The Stop Signal Stepping Task: how action cancellation commands disrupt step initiation in young and healthy older adults

Action cancellation - the ability to rapidly cancel an initiated movement in response to unexpected events - has been extensively studied in the upper limb using the stop signal task (SST). During gait, action cancellation is needed to stop and modify steps to avoid hazards and prevent falls. By adapting the SST to step initiation, this study investigated how the anticipatory postural adjustment (APA) and foot-lift phases of forward stepping were affected by action cancellation commands, and whether this changed with healthy ageing. The SST was performed in stepping, foot tap, and finger button conditions in 27 young (Mage = 28.7 years) and 29 healthy older adults (Mage = 70.1 years). Across conditions, older adults exhibited slower response speed compared to young adults and greater proactive slowing of responses when stop cues were anticipated. However, there was no significant difference in stopping speed between young and older adults. Stopping speed was fastest in the finger tap condition, and slowest in the step condition. When an APA was initiated in a step cancellation trial, the magnitude of the weight shift toward the step leg did not differ between successful and unsuccessful foot-lift cancellations. Foot-lift could be cancelled when stop cues were presented at similar phases of step preparation for young and older adults. These results suggest that the initial loading of the step leg is a ballistic process, however as weight is shifted toward the stance leg, action cancellation commands responding to external stimuli can decouple the APA and foot-lift step phases. Key PointsO_LIThe stop signal task (SST) - which allows an estimation of stopping speed independently of response speed - was applied to voluntary stepping in young and older adults. C_LIO_LIWhile response speed was slower for older than young adults, stopping speed was not significantly different between age groups in the upper limb, lower limb when seated, and during forward stepping. C_LIO_LIWhen stop cues were introduced, response speed slowed more in older than young adults, and more in the upper than the lower limb (i.e., Foot Tap and Step conditions). C_LIO_LIThe initial preparatory weight shift toward the stepping foot was not significantly different between successfully cancelled steps and normal steps, highlighting the ballistic nature of the early phase of step preparation. C_LIO_LIPrior to foot-lift, action cancellation commands could decouple the preparatory weight shift phase from foot-lift at similar stages of step initiation in young and healthy older adults. C_LI