Contemporary issues and theories of motor control, motor learning, and neuroplasticity

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Contemporary issues and theories of motor control, motor learning, and neuroplasticity

MARGARET L. ROLLER, PT, MS, DPT, ROLANDO T. LAZARO, PT, PhD, DPT, GCS, NANCY N. BYL, PT, MPH, PhD, FAPTA and DARCY A. UMPHRED, PT, PhD, FAPTA

The production and control of human movement is a process that varies from a simple reflex loop to a complex network of neural patterns that communicate throughout the central nervous system (CNS) and peripheral nervous system (PNS). Neural networks and motor pattern generators develop as the fetus develops in utero and are active before birth. These simple patterns become building blocks for more skillful, complex, goal-directed motor patterns as a person develops throughout life. New motor patterns are learned through movement, interactions with rich sensory environments, and challenging experiences that drive a person to solve problems. Personal desires and goals of the individual shape the process of learning new motor skills at all stages of life. If a condition exists or develops, or if an event occurs that damages the nervous system and prevents normal transmission, processing, and perception of information in the PNS and CNS, movement control becomes abnormal, slow, labored, uncoordinated, or weak, or movement may not be produced at all. The damaged nervous system is able to repair itself, change, and adapt to some extent by means of nerve regeneration and neuroplasticity. However, when nerve cells die and neural connections are not viable, alternative pathways within the nervous system exist to take the place of the normal process and provide some means of meeting the movement goal—whether it is to walk, use an arm to eat, or make a facial expression. This process of change, healing, or motor learning depends on many factors including inherent elements of the individual such as age, the extent of tissue damage, and other physiological and cognitive processes, as well as external factors such as interactions with sensory and motor system challenges, and goal-directed practice of meaningful, functional motor skills.

This chapter introduces the reader to basic concepts of motor control, motor learning, and neuroplasticity. Figures and tables are provided within each section to emphasize and summarize concepts. A patient case example is used to illustrate concepts in this chapter as they apply to the evaluation and management of people with neurological conditions. This chapter provides a foundation for chapters in Section II: Rehabilitation Management of Clients with Neurological System Pathology, and acts as a foundation for interacting with and treating patients in any clinical setting.

Motor control

Motor control is defined as “the systematic transmission of nerve impulses from the motor cortex to motor units, resulting in coordinated contractions of muscles.”1

This definition describes motor control in the simplest terms—as a top-down direction of action through the nervous system. In reality, the process of controlling movement begins before the plan is executed, and ends after the muscles have contracted. The essential details of a movement plan must be determined by the individual before the actual execution of the plan. The nervous system actively adjusts muscle force, timing, and tone before the muscles begin to contract, continues to make adjustments throughout the motor action, and compares movement performance with the goal and neural code (directions) of the initial motor plan. This extension of the definition takes into account that the body accesses sensory information from the environment, perceives the situation and chooses a movement plan that it believes to be the appropriate plan to meet the outcome goal of the task that the person is attempting to complete, coordinates this plan within the CNS, and finally executes the plan through motor neurons in the brain stem and spinal cord to communicate with muscles in postural and limb synergies, plus muscles in the head and neck that are timed to fire in a specific manner. The movement that is produced supplies sensory feedback to the CNS to allow the person to (1) modify the plan during performance, (2) know whether the goal of the task has been achieved, and (3) store the information for future performance of the same task-goal combination. Repeated performance of the same movement plan tends to create a preferred pattern that becomes more automatic in nature and less variable in performance. If this movement pattern is designed and executed well, then it is determined that the person has developed a skill. If this pattern is incorrect and does not efficiently accomplish the movement goal, then it is considered abnormal.

Theories and models of motor control

We begin this section with a summary and historical perspective of motor control theories (Table 4-1). The control of human movement has been described in many different ways. The production of reflexive, automatic, adaptive, and voluntary movements and the performance of efficient, coordinated, goal-directed movement patterns involve multiple body systems (input, output, and central processing) and multiple levels within the nervous system. Each model of motor control that is discussed in this section has both merit and disadvantage in its ability to supply a comprehensive picture of motor behavior. These theories serve as a basis for predicting motor responses during patient examination and treatment. They help explain motor skill performance, potential, constraints, limitations, and deficits. They allow the clinician to (1) identify problems in motor performance, (2) develop treatment strategies to help clients remediate performance problems, and (3) evaluate the effectiveness of intervention strategies employed in the clinic. Selecting and using an appropriate model of motor control is important for the analysis and treatment of clients with dysfunctions of posture and movement. As long as the environment and task demands affect changes in the CNS and the individual has the desire to learn, the adaptable nervous system will continue to learn, modify, and adapt motor plans throughout life.

