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
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
• 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
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
• 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
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
Hip Joint
Knee Joint Angle
Too Little Variation
Hip Joint
Knee Joint Angle
Too much variation
Hip Joint
Knee Joint Angle
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

similar documents