Rhythmic and discrete movements are frequently considered separately in motor control, probably because different techniques are commonly used to study and model them. Yet the increasing interest in finding a comprehensive model for movement generation requires bridging the different perspectives arising from the study of those two types of movements. In this article, we consider discrete and rhythmic movements within the framework of motor primitives, i.e., of modular generation of movements. In this way we hope to gain an insight into the functional relationships between discrete and rhythmic movements and thus into a suitable representation for both of them. Within this framework we can define four possible categories of modeling for discrete and rhythmic movements depending on the required command signals and on the spinal processes involved in the generation of the movements. These categories are first discussed in terms of biological concepts such as force fields and central pattern generators and then illustrated by several mathematical models based on dynamical system theory. A discussion on the plausibility of theses models concludes the work.
In the first part, the paper reviews experimental evidence for the existence of a motor primitive system located on the level of the spinal cord. In particular, the discussion is centred on the existence of central pattern generators and force fields (also: muscle synergies) defined in the spinal cord. Results showing the independence of these from cortical signals exist for animals up to the cat, or so. “In humans, the activity of the isolated spinal cord is not observable, […]: influences from higher cortical areas and from sensory pathways can hardly be excluded.”
The remainder of the article reviews dynamical systems that have been proposed as models for movement primitives. The models are roughly characterised according to the assumptions about the relationships between discrete and rhythmic movements. The authors define 4 categories: two/two, one/two, one/one and two/one, where a two means separate systems for discrete and rhythmic movements, a one means a common system, the number before the slash corresponds to the planning process (signals potentially generated as motor commands from cortex) and the number behind the slash corresponds to the execution system where the movement primitives are defined.
You would think that the aim of this excercise is to work out advantages and disadvantages of the models, but the authors mainly restrict themselves to describing the models. The main conclusion then is that discrete and rhythmic movements can be generated from movement primitives in the spinal cord while cortex may only provide simple, non-patterned commands. The proposed categorisation may help to discern models experimentally, but apparently there is currently no conclusive evidence favouring any of the categories (authors repeatedly cite two conflicting studies).