In this cross-sectional analysis of balance measures in a population-based cohort of older adults, we found that within the measures of COP-based static balance, the area of ellipse sway was much more strongly correlated with the ML than with AP measure of balance while sway speed was more strongly correlated with AP than with ML measures. Moreover, these static balance measures were weakly correlated with the clinic-based balance measures, while the clinic-based balance tests were strongly correlated with each other, including the one-leg stand. Correlations were higher in men than in women when comparing sway speed or AP sway with ML, ML RMS, area of ellipse, and clinic-based measures of balance, even after accounting for differences in height.
Some of the correlation coefficients among balance measures were modest (range: 0.221 to 0.997 among COP-based measures) though all of them were statistically significant, possibly owing to the large sample size of the study population. Such modest correlation coefficients between COP-based static balance and clinical balance measures suggest that static posture and clinic-based balance instruments may capture different aspects of balance. For example, postural sway may be more sensitive to sensorimotor function or impairment than clinical measures of balance (35
). Furthermore, Frykberg et al. (12
) found poor correlations between total Berg Balance Scale with the quiet stand COP-based force plate measures in a group of twenty subjects (mean age 50 years) who had experienced a stroke more than 6 months prior to study participation. Once Frykberg and colleagues separated the Berg Balance Scale into components of “maintaining a position” (including standing or sitting unsupported or standing with eyes closed) and “dynamic balance” (including transfers or picking up objects) the correlation coefficient between the “maintaining a position” and the average AP sway speed increased to 0.5. They reported that the “maintaining a position” component of Berg Balance Scale would better mirror the static balance measures of the force plate. Yet, the SPPB, which should be similar to the “maintaining a position” component of the Berg Balance Scale, did not show a higher correlation with the COP-based static balance measures in our study. These results imply that the lab- and clinic-based measures may possibly measure different aspects of balance, that they may complement each other, or that one may be a poorer measure of balance than the other. It is also possible that one or the other might not truly be measuring balance. Further investigation of this matter appears to be warranted.
However, it is also possible that standing barefoot for static balance measures versus wearing shoes for the clinical balance measures may explain some of the differences between these measures. Although bare feet might allow greater sensory feedback, most researchers believe that wearing shoes improves balance (36
) compared to bare feet; others believe it is the type of shoes that may improve balance (37
). We are uncertain as to whether going barefoot or wearing shoes can entirely explain the lower correlation between COP-based static and clinical balance measures.
The high correlations among the various clinic-based balance tests were not unexpected. First, two of the three SPPB balance items (side-by-side and tandem balance) were also part of the 14-item Berg Balance Scale (items 1 and 2). Also, Berg Balance Scale was highly correlated with the timed one-leg stand possibly because the one-leg stand was one of the most challenging tasks among all the Berg Balance Scale items (9
). It is entirely plausible that a participant’s ability to successfully complete the one-leg stand would enable a better overall BBS score.
It is unclear, however, why men had higher associations than women in our study when comparing AP or sway speed with ML and area of ellipse sways, as well as when comparing each of the clinical measures of balance with sway speed or ellipse sway, even after accounting for differences in height. While Bryant and colleagues (15
) found no statistically significant differences between men and women in their average COP-based ML and AP sways when the participants’ eyes were opened (whether or not the data were normalized by height) we found statistically significant differences between men and women in their COP-based AP and ML sways with eyes opened, even after taking height into account (data not shown). There are several differences between our study and that from Bryant et al. For each of our COP-based measure of balance, we used the average of 5 trials while Bryant used the average of 3 trials. Moreover, Bryant’s study included 44 men and 53 women while our study included 276 men and 489 women. Thus, our study may have more stable estimates and better power to detect significant differences between men and women for some of these measures. Nevertheless, there may not be any biologic mechanism underlying the observed differences between men and women in the correlations between AP or sway speed with ML and area of ellipse sways, or between each of the clinical measures of balance with sway speed or ellipse sway.
The MOBILIZE Boston population was a group with good balance on average compared to a hospital- or nursing home-based group of older adults. Thus it is possible that differences in variability or strength of associations between COP-based static balance and clinical balance measures could be observed in populations with clinical conditions. It is unclear what other aspects of balance could explain the significant but moderate correlations between the static and clinical balance measures, and possible sex differences in the correlations between sway speed or AP sway with many of the other measures of balance, as normalizing by height did not change the results. Future research should explore explanations for these differences.
Our study has several limitations. First, we chose the 5 COP-based sway measures because they have been reported to be highly correlated with falls in older adults (19
). It is possible that results would have been different had we included other more comprehensive static balance measures. Nevertheless, as indicated in a systematic review by Ruhe et al.(26
), no single COP measure is more reliable than the others, but any sway balance measures should include both a parameter of distance (e.g., area of ellipse) and time-distance (e.g., velocity or sway speed), as we did in our study. In addition, our measures of COP-based sway did not account for differences in base support or width of feet as these data were not collected; however, all participants were instructed to stand with legs approximately hip width apart so there were no extremes of the base of support. It is possible that not standing with feet together may account for the higher correlation between mediolateral laboratory measures, as the base of support affects ML sway differently from AP sway (39
). Finally, our study did not formally address measures of possible reduction in burden to participants regarding time and effort in using the SPPB or the one-leg stand as a sole clinical balance measure instead of the Berg Balance Scale or the COP-based measures. The SPPB or the one-leg stand may be preferable to many clinicians given that the one-leg stand involves a single task, and the balance component of the SPPB involves 3 items to complete compared to the 14 items in the Berg Balance Scale; however, it is unclear whether it can reliably measure balance in relation to an outcome. Reducing patient burden by limiting the number of tasks in a balance study is appealing in terms of time, cost and retention of study participants. Clinical measures are more complex and integrated measures of balance than laboratory-based measures (which are often used to investigate mechanisms). We view our results as in agreement with choosing a clinic-based measures of balance for our purposes in future epidemiologic studies. Nonetheless, it is up to researchers to choose the appropriate clinic-based balance measure that will be sufficient for their study needs. Future studies could compare the validity and reliability of using as few measures of balance as possible to reduce burden on elderly participants, in particular examining whether the one-leg stand can be the sole clinical measure of balance in relation to a specific outcome.
Our study concurrently examined several comprehensive classes of balance measures using validated instruments, providing a unique opportunity to examine the associations between clinical and laboratory-bases balance measures. Results suggest that there may be sex differences when comparing sway speed or AP sway with measures of sway in the ML direction and area of ellipse, and with each of the clinic-based measures of balance. Moreover, our study participants were from a population-based sample of community-dwelling older men and women. Results that are generalizable to healthy older adults aging in place may be more useful in the creation of a “gold standard” than findings from studies conducted in institutional settings.
The study results show strong agreements among clinic-based balance measures (Berg, SPPB and one-leg stand) and among lab-based balance measures (AP with sway speed, and ML with area of ellipse sway), respectively. However, agreements between clinic and lab-based measures were modest, suggesting these two types of measures may capture different aspects of balance and likely complement each other. Since there exist neither consensus nor any guidance on how to choose these measures for research, further investigation on the relationships among these measures seems warranted.