Human Motor Control

503 Pages·2009·12.771 MB·English

by  David A. Rosenbaum (Auth.)

P a r t I
PRELIMINARIES
C H A P T E R

Introduction
o u T l I n E
Understanding Human Motor Control 2 Behavioral Science, Cognitive Science,
and Human Factors �
Levels of Analysis 4
Physiology, Neuroscience, Medicine,
Fields Contributing to Research and Allied Fields �
on Human Motor Control 6
Organization of the Book 7
Physics 6
Engineering 66 Summary 9
Statistics ��
How do we control the movements of our bodies? How do we walk, talk, sing, and
smile? How do we manage to perform on the athletic field, play musical instruments, craft
tools, and produce 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,
our ability to move is impaired? How can movement disabilities be restored or, better yet,
prevented? And how can machines be made to carry out tasks that most people and animals
perform effortlessly?
As this list of questions suggests, understanding the control of body movements can have
significant effects in a wide range of endeavors. This is hardly surprising given that move-
ments occur in all walks of life. In sports, where moving skillfully is the very essence of
the activity, an understanding of motor control can permit heightened levels of competition.
In the fine arts, where performance on stage or in the studio permit self-expression, under-
standing how movements are controlled can enhance the quality of that expression and can
improve the training that leads to it. In medicine, where paralysis or lack of coordination can
sabotage the quality of life, rehabilitation can be refined through deeper appreciation of the
means by which the motor system functions. Finally, at home and in the workplace, where
machines and other appliances are used, such devices can be made safer and more efficient
through the application of principles gained through research on human motor control.

Human Motor Control, Second Edition © 2010, Elsevier Inc.
2 1. InTRoduCTIon
The question of how movements are controlled is just one of the questions in this field.
The other major one is how we maintain stability. Holding an object steady in the face of
changing wind conditions or while standing in a moving train are tasks that demand sta-
bilization. Without muscular control, such tasks would be impossible. Because stabilization
as well as movement are the jobs of the system covered here, we refer to that system as the
motor system, not just the movement system or the stabilization system.
The word “motor” has a connotation that is worth dismissing immediately. That conno-
tation is machinelike rigidity. Conventional motors churn away monotonously, performing
the same motions over and over again. By contrast, behavior is endlessly novel, reflecting
millions of years of evolution in which effective strategies developed for adaptation to ever-
changing conditions.
If you doubt the sophistication of the motor system, consider the state of robots today.
These devices embody much of what we know about motor control, but two-legged robots
can barely walk across uneven surfaces without toppling over. Getting a robot to make a
bed or fold laundry is out of the question.
The inability of robots to plan and control physical actions as flexibly as people and ani-
mals do means that people rather than robots are often called upon to perform in dangerous
situations. Astronauts rather than robots typically carry out repairs in outer space. Beneath
the earth, miners place themselves at risk as they pick away at unwieldy surfaces to extract
coal or other minerals. On earth’s surface, people defuse mines and sift through rubble after
earthquakes, floods, or bomb blasts. If we knew how people control their bodies, we could
endow robots with programs that enable the robots to move as people do and carry out dan-
gerous and boring tasks that people currently do.
UndeRStAnding HUMAn MOtOR COntROL
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 as obvi-
ous as they might seem, for under normal circumstances, movement and stability just seem
to happen. When things work well, it is easy to take them for granted. Indeed, a hallmark of
skilled performance is that it occurs without the need for attention. When one thinks about
performance, one’s performance often suffers (Wulf, 2007).
By contrast, in abnormal circumstances, skills may be disrupted. Due to an accident or
disease, the ability to move or stabilize the body may be impaired. A wide range of move-
ment disorders afflict people, many of which will be discussed in this book. Considering
these disorders and the factors that cause them can illuminate the factors that allow for nor-
mal performance. That knowledge can in turn contribute to our understanding of motor
control in general.
