Associated movements, postural adjustments and synergies: some comments about the history and significance of three motor concepts.

Associated movements, postural adjustments and synergies are three motor concepts directly related to each other, although their historical developments have followed relatively independent paths. We show in this study how some notions, often very old, are evolving and become more precise as the number of observations and experimental results increases. We show also how the relationships between these notions and the terms associated with them may vary depending on the historical period and on the authors. The various kinds of muscular activity related to a movement, which are the object of this review, present several aspects, each of them being often described by different terms, which are analyzed. On the other hand we study the various meanings of some terms or expressions, which may correspond to very different aspects of the motor system. The three concepts analyzed have in common a definition from final manifestations (essentially muscular and mechanical) of motor activity. This characteristic is probably due to the lack of enough experimental data to use more refined criteria for definition, like those based upon nervous structures implicated, mechanisms involved or functional significance. The situation may change in the near future, as this field of investigation is now entering a phase of rapid development.