and then looked at a number on a monitor for 30 seconds . in order to measure 
 postural sway , the subjects were asked to stand quietly in a comfortable upright position on 
 the force plate while looking straight ahead . all tests were performed with shoes removed . 
 according to the user manual , a distance of between 1 to 3  m is suitable for accurate 
 measurements . in our study , the distance between the subject and the monitor was set 1.5  m. 
 the following instruction was communicated to the subjects in order to ask them to move 
 their bodies as little as possible : two physical therapists participated as examiners in the reliability analysis based on 
 previous evidence14 . examiners acquired 
 data from all participants during two separate testing sessions ( sessions one and two ) . three repeats of each measurement sessions were performed , and the average was used in each 
 session . a rest period ( three minutes ) was provided between measurements in order to prevent 
 fatigue . after the first measurement was performed , the second measurement was performed one 
 week later . intraclass correlation coefficients [ icc(2,1 ) ] were used to determine the 
 test - retest reliability for postural sway between the two measurement sessions . icc values 
 less than or equal to 0.20 were considered poor ; 0.21 to 0.40 , 0.41 to 0.60 , 0.61 to 0.80 , 
 and 0.81 to 1.00 were considered to be fair , moderate , good , and very good values , 
 respectively15 . the paired t - test was 
 performed in order to test for any systematic differences between sessions . results were 
 considered significant at p<0.05 , and statistical analyses were performed using spss ver . 12.0 ( spss for windows ; spss inc , chicago , il , usa ) . a summary of the general characteristics of the subjects who completed the measurements 
 successfully is shown in table 1table 1 . general characteristics of the subjectsvariablesmale ( n=40)female ( n=20)overall ( n=60)paretic side left / right ( % ) 11/29 ( 27.5/72.5)5/15 ( 25/75)16/44 ( 26.2/72.1)etiology infarction / hemorrhage ( % ) 24/16 ( 60/40)13/7 ( 65/35)37/23 ( 60.7/37.7)age ( years)63.16.263.06.763.16.3height ( cm)165.36.8157.44.2162.77.1weight ( kg)62.38.056.35.660.37.8onset duration ( months)16.63.016.32.416.42.8bbs ( scores)38.84.839.33.538.94.4tug ( sec)24.13.824.13.724.13.7mmse - k ( scores)25.72.826.12.825.92.8values are expressed as n ( % ) or as the meansd . bbs , berg balance scale ; tug , timed 
 up and go test ; mmse - k , mini mental state examination - korean version . a summary of the results for intra - examiner reliability of postural sway for 
 the two sessions is shown for the two examiners in table 2table 2 . intra - examiner reliability of postural sway ( n=60)variablessession 1session 2icc95% cie1velocity ( mm / s)ap8.54.48.04.60.870.790.92ml13.14.512.74.90.690.530.80velocity moment ( mm)36.821.037.121.00.870.790.92e2velocity ( mm / s)ap8.84.18.44.10.930.890.96ml13.34.113.04.50.780.660.86velocity moment ( mm)37.520.236.519.20.930.890.95values are expressed as the meansd . e , examiner ; ap , anteroposterior ; ml , 
 mediolateral ; icc , intraclass correlation coefficient ; 95% ci , 95% confidence 
 interval . the icc ( 95% ci ) for all measures was good to very good , ranging from 0.69 to 
 0.93 ( 0.530.96 ) . a summary of the results for inter - examiner reliability of postural sway 
 for the two sessions between examiners is shown in table 3table 3 . inter - examiner reliability of postural sway ( n=60)variables1st test2nd testicc95% ciicc95% civelocity ( mm / s)ap0.970.950.980.920.880.95ml0.920.870.950.850.770.91velocity moment ( mm)0.960.940.980.960.930.97ap , anteroposterior ; ml , mediolateral ; icc , intraclass correlation coefficient ; 95% 
 ci , 95% confidence interval . the icc ( 95% ci ) was good to very good for all measures , ranging from 0.85 to 
 0.98 ( 0.770.99 ) . bbs , berg balance scale ; tug , timed 
 up and go test ; mmse - k , mini mental state examination - korean version values are expressed as the meansd . e , examiner ; ap , anteroposterior ; ml , 
 mediolateral ; icc , intraclass correlation coefficient ; 95% ci , 95% confidence 
 interval ap , anteroposterior ; ml , mediolateral ; icc , intraclass correlation coefficient ; 95% 
 ci , 95% confidence interval this study was conducted in order to determine the suitability of the good balance 
 system for measurement of postural sway in poststroke patients . our main results demonstrate good to very good inter- and intra - reliability of the good 
 balance system for estimating a stroke patient s postural sway 
