Fig 1.
(A) Experimental setup. The servo-motor was installed horizontally and applied perturbations to the crank, thus rotating the platform and footplates. A load cell measured left ankle torque. A potentiometer attached to the axis of rotation measured anteroposterior foot rotation. An accelerometer attached underneath the left footplate measured its acceleration. Two laser-reflex sensors placed at left mid-tibia and at the board tracked the anteroposterior shin and body tilt. Only left lower limb recordings were used for stiffness analysis. (B) Illustrative segment of stiffness-measuring data. Sample data of footplate (gray) and ankle (black) angle during a stiffness-measuring trial, with perturbations indicated by arrows. (C) Illustrative segment of body velocity data. Sample data of body velocity during the sine 0.4 condition. Platform oscillation is shown in black while body velocity is shown in light gray. A notch filter (0.05–0.15 Hz) was also applied to body velocity so that any direct contribution of the slow tilts to body sway was removed (dark gray).
Fig 2.
Univariate scatter plot of relevant data.
Black rectangles indicate mean values. Dotted lines connect data from each participant. (A) Intrinsic ankle stiffness (K). K estimates during normal, sine 0.2 and sine 0.4 conditions are shown for perturbations of 2 different sizes (0.2 & 0.9 deg). Dotted lines confirm that K reduced in all participants with larger perturbations and increased platform tilt. (B) Mean ankle torque. Same in all conditions. (C) RMS body velocity. Sway progressively increases with increased platform tilts. (**) indicates P<0.01 and (***) indicates P<0.001.
Fig 3.
Correlation between K (% mgh) and baseline ankle torque (%mgh).
Bivariate scatter plot with regression line and confidence interval band (95% CI) correlating K measured with 0.2 deg (top) and 0.9 deg (bottom) perturbations versus mean ankle torque. Significant correlations are highlighted in bold.
Fig 4.
Correlation between K (% mgh) and RMS body velocity (deg s-1).
Bivariate scatter plot with regression line and confidence interval band (95% CI) correlating K measured with 0.2 deg (top) and 0.9 deg (bottom) perturbations versus average RMS body velocity. Significant correlation is highlighted in bold.