Expert Motor Synergies Emerge Predominantly Offline During Early Skill Learning

Everyday skilled actions depend on the formation of coordinated motor synergies that integrate multiple effectors into stable, low-dimensional control units. Although initial practice of a new skill leads to rapid performance improvements, it is unclear whether the underlying movement kinematics reorganize on a similar timescale and in a way that directly relates to these gains. It also remains uncertain whether such reorganization occurs mainly during active practice or instead during brief rest breaks. Here, we tracked the temporal evolution of multi-digit synergy formation during early learning of a naturalistic keypress skill. Initial practice rapidly sculpted the motor repertoire toward higher-order, temporally compressed and overlapping multi-digit synergies. Highly stable synergies emerged after only minutes of practice and continued to be expressed across two training days. Notably, these expert synergies were primarily shaped during brief rest breaks and robustly predicted individual skill proficiency. Across learning, distinct synergy subtypes were evident, differing in their prevalence. Rarely expressed synergies reflected transient novice patterns, synergies expressed at intermediate levels indexed exploratory and trial-initiation strategies, and highly expressed synergies emerged later to dominate performance, reflecting the consolidation and expansion of skilled motor control. Together, these findings indicate that skilled performance is supported by the early formation of a compact repertoire of expert multi-digit synergies that emerge preferentially across rest periods and predict subsequent skill gains. They further raise the hypothesis that explicitly training such expert synergies-alongside task goals-could enhance learning in domains such as the arts, sport, and neurorehabilitation.