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

• 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

Knee Joint Angle

Hip Joint Angle

180

45

-1085

Too Little Variation

Knee Joint Angle

Hip Joint Angle

180

45

-1085

Too much variation

Knee Joint Angle

Hip Joint Angle

180

45

-1085

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