Reversals of anticipatory postural adjustments during voluntary sway in humans

Abstract

We describe reversals of anticipatory postural adjustments (APAs) with the phase of a voluntary cyclic whole-body sway movement. Subjects (n=9) held a standard load in extended arms and released it by a bilateral shoulder abduction motion in a self-paced manner at different phases of the sway. The load release task was also performed during quiet stance in three positions: in the middle of the sway range and close to its extreme forward and backward positions. Larger APAs were seen during the sway task as compared to quiet stance. Although the direction of postural perturbation associated with the load release was always the same, the direction of the APAs in the leg muscles reversed when the subjects were close to the extreme forward position as compared to the APAs in other phases and during quiet stance. The trunk muscles showed smaller APA modulation at the extreme positions but larger modulation when passing through the middle position, depending on the direction of sway, forward or backward. The phenomenon of APA reversals emphasizes the important role of safety in the generation of postural adjustments associated with voluntary movements. Based on these findings, APAs could be defined as changes in the activity of postural muscles associated with a predictable perturbation that act to provide maximal safety of the postural task component.

Figure 1. General EMG patterns in a representative subject during the quiet stance (QS) task

The load was released in the forward (A: QS_F), middle (B: QS_M) and backward (C: QS_B) positions. Time zero corresponds to the time of load release, and the two vertical lines indicate the 100 ms time interval over which indices of anticipatory postural activity (APA) were quantified. EMG signals for soleus, biceps femoris and erector spinae are inverted for better visualization.

Figure 2. EMG activity associated with an unperturbed cycle of voluntary sway (VS) in a representative subject

The top panel shows the moment about the frontal axis (M_y), where the highest positive value corresponds to the extreme forward position and the smallest negative value corresponds to the extreme backward position during VS. The remaining three panels show EMG patterns in the lower leg, thigh and trunk muscles, respectively. EMG signals for soleus, biceps femoris and erector spinae muscles are inverted for better visualization. Note that the ventral muscles (tibialis anterior, rectus femoris, rectus abdominis) show an increase in the activity during motion towards the backward position and the dorsal muscles (soleus, biceps femoris, erector spinae) show an increase in activity during motion towards the forward position.

Figure 3. General EMG patterns in a representative subject during the voluntary sway (VS) task

The load was released in the forward (A: VS_F), middle when moving backward (B: VS_MB), backward (C: VS_B) and middle when moving forward (D: VS_MF) positions. Time zero corresponds to time of load release and the vertical lines indicate the 100 ms time interval before load release over which indices of anticipatory postural activity (APA) were quantified. EMG signals for soleus, biceps femoris and erector spinae are inverted for better visualization.

Figure 4. Pattern of IEMG change for APAs of individual muscles of a representative subject over a cycle of voluntary sway (VS)

Ventral muscles: TA, tibialis anterior; RF, rectus femoris; RA, rectus abdominis. Dorsal muscles: SOL, soleus; BF, biceps femoris; ES, erector spinae. 0 and 100%, extreme forward position; 50%, extreme backward position; 25%, middle position when moving backward; and 75%, middle position when moving forward. Data were smoothed with a 5-point moving average window and fitted with a third-order polynomial. R² values are presented.

Figure 5. IEMG indices of individual muscles in the quiet stance (QS) and voluntary sway (VS) tasks in the extreme positions

Average across subjects IEMG indices in the extreme forward (open columns) and backward (filled columns) positions are shown with standard error bars. Muscle names are abbreviated as in Fig. 4. † Significant differences between the QS and VS tasks; * significant differences between the extreme forward and backward positions.

Figure 6. Average across subjects IEMG indices of individual muscles in the middle positions

Quiet stance middle position (QS_M; filled columns), voluntary sway middle position when swaying backwards (VS_MB; open columns) and when swaying forward (VS_MF; hatched columns) are shown with standard error bars. Muscle names are abbreviated as in Fig. 4. † Significant differences between the QS and VS tasks; * significant differences between VS_MB and VS_MF.