Differential time- and frequency-dependent structure of postural sway and finger tremor in Parkinson's disease

The time- and frequency-dependent patterns of standing balance centre of pressure (COP) and finger postural/resting tremor of 12 older individuals and eight age-matched Parkinsonian (PD) participants (on/off medication) were investigated. Tremor and COP data were analysed using measures of signal amplitude (RMS), time-dependent structure (approximate entropy, ApEn), time–frequency analysis and synchrony (Cross ApEn). Results showed that the PD individuals had significantly greater tremor amplitude and COP excursions in comparison to controls. Differences in the time-dependent structure were also observed between groups. In comparison to the elderly, the resting/postural tremor output of the PD subjects was more regular (lower ApEn). However, for the postural measures, a reciprocal pattern was observed with the COP being more complex (higher ApEn). All group differences were magnified when the PD individuals were off their medication. There was also greater synchrony between tremor and postural sway for the PD individuals, indicating a high degree of association between these motor outputs. These results are consistent with the view that the neural signal driving the enhanced limb tremor in PD is propagated throughout the motor system, consequently emerging within the postural sway dynamics. This commonality of motor output may be a contributing factor in the differential pattern in the dynamics of effector signal structure in PD as a function of task.