The calculated ICC values for the 6 balance measures across the 3 testing sessions are presented in Table 3 for men and women separately. Irrespective of balance measure, ICCs were ≥0.75 in women indicating an excellent intra and intersession reliability. In men, all but one parameter (CoP displacements in the mediolateral direction) showed ICC values ≥0.75 resulting in an excellent intra and intersession reliability. An ICC value of 0.59 was found for CoP displacements in mediolateral direction indicating a fair to good intersession reliability. In men and in women, CoP displacements in the anterior-posterior directions was the most reliable balance measure (ICC values between 0.84 and 0.97).
Bland-Altman plots for total CoP displacements obtained for intra and intersession comparisons are shown in Figure 3. In men, the charts illustrate that only 1/17 (5.9%) and 2/17 (11.8%) of the data points were beyond the mean ± 2SD lines for intra and intersession comparisons, respectively. In women, 1/22 (4.5%) of the data points was beyond the limits in both, intra and intersession comparisons. For the other balance measures, the numbers and percentages of data points, which were beyond the mean ± 2SD lines, were in the same range (data not shown).
This is the first study that examines intra and intersession reliability during 1-leg standing in young healthy adults using a computerized balance platform. The main finding was that irrespective of gender, nearly all investigated balance parameters showed an excellent intra and intersession reliability, which was qualitatively confirmed by the Bland-Altman plots. This finding varied from those that have been previously published (11,13,19,30). For example, fair to good intrasession reliability was reported using 1-leg (19) and 2-leg (11) standing as balance task, respectively. Differences in results between these 2 studies and our study could be because of the diverse testing duration applied in the studies. We acquired data for 30 seconds, whereas Goldie et al. (19) tested for 5 seconds and Doyle et al. (11) for 10 seconds only. Le Clair and Riach (24) reported that the longer the test duration, the better the reliability of the measure with a minimum of 30 seconds. In their study, ICC values for most of the investigated balance measures increased from poor for a 15-second trial to fair to good for a 30-second trial. This difference may be attributed to the proportion of the transient component of the CoP signal, which could be prevented by focusing on a sampling duration of <20 seconds (4). Furthermore, the contribution of the low-frequency components of the CoP displacements could be detected with longer but not with shorter sampling durations (4). In another study applying timed 1-leg balance tests, intersession reliability was reported to be also fair to good (13). However, tests were conducted either in eyes closed conditions on a firm and foam surface or on a foam surface with eyes open. This testing setup, in which the visual and proprioceptive input was manipulated, may have influenced intrasubject variation, which could have resulted in reduced ICC values. Furthermore, Santos et al. (30) reported fair to good intersession reliability for CoP speed during 2-leg standing. A rather small sample size (n = 12) was applied in their study for comparison, which may have caused bias because of insufficient statistical power.
Gender-specific balance performance was frequently reported in the literature with women showing better results than men did (12,16). In our study, gender differences in test-retest reliability were found during intersession comparison. More specifically, the observed ICC value for the displacements of the CoP in the mediolateral direction was excellent in women but fair to good in men. This finding can be interpreted as preliminary because it was observed in only one balance parameter. Further research using different standing or sensory conditions should reveal whether subjects' gender has an impact on intersession reliability during 1-leg standing.
From a methodological point of view, there is a controversy regarding the number of testing trials. In some studies, it is recommended to use the mean of ≥7 trials (9,23,30), whereas others conclude that 3 trials are sufficient (2,11,27). Furthermore, the applied test duration and sampling frequency vary between studies ranging from 10 to 120 seconds (4,11,23,24) and from 10 Hz to 100 Hz but not higher (2,9-11,28-30). Our finding of an excellent intra and intersession reliability was based on the mean of 3 consecutive trials, each acquired with a sampling duration of 30 seconds, which was in line with some of the above-cited studies. However, contrary to these studies, we used a much higher sampling frequency (400 Hz). Therefore, it is postulated that the level of sampling frequency could have an impact on test-retest reliability. Further research should reveal whether sampling frequency has an effect on intersession reliability of balance measures.
Another major controversial issue comprises the analysis of balance parameters. A large variety of balance measures was used in studies assessing test-retest reliability of static postural control with traditional variables such as length, range, speed, or area of the CoP being most often applied (2,11,23,30). Our finding of an excellent intra and intersession reliability was found during 1-leg standing using traditional balance measures such as total displacements of the CoP, the CoP displacements in mediolateral and anterior-posterior directions, and the CoP speed and area. Overall, care is needed, when generalizing our findings to different sample durations and frequencies, number of trials, and balance tasks or parameters. In addition, our results were obtained with healthy young adults. Therefore, assessment of intra and intersession reliability during 1-leg standing might vary significantly in populations not considered in this study (e.g., athletes, seniors, parkinsonians). In other words, care must be taken when transferring the present findings to other age groups or populations. Furthermore, there was a decrease in the geometric mean values from session 1 to sessions 2 (4-20%) and 3 (3-11%) depending on the respective balance measure considered, which could be because of a possible learning effect or could represent a random or systematic error. Yet, we used the mean of 3 consecutive trials to calculate each balance measure, which is recommended in the literature (27) as a sufficient number to obtain optimal results.
This study determined that frequently used balance parameters, such as total displacements of the CoP, the CoP displacements in mediolateral and anterior-posterior directions, and the CoP speed/area when measured during 1-leg standing, have an excellent intra and intersession reliability in healthy young adults. In this context, CoP displacements in anterior-posterior directions (i.e., forward/backward sways) proved to be the most reliable balance measure. Testings included only 3 trials over 30 seconds each, which were acquired with a high sampling rate (400 Hz). Consequently, if (a) a coach is attempting to track the progression of balance ability across season, (b) a clinician is attempting to assess lateral ankle trauma rehabilitation, or (c) a teacher is attempting to document BT effects during physical education, the 1-legged stance seems to be appropriate when using the presented test setup. Given the high validity when comparing testing equipments (i.e., computerized balance platforms) from a different manufacturer for the evaluation of balance performance (5,29), it is suggested that practitioners can use these data to identify the range in which the true value of a subject's score lies, irrespective of the computerized balance platform applied. Furthermore, researchers and clinicians can calculate a priori sample sizes for studies assessing balance ability during 1-leg standing recorded from a computerized balance platform (21). In addition, the test-retest reliability was examined using different time intervals (30 minutes and 1 week) that can be used by practitioners, researchers, and clinicians to document acute (i.e., within the same training session) and longer lasting (i.e., between training sessions) changes in balance performance in a reliable way.
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