Typical healthy walking displays greater variability in the mediolateral direction compared to the anteroposterior direction. This greater variability is thought to represent increased uncertainty in movement. As a result, it has been postulated that the mediolateral direction of gait requires more active control by the central nervous system while the anteroposterior direction is controlled through passive actions. However, this theory has only been tested on gait where progression occurs in the anteroposterior direction. Therefore, the purpose of this study was to investigate how the amount of variability is affected if progression occurs in the mediolateral direction using a lateral stepping gait. Results showed the anteroposterior direction had a significantly greater amount of variability than the mediolateral direction (p\u3c0.001). The results do not support current models of a partition of active control to different anatomical planes. Rather, it seems that other physical entities involved in motion, such as momentum and inertia, are able to decrease the dependence on active control from the central nervous system. In a lateral stepping gait, such physical entities were no longer assisting in the anteroposterior direction but had a larger impact in the mediolateral direction as it was the direction of progression. As a result variability in the anteroposterior direction increased. Thus, it is possible to infer increased reliance on active control from the central nervous system in the direction orthogonal to progression
