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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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THE PROBLEM OF MOTOR REDUNDANCY (DEGREE OF FREEDOM PROBLEM) Degrees of freedom (DOF) = Number of independent elements in a system (e.g. joints or muscles) Degrees of freedom problem = How dose CNS select a particular solution from infinite # of solutions.

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MOTOR VARIABILITY

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EXAMPLE – KINETIC VARIABLE F1F1 F2F2 + error

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EXAMPLE – KINETIC VARIABLE F1 F2 10N V Good V Bad F 1 + F 2 = 10N

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UNCONTROLLED MANIFOLD ANALYSIS (UCM) Task : F 1 + F 2 = 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 Basis vector for UCM space Basis vector for a subspace orthogonal to UCM

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UNCONTROLLED MANIFOLD ANALYSIS (UCM) Task : F 1 + F 2 + F 3 = 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 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 UCM coordinates PCA coordinates Uncontrolled Manifold Analysis (UCM) VS Principle Component Analysis (PCA)

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EXAMPLE – KINEMATIC VARIABLE

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MOTOR SYNERGIES Motor 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 UCM Greater 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 DoF1 DoF

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9 subjects (5 females & 4 males) Two experimental conditions 1) standing up onto a solid platform 2) standing up onto a narrow, padded base of support. Task :

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METHODS Task : Sit to stand movement Task equation : Basis vectors for UCM space Basis vector for Orthogonal space

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RESULTS Horizontal CM position The horizontal CM position: V UCM > V ORT The vertical CM position: V UCM ≈ V ORT Stronger synergies when Narrow BOS

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METHODS

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RESULTS

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