Synergic control of movement: From single muscles to the whole body

Within his multilevel scheme for the construction of movement, Nikolai Bernstein defined synergies as neural organizations of elements with two functions: (1) to group elemental variables and alleviate the problem of redundancy and (2) to ensure dynamical stability of salient performance variables. This definition has led to the emergence of two toolboxes, matrix factorization methods and analysis of covariation, in particular, within the uncontrolled manifold hypothesis. Over the past years, these methods have been applied to identify and quantify synergies across spaces of elemental variables, tasks, and populations. The concept of synergy is readily compatible with the idea of movement control using neural variables that define spatial referent coordinates (RCs) for the effectors at different levels of the presumed hierarchy, from whole-body movements, to limb movements, to actions of individual muscles, and to individual motor units. This chapter reviews studies of healthy persons and patients with various neurological disorders and demonstrates that the introduced framework is inherently rich and able to account for a variety of known facts as well as predict new nontrivial findings. Studies of synergies offer a potentially powerful toolbox to explore early impairments in the control of action stability, effects of treatment, and the role of supraspinal and spinal circuitry in stability of action.