Step initiation combines motor and cognitive processes involved in movement preparation and execution. Intracerebral recordings in the cat have shown that the pontomedullary reticular formation has a central role in:

– postural preparation prior to movement execution;

– the coupling between preparation and execution.

Furthermore, cortical control of the pontomedullary reticular structures modulates the release of postural adjustments. Imaging studies in humans have also shown that the prefrontal cortex has a particular role in postural adjustments. Accordingly, it has been hypothesized that attentional processes [which involve cortical structures] can also modulate the step initiation program. Investigation of this motor modulation provides a better understanding of the interaction between attention on the one hand and motor preparation and execution on the other. Postural modulation can be investigated by recording anticipated postural adjustments (APAs). An APA is essential for step initiation because it unloads the swing leg and thus creates the conditions required for progression. Indeed, the amplitude and duration of APAs are predictive of the subsequent peak step velocity. Recent literature showed that this APA can be modulated by general attentional alert processes, orientation processes or even executive control processes. When the subject is conditioned for gait initiation, an attentional stimulus provokes an involuntary release of the gait initiation program (referred to as “early APAs” or “pre-APAs”). This elevated involuntary release of a motor program might reflect failure to inhibit distracter stimuli and are more frequent during pathological gait initiation (as seen in Parkinson's disease patients, who are known to have attention deficits). The occurrence of APA errors also perturbs the gait initiation program. This corresponds to the correction of a step initiation APA when the initial direction of postural adjustment is not appropriate. The APA error then indicates a motor program error, which is efficiently corrected and prevents incorrect step initiation from taking place. It is known that APA errors are modulated by attention suggesting a link with delayed stimulus integration. Further research investigates cortical mechanisms implied in this interaction between motor programming and attention using event related potentials and time frequency analysis. Step initiation's biomechanical processes are useful for precisely studying how well-known attentional cortical process can modulate the different steps of automatized motor program. They are of clinical significance to understand gait initiation failure in elderly and patients with various neurological diseases including Parkinson's disease.