Hierarchies of Synergies in Human Movements

This brief review addresses the problem of motor redundancy, which exists at many levels of the neuromotor hierarchies involved in the production of voluntary movements. An approach to this problem is described based on the principle of abundance. This approach offers an operational definition for motor synergies using the framework of the uncontrolled manifold hypothesis. It is shown that hierarchical systems have inherent trade-offs between synergies at different control levels. These trade-offs have been demonstrated in experimental studies of human multi-finger pressing and prehension. They are likely to be present in other hierarchical systems, for example those involved in the control of large groups of muscles. The framework of the equilibrium-point hypothesis offers a physiologically based mechanism, which may form the basis for hierarchies of synergies.