Early postural adjustments in preparation to whole-body voluntary sway

Abstract

We studied postural adjustments associated with a quick voluntary postural sway under two conditions, self-paced and simple reaction-time. Standing subjects were required to produce quick discrete shifts of the center of pressure (COP) forward. About 400–500 ms prior to the instructed COP shift, there were deviations of the COP in the opposite direction (backwards) accompanied by changes in the activation levels of several postural muscles. Under the reaction-time conditions, the timing of those early postural adjustments did not change (repeated measures MANOVA: p>0.05) while its magnitude increased significantly (confirmed by repeated measures MANOVA: p<0.05). These observations are opposite to those reported for anticipatory postural adjustments under simple reaction time conditions (a significant change in the timing without major changes in the magnitude). We conclude that there are two types of feed-forward postural adjustments. Early postural adjustments prepare the body for the planned action and/or expected perturbation. Some of these preparatory actions may be mechanically necessary. Later, anticipatory postural adjustments generate net forces and moments of force acting against those associated with the expected perturbation. Both types of adjustments fit well the referent configuration hypothesis, which offers a unified view on movement-posture control.

Muscle synergies in preparation to a step made with and without obstacle

“…The second type of adjustment is the anticipatory synergy adjustments (ASAs) that optimize posture to facilitate a planned action and/ or an expected perturbation. These adjustments are seen about 200-250 ms prior to a self-paced action (Klous et al 2012) or prior to a predictable perturbation (Krishnan et al 2011a, b), which is similar to the timing of ΔV decline in our experiment. These adjustments to making a step are mechanically necessary to allow unloading the stepping leg and to create a moment of the vertical force rotating the body forward about the ankle joints.…”

“…Prior to the postural preparation to action and/or perturbation, there are two types of adjustments in the activity of the trunk and leg muscles (Klous et al 2012). The first one, anticipatory postural adjustments (APAs) generate forces and moments of force to mitigate or cancel postural disturbances associated with a predicted perturbation (reviewed in Massion 1992; Aruin and Latash 1995).…”

The effects of predictability on inter-limb postural synchronization prior to bouts of postural instability

“…Thus, the increased inter-limb synchrony in BS, representing efficient control, may be associated with lower cognitive demands and suggests that the other conditions required more attentional resources to prepare for instability resulting in decreased interlimb synchrony. Alternately, the reduction in inter-limb synchrony from QS to BS and from BS to the other conditions in the ML direction may reflect a decoupling of the COP during movement preparation to enable EPAs to effectively contribute to balance reactions [13]. The combinations of high inter-limb synchrony in the AP direction combined with the lower ML inter-limb synchrony may, therefore, represent the most efficient method to prepare for unpredictable perturbations of different magnitudes.…”

“…Previous work has demonstrated center of pressure (COP) shifts (i.e. early postural adjustments; EPA) prior to self-initiated forward postural sway [13] that were distinct from anticipatory postural adjustments. EPAs optimize postural control in preparation for volitional or reactive instability [13].…”

“…early postural adjustments; EPA) prior to self-initiated forward postural sway [13] that were distinct from anticipatory postural adjustments. EPAs optimize postural control in preparation for volitional or reactive instability [13]. Additionally, bilateral activation of postural muscles with comparable magnitude and onset in advance of self-initiated bilateral shoulder-flexion perturbations Anticipatory balance control optimizes balance reactions to postural perturbations.…”