Brief History of the Uncontrolled Manifold Hypothesis and Its Role in Motor Control

BACKGROUND: The apparent problem of motor redundancy was replaced by the principle of abundance and turned into a theoretical framework and associated toolbox for exploration of performance-stabilizing synergies. AIM and METHOD: We review briefly the development of the main methods within the UCM framework and some of the main findings, both basic and clinical. The UCM framework is naturally merged with the theory of hierarchical movement control with spatial referent coordinates. RESULTS: The UCM framework has established itself as a productive framework for the analysis of movement control, in particular as related to stability of salient performance variables. It led to the discovery of novel phenomena such as trade-offs within hierarchical systems, anticipatory synergy adjustments, synergies within systems of different complexity from single muscles to the whole body. It has also led to promising results offering sensitive biomarkers to various neurological disorders. Recent experiments suggest the existence of three main levels of organization of performance-stabilizing synergies tentatively associated with cortical, subcortical, and spinal circuitry. CONCLUSION: Currently, this approach is in its adulthood. Further progress may be expected in focusing on spaces of neural control variables, developing the method for analysis across species, and expanding the range and depth of clinical studies.