Relationship Between Linear and Nonlinear Measures of Postural Stability Measured Using a Nintendo Wii Balance Board
Dr. Douglas Powell
Introduction
Balance deficits are a cardinal symptom of many neurological injuries and pathologies. Thus, many studies have investigated the mechanisms underlying reduced postural stability in these injuries and conditions. Emerging literature has suggested that nonlinear dynamics may be more sensitive than traditional balance measures in detecting changes in the motor control underlying balance performance. The relationship between traditional and nonlinear measures of balance performance has not been well addressed in healthy adults. Therefore, the purpose of this study was to quantify the relationship between traditional and nonlinear measures of postural stability. It was hypothesized that a moderate to strong relationship between traditional and nonlinear measures of postural stability would be observed.
Methods
Thirty healthy young adults between the ages of 18 and 30 years participated in this study. All participants were free of neuro-musculoskeletal conditions that would negatively affect postural stability. Each participant performed five 30-second quiet standing trials while their balance was measured using a custom Nintendo Wii Balance Board (100 Hz, CSMi Solutions, Stoughton, MA). Center of pressure (CoP) data were filtered using a zero-lag fourth order Butterworth filter with a 20 Hz cut-off frequency. Traditional measures of postural stability included the root mean square distance (RMS) of the CoP trajectory in the mediolateral (ML) and anteroposterior (AP directions), and the resultant distance (RD). Traditional measures were calculated as described by Prietto (1996). Approximate entropy (ApEn) was calculated for the ML and AP CoP time-series, independently, as described by Pincus (1991). A correlation analysis was conducted to quantify the relationship between traditional measures of balance and ApEn values in the ML and AP series.
Results
The correlation analysis revealed moderate and strong negative relationships between RMS values and ApEn values in the AP (r = -0.593) and ML directions (r = -0.714), respectively. The RD had moderate and low correlations with ApEn values in the AP (r = -0.326) and ML directions (r = -0.484), respectively.
Discussion
The negative correlations demonstrate that larger sway magnitudes are associated with diminished sway variability. Traditional measures of postural stability suggest that large sway magnitudes are associated with reduced balance control. Further, nonlinear measures of postural stability suggest that individuals with reduced variability, as indicate by low ApEn values, exhibit diminished balance control. Therefore, these data suggest that traditional and nonlinear measures agree in the identification of individuals with balance deficits.