What does it mean to understand motor control? One understands a system if one can
predict and control it. We do not fully understand the weather, for example, because we
cannot predict it, except at a gross level, and we cannot control the weather in any practical
sense. On the other hand, we understand aeronautics well enough that we entrust our lives
to airplanes traveling at breakneck speeds and at dizzying heights.
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undERsTAndIng HumAn moToR ConTRol  
Virtually all neurologically normal individuals can control their movements reasonably
well, but none of these individuals can say in detail how they achieve that control. Most
adults can ride a bicycle, for example, but none can say how they do so. Even Albert Einstein
would have had to think for a while before saying how he managed to cycle.
The fact that the principles of motor control cannot be easily articulated does not mean
they can never be verbally expressed. The method for riding a bicycle has been written
down:
The rule observed by the cyclist is this. When he starts falling to the right he turns the handlebars to the
right so that the course of the bicycle is deflected along a curve toward the right. This results in a centrifugal
force pushing the cyclist to the left and offsets the gravitational force dragging him down to the right. This
maneuver presently throws the cyclist out of balance to the left, which he counteracts by turning the han-
dlebars to the left; and so he continues to keep himself in balance by winding along a series of appropriate
curvatures. A simple analysis shows that for a given angle of unbalance the curvature of each winding is
inversely proportional to the square of the speed at which the cyclist is proceeding [Polanyi, 1964, 49–50].
It is unlikely that Einstein drew on a verbal proposition like this one. The form of knowl-
edge he probably relied on, and the form of knowledge that the rest of us use, is tacit or
implicit knowledge (Polanyi, 1964).
By seeing that the knowledge underlying bike riding and other motor activities can be
verbalized, we see that understanding motor control involves being able to state the prin-
ciples of motor control. Ultimately, a theory of motor control will have to be expressed in a
form that lets the theory be communicated in books such as this one and in scientific articles.
In other words, it will have to verbalized. What form should that verbalized theory take?
It is possible to demonstrate one’s understanding of a system by simulating it. In the case
of motor control, such a demonstration can take the form of a robot that moves in a way
that is indistinguishable from the way neurologically normal people do. Such a robot would
pass the Turing test of motor intelligence. The term Turing test refers to a method of judg-
ing intelligence. The test was developed by Alan Turing, a mathematician and philosopher
who helped crack the secret code used by the Nazis in World War II. Turing proposed a test
for understanding verbally expressed intelligence. In the test, a human participant—call her
Elana—interacts with a computer or with another person via a keyboard and display. The
critical feature of the setup is that Elana does not know with whom she is dealing, a person
or a computer. Based on the interactions, she tries to determine whether the individual is a
computer or a person. If Elana cannot tell that she is interacting with a computer though her
correspondent is in fact a machine, one can say that the designer of the computer succeeded
in designing a machine that is as intelligent as a person. A Turing test of motor intelligence
would work in similar ways. If Elana watched a robot dancing and thought it was a human
being, she could say the designer of the dancing robot had captured, and truly understood,
the control of dance.
Are we near such a demonstration? Humanoid robots developed by Mitsuo Kawato and
his colleagues at the ATR Laboratories in Japan come close to passing the Turing test. One
of those robots is shown in Figure 1.1. The robot shown in this figure can play air hockey
as well as most people can. Even more interestingly, it does so in a way that is amazingly
lifelike. If you find and watch a video of this robot on the world-wide web, be mindful that
the person playing against the robot can do something the robot can’t. The person can walk
away, stroll down the hall, and, if he hears a child crying out for help in a room he passes, he
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4 1. InTRoduCTIon
FigURe . A humanoid robot playing air hockey with a person. From http://lasa.epfl.ch/events/events/
iros03workshop/AirHockey.jpg.
can enter the room and pull the child out from the shelf and pile of books that have landed
on him when he decided to climb up on his mother’s bookcase. The robot, by contrast, can
do just what it was programmed to do and no more.
