Online. Scientific Principles for Quantifying MotionPublished on Saturday, August 3, 1996 by Gideon Ariel
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.