Coupling between limb tremor and postural sway in Parkinson's disease

Increased tremor and postural instability are motor problems commonly associated with Parkinson's disease (PD). Despite the similarity between these oscillatory forms, little is known about the relation between them, especially for individuals with enhanced tremor. This study was designed to examine the nature of any relation between center of pressure (COP) excursions and postural/resting limb tremor of young, older individuals, and Parkinsonian participants in their different medication states. The resting and postural tremor for the PD participants was characterized by a single, prominent peak frequency between 4 and 7 Hz. The postural tremor for young/older participants contained smaller peaks between 1 to 4 and 7 to 12 Hz although no prominent peak was seen in their resting tremor. The AP and ML COP dynamics of all participants was characterized by a major peak between 0.1 and 0.5 Hz. An additional peak was observed in the COP output of the PD participants between 4 and 7 Hz. While no tremor-COP coupling was observed for the young/old groups, coherence analysis revealed a significant degree of coupling between COP motion and tremor between 4 and 7 Hz for PD participants. These results highlight that the amplified tremor in PD can manifest itself in COP dynamics. This finding may have implications for postural stability for this patient group.