Introduction to Motor Control TOPICS COVERED Motor control definition Four characteristics of human action Three theories of motor control Theories assessed relative to characteristics of human action
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What is motor control? - "the study of postures and movements and the mechanisms which underlie them" Two basic sciences contributed a great deal to the formation of the areas of motor control and learning. These were neurophysiology and psychology. There are four characteristics of
human actions which any theory of motor control must be able to account
for: flexibility, uniqueness, consistency, Flexibility - the ability to recruit different muscles and joints to achieve the same action Uniqueness - no two movements are ever performed in exactly the same way Consistency - temporal and spatial characteristics of a movement remain relatively stables from one performance to the next Modifiability - the ability to alter a movement pattern, even as it is being executed Theories of Motor Control Reflex - The reflex was considered to be the fundamental unit of motor control driving all voluntary movement. Hierarchical - Assumed that all aspects of movement planning and execution are the sole responsibility of one or more cortical centers. Dynamic Systems - Motor behavior results from the interactions of multiple subsystems. Reflex Model of Motor Control Basic Assumptions - 1) physical events occurring in the environment served as the stimulus for action, triggering a chain of individual reflex circuits that were responsible for producing a movement response - reflex chaining hypothesis 2) the individual was a passive recipient of externally produced sensory input Strengths of the reflex model - provided a simple explanation of how movements are controlled Weakness of the reflex model - 1) too simple to account for a person's ability to perform a wide variety of goal-directed actions 2) unable to account for movements performed in the absence of sensory feedback
Reflex Model's Ability to Account for Characteristics of Human Action Flexibility - no mechanism exists Uniqueness - no mechanism exists Consistency - there is a direct link between the stimulus and the response Modifiability - no mechanism exists
Hierarchical Models of Motor Control
Basic Assumptions - 1) all aspects of movement planning and execution are the sole responsibility of one or more cortical centers 2) movements are stored in memory in the form of plans or programs for movements - Motor Programs (Mps) 3) motor programs are prestructured sets of motor commands developed at the highest cortical level and sent to lower spinal centers
Historical contrast between hierarchical model and reflex model
Steven Keele's Definition of a Motor Program (1968)
"a set of muscle commands that are
structured before a movement sequence begins, and that allow the entire
sequence to be carried out uninfluenced by Reasons for the abandonment of the Motor Program concept
The Generalized Motor Program (GMP)
2) variant and invariant parameters of movements are applied to the GMP in order to specify how a movement pattern is to be expressed
3) accounts for our ability to achieve the same movement outcome using different muscle groups - motor equivalence 4) parameter specification accounts for the ability to perform novel movements
Strengths of Hierarchical Models
1) account for our ability to perform movements in the absence of sensory feedback 2) the GMP can be used to perform a variety of movements by applying different movement parameters 3) novel movements are possible
Hierarchical Model's Ability to Account for Characteristics of Human Action
Flexibility - different movement parameters are added to the same GMP Uniqueness - apply different movement parameters to GMP representing same class of actions Consistency - the application of invariant movement parameters to an action (e.g. relative time, relative force) Modifiability - it is possible to change the GMP being used, if time permits (e.g. more than one reaction time) Hypothesized Modes of Motor Control within the Context of Hierarchical Models open-loop control - ability to plan and execute a movement without the use of sensory feedback closed-loop control - ability to use sensory feedback to modify an ongoing movement if a movement error is detected Dynamic Systems Models of Motor Control Basic
Assumptions -1) planning of actions does not require elaborate
cognitive processing because information from the environment is
immediately 2) motor behavior emerges from the interaction of multiple subsystems (neurological, musculoskeletal, biological) Affordances - Properties of the environment taken with reference to the animal. An affordance is an invarient arrangement of surface/substance properties that permits a given animal a particular activity. Affordances II and trained perceivers Strengths of Dynamic System Models 1) acknowledges that actions are shaped by the intentions of the performer and the constraints imposed by the environment 2) the greater responsibility
afforded to the spinal cord and musculoskeletal levels accounts for our
ability to produce a number of highly sophisticated
I-45 and self-organizing systems Dynamic Systems Model's Ability to Account for Characteristics of Human Action Flexibility - self-organizing properties of the human system Uniqueness - muscle groups are only temporarily constrained to work together Consistency - invariance of coordinated muscle groups Modifiability - self-organizing properties of the human system
Coordination - the process by
which an individual constrains or condenses the available
degrees-of-freedom into the smallest number of Degree-of-Freedom - any of a limited number of ways in which a body may move or in which a dynamic system may change" Degrees of freedom and motor control
Two Mechanisms Available for the Coordination of Action
· muscle response synergy - a group of muscles constrained to act as a single behavioral unit · mechanical properties of the limbs 1) exploitation of mechanical
properties of muscles and their physical connections with ligaments to
generate force,
Control of Action
Control
- the process of manipulating the behavioral unit (i.e. set of muscle
response synergies). This process involves assigning movement
Skill - achieved when a performer can assign the optimal set of values or movement parameters to the behavioral unit
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MOTOR CONTROL
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