Chapter 5 Movement Models Jeffrey C. Ives Copyright © 2014 Wolters Kluwer Health| Lippincott...

Chapter 5

Movement Models

Jeffrey C. Ives

Copyright © 2014 Wolters Kluwer Health| Lippincott Williams & Wilkins

Motor Behavior: Connecting Mind and Body for Optimal Performance

Objectives and Questions

Copyright © 2014 Wolters Kluwer | Health Lippincott Williams & Wilkins

1. What is motor abundance and the degrees of freedom problem?

2. What is the purpose of movement models?

3. What are open- and closed-loop systems, and what models fit within these systems?

4. What do the terms generalized motor program, central pattern generator, schema, reflex model, and internal model all mean?

5. What are synergies and coordinative structures?

6. What are similarities among old and new models?

7. What are the systems model, constraints, affordances, and perception–action coupling?

The Need for Models

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins

• For any given movement, there are numerous ways the movement could be done.– This situation is called motor redundancy, which

enables a wide range of options.– Redundancy also poses a problem in selecting just one

solution, called the degrees of freedom problem.• Determining what and how the brain and body are trying to

control movement is theorized using models.• Models provide a “big picture” framework to explain how

the CNS and neuromuscular systems work to make movement.

Objectives 1, 2

Models


• Models provide a general framework of the processes and physiological systems contributing to the formation and execution of motor acts.

• Movement models serve two main purposes. – Provide a conceptual framework by which to

understand how movements are formulated and executed, and this enables prediction of change following interventions.

– Provide a framework for practical use to devise more effective programs for rehabilitation, practice, and training

Objective 2

Feedforward Versus Feedback Models


• Traditional models of motor control have been broadly described as open loop or closed loop. – Closed-loop models explain movement as an outcome of

feedback-initiated reflex actions and prepatterned neural systems.

• Does not require sophisticated commands from higher brain centers

– Open-loop models suggest a strict top–down hierarchy across CNS and neuromuscular structures in planning, executing, and initiating movement.

• The role of feedback in movement initiation and execution is minimized.

Objective 3

Feedforward Versus Feedback Models (cont.)

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & WilkinsObjective 3


Contemporary Hierarchical Versus Heterarchical Models


• Hierarchical models are similar to open loop, describing a systematic command structure from top to bottom.

Objectives 2, 3


Contemporary Hierarchical Versus Heterarchical Models (cont.)


• Heterarchical models are similar to closed loop, describing a distributed and balanced command and execution system.

Objectives 2, 3


Closed-Loop, Feedback-Based, and Heterarchical Models


• The simplest models of motor control are reflex models.– Movement stems from chaining together of reflex

actions that provide building blocks of complex behavior.

– In many animals, basic acts such as chewing, swallowing, and “fight or flight” actions are initiated by sensory feedback and executed by reflex movements.

– Reflex models are based on the presence of hardwired neural circuits and produce fixed movement patterns.

Objectives 3, 4

Closed-Loop, Feedback-Based, and Heterarchical Models (cont.)


• Hardwired circuits can also produce more complex stereotyped movements through central pattern generators (CPGs).

• CPGs are built-in movements initiated by CNS or sensory systems.

• Because they can run without complex commands or on sensory input only, they are considered closed loop.

Objectives 3, 4


CPGs in Locust Flying

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & WilkinsObjectives 3, 4


CPGs in the Spinal Cat

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & WilkinsObjectives 3, 4


Human Limb CPGs


• Circumstantial evidence for arm CPGs suggests each arm has its own pattern generator.

• CPGs can be reinforced by sensory feedback, though initiated and driven by nominal supraspinal commands.

Objectives 3, 4


Human CPGs for Walking


• Mounting evidence suggests walking CPGs in humans.

• CPGs may be exploited to improve walking performance in hemiparetic patients.

• Body-weight supported training is one therapeutic tool to engage the CPG.

• PBS video moving memories

Objectives 3, 4


Complex Heterarchical Models


• Complex closed-loop models include involvement of higher brain centers but still rely on feedback loops.

• Brain centers provide basic command to the next lower level, which in turn modifies and “re-commands” the signals and routes them out to the next lower levels. – Modification via sensory feedback is essential to fulfill

the command signals at each level.

Objectives 3, 4

Complex Heterarchical Models (cont.)


• Theories of what constitutes the motor commands vary.– Equilibrium point hypothesis suggests the commands

set stretch reflex thresholds.– Uncontrolled manifold hypothesis posits that the brain

activates series of synergistic muscle actions.– The brain may only offer “suggestions” to the next

lower levels.

