Summary measures of CoP trajectory, consisting of 95% confidence ellipse area (EA), root mean squared CoP excursion (RMS), and mean CoP velocity (MVEL), were selected to characterize patients’ postural stability. Detailed descriptions of the calculations for these balance parameters are provided elsewhere [23]. Briefly, EA provides a measure of spatial control of the CoP where it DC_AC50 defines the area of an ellipse that contains approximately 95% of the data. A larger EA indicates greater dispersion of the CoP and a less tightly controlled CoP position. RMS characterizes the root mean squared displacement of the CoP from its mean position. A higher RMS suggests that, on average, the CoP is an increased distance away from the mean position and towards a boundary of the base of support [23] and [26], which is style indicative of a decrease in postural stability. MVEL is the total distance traveled of the CoP divided by the duration of the trial. MVEL provides insight into the extent of corrective CoP actions taken during a balance task, where an increased MVEL indicates less appropriate CoP adjustments (i.e., overcorrecting) were made to maintain stability. EA, RMS, and MVEL have previously been used to distinguish elderly fallers from non-fallers [24] and [25]. These balance parameters were calculated for the 2-dimensional (planar) case as well as RMS and MVEL also being calculated for each 1-dimensional case (medial-lateral and anterior-posterior) to gain insight into whether changes were preferential to a given direction.
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