Motor Control Theories

1
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
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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

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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

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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

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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 helicopters 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?

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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

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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
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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

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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

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Motor Program-Based Theory, contd

• 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

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Motor Program-Based Theory, contd

• 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)

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GMP for Walking

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

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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

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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 1980s
• 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

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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

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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

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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)

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Order and Control Parameters, contd

• Control parameter
• 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

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Dynamic Pattern Theory Concepts Self-Organization

• 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

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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

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Attractors and Self-Organization for Movement
Coordination, contd

• 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

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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

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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

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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