Theories of Control - Motor Control

Theories of Control

How do we effectively control various muscles and joints to work effectively and efficiently?

Interaction between task, environment and individual. 

1. Four aspects of Motor Control

2. Reflex theories 

3. Heirarchical theories - Adams (71), Keele (68) and Schmidt (75)

4. Dynamical systems theories - Bernstein (67), Kelso (83) and Turvey (77)

5. Ecological theories - perception and action

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4 Aspects of Motor Control

1. Motor equivalence - the motor system has the ability to acheive functionally the same end result via different movements, involving different muscles and joints

2. Uniqueness of action - movements are never exactly repeated

3. Stability and consistency of action - most obvious feature of skilled performance is consistency and stability of temporal and spatial structure

4. Modifiability of action - skilled action is capable of amendment as a consquence of changes in information available to performer

This indicates the range of motor control problems the performer has to deal with .

Any theory of control must be able to account for these 4 aspects.

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Reflex theory

Consideres reflexes to be fundamental unit of control.

A stimulus creates a movement response - and they are combined to form movement of the individual.

Limitations:

1. Does not address Sheridan's model - cannot be modified depending on environment

2. Cannot explain fast movement

3. Cannot explain novel movement

4. Cannot explain how a single stimulus can result in various response

5. Does not explain movement with an absence of sensory stimuli

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Hierarchical theories

All aspects of movement planning and execution are the sole responsibility of the higehst levels of the central nervous system.

It's a top down model - higher cortical centres send commands to lower cortical centres and movement then takes place.

Feedback enable units to communicate, and therefore be controlled and adjusted. 

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Hierarchical - Adams' Open and Closed Loop (71)

The command centre issues command to effectors that is sufficient to initiate movement. 

The execution and completion is then dependant on feedback as it control the movement.

There are 2 modes of control - open (no feedback, preprogrammed) and closed (feedback)

For ballistic movements:

Rapid, no correction, preprogrammed, no feedback or control.

For controlled movements:

Slower, allow for correction, utilise feedback

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Hierarchical - Keele's Motor Program (68)

Motor program - set of muscle commands that are structured before a movement sequence begins.

Movements are stored in memories as motor programs.

Limitation:

This has major storage problems as there must be a limited capacity if all movements have a specific program of action.

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Hierarchical - Schmidt's GMP (75)

Generalised motor program - store famillies of movements rather than individual movements.

This is more adaptable and better explains control in a more varied environement.

Some examples of famillies of movements:

1. Locomotion skills

2. Upper limb function

3. Self help skills

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Dynamical systems - Bernstein (67)

Degrees of freedom - many structures that have to be controlled at any one time.

e.g. joints, muscles, alpha gamma linkages, motor units

How do we control them without overloading the system?

Linking them can reduce DOF.

Not that simple - movement can take place in changing contexts and conditions:

1. Anatomical factors

2. Mechanical variability

3. Physiological variability

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Dynamical systems - Kelso (83)

Synergies:

By linking 2 elements together, the positions of each are dependant on each other which reduces the number of degrees of freedom.

Causing them to act as a single functional unit.

Move hand from B to A and C to D sepately - different times to do each

Move hand from B to A and to C to D together - same times to do each

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Dynamical systems - Turvey (77)

Coordinative structures:

Group of muscles spanning several joints that are constrained to act as a single functional unit.

It helps control the number of degrees of freedom by reducing it. 

It is also self organising - they must be adjustable to control actions in different contexts.

Principles of coordinative structures:

1. Separate coordinative structures will integrate and act in unison as a single unit

2. System can automatically change from one level to another e.g. changes in gait

3. Self correcting - automatically adjust to acheive final desired state

4. Not context specific - can acquire desired outcome with varied inital conditions

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Perception-Action Coupling

The interaction between perceptual and movement variables that result in specific movement dynamics in accordance with the perceptual variable

Perception - the process of obtaining information from sensory stimulation and making it available for action.

Sensory receptors used to get information - vision, auditory and proprioception.

They do not work independantly - they constantly talk, inform and help each other to maintain accuracy.

Proprioception - muscles and tendons are filled with mechanical receptors that stretch and compress and tell us about body position, movement through space and postural control

1. Muscle spindles - stretch increases rate of discharge of afferent fibres to contract

2. Golgi tendon - activated by stretch to prevent excessive contractions

3. Joint receptors - fire at extreme angles 

4. Vestibular system - monitor angular/linear accelerations of the head

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