Objectives 3, 4

Complex Heterarchical Models and Synergies


• New models often rely on synergistic muscle and limb actions to simplify the CNS command structure.– Synergies are ensembles or groupings of muscles and

limbs that work together as a functional unit.– Actions of limbs or muscles constrain what actions can

happen at other limbs or muscles.• Synergies involve inherent neural pathways, muscle and

limb biomechanical properties, and learned behaviors. • Synergistic actions reduce degrees of freedom and

simplifies CNS planning.

Objectives 3, 4, 5

• Synergies among opposite limbs during bilateral movements are called coordinative structures.

• Synergies in muscle activation and timing are seen in wrist out-of-phase movements transitioning into in-phase movements during rapid movements.

Synergies and Coordinative Structures

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & WilkinsObjectives 3, 4, 5


Synergies and Coordinative Structures (cont.)


• In this experiment, based on the work of Kelso and colleagues, asymmetric movements assimilated the timing of one another such that each arm arrived at the target at the same time.

Objectives 3, 4, 5



Synergies and Coordinative Structures (cont.)

Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & WilkinsObjectives 3, 4, 5

• The experiment demonstrated here shows coupling among arms and legs.

Summary of Heterarchical Models and the Need for Hierarchical Control


• There is evidence for distributed control of motor actions from the CNS to peripheral systems such as CPGs and synergies.– Yet, heterarchical models do not explain nuances that

influence movement execution.– Heterarchical models cannot easily explain the widely

distributed and highly complex actions of the brain that accompany movement.

• Thus, the need for a centralized command system arising out of brain structures: hierarchical control

Objectives 3, 4, 5

Open-Loop and Hierarchical Models


• Hierarchical models suggest that motor skills arise from comprehensive sets of CNS commands.– Movements are considered centrally preprogrammed.– Precise manipulation of movement characteristics

comes from a continually involved CNS controller. • Feedback from sensory systems comes back into the

brain centers but is largely used to prepare or modify the next movement.

• The initiation of movement is purely open loop because there has been no preceding movement to provide feedback.

Objectives 3, 4

The Schema Theory


• The most long-standing hierarchical model is the schema theory.

• Schema theory posits a generalized motor program (GMP) and schemas.– GMPs are a general representation of various motor

actions, or a class of actions. – The schemas are separate memory components in

which movements are recognized and recalled, essentially the decision-making and learning processes for the GMP.

Objectives 3, 4

The Schema Theory (cont.)


• Stored in the GMP and schemata are invariant characteristics and parameters.– Invariant characteristics are

features of the GMP that do not change, for example, relative force, relative timing, and sequencing.

– Parameters are features that change within the GMP, for example, overall force, overall duration, and specific muscles.

Objectives 3, 4


Schema Theory Evidence


• Blocked movement shows similar movement pattern as normal movement.

• Suggests preprogrammed neural commands not influenced by feedback

Objectives 3, 4


Schema Theory Evidence (cont.)


• Other evidence for motor programs is found in bimanual transfer, for example, left and right handwriting similarities.

Objectives 3, 4


The Rise of Internal Models


• Schema theory criticized for:– Implausibility of the brain being able to store so much

information– GMPs do not explain how entirely novel movements

are created. – GMPs rely on an executive controller making never-

ending rapid fire decisions.– The concept of movement invariant characteristics may

not be so invariant.• Newer hierarchical models center on internal models.

Objectives 3, 4

Internal Hierarchical Models


• Internal model emphasize that the brain sends commands to the PNS and itself through efference copy. – Efference copy includes the movement plan and a

prediction of the sensory outcome.– Planning and initiation of the motor command are

based on prediction of outcomes.– Differences between the actual movement feedback

and the predicted feedback are used to refine subsequent motor commands.

– Motor commands are thus based on understanding the relationship between the original motor commands and the actual output.

Objectives 3, 4

Internal Model Schematic


• This forward internal model shows the planner (P) somewhere in the cortex sends a plan to the controller (CT) in the motor cortex.

• Plans are sent to the to the controlled object (CO), for example, spinal interneurons or motor units.

Objectives 3, 4


Internal Model Schematic (cont.)


• Efference copy goes to the forward model (FM) comparator, where it is compared to sensory feedback (FB).– Difference relayed

back to the controller • Visual cortex (VC)

information is relayed to the controller.

• Plans are revised based on comparator differences.

Objectives 3, 4


Model Consensus Points


• All the models find consensus on three points:– The nervous system is most concerned with

movement outcomes or effects than specific muscle actions.

– The nervous system must take into account psychological, physiological, and biomechanical properties of the body, the movement goals, and the environmental context.

– There exists hardwired, preformed, and synergistic movements that form building blocks for more complex movements.

