Transitions in persistence of postural dynamics depend on the velocity and structure of postural perturbations

Abstract

The sensorimotor system prefers sway velocity information when maintaining upright posture. Sway velocity has a unique characteristic of being persistent on a short time-scale and anti-persistent on a longer time-scale. The time where the transition from persistence to anti-persistence occurs provides information about how sway velocity is controlled. It is, however, not clear what factors affect shifts in this transition point. This research investigated postural responses to support surface movements of different temporal correlations and movement velocities. Participants stood on a force platform that was translated according to three different levels of temporal correlation. White noise had no correlation, pink noise had moderate correlation, and sine wave movements had very strong correlation. Each correlation structure was analyzed at five different average movement velocities (0.5, 1.0, 2.0, 3.0, and 4.0 cm·s), as well as one trial of quiet stance. Center of pressure velocity was analyzed using fractal analysis to determine the transition from persistent to anti-persistent behavior, as well as the strength of persistence. As movement velocity increased, the time to transition became longer for the sine wave and shorter for the white and pink noise movements. Likewise, during the persistent time-scale, the sine wave resulted in the strongest correlation, while white and pink noise had weaker correlations. At the highest three movement velocities, the strength of persistence was lower for the white noise compared to pink noise movements. These results demonstrate that the predictability and velocity of support surface oscillations affect the time-scale threshold between persistent and anti-persistent postural responses. Consequently, whether a feedforward or feedback control is utilized for appropriate postural responses may also be determined by the predictability and velocity of environmental stimuli. The study provides new insight into flexibility and adaptability in postural control. This information has implications for the design of rehabilitative protocols in neuromuscular control.

Persistence in postural dynamics is dependent on constraints of vision, postural orientation, and the temporal structure of support surface translations

“…Velocity responses were investigated on both short and long time scales. While, it is known that postural dynamics can characteristically demonstrate behavioral transitions between short and long time scales, based on the structure of support surface translations (Rand and Mukherjee, 2018), the purpose of this study however, was to specifically investigate the behavior within each time scale so the time scales were standardized to include only linear regions of the log/log plot for all participants. It was hypothesized that the persistence on the short time scale and antipersistence on the long time scale would be stronger in the ML compared to the AP direction, in the eyes closed compared to eyes open condition, and in conditions where the persistence of the structured translation was stronger.…”

“…However, it is important to emphasize that movements away from this optimal strength do not necessarily imply a degradation of the sensorimotor system. On the contrary, adjusting the strength of persistence and anti-persistence can be seen as a method of the sensorimotor system adapting to constraints imposed on the system for example when maintain postural control with changes in support surface velocity (Rand and Mukherjee 2018).…”

“…In our previous investigation into postural dynamics (Rand and Mukherjee, 2018), we had raised the possibility that our observation of the predictability and velocity of support surface oscillations affecting the time-scale threshold between persistent and anti-persistent postural responses was possibly indicative of factors critical for choosing feedforward versus feedback mechanisms for postural control. This study provides further insight into the flexibility and adaptability of postural control by extending the thought that such a choice could also be influenced by visual feedback and postural orientation.…”

“…There is also research hypothesizing that different time scales preferentially utilize position or velocity based on the frequency of movement (Gilfriche et al 2018). Center of pressure velocity (COP vel ) exhibits different behavior at different time scales (Delignieres et al 2011;Rand and Mukherjee 2018). On a short time-scale COP vel is persistent, when movement starts the velocity will continue to increase, then when it starts decreasing it will continue to decrease until movement is stopped.…”

“…With young subjects the temporal persistence of their COP response trended towards the temporal persistence of the support surface movements, indicating that the ability to adapt to the different environmental constraints in the anteroposterior direction. It has been demonstrated that as the velocity of movement increases, unpredictable stimuli result in a characteristic postural response different from a predictable stimuli (Rand and Mukherjee 2018). There was an increase in persistence in the short time scale when the stimuli were predictable and a decrease in persistence when the stimuli were unpredictable.…”