Asymmetries in reactive and anticipatory balance control are of similar magnitude in Parkinson's disease patients

Highlights

• PD patients can have asymmetrical static and reactive balance control.

• We investigated anticipatory balance control asymmetries in PD patients.

• PD patients controlled their anticipatory balance asymmetrically.

• Asymmetries in reactive and anticipatory balance control were of the same magnitude.

• Reactive and anticipatory balance asymmetries may share a common pathophysiology.

Abstract

Many Parkinson's disease (PD) patients show asymmetries in balance control during quiet stance and in response to perturbations (i.e., reactive balance control) in the sagittal plane. In addition, PD patients show a reduced ability to anticipate to self-induced disturbances, but it is not clear whether these anticipatory responses can be asymmetric too. Furthermore, it is not known how reactive balance control and anticipatory balance control are related in PD patients. Therefore, we investigated whether reactive and anticipatory balance control are asymmetric to the same extent in PD patients.

14 PD patients and 10 controls participated. Reactive balance control (RBC) was investigated by applying external platform and force perturbations and relating the response of the left and right ankle torque to the body sway angle at the excited frequencies.

Anticipatory postural adjustments (APAs) were investigated by determining the increase in the left and right ankle torque just before the subjects released a force exerted with the hands against a force sensor.

The symmetry ratio between the contribution of the left and right ankle was used to express the asymmetry in reactive and anticipatory balance control; the correlation between the two ratio's was investigated with Spearman's rank correlation coefficients.

PD patients were more asymmetric in anticipatory (p = 0.026) and reactive balance control (p = 0.004) compared to controls and the symmetry ratios were significantly related (ρ = 0.74; p = 0.003) in PD patients.

These findings suggest that asymmetric reactive balance control during bipedal stance may share a common pathophysiology with asymmetries in the anticipation of voluntary perturbations during, for instance, gait initiation.

Introduction

In patients with PD, motor symptoms (e.g., rigidity) typically manifest themselves asymmetrically [1], [2]. Recent studies have shown that balance control can also be asymmetrical in PD patients [3], [4], [5], [6]. Consequently, PD patients may be less able to control the torques needed to shift the body's center of mass in the lateral and sagittal directions; which is essential for the planning and preparation of actions such as gait initiation [7], [8].

Several studies have indicated that anticipatory postural adjustments (APAs), i.e. changes in postural muscle activity or joint torques prior to self-inflicted postural actions, are preserved in patients with PD, although slower and less pronounced in magnitude [8], [9]. It is unclear however, whether APAs in PD can also be asymmetric [8], [9].

Crucial for maintaining an upright posture is a) reactive balance control (the ability to respond to unpredictable perturbations), b) the ability to anticipate to a perturbation and c) the ability to make a step. It has been suggested that the coupling between posture (i.e., anticipatory balance control) and actual stepping, as normally seen in healthy subjects is disturbed in PD patients [11], [12]. It is however not clear whether reactive balance control and anticipatory balance control are abnormally related in PD patients.

Therefore, we investigated whether reactive and anticipatory balance control are asymmetric to the same extent in PD patients. We investigated, in a group of patients with PD, the asymmetries in APAs prior to a self-inflicted perturbation (i.e., anticipatory) and dynamic balance control during continuous unpredictable mechanical perturbations (i.e., reactive). A control group was included to investigate if asymmetries in the patient group were significantly larger than in healthy subjects.