11 General introduction Box 1. Biomechanical methods to assess dynamic balance Gait variability Gait variability is defined as the fluctuation in spatiotemporal characteristics that occur from step to step during walking.19 Variability is commonly determined for step length, step time, and step width, and expressed in a standard deviation or coefficient of variation (i.e., standard deviation divided by the mean). It is often assumed that a higher spatiotemporal gait variability reflects reduced gait stability.23-25 Margin of Stability In order to maintain balance during quiet stance, one has to be able to maintain one’s center of mass (CoM) within the area encompassed by both feet - the so-called base of support (BoS). In order not to fall in dynamic situations, for example during gait, a similar requirement exists where the so-called “extrapolated center of mass” (XCoM; a variable that takes both position and velocity of the CoM into account) needs to be maintained within the base of support (BoS).26 The distance between the XCoM and the BoS is called the margin of stability (MoS). If the MoS is negative (i.e., the XCoM exceeds the BoS), the person has to make an adjustment – for example take a step – to prevent a fall. A MoS value approaching nill, or an increase in the variability of the MoS are therefore considered to reflect gait instability.27 Foot Placement Deviation The foot placement deviation (FPD) reflects the adherence to the foot placement strategy. This strategy is based on the preposition that CoM position and CoM velocity at midstance can predict the ideal foot placement of the next step.28 To ensure that the actual foot is placed at this predicted ideal location, adjustments in the timing and location of the actual foot placement have to be controlled. The accuracy of the foot placement strategy is reflected in the root mean square error (RMSE) of the actual foot placement compared to the predicted foot placement. A higher RMSE indicates a lower accuracy of the foot placement strategy, which indicates reduced gait stablity.29 Local Dynamic Exponents Local dynamic exponents (LDEs) reflect the ability of a person to attenuate the effects of small perturbations during gait.19 For example, in an optimal condition, the variability of trunk displacements during consecutive steps is nil. However, due to small perturbations that arise during natural gait (e.g., during heel strike or small differences in floor height), trunk displacements will be somewhat different from one step to the other. If these differences are not attenuated, their impact will increase exponentially with time. A higher LDE implies that a person is less able to attenuate small gait perturbations, indicating less gait stability.30 1
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