Abstract
Motor control for forward step initiation begins with anticipatory postural adjustments (APAs). During APAs, the central nervous system controls the center of pressure (CoP) to generate an appropriate center of mass (CoM) position and velocity for various task requirements. In this study, we investigated the effect of required stepping accuracy on the CoM and CoP parameters during APA for a step initiation task. Sixteen healthy young participants stepped forward onto the targets on the ground as soon as and as fast as possible in response to visual stimuli. Two target sizes (small: 2 cm square and large: 10 cm square) and two target distances (short: 20% and long: 40% of the body height) were tested. CoP displacement during the APA and the CoM position, velocity, and extrapolated CoM at the timing of the takeoff of the lead leg were compared among the conditions. In the small condition, comparing with the large condition, the CoM position was set closer to the stance limb side during the APA, which was confirmed by the location of the extrapolated center of mass at the instance of the takeoff of the lead leg [small: 0.09 ± 0.01 m, large: 0.06 ± 0.01 m, mean and standard deviation, F(1, 15) = 96.46, p < 0.001, η2 = 0.87]. The variability in the mediolateral extrapolated center of mass location was smaller in the small target condition than large target condition when the target distance was long [small: 0.010 ± 0.002 m, large: 0.013 ± 0.004 m, t(15) = 3.8, p = 0.002, d = 0.96]. These findings showed that in the step initiation task, the CoM state and its variability were task-relevantly determined during the APA in accordance with the required stepping accuracy.
Highlights
When an individual stands at the edge of a pond and tries to step on a small stepping stone in the pond, he or she needs to accurately control his or her posture and foot placement at the same time
Through anticipatory postural adjustments (APAs), the center of mass (CoM) position and velocity must be appropriately set when the leading limb lifts from the ground because once the leading limb is no longer on the ground, the CoM motion is dominated by passive mechanics, and the CoM falls away from the supporting limb like an inverted pendulum (Lyon and Day, 1997; Pai and Patton, 1997; Hof et al, 2007)
The purpose of this study was to investigate the effect of required accuracy in a stepping initiation task on motor control during APAs
Summary
When an individual stands at the edge of a pond and tries to step on a small stepping stone in the pond, he or she needs to accurately control his or her posture and foot placement at the same time. The first biomechanical feature of forward step initiation is the transfer of the center of mass (CoM) forward and toward the supporting limb (Crenna and Frigo, 1991; Lyon and Day, 1997; Massot et al, 2019). This weight transfer is accomplished by a shift in the. CoM Variability in Forward Stepping center of pressure (CoP) backward and toward the side of the swing limb, which is often called an anticipatory postural adjustment (APA). For understanding the mechanisms of accurate step initiation, it is essential to investigate how the CoM state and its variability are controlled in an accurate step initiation task
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