Complexity-Based Measures Inform Effects of Tai Chi Training on Standing Postural Control: Cross-Sectional and Randomized Trial Studies

Abstract

Background: Diminished control of standing balance, traditionally indicated by greater postural sway magnitude and speed, is associated with falls in older adults. Tai Chi (TC) is a multisystem intervention that reduces fall risk, yet its impact on sway measures vary considerably. We hypothesized that TC improves the integrated function of multiple control systems influencing balance, quantifiable by the multi-scale "complexity" of postural sway fluctuations.

Objectives: To evaluate both traditional and complexity-based measures of sway to characterize the short- and potential long-term effects of TC training on postural control and the relationships between sway measures and physical function in healthy older adults.

Methods: A cross-sectional comparison of standing postural sway in healthy TC-naïve and TC-expert (24.5±12 yrs experience) adults. TC-naïve participants then completed a 6-month, two-arm, wait-list randomized clinical trial of TC training. Postural sway was assessed before and after the training during standing on a force-plate with eyes-open (EO) and eyes-closed (EC). Anterior-posterior (AP) and medio-lateral (ML) sway speed, magnitude, and complexity (quantified by multiscale entropy) were calculated. Single-legged standing time and Timed-Up-and-Go tests characterized physical function.

Results: At baseline, compared to TC-naïve adults (n = 60, age 64.5±7.5 yrs), TC-experts (n = 27, age 62.8±7.5 yrs) exhibited greater complexity of sway in the AP EC (P = 0.023), ML EO (P<0.001), and ML EC (P<0.001) conditions. Traditional measures of sway speed and magnitude were not significantly lower among TC-experts. Intention-to-treat analyses indicated no significant effects of short-term TC training; however, increases in AP EC and ML EC complexity amongst those randomized to TC were positively correlated with practice hours (P = 0.044, P = 0.018). Long- and short-term TC training were positively associated with physical function.

Conclusion: Multiscale entropy offers a complementary approach to traditional COP measures for characterizing sway during quiet standing, and may be more sensitive to the effects of TC in healthy adults.

Trial registration: ClinicalTrials.gov NCT01340365.

Figure 1. Flow of participants in randomization trial of healthy Tai Chi naïve adults.

Figure 2. Schematic illustration of the course-graining procedure for multiscale entropy.

x_i represents the original time-series samples. y_j represents the coarse-grained samples at the indicated scale. Adapted from Costa et al. 2002 .

Figure 3. Complexity-based and traditional measures of sway for age-matched Tai Chi naïve and Tai Chi expert older adults.

Mean value and standard errors for sway measured in the anterioposterior (AP) and mediolateral (ML) planes and during eyes-open (EO) and eyes-closed (EC) conditions. Statistical comparisons were adjusted for age.

Figure 4. Associations between changes in both complexity-based (A, B) and traditional measures (C, D) of sway vs. Tai Chi dosage (total hours practiced) among Tai Chi naïve individuals exposed to six months of training.

Percent changes in sway were assessed between the baseline and 6 month follow up visit for the anterioposterior (AP) and mediolateral (ML) planes and during eyes-open (EO) and eyes-closed (EC) conditions. Analyses excluding the largest x-axis value (i.e. potential outlier) reduced ΔAP EC p-value to a non-significant level (p = 0.423), however, the p-value for the ΔML EC was less affected (p = 0.037).

Figure 5. Measures of physical function for age-matched Tai Chi naïve and Tai Chi expert older adults.

Mean value and standard errors for A) Single leg stance time (SLST) and B) timed up and go (TUG). Comparisons were adjusted for age.

Figure 6. Associations between changes in physical function and Tai Chi dosage (total hours practiced) among Tai Chi naïve individuals exposed to six months of training.

Percent changes in function were assessed between the baseline and 6 month follow-up visit for single leg stance time (SLST) and timed up and go (TUG) parameters. Analyses excluding the largest x-axis value (i.e. potential outlier) reduced ΔSLST to a non-significant level (p = 0.286), however, the p-value for the ΔTUG was less affected (p = 0.0062).