STRUCTURE OF MOTOR VARIABILITY

Kyung Koh

BACKGROUND

Motor variability A commonly seen features in human movements Bernstein “repetition without repetition”

In the past, motor variability is thought to be the result of error.

Scholz and Schöner (2002) developed the uncontrolled manifold analysis (UCM) Variability which creates error Variability which does not

MOTOR VARIABILITY

EXAMPLE – KINETIC VARIABLE Task : F1 + F2 = 10N(= a line equation [1D])

+ error

F1 F2

+ error

where

EXAMPLE – KINETIC VARIABLETask : F1 + F2 = 10N(= a line equation [1D])

Good variability(which does not hurt performance)

Bad Variability (which does)

F1

F2

10N

10N

VGood

VBad

F1 + F2 = 10N

UNCONTROLLED MANIFOLD ANALYSIS (UCM)Task : F1 + F2 = 10N (= a line equation [1D])

Variability in a UCM space (task irrelevant space)

Variability in an orthogonal to UCM space (task relevant space)

F1

F2

10N

10NBasis vector for UCM space

Basis vector for a subspace orthogonal to UCM

UNCONTROLLED MANIFOLD ANALYSIS (UCM)Task : F1 + F2 + F3 = 10N(= a plane equation [2D])

Variability in a UCM space (task irrelevant space)

Variability in an orthogonal to UCM space (task relevant space)

F2

F3

10N

10N

Basis vectors for UCM space

Basis vector for orthogonal to UCM space

F1

10N

MOTOR SYNERGY

A linear transformation that transforms the data into a new coordinate system (NCS)

A method to measure variance of the data in NCS

F1

F2

10N

10N

UCM coordinates

PCA coordinates

Uncontrolled Manifold Analysis (UCM) VS Principle Component Analysis (PCA)

EXAMPLE – KINEMATIC VARIABLETask : Target (Tx,Ty)

By using jacobian Matraix, + error + error

where

error

MOTOR SYNERGIESMotor Synergies in UCM

Ratio of Vucm and Vorth are commonly used to measure synergies

STUDIES: MOTOR SYNERGIES

SUMMARY

There exists motor synergy

Task-specific co-variation of effectors with the purpose to stabilize a performance variable (or minimize task error) (Latash 2002).

The CNS uses all the available DOFs to generate families of equivalent solutions. DOFs work together to achieve a goal by compensating for each errors. (Gelfand and Tsetlin 1967).

BENEFITS OF HAVING GREATER VARIABILITY IN UCMGreater Variability in UCM space The system is redundant. More DoFs than necessary to perform a particular task (e.g., F1 + F2 = 10N).

During walking on an uneven surface, DOFs at the foot create variety of configuration to maintain stability.

Extra DOFs allows a system to be more flexible (e.g. when get injured)

24 DoF 1 DoF