LeveLS OF AnALySiS
Reflecting on the air-hockey-playing robot, you might say that the lifelike performance
of this robot is of only moderate interest, for human beings are made of flesh and blood,
whereas robots, or at least robots like the air-hockey-playing robot shown in Figure 1.1, are
made of synthetic materials. Doesn’t that difference make the comparison between people
and robots irrelevant to the understanding of human motor control?
The fact that one actor is real and the other is not does not mean that similarities between
them are irrelevant. Apart from the fact that it is fun to watch a machine behave as people
do, the deeper reason why robots and people are worth comparing is that different levels
of analysis characterize the study of human motor control. Understanding these levels of
analysis helps us appreciate why it useful to compare robots to people and animals.
David Marr (1982) proposed three levels of understanding for the study of such systems.
One level of analysis that Marr identified was the computational level. This level, according
to Marr, consists of a description, often expressed mathematically, of one or more functions
that a system is supposed to achieve. The cartoon in Figure 1.2 illustrates the computational
level. Here a cat prepares to jump onto a table. At some level, the cat’s planning can be rep-
resented in equations and diagrams of the sorts that physicists write, although of course
the cat does not explicitly use such equations anymore than people do, or, presumably, any
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lEvEls of AnAlysIs  
FigURe .2 The computational level of analysis in motor control. From http://www.thenewyorkerstore.com/
assets/1/46752_t.gif.
more than Einstein did when he rode his bicycle or carried out other physical activities.
Even if equations are not used explicitly, they are used implicitly in performance. The compu-
tational level of analysis is concerned with those implicit equations.
How the cat actually jumps onto the table is left out of the computational description.
The cat might crouch, leap up, prepare to land, and then use its sense of touch to check that
it has landed. These behavioral and cognitive states constitute the second of Marr’s levels,
what can be called the procedural level. This term is used because it concerns events that
occur in real time. (Marr actually used the term algorithmic level, but algorithms are guar-
anteed to work. Not all procedures do; getting a basketball to go through a net is not a sure
thing, though it relies on a procedure.)
The third of Marr’s levels is the implementation level. Here what counts is the physical
stuff that permits procedures to be physically achieved. In the case of the jumping cat, the
questions relevant to the implementation level are how muscles contract and relax, which
brain regions become active or inactive, and so on. If the cat were robotic rather than mam-
malian, the relevant issues would be how the hydraulics of the actuators should be used,
how the robot’s battery power should be drawn on, and so on.
Having identified the three levels in Marr’s triptych—the computational level, the pro-
cedural level, and the implementation level—we can now ask which of those three levels is
most important in the study of human motor control. One way to address this question is to
ask what would happen if any of the levels were excluded.
If the implementation level were excluded, most medical conditions involving motor con-
trol would be forgotten. No one would want that to happen, of course. We want advances in
motor control to help people overcome motor-control difficulties.
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6 1. InTRoduCTIon
If the algorithmic level were excluded, most studies of effective training procedures would
be left by the wayside, as would research in the field of human factors—designing safer
roads, smarter computer interfaces, and so on. Again, this is not what one would want.
Finally, if the computational level were excluded, most efforts to advance robotics would
be abandoned. This, too, would be undesirable, for even if you have qualms about robots—
perhaps you feel they will take over the jobs of too many people—computational work is
of great importance. Thus, the understanding of human motor control requires work at all
three levels. No level is more important than any other. All of the levels should be pursued
and respected.
FieLdS COntRiBUting tO ReSeARCH
On HUMAn MOtOR COntROL
If all three levels of analysis contribute to the understanding of human motor control, one
would expect fields that pursue any of the levels to contribute to this field of study. This is
indeed the case, making human motor control a truly interdisciplinary area.
Physics
As mentioned earlier, physics plays a key role in the study of human motor control. This
makes sense, considering that there are physical constraints on what we can do. We can only
jump so high, run so fast, push so hard, and so on. Physics helps determine the factors that
limit performance.
