The Consecutive Postural Adjustments (CPAs) that follow foot placement in single stepping

This research deals with the postural adjustments that occur after the end of a voluntary movement ("Consecutive Postural Adjustments": CPAs). With the aim of more fully characterizing the postural component of motor tasks, they are compared with those occurring before movement onset ("anticipatory postural adjustments": APAs). Ten male adults were asked to perform a single step as quickly as possible to a target marked on the ground (STEP). A force plate measured the resultant reaction forces along the antero-posterior axis (Rx) and the Centre of Pressure (CoP) displacements along the antero-posterior and lateral axes (Xp and Yp). The velocity of the Centre of Gravity (COG) along the antero-posterior axis was calculated and the peak velocity (termed "progression velocity": VxG) was measured. Antero-posterior linear impulses (∫Rxdt) were also calculated. Two step lengths were considered: a short and a long one (SS and LS conditions). Each session consisted of ten steps. The antero-posterior force time course presented two phases: a positive one that included the APA and the STEP periods, then a negative one corresponding to CPA. The corresponding backward impulse (CPIx) was equal to the forward one (BPIx), which identifies CPA as a counter-perturbation, that is, a process by which the central nervous system controls and stops a forward impulse. CPA durations and peak amplitudes (dCPA and pCPA) were significantly greater than the corresponding APA values (dAPA and pAPA). Moreover, when the step length was greater, that is, when the progression velocity was faster, pCPAs, like pAPAs, increased, suggesting that CPAs and APAs are parts of the same motor program. In addition, CPA duration and time to peak amplitude do not vary with progression velocity, which could be explained by the movement braking constraints. Finally, CPA can be viewed as a means of achieving a new "static" postural equilibrium as soon as possible after foot contact, and to prevent the risk of falling. The outcome provides additional knowledge on how a stable posture is achieved at the end of a task movement, and is discussed from a postural control perspective.