Scientific Principles for Quantifying Motion
Human movement has fascinated men for centuries including
some of the world's greatest thinkers such as Leonardo da Vinci, Giovanni Borelli, Wilhelm
Braune, and others. Many questions posed by these stellar geniuses have been or can be
addressed by the relatively new area of Biomechanics. Biomechanics is the study of the
motion of living things, primarily, and it has evolved from a fusion of the classic
disciplines of anatomy, physiology, physics, and engineering. "Bio" refers to
the biological portion, incorporating muscles, tendons, nerves, etc. while
"mechanics" is associated with the engineering concepts based upon the laws
described by Sir Isaac Newton. Human bodies consist of a set of levers which are powered
by muscles. Quantification of movements, whether human, animal, or inanimate objects, can
be treated within biomechanics according to Newtonian equations. It may seem obvious, with
the perfect vision of hind sight, that humans and their activities, such as wielded as
tools (hammer, ax) or implements (baseball bat, golf club, discus), must obey the
constrains of gravitational bodies just as do bridges, buildings, and cars. For some
inexplicable reason, humans and their activities had not been subjected to the appropriate
engineering concepts which architects would use when determining the weight of books to be
housed in a new library nor engineers would apply to designing a bridge to span a wide,
yawning abyss. It was not until Newton's apple fell again during the 20th Century that
Biomechanics was born.
Biomechanics, then, is built on a foundation of
knowledge and the application of the basic physical laws of gravitational effects as well
as those of anatomy, chemistry, physiology, and other human sciences. Early quantification
efforts of human movement organized the body as a system of mechanical links. Activities
were recorded on movie film which normally consisted of hundreds of frames for each of the
desired movement segment. Since each frame of the activity had to be processed
individually, the task was excessively lengthy, tedious, and time intensive. The hand
calculations of a typical 16 segment biomechanical "human" required many hours
for each frame necessitating either numerous assistants or an individual investigator's
labor-of-love and, frequently, both. Unfortunately, these calculations were susceptible to
numerical errors. The introduction of large, main-frame computers improved reliability and
reasonableness of the results replacing much of the skepticism or distrust associated with
the manually computed findings. Computerization accelerated the calculations of a total
movement much more rapidly than had been previously possible but presented new
difficulties to overcome. Many of the early biomechanical programs were cumbersome, time
intensive main-frame endeavors with little appeal except to the obsessed, devotee of
computers and movement assessment. However, even these obstacles were conquered in the
ever expanding computerization era.
The computerized hardware/software system provides a means
to objectively quantify the dynamic components of movement in humans regardless of the
nature of the event. Athletic events, gait analyses, job-related actions as well as motion
by inanimate objects, including machine parts, air bags, and auto crash dummies are all
reasonable analytic candidates. Objectivity replaces mere observation and supposition.
One of the most important aspects included in the
"Bio" portion of Biomechanics is the musculoskeletal system. Voluntary human
movement is caused by muscular contractions which move bones connected at joints. The
neuromuscular system functions as a hierarchical system with autonomic and basic, life
sustaining operations, such as heart rate and digestion, controlled at the lowest,
non-cognitive levels and with increasing complexities and regulatory operations, such as
combing the hair or kicking a ball, controlled by centers which are farther up the nervous
system. Interaction of the various control centers is regulated through two fundamental
techniques each governed like a servosystem. The first technique equips each level of
decision making with subprocessors which accept the commands from higher levels as well as
accounting for the inputs from local feedback and environmental information sensors. Thus,
a descending "pyramid" of processors is defined which can accept general
directives and execute them in the presence of varying loads, stresses, and other
perturbations. This type of input-output control is used for multimodal processes, such as
maintaining balance while walking on an uneven terrain, but would be inappropriate for
executing deliberate, volitional, complex tasks like the conductor using the baton to
coordinate the music of the performing musicians.
The second technique which the brain utilizes to control
muscular contractions applies to the operation of higher level systems which generate
output strategies in relation to behavioral goals. These tasks use information from
certain sensory inputs including joint angle, muscle loading, and muscular extension or
flexion which are assessed, transmitted to higher centers for computation, and then
executes the set of modified neural transmissions received. Cognitive tasks requiring the
type of informational input which influences actions are the ones with which humans are
most familiar since job execution requires more thought than breathing or standing
upright.
One of the most important concepts which is frequently
misunderstood is that limb movement is possible only through contractions of individual
muscle fibers. For most cases of voluntary activity, muscles work in opposing pairs with
one set of muscles opening or extending the joint (extensors) while the opposite muscle
group closes or flexes the joint. The degree of contraction is proportional to the
frequency of signal from the nerve as signaled from the higher centers. Movement control
is provided by a programmable mechanism so that when flexors contract, the extensors
relax, and vice versa. The motor integration "program" generated in the higher,
cognitive levels regulates not only the control of the muscle groups about a joint but
also those necessary actions by other muscles and limbs to redistribute weight, to
counteract shifts in the center of gravity, etc.
The importance of the nervous system in the control and
regulation of coordinated movement cannot be underemphasized. When a decision is made to
move a body segment, the prime muscles or "agonists" receive a signal to
contract. The electrical burst stimulates the agonist muscular activity causing an
acceleration of the segment in the desired direction. At the same time, a smaller signal
is transmitted to the opposite muscle group, or antagonist, which causes it to function as
a joint stabilizer. With extremely rapid movements, the antagonist is frequently
stimulated to slow the limb in time to protect the joint from injury. It is the strength
and duration of the electrical signal to both the agonist and antagonist which govern the
desired action.
