Variation and Variability

1. Variation and Variability • Variation- a broad repertoire of behaviors for a specific motor function • Variabilitythe capacity to select from the repertoire the best motor strategy for the context

2. Variability and Error • Old view was that variability = error • Hypothesis: one correct form of movement • “mature form”

3. Contemporary Perspective • Variability is good!

4. Good Variability • Variability that preserves function • several ways to get out of bed • all support the goal task of independence

5. Good Variability • Ability to vary force and speed within a strategy to meet environmental conditions

6. 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

7. Bad Variability • Inability to constrain force or speed • leads to misses, falls, collisions • Ataxia

8. Neutral Variability • Some variations don’t matter • If functional goal is preserved

9. Contemporary Approaches to Study Variability • When is Variability Good or Bad?

10. Most Variability is Good • person specific(anthropometric characteristics, age) • task-specific (individual’s goal or motivation) • context –specific (situation specific performances)

11. Discerning Good vs Bad • separating variability • seen in outcome • seen during execution

12. Outcome Variability • As long as goal task is accomplished – no harm done in outcome variability

13. Execution Variability • variability during execution, • if goal accomplished… • elements that vary are considered part of a “synergy”

14. Synergy • parts working together for the common good

15. Manifold Concept • The combination of all configurations seen during execution that produce the solution to the task • concept borrowed from contemporary physics

16. Manifold Concept Research • Examine variability in repeated trials of 1 task • gripping a cup – how do the individual digits contribute to successful performance

17. 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

18. 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

19. Examining Movement Trajectories 45 Hip Joint Angle -10 85 180 Knee Joint Angle

20. Too Little Variation 45 Hip Joint Angle -10 85 180 Knee Joint Angle

21. Too much variation 45 Hip Joint Angle -10 85 180 Knee Joint Angle

22. Developmental Change in Variability • Increased Consistency • Increased Flexibility

23. Increased consistency • Practice leads to movements that are • smoother • more efficient • more accurate •  more consistent task outcomes

24. How to Structure Practice? • Variable practice is best!

25. Variability between individuals • Characteristics of the individual • anthropometrics • different experiences • temperament • motivation

26. Developmental Change in Variability • Increased Flexibility in performance • practice in different contexts • scale control variables up and down • build capacity to vary