The sway-density curve and the underlying postural stabilization process

The sway-density curve (SDC) is computed by counting, for each time instant, the number of consecutive samples of the statokinesigram falling inside a circle of "small" radius R. The authors evaluated the sensitivity of the curve to the variation of R and found that in the range 3-5 mm the sensitivity was low, indicating that SDC is a robust descriptor of posturographic patterns. In addition, they investigated the relationship between SDC and the underlying postural stabilization process by decomposing the total ankle torque into three components: a tonic component (over 69% of the total torque), an elastic torque caused by ankle stiffness (about 19%), and an anticipatory active torque (about 12%). The last component, although the smallest in size, is the most critical for the overall stability of the standing posture and appears to be correlated with the SDC curve.