Bernstein's construction of movements reassessed: Part I: Levels A and B, and introduction to level C

This is the first part of the planned series of two papers with the purpose to update and reassess the classical hierarchical scheme for the construction of movements introduced by Nikolai Bernstein. Here, we discuss the two lower levels. In particular, a definition of tone is offered within the theory of movement control with changes in the stretch reflex threshold. We suggest splitting the lowest level of control (Level of Tone, A) into two sub-levels, A1 and A2, with the former involved with the stretch reflex and the latter reflecting the basic commands parametrizing the stretch reflex, from motor units to muscles and to multi-muscle effectors. The next level (Level of Synergies, B) is also split into two sub-levels, B1 and B2, related to the formation of robust groups (modes) of muscles or effectors and to ensuring dynamical stability of salient performance variables in spaces of such modes, respectively. We introduce the Level of Spatial Field (Level C) and plan to discuss it in more detail, as well as the two higher levels, within the follow-up paper. A new hierarchical scheme based on the ideas of Bernstein is suggested to incorporate new data and theoretical advances.