Physics can also indicate the factors that enable performance. A skillful baseball batter can
learn how to use the momentum of her bat to maximize the force she exerts on the oncom-
ing ball, and she can learn to hit the ball on the bat’s “sweet spot,” where bounce-back is
maximized. Similarly, a cat preparing to jump onto a table can learn how to drop on its
haunches before leaping up, exploiting elastic energy (Alexander, 1992).
engineering
The computational style of analysis used by physicists in their theoretical work has also
been used by engineers in their applied work. Men and women in engineering use physical
principles to solve practical problems such as designing bridges that don’t fall or designing
computers that don’t forget. Engineers also work on issues related to motor control such as
robotics and prostheses (artificial appendages used by amputees).
One issue that engineers have worked on a great deal is how to use feedback effectively.
Feedback is crucial for effective motor control, whether the performance entails standing on
a moving train, chasing a fly ball, or driving an automobile. The approach to the analysis of
feedback that engineers have developed is called control theory.
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oRgAnIzATIon of THE book     7
Statistics
If you take a pencil and move the tip back and forth as quickly as possible between two
target points, always lifting the pencil between the hits, you will end up with a cloud of
points near each target. These clouds show that, despite your best intentions, your perform-
ance is not perfect. No one’s is. Everyone’s performance is variable to some degree.
The study of variability as well as other features of data is the focus of statistics. Some
important recent advances in the understanding of human motor control have been made
possible through the application of statistics to motor control research.
Behavioral Science, Cognitive Science, and Human Factors
Researchers who study the procedural level of human motor control work in the fields of
behavioral science, cognitive science, and human factors. Behavioral scientists study overt
physical activity to predict and, on occasion, control behavior. Cognitive scientists are also
interested in the procedural level, but are especially interested in the internal processes that
allow behavior to unfold. Cognitive scientists often say they are interested in the software
that makes behavior possible. Human factors specialists are interested in the applications of
behavioral and cognitive science to practical problems such as designing easy-to-understand
road signs, easy-to-use computer interfaces, and easy-to-manipulate hand tools. Scientists in
these fields work in academic departments, such as departments of psychology, industrial
engineering, physical education, or kinesiology, or in companies or governmental agencies.
Chapter 4 provides an overview of behavioral science, cognitive science, and human factors
approaches to human motor control.
Physiology, neuroscience, Medicine, and Allied Fields
People who study the implementation level of motor control in biological systems are most
often associated with physiology. Physiologists interested in motor control focus on mus-
cle, bones, and joints, as well as the main regulator of muscle activity, the nervous system,
which is the province of neuroscience. The practitioners who apply this information in the
clinic are 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 craft and fit
artificial limbs for people with amputations). Rudiments of physiology, neuroscience, medi-
cine, and allied fields that pertain to human motor control will be presented in Chapter 3.
ORgAnizAtiOn OF tHe BOOk
This book has three main parts. The first is introductory. It contains, besides this chap-
ter, Chapter 2 (Core Problems), Chapter 3 (Physiological Foundations), and Chapter 4
(Psychological Foundations).
HumAn moToR ConTRol, 2 Ed
 1. InTRoduCTIon
The second part is organized by activity. Within it, Chapter 5 is concerned with walk-
ing and related forms of locomotion. Chapter 6 is concerned with looking, or more specifi-
cally, the control of eye and head movements. Chapter 7 focuses on reaching and grasping.
Chapter 8 treats the control of drawing and writing. Chapter 9 covers keyboarding (i.e., the
control of typewriting, text-messaging, piano playing, and other finger-movement tasks).
Chapter 10 covers the control of speaking and singing. Chapter 11, the last chapter in Part II,
is concerned with smiling and other forms of facial expression.
The third part of the book has a single chapter, Moving On. This chapter presents new
advances and new challenges that newcomers to the field may wish to take up.
You should know why the book is organized as it is and why the book has the title it has.
