Transcript Session III, part I (Variation and Variability)

Variation and Variability
• Variation - a broad repertoire of behaviors for
a specific motor function
• Variability the capacity to select from the
repertoire the best motor strategy for the
context
Variability and Error
• Old view was that variability = error
• Hypothesis: one correct form of movement
• “mature form”
Contemporary Perspective
• Variability is good!
Good Variability
• Variability that preserves function
• several ways to get out of bed
– all support the goal task of independence
Good Variability
• Ability to vary force and speed within a
strategy to meet environmental conditions
Good Variability
• Less noticeable
– variation across synergistic muscles that enables
one to adjust forces across components of a
movement pattern to preserve the outcome
– eg. consistent foul shot despite small variations in
muscle forces
Bad Variability
• Inability to constrain force or speed
– leads to misses, falls, collisions
• Ataxia
Neutral Variability
• Some variations don’t matter
• If functional goal is preserved
Contemporary Approaches to Study
Variability
• When is Variability Good or Bad?
Most Variability is Good
• person specific (anthropometric
characteristics, age)
• task-specific (individual’s goal or motivation)
• context –specific (situation specific
performances)
Discerning Good vs Bad
• separating variability
– seen in outcome
– seen during execution
Outcome Variability
• As long as goal task is accomplished – no harm
done in outcome variability
Execution Variability
• variability during execution,
– if goal accomplished…
– elements that vary are considered part of a
“synergy”
Synergy
• parts working together for the common good
Manifold Concept
• The combination of all configurations seen
during execution that produce the solution to
the task
• concept borrowed from contemporary physics
Manifold Concept Research
• Examine variability in repeated trials of 1 task
– gripping a cup – how do the individual digits
contribute to successful performance
Application to Clinical Practice
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Examine children with DS on a treadmill,
show variability of action across LE joints
yet stable outcome (no falls, etc)
suggests capacity to vary is functional
Another way to analyze variability
• Analyze the structure of performance across
several trials of a task
– a time series analysis
– examine trajectories of a movement and see if the
patterns strictly overlap
Examining Movement Trajectories
45
Hip Joint
Angle
-10
85
Knee Joint Angle
180
Too Little Variation
45
Hip Joint
Angle
-10
85
Knee Joint Angle
180
Too much variation
45
Hip Joint
Angle
-10
85
Knee Joint Angle
180
Developmental Change in Variability
• Increased Consistency
• Increased Flexibility
Increased consistency
• Practice leads to movements that are
– smoother
– more efficient
– more accurate
•  more consistent task outcomes
How to Structure Practice?
• Variable practice is best!
Variability between individuals
• Characteristics of the individual
– anthropometrics
– different experiences
– temperament
– motivation
Developmental Change in Variability
• Increased Flexibility in performance
– practice in different contexts
– scale control variables up and down
– build capacity to vary