Postural Control Evaluation via COP Trajectory

Postural control was evaluated using the path of COP. COP was measured on a force plate (9281E, Kistler, Winterthur) under three conditions: double-leg standing with eyes open (DEO); double-leg standing with eyes closed (DEC); and single-leg standing with eyes open (SEO). The order of each condition tested was chosen randomly. The dominant leg was selected for the SEO task. Leg dominance was determined after performing three trials of three functional tests [23 (link)]. First, subjects were asked to step onto a 40-cm box; the leg used to perform the step-up was identified as the dominant leg. Next, subjects were pushed from behind, and the leg that stepped out was identified as dominant. Then, subjects were asked to kick a soccer ball, and the leg used to kick the ball was recorded as the dominant leg. The leg that was dominant in two out of the three tests was considered the dominant leg for this study.
To attain the double- and single-leg stance position, subjects were instructed to keep their standing leg still, with their arms by their sides. For single-leg standing, they maintained the non-weight-bearing leg in a position of 90 degrees of knee flexion, keeping their thighs vertical. Before the test measurements were conducted, subjects practiced 3–5 trials of each test position. For the test measurements in each position, subjects were asked to stand for as long as possible, up to 30 seconds. The test was stopped when subjects were unable to maintain the requirements of the test position. The standing test measurements were performed once in each test position with a 10-second rest between each position, which were performed in a random order.
To process the data, the output from the force plate was introduced to the computer through an analog-to-digital converter (PH-770, DKH, Tokyo) at a sampling frequency of 1 kHz., and the COP trajectory was analyzed for 20 seconds, excluding the first 5 and last 5 seconds during double- and single-leg standing tasks. Fig 2 shows typical examples of the COP trajectory before and after the exercises. The following variables were used to describe the movement of the COP that were analyzed by TRIAS software (DKH Corp., Tokyo): total length (TL); mean velocity (MV = TL / total time); sway area (SA); maximum range of anteroposterior sway (AP range); maximum range of mediolateral sway (ML range); mean velocity of anteroposterior sway (AP velocity = AP length / total time); and mean velocity of mediolateral sway (ML velocity = ML length / total time). TL and SA were calculated with the following equation [24 (link)]:
$\mathrm{TL}={\displaystyle \sum _{n=1}^{N-1}\sqrt{{(A{P}_{n+1}-A{P}_n)}^2+{(M{L}_{n+1}-M{L}_n)}^2}}$
$\mathrm{SA}=\frac{1}{2}{\displaystyle \sum _{n=1}^N\left|A{P}_{n+1}M{L}_n-A{P}_{n}M{L}_{n+1}\right|}$
where N is the number of data points included in the analysis (20000 points) and n is the COP time series. The COP (AP and ML) time series were passed through a Butterworth low-pass digital filter with a 6 Hz cut-off frequency. TL is the total length of the COP trajectory; i.e., the sum of the distance between consecutive points of the COP trajectory. SA estimated by the area of a convex hull; the sum of the triangulation formed by two points on the COP trajectory necessary for calculating the convex hull. AP and ML ranges were the distance between the anterior and posterior peak displacements and the distance between the medial and lateral peak displacements.