Regarding organization, and in particular the devotion of much of the book to distinct activ-
ities (Chapters 5–11), this approach has several advantages. One is that research in motor
control, like research in most fields today, has become specialized. It is not really a carica-
ture of the field to say that there are people who work entirely on eye movements, others
who work entirely on reaching and grasping, others who work entirely on speech, and so
forth. The proliferation of subspecialties within motor-control research derives partly from
the practical and theoretical interests of workers in the field. Practical concerns with particu-
lar tasks sometimes compel investigators to pursue particular task domains at the expense
of others. A researcher working for a computer company, for example, is likely to be more
interested in keyboarding than in singing. Theoretical interests also place some investigators
on circumscribed research paths. Students of vision, for example, are naturally more inter-
ested in oculomotor (eye movement) control than in speech.
The second reason for devoting separate chapters to separate activities is that many of
the specialty areas have developed their own problems and methodologies. The twists and
turns in one area do not always map easily onto the lines of study in another area, so rather
than risk losing the richness of particular areas of study, each area is paid its due by being
considered on its own.
In treating the subsystems separately, it is critical that the treatment not become too
parochial. Areas of common concern need to be highlighted, as does work on the coor-
dination of different motor tasks. Being on the lookout for similarities between the motor
subsystems puts one in an advantageous position for detecting differences between them.
That there might be differences is a real and intriguing possibility, given that some proc-
esses may be controlled by independent modules with their own rules of operation (Fodor,
1983). If modularity applies to the motor system, different motor activities might be con-
trolled in wholly different ways. Treating the motor activities separately makes it possible
to identify those differences if they exist or note their absence if they might otherwise be
expected.
Regarding the title of the book, Human Motor Control, many but not all of the activities
that are discussed here can be carried out only by people, but many of the activities can
also be carried out by non-human animals. The word “human” is included in the title even
though animal research is discussed throughout the volume. Animal work is important to
the understanding of human motor control (and vice versa).
The use of the term “motor control” also requires justification, for one might wonder
whether there really is a system responsible for motor activity, separate from other systems
HumAn moToR ConTRol, 2 Ed
summARy  9
responsible for perception or cognition. There is no such separate system, and no claim is
made here that there is. Demarcating systems need not imply that the systems are isolated.
It is helpful, if only for educational purposes, to treat motor control as a topic in its own
right, just as it is helpful to analyze perceptual functions such as vision or audition on their
own. Ultimately, all the seemingly separate systems work together. The extent to which they
do provides clues about the integration of function that is critical for adaptive performance
in everyday life.
SUMMARy
1. The study of human motor control can have practical benefits in sports, in medicine, in
the home, and in the workplace.
2. Research on human motor control focuses on the control of movement as well as the
control of stability.
3. Robots reflect our understanding of human motor control. Robots are still relatively
primitive. They cannot exhibit the endless novelty of motor behaviors that people can
exhibit.
4. The goal of studying human motor control is to understand it. A measure of
understanding of human motor control is the ability to verbalize the principles of such
control and the ability to simulate such control in robots. If robots act in ways that are
indistinguishable from the way people act, one can say that developers of robots truly
understand human motor control, at least with respect to the acts being performed by
the machines they develop.
5. Different levels of analysis are pursued in the study of human motor control. These
are the computational level, the procedural level, and the implementation level. The
computational level pertains to abstract, typically mathematical, levels of description.
The procedural level pertains to behavioral and cognitive methods used in performance.
The implementation level pertains to the material substrates—the neurons, muscles, or,
in the case of robots, motors, wires, and so on. All three levels are necessary for analysis.
6. Many fields contribute to the study of human control: physics, statistics, engineering,
behavioral science, cognitive science, human factors, physiology, neuroscience, medicine,
and allied fields.
7. This book has three sections. The first is introductory, with chapters on core problems,
physiological foundations, and psychological foundations. The second section is
concerned with different motor activities, with chapters on walking, looking, reaching,
drawing and writing, keyboarding, speaking and singing, and smiling. The third section
is concerned with recent advances and future challenges.
8. This book is called Human Motor Control even though research with animals is discussed
and even though the system responsible for motor control is not separate from the
system responsible for perception or the system responsible for learning. Distinctions
between these systems are drawn for educational purposes only.
HumAn moToR ConTRol, 2 Ed