Table 1.

Center-of-Pressure Calculations.

COPvel: The mean value of the instantaneous velocity of the COP in a given direction during a given time period	$\begin{array}{l}\text{A/P COPvel}=\frac{{\displaystyle \sum _{\text{t}=1}^{\text{T}}\left|\frac{{\text{x}}_{\text{cop,t}}-{\text{x}}_{\text{cop,t}-1}}{\Delta \text{t}}\right|}}{\text{T}-1}\hfill \\ \text{M/L COPvel}=\frac{{\displaystyle \sum _{\text{t}=1}^{\text{T}}\left|\frac{{\text{y}}_{\text{cop,t}}-{\text{y}}_{\text{cop,t}-1}}{\Delta \text{t}}\right|}}{\text{T}-1}\hfill \end{array}$
M/L COPsd: Overall standard deviation of sway in the M/L direction in a given time period for a given number of trials	$\begin{array}{c}\text{M/L COPsd}=\sqrt{\frac{{\displaystyle \sum _{\text{n}=1}^{\text{N}}{\displaystyle \sum _{\text{t}=0}^{\text{T}}{\text{Sway}}_{\text{M}/\text{L,t,n}}^2}}-\frac{{\left({\displaystyle \sum _{\text{n}=1}^{\text{N}}{\displaystyle \sum _{\text{t}=0}^{\text{T}}{\text{Sway}}_{\text{M/L,t,n}}}}\right)}^2}{\text{N}(\text{T}-1)}}{\text{NT}-1}}\\ \ast {\text{Sway}}_{\text{M/L}}=\frac{{\displaystyle \sum _{\text{t}=0}^{\text{T}}\left|{\text{COP}}_{\text{M/L,t}}-{\text{COP}}_{\text{M/L,mean}}\right|}}{\text{T}}\end{array}$
COParea: An area defined by the maximum (max) anterior (ant), posterior (post), medial (med), and lateral (lat) sways during a given time period.	$\begin{array}{c}\text{COParea}=\frac{({\text{Sway}}_{\text{max,ant}}+{\text{Sway}}_{\text{max,post}})\times ({\text{Sway}}_{\text{max,med}}+{\text{Sway}}_{\text{max,lat}})}{\text{T}}\\ \ast {\text{Sway}}_{\text{max,direction}}=\frac{{\displaystyle \sum _{\text{t}=0}^{\text{T}}\left|{\text{COP}}_{\text{direction,t}}-{\text{COP}}_{\text{direction,mean}}\right|}}{\text{T}}\end{array}$
M/L COPmax: Maximum distance between the instantaneous COP position and the average COP position during a given time period.	$\text{M/L COPmax}=\frac{{\displaystyle \sum _{\text{t}=0}^{\text{T}}\left|{\text{COP}}_{\text{max,M/L}}-{\text{COP}}_{\text{M/L,mean}}\right|}}{\text{T}}$

Calculations for the following postural stability measures are presented in Table 1: Anterior/Posterior Center-of-Pressure Velocity (A/P COPvel); Medial/Lateral Center-of-Pressure Velocity (M/L COPvel); Medial/Lateral Center-of-Pressure Standard Deviation (M/L COPsd); Center-of-Pressure Area (COParea); and Medial/Lateral Center-of-Pressure Maximum Excursion (M/L COPmax). t = A given time point; T = Number of data points per trial; N = Number of trials