Structure of motor variability
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STRUCTURE OF MOTOR VARIABILITY
Kyung Koh
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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
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MOTOR VARIABILITY
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EXAMPLE – KINETIC VARIABLE Task : F1 + F2 = 10N(= a line equation [1D])
+ error
F1 F2
+ error
where
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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
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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
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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
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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)
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EXAMPLE – KINEMATIC VARIABLETask : Target (Tx,Ty)
By using jacobian Matraix, + error + error
where
error
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MOTOR SYNERGIESMotor Synergies in UCM
Ratio of Vucm and Vorth are commonly used to measure synergies
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STUDIES: MOTOR SYNERGIES
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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).
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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