TABLE 4-1 image

THEORIES OF MOTOR CONTROL

MOTOR CONTROL THEORY AUTHOR AND DATE PREMISE
Reflex Theory Sherrington 1906244 Movement is controlled by stimulus-response. Reflexes are combined into actions that create behavior.
Hierarchical Theories Adams 1971245 Cortical centers control movement in a top-down manner throughout the nervous system.Closed-loop mode: sensory feedback is needed and used to control the movement.
Open-loop mode: movements are preprogrammed and no feedback is used.
Dynamical Systems Theory Bernstein 196710Turvey 1977246
Kelso and Tuller 1984247
Thelen 1987248
Movement emerges to control degrees of freedom.Patterns of movements self-organize within the characteristics of environmental conditions and the existing body systems of the individual. Functional synergies are developed naturally through practice and experience and help solve the problem of coordinating multiple muscles and joint movements at once.
Motor Program Theory Schmidt 1976249 Adaptive, flexible motor programs (MPs) and generalized motor programs (GMPs) exist to control actions that have common characteristics.
Ecological Theories Gibson and Pick 2000250 The person, the task, and the environment interact to influence motor behavior and learning. The interaction of the person with any given environment provides perceptual information used to control movement. The motivation to solve problems to accomplish a desired movement task goal facilitates learning.
Systems Model Shumway-Cook 200735 Multiple body systems overlap to activate synergies for the production of movements that are organized around functional goals. Considers interaction of the person with the environment.

Motor programs and central pattern generators

A motor program (MP) is a learned behavioral pattern defined as a neural network that can produce rhythmic output patterns with or without sensory input or central control.2 MPs are sets of movement commands, or “rules,” that define the details of skilled motor actions. An MP defines the specific muscles that are needed, the order of muscle activation, and the force, timing, sequence, and duration of muscle contractions. MPs help control the degrees of freedom of interacting body structures, and the number of ways each individual component acts. A generalized motor program (GMP) defines a pattern of movement, rather than every individual aspect of a movement. GMPs allow for the adjustment, flexibility, and adaptation of movement features according to environmental demands. The existence of MPs and GMPs is a generally accepted concept; however, hard evidence that an MP or a GMP exists has yet to be found. Advancements in brain imaging techniques may substantiate this theory in the future.2,3

In contrast to MPs, a central pattern generator (CPG) is a genetically predetermined movement pattern.4 CPGs exist as neural networks within the CNS and have the capability of producing rhythmic, patterned outputs resembling normal movement. These movements have the capability of occurring without sensory feedback inputs or descending motor inputs. Two characteristic signs of CPGs are that they result in the repetition of movements in a rhythmic manner and that the system returns to its starting condition when the process ceases.5 Both MPs and CPGs contribute to the development, refinement, production, and recovery of motor control throughout life.

The person, the task, and the environment: an ecological model for motor control

Motor control evolves so that people can cope with the environment around them. A person must focus on detecting information in the immediate environment (perception) that is determined to be necessary for performance of the task and achievement of the desired outcome goal. The individual is an active observer and explorer of the environment, which allows the development of multiple ways in which to accomplish (choose and execute) any given task. The individual analyzes a particular sensory environment and chooses the most suitable and efficient way to complete the task. The person consists of all functional and dysfunctional body structures and functions that exist and interact with one another. The task is the goal-directed behavior, challenge, or problem to be solved. The environment consists of everything outside of the body that exists, or is perceived to exist, in the external world. All three of these motor control constructs (person, task, environment) are dynamic and variable, and they interact with one another during learning and production of a goal-directed, effective motor plan.

Body structures and functions that contribute to the control of human posture and movement

Keen observation of motor output quality during the performance of functional movement patterns helps the therapist determine activity limitations and begin to hypothesize impairments within sensory, motor, musculoskeletal, cardiopulmonary, and other body systems. The following section presents and defines some of these key factors, including sensory input systems, motor output systems, and structures and functions involved in the integration of information in the CNS.

Role of sensory information in motor control

Sensory receptors from somatosensory (exteroceptors and proprioceptors), visual, and vestibular systems and taste, smell, and hearing fire in response to interaction with the external environment and to movement created by the body. Information about these various modalities is transmitted along afferent peripheral nerves to cells in the spinal cord and brain stem of the CNS. All sensory tracts, with the exception of smell, then synapse in respective sensory nuclei of the thalamus, which acts as a filter and relays this information to the appropriate lobe of the cerebral cortex (e.g., somatosensory to parietal lobe, visual to occipital lobe, vestibular, hearing, and taste to temporal lobe). Sensory information is first received and perceived, then associated with other sensory modalities and memory in the association cortex. Once multiple sensory inputs are associated with one another, the person is then able to perceive the body, its posture and movement, the environment and its challenges, and the interaction and position of the body with objects within the environment. The person uses this perceptual information to create an internal representation of the body (internal model) and to choose a movement program, driven by motivation and desire, to meet a final outcome goal. Although the sensory input and motor output systems operate differently, they are inseparable in function within the healthy nervous system. Agility, dexterity, and the ability to produce movement plans that are adaptable to environmental demands reflect the accuracy, flexibility, and plasticity of the sensory-motor system.

