The control of multi-joint movements relies on detailed internal representations

Abstract

This paper addresses the question what level of detail is required in internal representa tions used in control of multi-joint movements, focusing on contact control tasks. Following Bernstein, we define the central problem to be which strategies are used in the nervous system in order to control the vastly redundant musculoskeletal system. Simplifications based on equilibrium point theories are rejected on the basis that when they are simple they do not lead to adequate behaviour, whereas when they are complex they implicitly introduce the detailed internal representations that they were meant to dispense with. Based on both experimental data and on simulation results, it is argued that timing of muscle activation needs to be precisely tuned to the task at hand and the environmental conditions. It is argued that it is impossible to achieve this without detailed internal representations of the properties of the effector system in relation to the environment. It is attempted to link Bernstein's notion of a hierarchical organization of the nervous system in which tasks are delegated to subsystems as low as possible in the hierarchical structure of the central nervous system, to recent advances in neuroscience. G.. I van Ingen Schenau et al./H um an Movement Science 14 (1995) 1, IntroductionMulti-joint movements are not realized by simply stringing together the movements in the joints involved. D ue to mechanical coupling and the action of poly-articular muscles, motions in one joint affect those in many others as well. Moreover, a human subject moving on one or two feet in a gravitational field can be considered as a highly unstable interlinked inverted pendulum controlled by a redundant set of actuators which are far from ideal force generators. This enormous complexity o f our action system was convincingly discussed already half a century ago by Bernstein (e.g., Bernstein, 1967). Based on his inspiring work and further stimulated by Gibson (e.g., Gibson, 1979) who stressed the guiding role of environmental information on the control of actions, many have tried to identify simplify ing principles in the organisation of movement control.Especially among ecological psychologists the idea o f detailed internal representations or motor programs has been disputed over the past decade (see Meijer and Roth, 1988 for discussion of this and related issues in the motor-action controversy). Since, indeed, it appears very unlikely that the central nervous system contains motor programs that deal with all details necessary to realize all variants of any movement observed in various environmental conditions, we sympathise with what has been defined by Turvey as "Round I of theorising on Bernstein's problem: how to minimize the executive responsibility of the CNS" (Turvey, 1990). We have consider able problems, however, with his " Round II" of theorising where selforganisation is proposed as a general principle underlying the organisation of all types of movement (Turvey, 1990; see also Kelso and Tuller, 1984; Schö...

Dynamical models of movement coordination

“…In a sense, Van Ingen Schenau et al (1995) are correct in stating that the discussed phase transitions in interlimb coordination require an inter nal representation, at least when we assume that they use this term in a broad sense (as we understand they do) to refer to the structural properties of neurons and groups of neurons (or "C P G 's") that define the necessary conditions for phase transitions to occur, such as connectivity, synaptic strengths and activity thresholds, This does not imply, however, that the concept of a motor programme is a useful explanatory construct, because this is a very different kind of internal representation, implying a very different approach. Rather than attempting to substitute or complement one phenomenological construct that captures the main features of phase transitions quite well with another construct that captures it less well, it seems a more productive strategy to attempt to uncover the processes that underlie or give rise to specific dynamics.…”

Parameter interdependence and success of skeletal muscle modelling

“…the processes that are responsible for the timing and fine tuning of forces and moments exerted by different muscles (c.f. several chapters of Winters and Woo, 1990;Jacobs and van Ingen Schenau, 1992a;Jacobs and van Ingen Schenau, 1992b;van Ingen Schenau et al, 1994;van Ingen Schenau et al, 1995). In that type of work attention is usually not focused on the properties of individual muscle but more on the interactions of the effects of forces they exert.…”

“…Work on intermuscular coordination (e.g., Jacobs and van Ingen Schenau, 1992a; Jacobs and van Ingen Schenau, 1992b;van Ingen Schenau et al, 1994;van Ingen Schenau et al, 1995) indicates that, to be able to perform a movement in the context of specified tasks in a particular environment, specified net moment needs to be exerted at specified joints and that particular combinations of activity of certain muscles is required. In other words specific muscle will have to exert specific forces at given times in order to create the required net moment in the joint they cross as well the reaction force vector exerted by the earth needed to be able to perform the specified task.…”

Coordination of multi-joint movements: An introduction to emerging views

“…Gielen et al (1995) provide an overview of constraints that have been proposed to be operating. Some of these are thought to be implemented neurally (e.g., task-specific relative activation of muscles; using only two rotational degrees of freedom to control subsystems with three rotational degrees of freedom); others are thought to arise from physical principles, in particular from musculo-skeletal mechanics (e.g., the tight coupling between joint rotations and translation of particular points on the skeleton; the tight relation between net joint torques and the force exerted on the environment; see also Van Ingen Schenau et al, 1995). Identification of such constraints is a prerequisite for proper understanding of the coordination of multi-joint movements.…”

“…In contrast, Bernstein (1967) argued that top-level commands cannot fully specify what will happen kinematically. As internal state variables exist at different hierarchical levels in the neuro-musculoskeletal system, the kinematics of the movement must rather be conceptualized as emerging from the interaction between top-level commands, numerous internal states and kinetic as well as informational couplings to the environment (see also Van Ingen Schenau et al, 1995). According to Bernstein, then, the central problem in the study of the coordination of multi-joint movements can be formulated thus: "how can we conceive of a theory of human action which explains that humans execute motor tasks in a highly consistent way, both intra-and inter-individually, notwithstanding the fact that the human action system contains a much larger number of neural, muscular and skeletal degrees of freedom than required by environment-related task goals".…”