Random walk and drifts within the uncontrolled manifold during multi-finger force production tasks

We explored processes that take place in the uncontrolled manifold (UCM) for the task of two-hand accurate force production. Based on previous studies of structured motor variability, we hypothesized that these processes would represent a superposition of a random walk within a shallow potential field and a slow drift of the potential field. Young healthy participants performed accurate force production over 60 s with visual feedback on the force magnitude, starting from different initial sharing of the total force between the hands. No feedback on force sharing was available after the first 5 s. Trajectories within the UCM demonstrated a random walk with the characteristic step time of 50–80 ms and a slow drift, primarily toward the 50:50 sharing pattern, although drifts away from 50:50 and with reversals were also observed. The Hurst exponent of the diffusion plots showed a trend toward persistent fluctuations at times under 0.2 s and anti-persistent fluctuations at times over 0.5 s. The characteristic drift times were between 5 and 15 s. The drift magnitudes were large for the initial sharing, different from 50:50, and small for the 50:50 sharing. The Hurst exponent for longer times correlated significantly with the peak-to-peak drift magnitude. We conclude that natural processes within the UCM represent a superposition of random walk and slow drift and can cause drifts in performance variables reported earlier. Spinal circuitry is a likely major contributor to the random walk. These results suggest direct application to clinical studies of impaired movement stability.