Postural adjustments to self-triggered perturbations under conditions of changes in body orientation

Abstract

We studied anticipatory and compensatory postural adjustments (APAs and CPAs) associated with self-triggered postural perturbations in conditions with changes in the initial body orientation. In particular, we were testing hypotheses on adjustments in the reciprocal and coactivation commands, role of proximal vs. distal muscles, and correlations between changes in indices of APAs and CPAs. Healthy young participants stood on a board with full support or reduced support area and held a standard load in the extended arms. They released the load in a self-paced manned with a standard small-amplitude arm movement. Electromyograms of 12 muscles were recorded and used to compute reciprocal and coactivation indices between three muscle pairs on both sides of the body. The subject's body was oriented toward one of three targets: straight ahead, 60° to the left, and 60° to the right. Body orientation has stronger effects on proximal muscle pairs compared to distal muscles. It led to more consistent changes in the reciprocal command compared to the coactivation command. Indices of APAs and CPAs showed positive correlations across conditions. We conclude that the earlier suggested hierarchical relations between the reciprocal and coactivation command could be task-specific. Predominance of negative or positive correlations between APA and CPA indices could also be task-specific.

Keywords: Anticipatory postural adjustments; Coactivation; Compensatory postural adjustments; Postural control; Reciprocal activation; Referent coordinate.

Fig. 1

A A schematic illustration of the control of vertical posture in a sagittal plane. Posture is controlled with respect to a time-varying spatial coordinate (rambling). Referent orientation of the body with respect to that coordinate is defined by the reciprocal command (R-command). The difference between the actual and referent body orientations produces moment of force balancing that of the gravity force. This transformation is mediated by the coactivation command (C-command). B The R- and C-commands at the task (body) level are mapped on {R; C} commands for the joints (H hip, K knee, A ankle), which project of control variables for the muscles (λ) for the flexor (F) and extensor (E) muscles. Note that all these transformations are abundant with more variables at lower levels compared to those at higher levels

Fig. 2

Schematic representation of the experimental setup. Participants were asked to stand comfortably on one of the two boards (the “Unstable” board is illustrated), with arms stretched forward parallel to the ground and fingers outstretched, holding the sphere between their palms. They were instructed to turn the body without moving their feet toward one of the three directions as shown in the figure and drop the sphere at their own rate by quickly abducting bilaterally both upper arms along the horizontal plane with a small-amplitude action

Fig. 3

Filtered EMG traces (six trials averaged) for a representative participant in the -60° direction and Unstable condition for Left and Right Body Side. The vertical dashed line corresponds to the release of the load. EMG in Volt units. Anticipatory postural adjustments (APA) and compensatory postural adjustments (CPA) are highlighted in light and dark gray, respectively

Fig. 4

Filtered EMG traces (6 trials averaged) of RF, BF, RA, and ES for a better representation of the APA and CPA timing. The figure shows the traces for a representative participant in the -60° direction and Unstable condition for Right Body Side. The vertical dashed line corresponds to the release of the load. EMG in Volt units. Anticipatory postural adjustments (APA) and compensatory postural adjustments (CPA) are, respectively, highlighted in light and dark gray

Fig. 5

R-index (upper row) and C-index (bottom row) values related to anticipatory postural adjustments (APA) for the agonist–antagonist pairs acting at the ankle (TA/SO, dark gray), knee (RF/BF, light gray), and hip (RA/ES, white) joints for the Stable Condition (left column) and Unstable Condition (right column). Histograms depict the mean value and the standard error. *significant effects at p < 0.05, ** significant effects at p < 0.01. TA tibialis anterior; SO soleus, RF rectus femoris, BF biceps femoris, RA rectus abdominis, ES erector spinae

Fig. 6

R-index values as a function of the release angle of the sphere (-60°, 0°, and 60°) for anticipatory postural adjustments (APA, upper row) and compensatory postural adjustments (CPA, bottom row) for the Stable Condition (left column) and Unstable Condition (right column). Gray and white plots represent left and right body sides, respectively. Histograms depict the mean value and the standard error. *significant effects at p < 0.05, **significant effects at p < 0.01

Fig. 7

R-index (upper row) and C-index (bottom row) values related to compensatory postural adjustments (CPA) for the agonist–antagonist pairs acting at the ankle (TA/SO, dark gray), knee (RF/BF, light gray), and hip (RA/ES, white) joints for the Stable Condition (left column) and Unstable Condition (right column). Histograms depict the mean value and the standard error. *significant effects at p < 0.05, **significant effects at p < 0.01. TA tibialis anterior, SO soleus, RF rectus femoris, BF biceps femoris, RA rectus abdominis, ES erector spinae

Fig. 8

Linear correlations between the indices computed over the anticipatory postural adjustments (APA) and compensatory postural adjustments (CPA) for each subject (12 subjects in total). All regression lines have positive slopes as depicted in each panel. Dots in each panel include the three agonist–antagonist pairs, the two stable–unstable conditions, and the three angles of ball release for a total of 18 points

Fig. 9

C-index (upper row) and R-index (bottom row) values related to EMG-based for the agonist–antagonist pairs across all joints (ankle, knee, and hip) and across Stable and Unstable Conditions. Histograms depict the mean value and the standard error. *significant effects at p < 0.05, **significant effects at p < 0.01

Fig. 10

A Schematic illustration of the control of vertical posture with the reciprocal and coactivation (R and C) commands. The former defines referent body orientation (RO) in space. The latter defines the range where agonist and antagonist muscles are active simultaneously. At the level of mechanics, it defines apparent stiffness coefficient, which translates the angular units of the difference between RO and actual body orientation (gray image) into moment of force units counteracting effects of gravity. B Referent joint configurations (black lines, RC_A1 and RC_A2) associated with the “ankle strategy”. C Referent joint configurations associated with the “hip strategy” (black lines, RC_H1 and RC_H2)