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

Too Little Variation Knee Joint Angle Hip Joint Angle

Too much variation Knee Joint Angle Hip 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