The movement of agonists and antagonists, whether a
cognitive process such as throwing a ball or an acquired activity such as postural
control, is controlled by the nervous system. Many ordinary voluntary human activities
resulting from agonist-antagonist muscular contraction are classified by different terms,
"isotonic", "variable resistance", and "ballistic". Slower,
tracking movements demonstrate smaller, more frequent, electrical signal alterations and
controls for both agonist and antagonist. These types of motion are "tracking"
movements.
One control mechanism available involves the process of
information channeled between the environment and the musculature. Closed-loop control
involves the use of feedback whereby differences between actual and desired posture are
detected and subsequently corrected, whereas open-loop control utilizes feedforward
strategies that involve the generation of a command based on prior experience rather than
on feedback. Braitenberg and Onesto proposed a network for converting space into time by
providing that the position of an input would determine the time of the output. This
"open loop" system would trigger a preset signal from the nervous system to the
muscle generating a "known" activity. Kicking a ball, walking, throwing a
baseball, swinging a golf club, and hand writing are considered ballistic movements.
When a limb moves, a sophisticated chain of events occurs
before, during, and after the movement is completed. The fineness of control depends upon
the number of motor nerve units per muscle fiber. The more neurons, the finer the ability
to maneuver, as with eye movements or delicate hand manipulations. In contrast to the high
innervation ratio of the eye, the biceps of the arm has a very low rate of nerve-to-muscle
fiber resulting in correspondingly more coarse movements. While the amount of nervous
innervation is important when anticipating the precision of control, the manner of
interaction and timing between muscles, nerves, and desired outcome is probably more
important when evaluating performance.
Recognizable actions elicit execution of patterned,
synchronous nervous activity. Frequently repeated movements are usually performed crudely
in the beginning stages of learning but become increasingly more skilled with use and/or
practice. Consider the common activity of handwriting and the execution of one's own
signature. The evolution from a child's irregular, crude printing to an adult's
recognizable, consistently repeatable signature is normal. Eventually, the individual's
signature begins to appear essentially the same every time and is uniquely different from
any other person. Not only can the person execute handwritten signatures consistency but
can use chalk to sign the name in large letters on a blackboard producing a recognizably
similar appearance. The individuality of the signature remains whether using the fine
control of the hand or the recruiting the large shoulder and arm muscles not normally
required for the task. Reproduction of recognizable movements occurs from preprogrammed
control patterns stored in the brain and recruited as necessary. Practicing a golf swing
until it results in a 300 hundred yard drive down the middle of the fairway, getting the
food-laden fork from the plate into the mouth, and remembering how to ride a bike after a
30 year hiatus illustrate learned behavior that has become "automatic" with
practice and can be recalled from the brain's storage for execution.
Volitional tasks require an integration of neurological,
physiological, biochemical, and mechanical components. There are many options available
when performing a task, such as walking, but eventually, each person will develop a
pattern that will be recognizable as that skill, repeatable, and with a certain uniqueness
associated with that particular individual. Although any person's movement could be
quantified with biomechanical applications and compared to other performers in a similar
group, e.g. the Gold, Silver, and Bronze medalists in an Olympic event, perhaps it will be
the ability to compare one person to him/herself that will provide the most meaningful
assistance in the assault on aging.
There are many areas of daily living in which biomechanical
analyses could be useful. Biomechanics could be utilized to design a house or chair to
suit the body or to lift bigger, heavier objects with less strain. This science could be
useful in selecting the most appropriate athletic event for children or for improving an
adult's performance.
With increasing international interest in competitive
athletics, it was inevitable that computers would be used for the analysis of sports
techniques. Computer calculations can provide information which surpass the limits of what
the human eye can see and intuition can deduce. Human judgment, however, is still
critically important. As in business and industry, where decisions are based ultimately
upon an executive's experience and interpretive ability, the coach or trainer is and will
remain the ultimate decision maker in athletic training. Rehabilitation and orthopedic
specialists can assess impaired movement relative to normal performance and/or apply
computerized biomechanical techniques to the possibilities of achieving the restoration of
normal activities. With the increase in the population of older citizens, gerontological
applications will increase. The computer should be regarded as one more tool, however
complex, which can be skillfully used by humans in order to achieve a desired end.
One factor which man has lived with is change. The
environment in which we live is changing during every one of the approximately 35 million
minutes of our lives. The human body itself changes from birth to maturity and from
maturity to death. The moment man first picked up a stone to use as tool, the balance
between humans and the environment was altered. After that adaptation, the ways in which
the surrounding world changed resulted in different effects and these were no longer
regular or predictable. New objects were created from things which otherwise would have
been discounted. These changes were made possible by humans due to the invention of tools.
The more tools man created, the faster was the rate of environmental change. Today, the
rate of change due to tools has reached such a magnitude that there is a great danger to
the whole environment and frequently to the people who use the tools such as were
discovered during the Industrial Revolution or even today with such problems as carpal
tunnel syndrome.
Human beings seem to have become so infatuated with their
ability to invent things that they have concentrated almost exclusively upon improving the
efficiency, safety, durability, cost, or aesthetic appeal of the device. It is ironic that
with all of the innovative development, little consideration has been given to the most
complex system with the most sophisticated computer in the world -- the human body.