Submovements in manual tracking: people with Parkinson’s disease produce more submovements than age-matched controls

Background: In general, people are unable to produce slow, smooth movements - as movements become slower (i.e., with longer durations), they become jerkier. A hallmark feature of Parkinson’s disease is bradykinesia - slowness of movement. In this study, we investigate the intersection of these two observations - how do people with Parkinson’s disease (PwP) perform in a slow tracking task, and how does it vary as a function of movement frequency? On the one hand, as PwP move more slowly in day-to-day life, they may be better in a slow tracking task. On the other hand, their general impairment in movement production may lead to worse tracking outcomes. Methods: We used a well-tested tracking task known as the one-person mirror game, where participants control the left-right movement of an ellipse on a graphics tablet. They did so using a stylus and were instructed to match the horizontal location of a stimulus, an ellipse moving in a sinusoidal fashion at different movement frequencies and peak velocities. We calculated the submovement rate, identifying both type 2 (acceleration zero crossings) and type 3 (jerk zero crossings) from the trajectories, as well as relative position error (dX) and mean timing error (dT). To account for age-related performance decline, we tested three groups: PwP (N = 31), age-matched controls (OC; N = 29), and younger controls (YC; N = 30) in a cross-sectional study, and used mixed-design ANOVAs to compare across groups and movement frequencies. Results: We reproduced earlier results showing that slow movements (i.e., with lower frequencies) require more submovements to track. PwP also generally performed more submovements than the other two groups, but only for type 3 submovements, whereas OC and YC performed submovements at a similar rate. Younger controls (YC) performed fewer tracking errors than older participants (both PwP and OC), and OC performed better than PwP. Conclusions: The ability to smoothly track showed an age-related decline, with PwP producing more errors and using more submovements. This may be due to reduced automaticity in movement production. The findings of the study can be used to guide optimal movement frequencies for motor training for older adults and PwP.