The role of anticipatory postural adjustments in compensatory control of posture: 2. Biomechanical analysis

Abstract

The central nervous system (CNS) utilizes anticipatory (APAs) and compensatory (CPAs) postural adjustments to maintain equilibrium while standing. It is known that these postural adjustments involve displacements of the center of mass (COM) and center of pressure (COP). The purpose of the study was to investigate the relationship between APAs and CPAs from a kinetic and kinematic perspective. Eight subjects were exposed to external predictable and unpredictable perturbations induced at the shoulder level while standing. Kinematic and kinetic data were recorded and analyzed during the time duration typical for anticipatory and compensatory postural adjustments. When the perturbations were unpredictable, the COM and COP displacements were larger compared to predictable conditions with APAs. Thus, the peak of COM displacement, after the pendulum impact, in the posterior direction reached 28+/-9.6mm in the unpredictable conditions with no APAs whereas it was 1.6 times smaller, reaching 17+/-5.5mm during predictable perturbations. Similarly, after the impact, the peak of COP displacement in the posterior direction was 60+/-14 mm for unpredictable conditions and 28+/-3.6mm for predictable conditions. Finally, the times of the peak COM and COP displacements were similar in the predictable and unpredictable conditions. This outcome provides additional knowledge about how body balance is controlled in presence and in absence of information about the forthcoming perturbation. Moreover, it suggests that control of posture could be enhanced by better utilization of APAs and such an approach could be considered as a valuable modality in the rehabilitation of individuals with balance impairment.

Figure 1

Temporal evaluation (from −200 ms to + 400 ms in relation to T⁰) of the ankle, knee, hip, spine, thorax, and head displacements during predictable and unpredictable conditions. Each point represents the angular displacements in the sagittal plane (flexion (+) and extension (−) of these variables averaged over a 50 ms interval (−201 to −150 ms, −151 to −100, and so on) and its standard error. The 4 time epochs of 150 ms used for the analysis are represented by the brackets on the bottom (APA1, APA2, CPA1, and CPA2). The dotted vertical line shows the moment of body perturbation (T₀).

Figure 2

Temporal evaluation (from −350 ms to +1000 ms in relation to T₀) of the COMy, COMz, and COP displacements during predictable and unpredictable conditions. Each point represents the mean COMy and COP displacements in anterior (−) and posterior (+) directions and COMz displacements towards downward (−) and upward (+) directions averaged over a 50 ms intervals (−351 to −300 ms, −301 to −250, and so on) and its standard error across 8 subjects. The dotted vertical line shows the moment of body perturbation (T₀).

Figure 3

Differences between predictable and unpredictable conditions in the maximal/peak COMy, COMz, and COP displacements (mean and standard error) after the perturbation. ^* denotes significant difference at alpha level of 0.05.

Figure 4

Times at which COMy (TCOMy), COMz (TCOMz), and COP (TCOP) reached their maximal/peak displacements (mean and standard error) after the perturbations for predictable and unpredictable conditions.