Motor control theory Theory and Professional Practice What

Motor control theory

Theory and Professional Practice • What is theory? – Accurately describes a large class of observations – Make definite predictions about results of future observations (Hawking, 1996) • With motor learning and control, theories focus on: – Explaining human behavior – Providing explanations about why people perform skills as they do – Identifying variables through observations of these performances

Motor Control Theory • Describes and explains how the nervous system produces coordinated movement of motor skill in a variety of environments • Two issues of importance: – Coordination – Degrees of freedom

Coordination • Patterning of body and limb motions relative to the patterning of environmental objects and events (Turvey, 1990) • Two parts to consider: – Movement pattern of a skill in relationship at a specific point of time – Context of the environment of the head, body, and/or limb movements so the actions can be accomplished

Degrees of Freedom • Number of independent elements in a central system and the ways each component can act • Degree of Freedom problem: – Control problem – Occurs in the designing of a complex system that produces a specific result – Involves determining how many degrees of freedom is required to produce the result

Open Loop and Closed Loop Systems • Models of basic descriptions that show the CNS and PNS initiate and control action • Each has a central center (executive) • Executive generates and issues movement instructions to effectors (muscles/joints) • Both also contain movement instructions from control center to effectors

Differences Between the Systems n Two differences Open Loop Closed Loop • Does not use • Uses feedback • Control center issues • Control center information to provides all the effectors sufficient information for only to initiate effectors to carry out movement

Two Theories of Motor Control 1. Motor Program-based theory: memorybased construct that controls coordinated movement 2. Dynamic Pattern theory: approach to describing and explaining the control of coordinated movement that emphasize the role of information in the environment and properties of the body/limbs

Motor Program-Based Theory • Based on the work of Richard Schmidt (1988) • Hypothesized generalized motor program: mechanism accounting for adaptive and flexible qualities of human movement • Proposed that generalized motor program controls class of actions vs. specific movement

• These “invariant factors” are the signatures of the generalized motor program (does not vary from one performance to another) • Parameters are specific movement features that can vary from one performance of a skill to another

Schmidt’s Schema Theory • Schema: rules that serve to provide the basis for a decision based from related experiences • Described two control components: – Generalized motor program – control mechanism responsible for controlling a class of actions, i. e. , kicking – Motor response schema – responsible for providing the specific rules governing an action in a given situation

• This theory explains how a person can adapt to new situations or environmental contexts • Solves the degree of freedom problem

Dynamic Pattern Theory • Describes the control of coordinated movement that emphasizes the role of information in the environment and dynamic properties of the body/limbs • Seen from the perspective of nonlinear dynamics (behavioral changes are abrupt)

Dynamical Systems theory • Bernstein-characteristics of acting systems, external and internal forces. • Distributed model of motor control-no need for higher center of control. • Interaction of elements, physical and neural components. • Limitations

Systems Theory: Bernstein’s Degree of Freedom Problem • How does the CNS select solution from an infinite number of possibilities for task? a a • Solution – Higher levels activate lower levels while lower levels activate synergies, i. e. groups of muscles that are constrained to act together as a unit

Stability and Attractors • Stability: behavioral steady state of a system • Attractors: stable behavioral steady states of systems – Preferred behavioral states – Energy efficient states

Order and Control Parameters • Order parameter – Called collective variables – Functional specific and abstract variables that define the overall behavior of the system – Enable a coordinated pattern of movement that can be reproduced and distinguished from other patterns – Relative phase is the most prominent of order parameters which represents the movement relationship between two movement segments

• Control parameter – Variable, when increased or decreased, will influence the stability and character of the order parameter – Is important to identify since it becomes the variable to manipulate in order to assess the stability of the order parameter – Provides the basis for determining attractor states for patterns of limb movement

• Self-Organization – When certain conditions characterize a situation, a specific pattern of limb movement emerges – This pattern of movement self-organizes within the characteristic of environmental conditions and limb dynamics

• Movement emerges as a result of interacting elements. No needs for specific neural commands or motor programs. • Variability of movement is normal. Optimal amount of variability allows for flexible, adaptive strategies to meet the environmental demand

Dynamic Systems Theory: Principle of Self-Organization A new movement emerges when a control parameter reaches a critical value

Systems Theory: Bernstein’s Degree of Freedom Problem • Viewed body as a mechanical system, involving the interaction between mass, external force (e. g. gravity), internal force • “Coordination of movement is the process of mastering the redundant degrees of freedom of the moving organism” (Bernstein, 1967)

Coordinated Structures • Functional synergies of muscles and joints of a person’s nervous system which act cooperatively to produce an action • Develops through practice, experience, or naturally • Can be intrinsic (walking) or developed through practice

Perception and Action Coupling • Dynamic pattern theory • Essential element in accounting for skillful performance of open skills • The perception part of the interaction detects and uses critical invariant information in the environment

• The action part involves the setting and regulating of movement control features that enable action goal achievement • For example, Greek letter “tau” ( )

Limitation of Systems Theory • Nervous system is fairly unimportant • How do we apply mathematics and body mechanics to clinical practice?

