Extrinsic variability but not intrinsic variability affects the search for synergies in de-novo motor learning

De-novo motor learning involves emergence of novel task-specific joint angle coordination patterns, called synergies, through structured change in joint angle variability, called search. Search implies variability in joint angles and task performance, but the understanding of the relationship between variability and learning is still unclear. This study examines the contributions of extrinsic variability - differing practice schedules in the task (blocked vs. random) and intrinsic variability - flexibility in individual's movements - on learning, search and synergy formation. Participants were first categorized based on their intrinsic variability (i.e., joint angle covariation in an adaptive reaching task). Learning was then evaluated on a virtual lateral interception task with a redundant mapping between joint angles and paddle position that necessitated forming new synergies. Overall, search reduced, and synergies emerged with practice. Intrinsic variability did not affect learning, search or synergy formation. Extrinsic variability increased search behaviour but did not interact with emergence of synergies. During practice, the variability orthogonal to the uncontrolled manifold (solution sub-space within the joint space) reduced rapidly in the initial phases while the change in variability along the uncontrolled manifold persisted over practice. We demonstrate search in joint space during learning is structured and extrinsic variability increases search behaviour.