The CNS uses sensory information in a variety of ways to regulate posture and movement. Before movement is initiated, information about the position of the body in space, body parts in relation to one another, and environmental conditions is obtained from multiple sensory systems. Special senses of vision, vestibular inputs that respond to gravity and movement, and visual-vestibular interactions supply additional information necessary for static and dynamic balance and postural control as well as visual tracking. Auditory information is integrated with other sensory inputs and plays an important role in the timing of motor responses with environmental signals, reaction time, response latency, and comprehension of spoken word. This information is integrated and used in the selection and execution of the movement strategy. During movement performance, the cerebellum and other neural centers use feedback to compare the actual motor behavior with the intended motor plan. If the actual and intended motor behaviors do not match, an error signal is produced and alterations in the motor behavior are triggered. In some instances, the control system anticipates and makes corrective changes before the detection of the error signal. This anticipatory correction is termed feed-forward control. Changing one’s gait path while walking in a busy shopping mall to avoid a collision is an example of how visual information about the location of people and objects can be used in a feed-forward manner.

Another role of sensory information is to revise the reference of correctness (central representation) of the MP before it is executed again. For example, a young child standing on a balance beam with the feet close together falls off of the beam. An error signal occurs because of the mismatch between the intended motor behavior and the actual motor result. If the child knows that the feet were too close together when the fall occurred, then the child will space the feet farther apart on the next trial. The information about what happened, falling or not falling, is used in planning movement strategies for balancing on any narrow object such as a balance beam, log, or wall in the future.

Sensory information is necessary during the acquisition phase of learning a new motor skill and is useful for controlling movements during the execution of the motor plan.68 However, sensory information is not always necessary when performing well-learned motor behaviors in a stable and familiar context.6,7 Rothwell and colleagues7 studied a man with severe sensory neuropathy in the upper extremity. He could write sentences with his eyes closed and drive a car with a manual transmission without watching the gear shift. He did, however, have difficulty with fine motor tasks such as buttoning his shirt and using a knife and fork to eat when denied visual information. The importance of sensory information must be weighed by the individual, unconsciously filtering and choosing appropriate and accurate sensory inputs to use to meet the movement goal.

Sensory experiences and learning alter sensory representations, or cortical “maps,” in the primary somatosensory, visual, and auditory areas of the brain. Training, as well as use and disuse of sensory information, has the potential to drive long-term structural changes in the CNS, including the formation, removal, and remodeling of synapses and dendritic connections in the cortex. This process of cortical plasticity is complex and involves multiple cellular and synaptic mechanisms.9 Plasticity in the nervous system is discussed further in the third section of this chapter.

Coordination

The movement plan is customized by communications among the frontal lobes, basal ganglia, and cerebellum, with functional connections through the brain stem and thalamus. During this process specific details of the plan are determined. Postural tone, coactivation, and timing of trunk muscle firing are set for proximal stability, balance, and postural control. Force, timing, and tone of limb synergies are set to allow for smooth, coordinated movements that are accurate in direction of trajectory, order, and sequence. The balance between agonist and antagonist muscle activity is determined so that fine distal movements are precise and skilled. This process is complicated by the number of possible combinations of musculoskeletal elements. The CNS must solve this “degrees of freedom” problem so that rapid execution of the goal-directed movement can proceed and reliably meet the desired outcome.10 Once these movement details are complete the motor plan is executed by the primary motor area in the precentral gyrus of the frontal lobe.

Execution

Pyramidal cells in the corticospinal and corticobulbar tracts execute the voluntary motor plan. Neural impulses travel down these central efferent systems and communicate with motor neurons in the brain stem and spinal cord. The corticobulbar tract communicates with brain stem motor nuclei to control muscles of facial expression, mouth and tongue for speaking and eating, larynx and pharynx for voice and swallow, voluntary eye movements for visual tracking and saccades, and muscles of the upper trapezius for shoulder girdle elevation. The corticospinal tract communicates with motor neurons in the spinal cord. The ventral corticospinal tract system communicates primarily with proximal muscle groups to provide the appropriate amount of activation to stabilize the trunk and limb girdles, thus allowing for dexterous distal limb movements. The lateral corticospinal tract system communicates primarily with muscles of the arms and legs—firing alpha motor neurons in coordinated synergy patterns with appropriate activity in agonist and antagonist muscles so that movements are smooth and precise. Other motor nuclei in the brain stem are programmed to fire just before corticospinal tract activity in order to supply postural tone. These include lateral and medial vestibular spinal tracts, reticulospinal tract, and rubrospinal tract systems. Adequate and balanced muscle tone of flexors and extensors in the trunk and limbs occurs automatically, without the need for conscious control. These brain stem nuclei have tonic firing rates that are modulated up or down to effectively provide more or less muscle tone in body areas depending on stimulation from gravity, limbic system activity, external perturbations, or other neuronal activity.

