Motor Control Theories Chapter 5 Motor Control Theories Concept: Theories about how we control coordinated movement differ in terms of the roles of central and environmental features of a control system

Theory and Professional Practice What is a theory? Accurately describes a large class of observations Make definite predictions about results of future observations (Hawking, 1996) Theories of motor learning and control focus on: Explaining human movement behavior Providing explanations about why people perform skills as they do Does a theory have relevance to professional practice? Provides the “why” basis for what practitioners do [See Figure 5.1]

Motor Control Theory Describes and explains how the nervous system produces coordinated movement of motor skill in a variety of environments Two important terms: Coordination The degrees of freedom problem

Coordination Patterning of body and limb motions relative to the patterning of environmental objects and events (Turvey, 1990) Two parts to consider: Movement pattern of a skill in relation to a specific point of time Context of the environment of the head, body, and/or limb movements so the actions can be accomplished

Degrees of Freedom Problem Degrees of freedom (df) = Number of independent elements in a system and the ways each element can act Degrees of freedom problem = How to control the df to make a complex system act in a specific way e.g. The control of a helicopter’s flight (described in the textbook) Degree of freedom problem for the control of movement: How does the nervous system control the many df of muscles, limbs, and joints to enable a person to perform an action as intended?

Two General Types of Control Systems Open- and Closed-Loop Control Systems [See Figure 5.3] Incorporated into all theories of motor control Models of basic descriptions to show different ways the CNS and PNS initiate and control action Each has a central control center (executive) Function to generate and forward movement instructions to effectors (i.e., muscles) Each includes movement instructions from control center to effectors Content of the instructions differs between systems

Differences Between the Systems Open-Loop Does not use feedback Control center provides all the information for effectors to carry out movement Does not use feedback to continue and terminate movement Movement instructions Movement control center Movement effectors Closed-Loop Uses feedback Control center issues information to effectors sufficient only to initiate movement Relies on feedback to continue and terminate movement Movement instructions Movement Control center Movement effectors

Two Theories of Motor Control Motor Program-based theory: Memory-based mechanism that controls coordinated movement Dynamic Pattern theory (a.k.a. Dynamical Systems): Describes and explains coordinated movement control by emphasizing the role of information in the environment and mechanical properties of the body and limbs

Motor Program-Based Theory Best example comes from “Schema Theory” by Schmidt (1988) Generalized motor program (GMP): Hypothesized memory-based mechanism responsible for adaptive and flexible qualities of human movement Proposed that each GMP controls a class of actions that have common invariant characteristics

Motor Program-Based Theory, cont’d GMP Function To serve as the basis for generating movement instructions prior to and during the performance of an action GMP Characteristics Invariant features Characteristics of the GMP that do not vary across performances of a skill within class of actions The identifying signature of a GMP Parameters Specific movement features added to the invariant features to enable the performance of a skill in a specific situation Characteristics can vary from one performance of a skill to another

Motor Program-Based Theory, cont’d Invariant features and parameters Example of an invariant feature Relative time of the components of an action (i.e. % of total time each component uses during performance) Example of a parameter Overall time (i.e.) for performing a skill An Analogy from Music and Dance Relative time = Rhythm (beat) of the music, e.g. The 3 beats to a measure for a waltz Overall time = Tempo (The speed at which you waltz) Regardless of how fast or slow you waltz, the rhythm remains the same (i.e. invariant)

GMP for Walking Invariant Parameter Relative time for gait cycle phases - Parameter Walking speed

Motor Program-Based Theory: Testing Relative Time Invariance Experiment by Shapiro et al. (1981) Used gait characteristics to test prediction of relative time invariance for a class of actions controlled by a GMP: Are walking and running one or two classes of action? Assessed 4 components of 1 step cycle Calculated relative time for each component at 9 different speeds (3 – 12 km/hr) Relative time = % of total time each component required for 1 step cycle Results: Relative time similar within speeds when walking but different from speeds when running (similar within speeds when running) [See Figure 5.5]

Dynamic Pattern Theory (a.k.a., Dynamical Systems) Describes the control of coordinated movement that emphasizes the role of information in the environment and dynamic properties of the body/limbs Began to influence views about motor control in early 1980’s Views the process of human motor control as a complex system that behaves like any complex biological or physical system Concerned with identifying laws (natural and physical) that govern changes in human coordination patterns

Dynamic Pattern Theory Concepts Motor control system operates on the basis of non-linear dynamics: Behavioral changes are not always continuous, linear progressions but often make sudden and abrupt changes Behaviors specified by environmental and task characteristics/conditions Behaviors are self-organized

Dynamic Pattern Theory Concepts: Attractors Attractor – A stable state of the motor control system that leads to behavior according to preferred coordination patterns (e.g. walking) Characteristics of an attractor: Identified by order parameters (e.g., relative phase) Control parameters (e.g., speed) influence order parameters Minimum trial-to-trial performance variability Stability – Retains present state despite perturbation Energy efficient

Dynamic Pattern Theory Concepts: Order and Control Parameters Order parameters Also called collective variables Functionally specific and abstract variables that define the overall behavior of the system Enable a coordinated pattern of movement that can be reproduced and distinguished from other patterns Relative phase is the most prominent of order parameters which represents the movement relationship between two movement segments (see chapter 2)

Order and Control Parameters, cont’d A variable, when increased or decreased, will influence the stability and character of the order parameter Is important to identify since it becomes the variable to manipulate in order to assess the stability of the order parameter Provides the basis for determining attractor states for patterns of limb movement

Dynamic Pattern Theory Concepts: Self-Organization When certain conditions characterize a situation, a specific pattern of limb movement emerges This pattern of movement self-organizes within the characteristic of environmental conditions and limb dynamics

Attractors and Self-Organization for Movement Coordination Gait Transitions Research (to be discussed more in ch. 7) shows that if a person begins walking on treadmill at slow speed Treadmill speed increases every few minutes Person begins to run at a certain speed [not same speed for all people] Same effect if person begins running on treadmill - Begins to walk at certain speed Swim Stroke Transitions Research in France (2004) 14 elite male swimmers Each trial involved a swim velocity increase [began at preferred velocity] Arm-stroke analysis showed 2 distinct arm movement coordination modes Began in one mode but abruptly began 2nd mode at a specific swim velocity

Attractors and Self-Organization for Movement Coordination, cont’d Discuss how the two research examples on the previous slide demonstrate the dynamic pattern theory concepts of: Self-organization Control parameter Attractors (i.e., stable coordination states) Non-linear behavior change

Dynamic Pattern Theory Concepts: Coordinative Structures Functional synergies (i.e. cooperative groups) of muscles and joints that act cooperatively to produce an action If a perturbation stops one set of muscles from working, another works in its place e.g. walking with a leg cast Develop through practice, experience, or naturally

Dynamic Pattern Theory Concepts: Perception and Action Coupling The linking together (i.e. coupling) of movement to environmental information The perception part The detection of critical invariant information in the environment The action part The movement that becomes associated with what is specified by the environmental information An example When walking, the time to contact an object in your pathway (specified by the perception of the changing size of the object) determines when you initiate stepping over the object i.e. Your stepping action is “coupled” with your visual perception of the object

Present State of the Control Theory Issue Currently, both the motor program-based theory and dynamic pattern theory predominate Research investigating each has shown that a theory of motor control cannot focus exclusively on movement information specified by the CNS Task and environmental characteristics must be also be taken into account Speculation exists that a hybrid of the two theories as a compromise theory could emerge to explain the control of coordinated movement