Motor synergies during manual tracking differ between familiar and unfamiliar trajectories

Synergistic control of the effector space allows high precision in task-relevant degrees of freedom, while noise is limited to task-irrelevant degrees of freedom. The present study investigates whether this typical structure of the variance-covariance matrix of the joint angles during manual tracking differs between familiar and unfamiliar trajectories. Subjects tracked a target moving in 2D on a graphics tablet with a hand-held pen, while their arm movements were not restricted. Subjects familiarized themselves with one target trajectory during an initial training block with 40 periodic trials. In the following test block, this familiar trajectory and several unfamiliar trajectories were presented in a mixed-block design to study prediction effects at the level of endpoint and joint trajectories. The differences in the synergistic control of arm movements were analyzed using the "uncontrolled manifold method." The results showed smaller variances and weaker motor synergies during tracking of familiar trajectories than during tracking of unfamiliar trajectories. The decrease in the synergy index was due to a stronger decrease in the variance irrelevant than of the variance relevant for pen position. In the context of motor control theory, these results suggest that tracking movements on familiar and unfamiliar target trajectories do not only differ in the available knowledge about target location but also apply different strategies to control the effector space.