Representative sample of postural acceleration in the antero-posterior direction during standing on level surface with eyes open. The root-mean-square (RMS) is the standard deviation of the points, referenced to the mean value of the entire acceleration curve. The normalized path length (NPL) is the sum of the absolute value of the individual path length distances (as shown in the inset figure, PL1, PL2, …, PL6), divided by the length of time that it takes to travel this distance. This is performed for the entire length of time for the trial.