Objectives 3, 4, 6

The Systems Model and Approach


• The systems model describes the production of skilled movement as a natural outcome of the person interacting with the environment. – Task goals and characteristics of the individual interact

with the characteristics of the environment. – Task requirements and the environmental context

cannot be separated from movement planning, initiating, and executing.

Objective 7


The Systems Model and Approach (cont.)



The Systems Model and Approach (cont.)


• The individual, the task, and the environment are systems that interact and each of these systems contain multiple subsystems that also interact.– Systems are assemblies or groups of components that

together have certain features or characteristics that are task specific.

– The dynamic interplay among these systems identifies them as dynamic systems.

Objective 7

Systems Model and Constraints


• The features and characteristics of systems impose constraints to movement.– A constraint is a barrier or restriction that must be

used, avoided, or overcome for effective movement to take place.

• Constraints may be task, environmental, or individual.– Task constraints and environmental constraints are

considered external.– Environmental constraints may be regulatory or

nonregulatory and physical or sociocultural.– Individual systems produce internal constraints.

Objective 7

Behavior of Biological Systems


• Dynamic systems, working as individual units and interacting with other systems, have self-organizing properties.– The system tries to maintain a stable and patterned

state of operation, called an attractor state.– The stable state is resistant to change but does

naturally fluctuate within the stable state.– If knocked out of the stable state, the system will try

and find a new stable (attractor) state given the new set of circumstances and dynamic properties.

Objective 7

Behavior of Biological Systems (cont.)


• Changing from one stable state to another is called a transition phase.– Example: Walking to running demonstrates that high

speeds or workloads destabilize the stable walking pattern and forces a transition to running

– Speed is a control parameter, which are those factors that when they change may cause a wholesale change throughout the entire system.

– The rest of the system components that follow suit are called order parameters.

• Coordination, gait repatterning, and vertical center of mass movement are order parameters.

Objective 7

Transitions in Dynamic Systems


• The walk to run transition is marked by changes in limb velocity as a control parameter.

• Characteristics such as range of motion, “flight phase,” and center of mass movement are order parameters.


Destabilizing Dynamic Systems


• It is often necessary to destabilize the system in order to promote better functioning at a new stable state. – Example: Strength training aims to cause tissue

breakdown to promote new tissue growth. • Destabilization does not always have positive outcomes.

– Changing one variable, even for the “better,” may have a minimal impact on overall performance.

– Sometimes, such as in overtraining syndrome, the system adapts to a poorly functioning stable state.

Stabilizing Dynamic Systems with Perception–Action Coupling


• Constraint-based information from the environment continually merges with internal sensory information. – Given a task, the system wants to find a stable

movement solution.• Perceptual systems determine ways to take constraints

into account for movement planning and execution. – This process is known as searching for affordances.– Affordances link what is perceived and what action

may take place; a process termed perception–action coupling.


Perception–Action Coupling


• Perception–action coupling makes precise motor programs unnecessary.

• Planning and action information are part of the environment and revealed when interacting with the environment.

• Consider the wall-climbing actions afforded each person in the picture.

Perception–Action Coupling (cont.)


• This experimental setup could change the virtual reality hallway “speed” to match or mismatch treadmill speed.

– Resulted in optic flow perceived by the person that he were walking faster or slower than in reality.

• Mismatched virtual reality hallway speed caused the person to walk faster or slower to match the environment.

– Clear example of perception–action coupling

Objective 7


Perception–Action Coupling (cont.)


Permission from Mohler et al. (2007)


Applying the Systems Approach


• The systems approach provides a framework from which to address movement-related problems.– How individuals interact within the environment with

their own constraints and capabilities leads to individual-specific assessments and interventions.

– Understanding environmental constraints enables the practitioner to bring those situation-specific constraints into the practice and training environment.

Summary


• The benefit of motor abundance also brings on the problem of degrees of freedom. – Movement models attempt to explain the overarching

planning, initiation, and execution process.• Models fall into open- (hierarchical) and closed-loop

(heterarchical) systems.– Hierarchical models include the schema theory and

internal models.– Heterarchical models range from reflex models to

dynamic systems.

Summary (cont.)


• The systems theory and approach views movement from the perspective of what factors influence movement production.– Involves dynamic systems interacting, including

individual, task, and environmental systems– Dynamic systems include synergies and other

interacting components that set constraints upon one another.

• Interactions among the human operator and the task and the environment are worked out based on the ideas of constraints, affordances, and perception–action coupling.

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Chapter 5 Movement Models Jeffrey C. Ives Copyright © 2014 Wolters Kluwer Health| Lippincott...

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