Accuracy in control of center of mass during anticipatory postural adjustments prior to forward step

We aimed to clarify the speed and accuracy in center of mass (COM) control during anticipatory postural adjustment (APA) and step phases in a step task where different levels of accuracy were required. Twelve subjects stepped onto stayed on bars with 2, 4, 6, and 8 cm width. The ground reaction forces and moments and 3D motion data were measured. The duration of the APA and step phases, and the accuracy of the COM state at the end of each phase were analyzed. The SD of the COM position and velocity was smaller in the 2 cm condition than 8 cm condition at the end of the APA, whereas the APA duration was not different between the conditions. The results suggest that the APA was controlled according to the required level of COM accuracy, however, this was not explained by the simple speed-accuracy tradeoff principle.