Disruptive influences of a cued voluntary shift on coordinated movement in Parkinson's disease.

A temporary and/or involuntary stoppage of movement is identifiable in the execution phase of writing, walking, and turning movements in individuals with Parkinson's disease (PD) and may be referred to as freezing. However, the unpredictability of such akinetic impairments has made it difficult to study experimentally. The present study compared PD and age-matched control groups in their ability to coordinate continuous and simultaneous upper limb movements in trials involving two parts. In the first part of each trial, participants performed either in-phase movements (symmetric, simultaneous movement toward and away from the midline of the body), or anti-phase movements (isodirectional). At the midpoint of the trial, they were signaled by an auditory metronome to execute an intentional and voluntarily switch from the coordination currently being performed to the opposite coordination pattern. In the second half of the trial participants were required to maintain performance in the other coordination mode. All trials were paced by an auditory metronome at one of three different speeds (0.75, 1.25, 1.75 Hz). Measures of temporal coordination (relative phase) indicated that overall, participants with PD required significantly longer periods of time to achieve a switch between coordination patterns compared to healthy controls, and experienced greater difficulty changing from the in-phase to anti-phase mode of coordination. As well, movement stoppage was observed in 53.9% of the in-phase to anti-phase switch trials, but in only 15.5% of the anti-phase to in-phase trials. We conclude that interruptions to movement execution are most common when switching to coordinated movements that impose greater motor demands in individuals with PD.