Adaptation

Adaptation is the process of using sensory inputs from multiple systems to adapt motor plans, decrease performance errors, and predict or estimate consequences of movement choices. The goal of adaptation is the production of consistently effective and efficient skilled motor actions. When all possible body systems and environmental conditions are considered in the motor control process, it is easy to understand why there is often a mismatch between the movement plan that is chosen and how it is actually executed. Errors in movements occur and cause problems that the nervous system must solve in order to deliver effective, efficient, accurate plans that meet the task goal. To solve this problem the CNS creates an internal representation of the body and the surrounding world. This acts as a model that can be adapted and changed in the presence of varying environmental demands. It allows for the ability to predict and estimate the differences between similar situations. This ability is learned by practicing various task configurations in real-life environments. Without experience, accurate movement patterns that consistently meet desired task goals are difficult to achieve.11

Flexibility

A person should have enough flexibility in performance to vary the details of a simple or complex motor plan to meet the challenge presented by any given environmental context. This is a beneficial characteristic of motor control. When considering postural control, for example, a person will typically display a random sway pattern during standing that may ensure continuous, dynamic sensory inputs to multiple sensory systems.12 The person is constantly adjusting posture and position to meet the demand of standing upright (earth vertical), as well as to seek information from the environment. Rhythmic, oscillating, or stereotypical sway patterns that are unidirectional in nature are not considered flexible and are not as readily adaptable to changes in the environment. Lack of flexibility or randomness in postural sway may actually render the person at greater risk for loss of balance and falls.

Control of voluntary movement

Table 4-2 shows the body system processes involved in motor control, their actions, and the body structures included. The following section explains these processes in more detail.

TABLE 4-2 image

COMPONENTS OF MOTOR CONTROL: BODY SYSTEM PROCESSES INVOLVED IN MOTOR CONTROL, THEIR ACTIONS, AND THE BODY STRUCTURES INCLUDED

PROCESS ACTION BODY STRUCTURES INVOLVED
Sensation Sensory information, feedback from exteroceptors and proprioceptors Peripheral afferent neurons, brain stem, cerebellum, thalamus, sensory receiving areas in the parietal, occipital, and temporal lobes
Perception Combining, comparing, and filtering sensory inputs Brain stem, thalamus, sensory association areas in the parietal, occipital, visual, and temporal lobes
Choice of movement plan Use of the perceptual map to access the appropriate motor plan Association areas, frontal lobe, basal ganglia
Coordination Determining the details of the plan including force, timing, tone, direction, and extent of the movement of postural and limb synergies and actions Frontal lobe, basal ganglia, cerebellum, thalamus
Execution Execution of the motor plan Corticospinal and corticobulbar tract systems, brain stem motor nuclei, and alpha and gamma motor neurons
Adaptation Compare movement with the motor plan and adjust the plan during performance Spinal neural networks, cerebellum

Role of the cerebellum

The primary roles of the cerebellum are to maintain posture and balance during static and dynamic tasks and to coordinate movements before execution and during performance. The cerebellum processes multiple neural signals from (1) motor areas of the cerebral cortex for motor planning, (2) sensory tract systems (dorsal spinal cerebellar tract, ventral spinal cerebellar tract) from muscle and joint receptors for proprioceptive and kinesthetic sense information resulting from movement performance, and (3) vestibular system information for the regulation of upright control and balance at rest and during movements. It compares motor plan signals driven by the cortex with what is received from muscles and joints in the periphery and makes necessary adjustments and adaptations to achieve the intended coordinated movement sequence. Movements that are frequently repeated “instructions” are stored in the cerebellum as procedural memory traces. This increases the efficiency of its role in coordinating movement. The cerebellum also plays a role in function of the reticular activating system (RAS). The RAS network exists in the brain stem tegmentum and consists of a network of nerve cells that maintain consciousness in humans and help people focus attention and block out distractions that may affect motor performance. Damage to the cerebellum, its tract systems, or its structure creates problems of movement coordination, not execution or choice of which program to run. The cerebellum also plays a role in language, attention, and mental imagery functions that are not considered to take place in motor areas of the cerebral cortex (see Table 4-2).

The cerebellum plays four important roles in motor control13:

1. Feed-forward processing: The cerebellum receives neural signals, processes them in a sequential order, and sends information out, providing a rapid response to any incoming information. It is not designed to act like the cerebral cortex and does not have the capability of generating self-sustaining neural patterns.

2. Divergence and convergence: The cerebellum receives a great number of inputs from multiple body structures, processes this information extensively through a structured internal network, and sends the results out through a limited number of output cells.

3. Modularity: The cerebellum is functionally divided into independent modules—hundreds to thousands—all with different inputs and outputs. Each module appears to function independently, although they each share neurons with the inferior olives, Purkinje cells, mossy and parallel fibers, and deep cerebellar nuclei.

