Motor Synergies and Their Changes with Practice

We review a series of studies that used the framework of the uncontrolled manifold (UCM) hypothesis to quantify changes in motor synergies with practice. The UCM hypothesis states that control of a multi-element action, at any time, may be associated with creation of a subspace (a UCM) within the state space of the elements. This subspace corresponds to a stable value of an important performance variable or several variables. Strength of a synergy may be estimated quantitatively as proportion of the total variance of its elements, e.g. across several trials at a task, that lies within the UCM. Quantitative analysis of covariation patterns of kinematic and kinetic elemental variables with respect to stabilization of different, task -specific performance variables allowed to monitor changes in motor synergies with practice. The studies have demonstrated two stages in practice-related effects. Early in practice of novel tasks, synergies stabilizing important performance variables emerged and strengthened. Later, in some instances, variability of elemental variables which did not affect the performance variables decreased more rapidly leading to the synergies becoming seemingly weaker. Experiments with transcranial magnetic stimulation applied to the primary motor cortex have suggested that practice led to plasticchanges in neural structures mediating motor response to the stimulation.