 measurement . postural sway measurements using a force plate record vertical strength and can be used to 
 explain postural control ability using parameters such as postural sway velocity and 
 velocity moment16 . the amount of postural 
 sway is usually divided into the anteroposterior direction and the mediolateral 
 direction17 . in general , postural sway 
 is known to increase with age , and the frequency of falls increases as sway increases18 . in addition , postural sway is 11.5 times 
 higher in stroke patients than in elderly people19 . the ability to maintain body balance against externally produced 
 perturbations is achieved by the postural control system . thus , impairment in this system 
 could lead to increased postural sway as observed in stroke patients20 . in other words , accurate evaluation for postural sway in 
 stroke patients is important for successful rehabilitation and return to the home and 
 community . many previous studies targeting elderly people have reported good reliability of postural 
 sway measured using a force platform . for example , lafond et al.21 assessed the intra - session reliability of cop measurement 
 of postural steadiness in elderly . in addition , benvenuti et al.22 found good reliability ( icc=0.74 ) based on two consecutive trials 
 for cop mean velocity . however , the reliability of postural sway outcome in stroke patients 
 has been addressed in only a few studies , and the results were not consistent . liston et 
 al.4 investigated reliability of 
 measures obtained from 20 stroke patients using the balance master and reported its 
 reliability for both movement path ( icc=0.84 ) and time ( icc=0.88 ) . in contrast , helbostad et 
 al.23 reported that two repeated 
 measures gave adequate repeatability for elderly people but not for stroke patients when 
 they were tested with their eyes open . because these findings were obtained from relatively 
 small sample size ( 12 and 23 stroke patients ) , the results of these studies are not 
 unequivocal . therefore , in the current study , we investigated the reliability in a larger 
 sample size ( 60 stroke patients ) , and the results demonstrated good to very good intra- and 
 inter - examiner reliability of the good balance system for 
 measurement of postural sway . therefore , we suggest that measurement of postural sway using 
 the good balance system would be useful for clinical assessment in 
 poststroke patients . the current study showed that the measurements of postural sway obtained from the good 
 balance system were reliable in the intra- and inter - reliability 
 analysis , but there are some limitations . in the current study , we determined that the good 
 balance system was reliable equipment for measurement of postural 
 sway . however , the findings of previous studies targeting stroke patients were not 
 consistent ; therefore , there is still the potential for controversy . thus , we suggest that 
 further study on the reliability of postural sway measurements using a force plate targeting 
 stroke patients is needed . another limitation of the current study is the small number of 
 measurements for each session ( only three measurements ) . conduct of a larger number of 
 trials might contribute to the reliability of postural sway measurement .
[ purpose ] the purpose of this study was to examine test - retest reliability of the good 
 balance system for measurement of postural sway in poststroke 
 patients . 
 [ subjects ] sixty chronic stroke patients ( 40 men and 20 women ; age 63.08  years ; 
 stroke duration 16.45 months ) participated in this study . 
 [ methods ] postural sway was 
 evaluated using a force platform system ( good balance system , metitur oy , jyvaskyla , 
 finland ) . 
 two examiners measured postural sway for all participants during two separate 
 testing sessions . 
 the second measurement was performed one week after the first 
 measurement . 
 intraclass correlation coefficients [ icc(2,1 ) ] were used for 
 estimation of reliability . [ results ] 
 the icc ( 95% ci ) for intra - examiner reliability was 
 good to very good , ranging from 0.69 to 0.93 ( 0.530.96 ) , and the icc for inter - examiner 
 reliability was good to very good , ranging from 0.85 to 0.98 ( 0.770.99 ) . 
 [ conclusion ] the 
 results of the current study indicated that the intra- and inter - examiner reliability of 
 the good balance system for measurement of postural sway was good 
 to very good . 
 therefore , we suggest that measurement of postural sway using the good 
 balance system would be useful for clinical assessment in 
 poststroke patients .