Clinical Implications of Systems Theory • Body is a mechanical system. Consider musculoskeletal factors underlying a patient’s movement problem • Changes in movements may not necessarily result from neural changes, e. g. faster vs. slow gait, speed during sit to stand • Encourage the patient to explore variable movements

Present State of the Control Theory Issue • Motor program-based theory and dynamic pattern theory are the predominant behavioral theories addressing how the nervous system produces coordinated movement • Theory of control cannot focus exclusively on the movement information that is specified by the CNS

• Task and environmental characteristics must be taken into account (Newell, 1986) • Speculation of hybrid on a compromise theory could emerge, to explain the control of coordinated movement

Reflex Theory Reflexes are the building blocks of complex motor behaviors or movements

Stimulus Response Sensory receptor Response Effector(muscle) • Classified the major responses to stimuli, and believed that most of the voluntary movements resulted from these fundamental reflexes.

Limitations of Reflex Theory • Unable to explain – Spontaneous and voluntary movements – Movement can occur without a sensory stimulus – Fast sequential movements, e. g. typing – A single stimulus can trigger various responses (reflexes can be modulated) – Novel movements can be carried out.

Information processing model Input (Signals) Processing (The human) Output (Motor response)

Information processing model Stimulus (Input) iden. Response selection progra. REACTION TIME THE HUMAN Movement output

Hierarchical Theory • Top - down structure • Reflexes are part of this hierarchy, normally higher centers inhibit them • Motor control emerges from reflexes, later on integrated with higher control levels. – Limitations

Hierarchical Theory

Hierarchical Theory • Higher centers are always in control of lower centers • Higher centers inhibit the reflexes controlled by lower centers • Reflexes controlled by lower centers are present only when higher centers are damaged • Neuromaturational theory of development – The brain determines infant behavior!

Hierarchical Theory

Current Concepts Related to Hierarchical Theory • Each level of the motor system can act on other levels • Reflexes are one of many processes of motor control

Limitations of Hierarchical Theory • Environment and other non-CNS factors can affect movement, e. g. Thelen’s experiments showed that baby’s stepping response reemerges with body weight support • Normal adults exhibit lower level reflexes, e. g. flexor withdrawal

Motor Programming Theories • Concept of a central motor pattern or motor program – Many studies found that movement is possible even in the absence of stimuli or sensory input – Sensory inputs are not required to produce a movement but they are important in adapting and modulating the movement

General setup for studies of locomotion in cats with spinal lesions Rossignol, 2011

Evidence of a Motor Program: Central Pattern Generator (CPGs) • CPGs are spinal networks capable of generating bilateral rhythmic movements, such as swimming or walking, in the absence of descending and sensory inputs • CPGs are network of interneurons that alternatively activate flexors and extensors on one side, and coordinate with CPGs on the other side

Motor Programming Theories • Motor programs are – Hardwired and stereotyped neural connections such as central pattern generators (CPGs) – Abstract rules for generating movements at the higher level • Motor program can be activated by sensory stimuli or by central processes

• Synergy is a task-specific covariation of elemental variables with the purpose to stabilize a performance variable, i. e. minimize errors of a performance variable – Reaching: joint rotation angle stabilize hand position – Grasping: individual finger force stabilize total grasp force – Standing stability: postural muscle activation stabilize COP

Motor Programming Theories Writing

Limitations of Motor Programming Theories • Does not consider that the nervous system must deal with both musculoskeletal and environmental variables to produce movements – e. g. identical neural commands to elbow flexors can produce different movements depending on the initial position of the arm and the force of gravity

Clinical Implications of Motor Programming Theories • Movement problems are caused by abnormal CPGs or higher level motor programs • It is important to help patients relearn the correct rules for action • Focus on retraining movements that are critical to a functional task, not just specific muscles in isolation

Motor control models Reflex Hierarchical Systems Neurotherapeutic facilitation Contemporary task-oriented

Task oriented theories • Control of movement is organized around goal -directed functional behaviors such as walking or talking.

Task-Oriented Approach (motor control of motor learning approach)

Task-Oriented Approach • Movement is organized around a behavioral goal and is constrained by the environment • Patients learn by actively attempting to solve the movement problem rather than by repetitively practicing normal patterns of movement.