1. Our previous study showed that two distinct postural modifications occurred when subjects were instructed to step, rather than maintain stance, in response to a backward surface translation: 1) the automatic postural responses to the surface perturbation were reduced in magnitude and 2) the anticipatory postural adjustments promoting foot off were shortened in duration. This study investigates the extent to which task instruction, prediction of perturbation velocity, and afferent sensory information related to perturbation velocity are responsible for these postural modifications. 2. Eleven human subjects were instructed in advance, to either maintain stance or step forward in response to a backward surface translation. Four different velocities of translation were used to perturb equilibrium. To assess the influence of predicted versus actual velocity information, the surface translations were presented in both a blocked order of increasing perturbation velocity (predictable) and a random order (unpredictable). Lower-extremity electromyographs (EMGs), ground reaction forces, and movement kinematics were quantified for both the automatic postural responses to perturbation and the anticipatory postural adjustments for step initiation. 3. The instruction to step was not solely responsible for the suppression of the automatic postural response. Prediction of perturbation velocity was required for significant suppression of the early automatic postural response when subjects stepped in response to the perturbation. When compared with the stance condition, the magnitude of the initial 50 ms of the automatic response in bilateral soleus and the left limb gastrocnemius (initial stance limb) was significantly reduced only when the perturbation velocities were presented in a blocked order. The magnitude of the automatic response was not reduced in the gastrocnemius of the right limb, which was always the initial swing limb and recruited for heel-off in the step conditions. This asymmetrical reduction of the gastrocnemius suggests that modification of the response was specific to the instruction, rather than a general decrease in the extensor muscle excitability. 4. The suppression of the early automatic postural response involved a change in the bias of the response. Despite the reduced magnitude during the predictable velocity step condition, the slope (i.e., gain) of the response with increasing velocities was not different from that of the stance condition. Thus the excitability of the automatic response was reduced by a relatively constant amount for each velocity when the perturbation velocity was predictable. 5. In contrast to the importance of velocity prediction for modification of the automatic postural response, actual velocity information was used for modification of the anticipatory postural adjustments when step was initiated in response to the surface perturbation. Regardless of whether the perturbation velocities were presented in a blocked or random order, the anticipatory postural adjustments were rapidly initiated and the duration of the postural adjustments for step initiation was shortened as the velocity of perturbation increased. 6. We conclude that the CNS uses prediction of perturbation velocity to modify the excitability of early automatic postural responses when the postural goal changes. In contrast, actual afferent velocity information can be used to modify the duration of the anticipatory postural adjustments for a voluntary step in response to perturbation. Thus the CNS utilizes feed-forward prediction to modify peripherally triggered postural responses, and utilizes immediate afferent information to modify the centrally initiated postural adjustments associated with voluntary movement.
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