88 Chapter 6 Spatiotemporal gait variability Gait variability was expressed in the coefficient of variation (CoV) for step length, step time and step width using the following equation: = ∗100% (1) = − (2) = ∗ + ∗ ′ + (3) (1) Medio-lateral Margin of Stability MoS was calculated in the mediolateral direction using the following equation26, and defined as its minimum value during single stance: = ∗100% (1) = − (2) = ∗ + ∗ ′ + (3) (2) The base of support (BoS) was defined as the position of the mediolateral center of pressure (CoP) using the force plate recordings. 22 The extrapolated center of mass (XCOM) was calculated according to Hof et al. 26 for which the position of the center of mass (CoM) was extracted from VICON. The mean MoS was calculated over the valid left and right steps collected during the three-minute walking trials, whereas variability of MoS was calculated using standard deviation. Foot Placement Deviation A previous study showed that the optimal mediolateral foot placement of the next step can be predicted based on the mediolateral CoM position and CoM velocity at mid stance. 27 To evaluate whether people with HSP can adhere to this optimal foot placement strategy, we calculated foot placement deviations using the root mean square error (RMSE) between the predicted and actual foot placements. 23 To this end, we calculated the predicted foot placement in mediolateral direction based on the medio-lateral CoM position and velocity at heel strike using the following equation: 23 = ∗100% (1) = − (2) = ∗ + ∗ ′ + (3) (3) where the = ∗100% (1) = − (2) = ∗ + ∗ ′ + (3) and = ∗100% (1) = − (2) = ∗ + ∗ ′ + (3) are defined as the regression coefficients, and = ∗100% (1) = − (2) = ∗ + ∗ ′ + (3) as the model error. The actual foot placement was calculated as the centered medio-lateral distance (i.e., subtracting the general mean from each individual step) between the left and right heel markers at midstance. 23 Local Dynamic Stability To assess local dynamic stability, the revised Rosenstein algorithm was used to calculate maximum local divergence exponents (LDEs). 8, 24 LDEs were calculated for the trunk and pelvis over one step (i.e., the so-called short-LDE) in the anteriorposterior, medio-lateral and vertical directions. For this purpose, the trunk was defined as the midpoint between the processus spinosus of the 7th cervical
RkJQdWJsaXNoZXIy MjY0ODMw