Human Motor Control

David A. Rosenbaum

• Publisher: Academic Press
• Year: 2014
• ISBN:
  9780080571089
• Topic: Psicología
• Subtopic: Psicología cognitiva y cognición

Human Motor Control is a elementary introduction to the field of motor control, stressing psychological, physiological, and computational approaches. Human Motor Control cuts across all disciplines which are defined with respect to movement: physical education, dance, physical therapy, robotics, and so on. The book isorganized around major activity areas.

• Acomprehensive presentation of the major problems and topics in human motor control
• Incorporates applications of work that lie outside traditional sports or physical education teaching

PART I

PRELIMINARIES

Outline

Chapter 1: INTRODUCTION

Chapter 2: PHYSIOLOGICAL FOUNDATIONS

Chapter 3: PSYCHOLOGICAL FOUNDATIONS

1

INTRODUCTION

Publisher Summary

Motor control is essential for virtually all aspects of life. It allows communication, manipulation of objects, transportation from place to place, eating, breathing, and reproduction. The central issues in motor control research are twofold: (1) making movements and (2) maintaining stability. Two principal kinds of analyses have been pursued in the study of human motor control. One is tied to the physical mechanisms responsible for movement and stability. This sort of analysis has been pursued chiefly by physiologists. The other kind of analysis is concerned with functional aspects of motor control and can be carried out without necessary regard for the physical underpinnings of behavior. This sort of analysis has been pursued chiefly by psychologists. One of the major issues in the field of human motor control is the degrees-of-freedom problem. The chapter discusses the way in which particular movements are selected, given that there are more degrees of freedom in the muscles and joints than in the description of the task to be performed. One way of solving the degrees-of-freedom problem has been to propose that efficiency is taken into account in selecting movements. One possible efficiency constraint is minimizing mean squared jerk. Another approach has been to identify motor synergies—dependencies among effector elements, seen, for example, in the functional coupling of the two arms. These dependencies effectively reduce the degrees of freedom that must be controlled. A third approach is to rely on the biomechanical properties of the motor system. By relying on the effects of gravity, for example, or on the effects of the elasticity of the muscles and tendons, it may be unnecessary to compute detailed aspects of movement trajectories.

How do we move? How do we walk, talk, sing, and smile? How do we perform on the athletic field, play musical instruments, craft tools and works of art? How do we learn to carry out these activities, and why are some of us better at them than others? What goes wrong when, through accident or disease, the ability to move is impaired? How can movement disabilities be restored or, better yet, prevented? And how can machines be made to carry out the tasks that most people (and animals) perform effortlessly?

As this list of questions suggests, understanding human motor control can have significant effects in a wide range of endeavors. This is hardly surprising given that movement occurs in virtually all walks of life. In sports, where rapid coordinated action can make the difference between victory and defeat, an understanding of motor control can allow for more victories or heightened levels of competition. In the fine arts, where performance on the stage or in the studio allows for aesthetic expression, understanding how we control the movements of our bodies can enhance the quality of expression as well as the training that leads to it. In medicine, where paralysis, lack of coordination, or weakness can sabotage the quality of life, rehabilitation can be improved through a deeper appreciation of the means by which the motor system functions. Finally, at home and in the workplace, the use of machines or appliances can be made safer or more efficient through the application of principles gained through motor control research.

Two fundamental questions lie at the heart of this field of study. One is how we control our movements; the other is how we maintain stability. Holding an object steady in changing wind conditions or standing still in a subway are tasks that demand stabilization. Without muscular control, such tasks would be hopeless—as hopeless, in fact, as moving. Because stabilization as well as movement must be achieved by the system we will be studying, we will not refer to it as the movement system or the stabilization system, but rather the motor system.

The word motor has some unfortunate connotations. One is that of machinelike rigidity. Conventional motors churn away monotonously, performing the same motions over and over again. By contrast, behavior is endlessly novel, at least under normal conditions. The novelty of behavior could only occur if the motor system allowed for the generation of continually changing patterns of muscle activity. It does so by relying on a rich configuration of neuromuscular assemblages that have evolved over millions of years. If you doubt the sophistication of the motor system, consider modern robots. These devices embody much of what we currently know about motor control, yet they can barely walk across uneven surfaces without toppling over, or engage in such mundane activities as tying a Boy Scout knot. Given the relatively mediocre performance of state-of-the-art robots, our ignorance of motor control is painfully obvious. A robot may run with motors—the other connotation of “motor” control—but the human body does not, at least not with conventional motors made of axles and magnetic coils. The motive forces for behavior are controlled in more subtle and sophisticated ways. Understanding how these forces are governed and physically realized can help us develop more effective robots. In addition, and perhaps more importantly, it can help us appreciate how we function as active, intelligent agents.

PHYSIOLOGICAL AND PSYCHOLOGICAL EXPLANATION

What does it mean to understand human motor control? What is to be understood, and what form should the understanding take? The answers to these questions are not obvious, for under normal circumstances movement and stability just seem to happen. When things work well, it is often unclear what their underlying components are. A hallmark of skilled performance, in fact, is that it occurs effortlessly. Thinking about motor skills can often prevent them from happening.

In abnormal circumstances skills may be disrupted. As a result of accident or disease, one’s ability to move or stabilize the body may be drastically impaired. A wide range of motor disorders afflict people; many will be discussed here. Considering these disorders and the factors that cause them helps illuminate the substrates of normal performance.

It is possible to study the motor system in many ways. Understanding the physical components of the system is a task of physiologists—people who investigate the functions served by the physical structures of the body. Physiologists interested in motor control focus on muscles, bones, and joints, as well as the nervous system, the neural network that governs how muscles act. The practitioners who apply this information in the clinic include neurologists, who diagnose and treat ailments of the nervous system, orthopedists, who diagnose and treat disorders of bones and joints, physical therapists, who help restore motion and stability through behavioral rehabilitation, and prostheticians, who design and fit artificial limbs (prostheses) for people with amputations. Rudiments of motor physiology will be described in Chapter 2, Physiological Foundations.

Besides analyzing motor control in physical terms, another useful approach is psychological. This approach is described in Chapter 3, Psychological Foundations. Theories in psychology are not restricted to effects of personality, mental illness, or conscious thought. They also focus on mental functions—conscious or unconscious—underlying performance. Psychologists do not usually deny physical causes of behavior; in fact, they are usually pleased if their models find physiological support. However, the explanations that psychologists pursue usually do not require one-to-one mappings of identified biological mechanisms to behavioral or mental phenomena. Psychologists accept the fact that perception, thought, and action may emerge from the collective effects of many biological mechanisms. Identifying those mechanisms or the way they work is of less concern than understanding the emergent properties of the system as a whole.

Both for psychologists and for physiologists, four major problems occupy the core of motor control research. These are (1) the degrees-of-freedom problem, (2) the serial-order problem, (3) the perceptual-motor integration problem, and (4) the skill-acquisition problem. The next sections introduce each of these problems in turn.

THE DEGREES-OF-FREEDOM PROBLEM

Most physical tasks can be performed in an infinite number of ways. This has some advantages. One is obstacle avoidance (Cruse, 1986). If you need to reach for an object and there are obstacles in the way, it is helpful to have more than one way to reach for it. Another advantage is that the limbs that normally perform the task may not always be available for doing so. Holding a heavy package, for example, may make it impossible for you to turn on a light switch the way you usually do (with your hand). Nevertheless, you can turn on the light switch with your chin, even if you have never done so before. Similarly, ...