4. Plasticity: Synapses within the cerebellar system (between parallel fibers and Purkinje cells, and synapses between mossy fibers and deep nuclear cells) are susceptible to modification of their output strength. The influence of input on nuclear cells is adjustable, which gives great flexibility to adjust and fine-tune the relationship between cerebellar inputs and outputs.

Information processing

The processing of information through the sensory input, motor output, and central integrative structures occurs by various methods to produce movement behaviors. These methods allow us to deal with the temporal and spatial components necessary for coordinated motor output and allow us to anticipate so that a response pattern may be prepared in advance. Serial processing is a specific, sequential order of processing of information (Figure 4-1) through various centers. Information proceeds lockstep through each center. Parallel processing is processing of information that can be used for more than one activity by more than one center simultaneously or nearly simultaneously. A third and more flexible type of processing of information is parallel-distributed processing.14 This type of processing combines the best attributes of serial and parallel processing. When the situation demands serial processing, this type of activity occurs. At other times parallel processing is the mode of choice. For optimal processing of intrinsic and extrinsic sensory information by various regions of the brain, a combination of both serial and parallel processing is the most efficient mode. The type of processing depends on the constraints of the situation. For example, maintaining balance after an unexpected external perturbation requires rapid processing, whereas learning to voluntarily shift the center of gravity to the limits of stability requires a different combination of processing modes.

In summary, information processing reinforces and refines motor patterns. It allows the organism to initiate compensatory strategies if an ineffective motor pattern is selected or if an unexpected perturbation occurs. And, most important, information processing facilitates motor learning.

Movement patterns arising from self-organizing subsystems

Coordinated movement patterns are developed and refined via dynamic interaction among body systems and subsystems in response to internal and external constraints. Movement patterns used to accomplish a goal are contextually appropriate and arise as an emergent property of subsystem interaction. Several principles relate to self-organizing systems: reciprocity, distributed function, consensus, and emergent properties.15

Reciprocity implies information flow between two or more neural networks. These networks can represent specific brain centers, for example, the cerebellum and basal ganglia (Figure 4-2). Alternatively, the neural networks can be interacting neuronal clusters located within a single center, for example, the basal ganglia. One model to demonstrate reciprocity is the basal ganglia regulation of motor behavior through direct and indirect pathways to cortical areas. The more direct pathway from the putamen to the globus pallidus internal segment provides net inhibitory effects. The more indirect pathway from the putamen through the globus pallidus external segment and subthalamic nucleus provides a net excitatory effect on the globus pallidus internal segment. Alteration of the balance between these pathways is postulated to produce motor dysfunction.16,17 An abnormally decreased outflow from the basal ganglia is postulated to produce involuntary motor patterns, which produce excessive motion such as chorea, hemiballism, or nonintentional tremor. Alternatively, an abnormally increased outflow from the basal ganglia is postulated to produce a paucity of motions, as seen in the rigidity observed in individuals with Parkinson disease (see Chapter 20).

Distributed function presupposes that a single center or neural network has more than one function. The concept also implies that several centers share the same function. For example, a center may serve as the coordinating unit of an activity in one task and may serve as a pattern generator or oscillator to maintain the activity in another task. An advantage of distributing function among groups of neurons or centers is to provide centers with overlapping or redundant functions. Neuroscientists believe such redundancy is a safety feature. If a neuronal lesion occurs, other centers can assume critical functional roles, thereby producing recovery from CNS dysfunction.1822

Consensus implies that motor behavior occurs when a majority of brain centers or regions reach a critical threshold to produce activation. Also, through consensus extraneous information or information that does require immediate attention is filtered. If, however, a novel stimulus enters the system, it carries more weight and receives immediate attention. A novel stimulus may be new to the system, may reflect a potentially harmful situation, or may result from the conflict of multiple inputs.

Emergent properties may be understood by the adage “the whole is greater than the sum of its parts.” This concept implies that brain centers, not a single brain center, work together to produce movement. An example of the emergent properties concept is continuous repetitive activity (oscillation). In Figure 4-3, A, a hierarchy is represented by three neurons arranged in tandem. The last neuron ends on a responder. If a single stimulus activates this network, a single response occurs. What is the response if the neurons are arranged so that the third neuron sends a collateral branch to the first neuron in addition to the ending on the responder? In this case (Figure 4-3, B), a single stimulus activates neuron No. 1, which in turn activates neurons No. 2 and No. 3, causing a response as well as reactivating neuron No. 1. This neuronal arrangement produces a series of responses rather than a single response. This process is also termed endogenous activity.

image
Figure 4-3 image Emergent property.

