Anticipatory postural adjustments in conditions of postural instability

Abstract

Objectives: The purpose of this study was to investigate anticipatory postural adjustments (APAs) in standing subjects who performed a standard motor action triggering a standard postural perturbation (releasing a 2.2 kg load from extended arms) in conditions of different stability requirements.

Methods: The degree of stability was varied either by balancing on special boards with long and narrow support beams or by instructions to the subjects. In the first series of experiments 13 subjects stood on the board facing either perpendicular to the beam (instability in a sagittal plane) or along the beam (instability in frontal plane); different widths of the beam were used to vary the degree of instability. During the second series of experiments (6 subjects) inclined and one-legged postures were used to induce instability in sagittal and frontal planes respectively. EMG activity of rectus abdominis, erector spinae, rectus femoris, biceps femoris, tibialis anterior, and soleus muscles were recorded. Statistical methods included repeated measures analysis of variance (ANOVA) with direction of instability and level of instability being major factors, descriptive statistics, and post hoc Student's t tests.

Results: The integral measure of changes in the background electromyographic activity of postural muscles during APAs depended on two factors related to the postural task: (1) standing on a platform with a narrow support area led to an attenuation of the APAs; and (2) these effects were stronger when instability was in a sagittal rather than in the frontal plane. The anticipatory component in the displacement of the center of pressure did not show a clear attenuation that would depend on the direction of instability.

Conclusions: We suggest a hypothesis that, in conditions of high stability demands, the central nervous system may suppress APAs as a protection against their possible destabilizing effects. These effects are more pronounced when the direction of an expected perturbation is in the plane of instability.

Introduction

Anticipatory postural adjustments (APAs) can be seen in postural muscles prior to self-triggered or self-inflicted postural perturbations. Their assumed role is to counteract the expected mechanical effects of the perturbation in a feedforward manner (Bouisset and Zattara, 1990; Massion, 1992). The process of generation of APAs is likely to be affected by 3 major factors: (1) expected magnitude and direction of the perturbation; (2) voluntary action associated with the perturbation; and (3) postural task.

Effects of the first two factors, related to the forthcoming perturbation and to the motor action, have been studied relatively extensively. In particular, it has been shown that the intensity of APAs is graded as a function of the magnitude and direction of an expected postural perturbation (Horak et al., 1984; Dick et al., 1986; Lee et al., 1987; Bouisset and Zattara, 1990; Aruin and Latash, 1995a). Experiments with load manipulations let the experimenters separate the effects of the magnitude of motor action and of the magnitude of perturbation upon the APAs. Such studies have suggested that both the magnitude of the action and the magnitude of the perturbation can affect APAs independently (Dufosse et al., 1985; Paulignan et al., 1989; Aruin and Latash, 1995b, Aruin and Latash, 1996).

Reports on the dependence of APAs on the stability demands of the postural task have been somewhat conflicting, suggesting that the dependence may be non-monotonic. In particular, APAs associated with voluntary movements were attenuated or absent when the posture was unstable (Nouillot et al., 1992) as well as when it was very stable (Nardone and Schieppati, 1988). In experiments with backward bending, the anticipatory EMG burst in the calf muscles was reduced when the subjects performed the same task while standing on a small plate placed on a narrow support (Pedotti et al., 1989).

Based on these reports, we formulated the following hypothesis: Since, by their own nature, APAs can provide only an approximate compensation for an expected perturbation, the central nervous system (CNS) may consider them as additional potential sources of postural perturbation. Thus, in conditions of postural instability, the CNS may use an adaptive strategy in order not to subject the unstable equilibrium to additional, potentially perturbing forces. Such a strategy may lead to a reorganization of APAs which may include, in particular, an attenuation of APAs reflected in electromyographic and biomechanical characteristics, as well as other changes in the timing and level of activity in some postural muscles and shifts of the center of pressure. To test this hypothesis, we used an earlier paradigm (Aruin and Latash, 1995b) which includes releasing a standard load from the extended arms with a standard motor action. Within this paradigm, stability requirements were manipulated while both motor action and parameters of the perturbation remained unchanged.