Impaired synergic control of posture in Parkinson's patients without postural instability

Abstract

Background: Postural instability is one of most disabling motor symptoms in Parkinson's disease. Indices of multi-muscle synergies are new measurements of movement and postural stability.

Objectives: Multi-muscle synergies stabilizing vertical posture were studied in Parkinson's disease patients without clinical symptoms of postural instability (Hoehn-Yahr ≤ II) and age-matched controls. We tested the hypothesis that both synergy indices during quiet standing and synergy adjustments to self-triggered postural perturbations would be reduced in patients.

Methods: Eleven Parkinson's disease patients and 11 controls performed whole-body tasks while standing. Surface electromyography was used to quantify synergy indices stabilizing center of pressure shifts in the anterior-posterior direction during a load-release task.

Results: Parkinson's disease patients showed a significantly lower percentage of variance in the muscle activation space accounted for by the first four principal components, significantly reduced synergy indices during steady state, and significantly reduced anticipatory synergy adjustments (a drop in the synergy index prior to the self-triggered unloading).

Conclusions: The study demonstrates for the first time that impaired synergic control in Parkinson's disease can be quantified in postural tasks, even in patients without clinical manifestations of postural instability. Synergy measurements may provide a biomarker sensitive for early problems with postural stability in Parkinson's disease.

Keywords: Anticipatory synergy adjustments; Muscle mode; Parkinson's disease; Posture; Synergy.

Figure 1

Normalized EMG time profiles of a few selected muscles across the load-release (LR) trials for a representative PD subject: tibialis anterior (TA), soleus (SOL), gastrocnemius medialis (GM), biceps femoris (BF), rectus femoris (RF), and thoracic erector spinae (EST). The solid lines are the averages of the corresponding muscle activity across trials while the shaded areas shows one standard error. The vertical dashed lines represent the moment when the subject released the load. Arrows in the EMG panels mark the onset of APAs (t_APA).

Figure 2

A: Time profiles of the synergy index (ΔV_Z), averaged across patients (dashed line) and controls (solid line), during the load-release (LR) task. SS₁ and ≈T₀ are the time intervals selected for statistical analysis. Note the significantly higher ΔV_Z in CO and the drop in this index before T₀. B: Z-transformed synergy indices (ΔV_Z) for the PD (black bar) and CO (white bar) groups over the SS₁ and ≈T₀ phases. Mean ± SE values across subjects within each group are shown. Statistically significant differences are marked with a star.