Another example of an emergent property is the production of motor behavior. Rather than having every MP stored in the brain, an abstract representation of the intended goal is stored. At the time of motor performance, various brain centers use the present sensory information, combined with past memory of the task, to develop the appropriate motor strategy. This concept negates a hardwired MP concept. If MPs were hardwired and if an MP existed for every movement ever performed, the brain would need a huge storage capacity and would lack the adaptability necessary for complex function.

Controlling the degrees of freedom

Combinations of muscle and joint action permit a large number of degrees of freedom that contribute to movement. A system with a large number of degrees of freedom is called a high-dimensional system. For a contextually appropriate movement to occur, the number of degrees of freedom needs to be constrained. Bernstein10 suggested that the number of degrees of freedom could be reduced by muscles working in synergies, that is, coupling muscles and joints of a limb to produce functional patterns of movement. The functional unit of motor behavior is then a synergy. Synergies help to reduce the degrees of freedom, transforming a high-dimensional system into a low-dimensional system. For example, a step is considered to be a functional synergy pattern for the lower extremity. Linking together stepping synergies with the functional synergies of other limbs creates locomotion (interlimb coordination).

Functional synergy implies that muscles are activated in an appropriate sequence and with appropriate force, timing, and directional components. These components can be represented as fixed or “relative” ratios, and the control comes from input given to the cerebellum from higher centers in the brain and the peripheral or spinal system and from prior learning (see Chapter 21).20,22,23 The relative parameters are also termed control parameters. Scaling control parameters leads to a change in motor behavior to accomplish the task. For example, writing your name on the blackboard exemplifies scaling force, timing, and amplitude. Scaling is the proportional increase or decrease of the parameter to produce the intended motor activity.

Coordinated movement is defined as an orderly sequence of muscle activity in a single functional synergy or the orderly sequence of functional synergies with appropriate scaling of activation parameters necessary to produce the intended motor behavior. Uncoordinated movement can occur at the level of the scaling of control parameters in one functional synergy or inappropriate coupling of functional synergies. The control parameter of duration will be used to illustrate scaling. If muscle A is active for 10% of the duration of the motor activity and muscle B is active 50% of the time, the fixed ratio of A/B is 1:5. If the movement is performed slowly, the relative time for the entire movement increases. Fixed ratios also increase proportionally. Writing your name on a blackboard very small or very large yields the same results—your name.

Timing of muscle on/off activation for antagonistic muscles such as biceps and triceps, or hamstrings and quadriceps, needs to be accurate for coordination and control of movement patterns. If one muscle group demonstrates a delayed onset or maintains a longer duration of activity, overlapping with triceps “on” time, the movement will appear uncoordinated. Patients with neurological dysfunction often demonstrate alterations in the timing of muscle activity within functional synergies and in coupling functional synergies to produce movement.24,25 These functional movement synergies are not hardwired but represent emergent properties. They are flexible and adaptable to meet the challenges of the task and the environmental constraints.

Finite number of movement strategies

The concept of emergent properties could conceivably imply an unlimited number of movement strategies available to perform a particular task. However, limiting the degrees of freedom decreases the number of strategies available for selection. In addition, constraints imposed by the internal environment (e.g., musculoskeletal system, cardiovascular system, metabolic activity, cognition) and external environment (e.g., support surface, obstacles, lighting) limit the number of movement strategies. Horak and Nashner26 observed that a finite number of balance strategies were used by individuals in response to externally applied linear perturbations on a force plate system. With use of a life span approach, VanSant27 identified a limited number of movement patterns for the upper limb, head-trunk, and lower limb for the task of rising from supine to standing.

The combination of these strategies produces the necessary variability in motor behavior. Although an individual has a preferred or modal profile, the healthy person with an intact neuromuscular system can combine strategies in various body regions to produce different movement patterns that also accomplish the task. Persons with neurological deficits may be unable to produce a successful, efficient movement pattern because of their inability to combine strategies or adapt a strategy for a given environmental change (e.g., differing chair height for sit-to-stand transitions).

Variability of movements implies normalcy

A key to the assessment and treatment of individuals with neurological dysfunction lies in variability of movement and in the notion that variability is a sign of normalcy, and stereotypical behavior is a sign of dysfunction.

Age, activity level, the environment, constraints of a goal, and neuropathological conditions affect the selection of patterns available for use during movement tasks. When change occurs in one or more of the neural subsystems, a new movement pattern emerges. The element that causes change is called a control parameter. For example, an increase in the speed of walking occurs until a critical speed and degree of hip extension are reached, thereby switching the movement pattern to a run. When the speed of the run is decreased, there is a shift back to the preferred movement pattern of walking. A control parameter shifts the individual into a different pattern of motor behavior.

This concept underlies theories of development and learning. Development and learning can be viewed as moving the system from a stable state to a more unstable state. When the control variable is removed, the system moves back to the early, more stable state. As the control variable continues to push the system, the individual spends more time in the new state and less time in the earlier state until the individual spends most of the time in the new state. When this occurs, the new state becomes the preferred state. Moving or shifting to the new, preferred state does not obviate the ability of the individual to use the earlier state of motor behavior. Therefore new movement patterns take place when critical changes occur in the system because of a control parameter but do not eliminate older, less-preferred patterns of movement.

