The posterior shift anticipatory postural adjustment in choice reaction step initiation

• Simple (SRST) and choice (CRST) reaction stepping tasks were analyzed.
• The posterior shift APA (PS APA) pattern was identified.
• Stimulus evaluation during CRST was not completed before the movement execution.
• PS APA is a potentially advantageous adaptive action for a fast forward step.

The ability to step quickly in response to a perturbation has been shown to be critical for prevention of falls. The cognitive processing, weight shifting, and locomotion must be well timed to execute a successful step. The purpose of this study was to compare the response preparation and response execution processes between a simple (SRST) and a choice reaction stepping task (CRST). Nine healthy young subjects were recruited to participate in this study. Subjects were required to stand on a forceplate and maintain their balance, and step forward on a second forceplate with either the left or right foot after hearing an auditory tone. The center of pressure (COP) was analyzed to determine the types of anticipatory postural adjustment (APA) prior to a step. The APA phase and stepping phase timing was calculated based on the COP trajectory. Findings showed that reaction time (RT), APA phase and overall stepping latency were slower for CRST than for SRST. We also identified an intermediate type of APA response (posterior shift APA) in addition to the correct and error APA response, and found the posterior shift APA response had the fastest execution time for CRST, and may be beneficial for falls prevention.