Motivation to accomplish a task in spite of functional limitations and neuropathological conditions can also shift the individual’s CNS to select different patterns of motor behavior. The musculoskeletal system, by nature of the architecture of the joints and muscle attachments, can be a constraint on the movement pattern. An individual with a functional contracture may be limited in the ability to bend a joint only into a desired range, thereby decreasing the movement repertoire available to the individual. Such a constraint produces adaptive motor behavior. Dorsiflexion of the foot needs to meet a critical degree of toe clearance during gait. If there is a range of motion limitation in dorsiflexion, then biomechanical constraints imposed on the nervous system will produce adaptive motor behaviors (e.g., toe clearance during gait). Changes in motor patterns during the task of rising from supine to standing are observed when healthy individuals wear an orthosis to limit dorsiflexion.28 The inability to easily open and close the hand with rotation may lead to adaptations that require the shoulder musculature to place the hand in a more functional position. This adaptation uses axial and trunk muscles and will limit the use of that limb in both fine and gross motor performance. Refer to Chapter 23.

Preferred, nonobligatory movement patterns that are stable yet flexible enough to meet ever-changing environmental conditions are considered attractor states. Individuals can choose from a variety of movement patterns to accomplish a given task. For example, older adults may choose from a variety of fall-prevention movement patterns when faced with the risk of falling. The choice of motor plan may be negatively influenced by age-related declines in the sensory input systems or a fear of falling. For example, when performing the Multi-Directional Reach Test,29 an older adult may choose to reach forward, backward (lean), or laterally without shifting the center of gravity toward the limits of stability. This person has the capability of performing a different reaching pattern if asked, but prefers a more stable pattern.

Obligatory and stereotypical movement patterns suggest that the individual does not have the capability of adapting to new situations or cannot use different movement patterns to accomplish a given task. This inability may be a result of internal constraints that are functional or pathophysiological. The patient who has had a stroke has CNS constraints that limit the number of different movement patterns that can emerge from the self-organizing system. With recovery, the patient may be able to select and use additional movement strategies. Cognition and the capability to learn may also limit the number of movement patterns available to the individual and the ability of the person to select and use new or different movement patterns.

Obligatory and stereotypical movement patterns also arise from external constraints imposed on the organism. Consider the external constraints placed on a concert violin player. These external constraints include, for example, the length of the bow and the position of the violin. Repetitive movement patterns leading to cumulative trauma disorder in healthy individuals can lead to muscular and neurological changes.3033 Over time, changes in dystonic posturing and changes in the somatosensory cortex have been observed. Although one hypothesis considers that the focal dystonia results from sensory integrative problems, the observable result is a stereotypical motor problem.

To review, the nervous system responds to a variety of internal and external constraints to develop and execute motor behavior that is efficient to accomplish a specific task. Efficiency can be examined in terms of metabolic cost to the individual, type of movement pattern used, preferred or habitual movement (habit) used by the individual, and time to complete the task. The term attractor state is used in dynamical systems theory to describe the preferred pattern or habitual movement.

Individuals with neurological deficits may have limited repertoires of movement strategies available. Patients experiment with various motor patterns in order to learn the most efficient, energy-conscious motor strategy to accomplish the task. Therapists can plan interventions that help to facilitate refinement of the task to match the patient’s capability, allowing the task to be completed using a variety of movement strategies rather than limited stereotypical strategies, leading to an increase in function.

Errors in motor control

When the actual motor behavior does not match the intended motor plan, an error in motor control is detected by the CNS. Common examples of errors in motor control are loss of balance; inappropriate scaling of force, timing, or directional control; and inability to ignore unreliable sensory information, resulting in sensory conflict. Any one or combination of these errors may be the cause of a fall or error in performance accuracy.

Errors also occur when unexpected factors disrupt the execution of the program. For example, when the surface is unreliable (sand, unstable, moving), this will force the individual to adapt motor responses to meet the demand of the environment. Switching between closed environments (more stable) and open environments (more unpredictable) will challenge the individual to adapt motor responses. When an individual steps off of a moving sidewalk, a disruption in walking occurs. The first few steps are not smooth because the person needs to switch movement strategies from one incorporating a moving support surface to one incorporating a stationary support surface.

Errors occur in the perception of sensory information, in selection of the appropriate MP, in selection of the appropriate variable parameters, or in the response execution. Patients with neurological deficits may demonstrate a combination of these errors. Therefore an assessment of motor deficits in clients includes analysis of these types of errors. If a therapist observes a motor control problem, there is no guarantee that the central problem arises from within the motor system. Somatosensory problems can drive motor dysfunction; cognitive and emotional problems express themselves through motor output. Thus it is up to the movement specialist to differentiate the cause of the problem through valid and reliable examination tools (see Chapter 8). Once the cause of the motor problem has been identified, selection of interventions should lead to more outcomes.

All individuals, both healthy and those with CNS dysfunction, make errors in motor programming. These errors are assessed by the CNS and are stored in past memory of the experience. Errors in motor programming are extremely useful in learning. Learning can be viewed as decreasing the mismatch between the intended and actual motor behavior. This mismatch is a measure of the error; therefore a decrease in the degree of the error is indicative of learning. Errors, then, are an important part of the rehabilitation process. However, this does not mean that the therapist allows the client to practice errors over and over. The ability of the patient to detect an error and correct it to produce appropriate and efficient motor behavior is one key to recovery and an important consideration when intervention strategies are developed. This will be discussed further in the next section of this chapter.

Motor control section summary

Motor control theories have been developed and have evolved over many years as our understanding of nervous system structure and function has become more advanced. The control of posture and movement is a complex process that involves many structures and levels within the human body. It requires accurate sensory inputs, coordinated motor outputs, and central integrative processes to produce skillful, goal-directed patterns of movement that achieve desired movement goals. We must integrate and filter multiple sensory inputs from both the internal environment of the body and the external world around us to determine position in space and choose the appropriate motor plan to accomplish a given task. We combine individual biomechanical and muscle segments of the body into complex movement synergies to deal with the infinite “degrees of freedom” available during the production of voluntary movement. Well learned motor plans are stored and retrieved and modified to allow for flexibility and variety of movement patterns and postures. When the PNS or CNS is damaged and the control of movement is impaired, new, modified, or substitute motor plans can be generated to accomplish goal-directed behaviors, remain adaptable to changing environments, and produce variable movement patterns. The process of learning new motor plans and refining existing behaviors by driving neuroplastic changes in the nervous system is discussed in the next sections of this chapter. The control of posture and balance is also discussed in Chapter 22.

Motor learning

Therapeutic interventions that are focused on restoring functional skills to individuals with various forms of neurological problems have been part of the scope of practice of physical therapists (PTs) and occupational therapists (OTs) since the beginning of both professions. These two professions have emerged with a complementary background to examine, evaluate, determine a prognosis, and implement interventions that empower clients to regain functional control of activities of daily living (ADLs) (e.g., getting out of bed, bathing, walking, and eating, as well as working, playing, and socially interacting) and resume active participation in life after neurological insult. These two professions specialize in the analysis of movement and possess knowledge of the scientific background to understand why the movement is occurring, what strengths and limitations exist within body systems to produce that movement, and how different therapeutic interventions can facilitate or enhance functional movement strategies that remediate dysfunction and ultimately carry over into improved performance of daily activities and participation in life of an individual. PTs and OTs are also knowledgeable about diseases of body systems (neurological, musculoskeletal, integumentary, cardiopulmonary, and integumentary systems) and how the existence or progression of these pathological states affects motor performance and quality of life. Consideration and training of individuals who give assistance and support needed to help clients maintain functional skills during transitional disease states is also a component of practice and of treating the client in a holistic manner.

It is therefore important for clinicians to understand how individuals learn or relearn motor tasks and how learning of motor skills can best be achieved to optimize outcomes.

Motor learning results in a permanent change in the performance of a skill because of experience or practice.34 The end result of motor learning is the acquisition of a new movement, or the reacquisition and/or modification of movement.35 The patient must be able to prepare and carry out a particular learned movement36 in a manner that is efficient (optimal movement with the least amount of time, energy, and effort),37 consistent (same movement over repeated trials),38 and transferrable (ability to perform movement under different environments and conditions) to be considered to have learned a skill.

Long-term learning of a particular motor task allows the patient to use this particular skill to optimize function. This type of learning is expressed in declarative and procedural memory. Declarative or explicit memory is expressed by conscious recall of facts or knowledge. An example of this could be the patient verbally stating the steps needed when going up the stairs with the use of crutches. This is opposed to procedural (or nondeclarative) learning, in which movement is performed without conscious thought (e.g., riding a bike or rollerblading). The interplay of conscious (cognitive and emotional) and unconscious memory affects ultimate learning and may decrease the time needed to learn or relearn a functional movement and its use in everyday activity.

The ability of an individual to have learned a motor skill is measured indirectly by testing the ability of a patient to perform a particular task or activity both over time and in different environmental contexts (performance). The testing must be done over a period of time to determine long-term learning and minimize the temporary effects of practice. In retention tests, the patient performs the task under the same conditions in which the task was practiced. This type of test evaluates the patient’s ability to learn the task. This is in contrast to transfer tests, in which the patient performs the activity under different conditions from those in which the skill was practiced. This evaluates the ability of the patient to use a previously learned motor skill to solve a different motor problem.

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