Chapter 1. What Is Biomechanics?

After completing this chapter, you will be able to:

• Define the terms biomechanics, statics, dynamics, kinematics, and kinetics, and explain the ways in which they are related.
• Describe the scope of scientific inquiry addressed by biomechanists.
• Distinguish between qualitative and quantitative approaches for analyzing human movement.
• Explain how to formulate questions for qualitative analysis of human movement.
• Use the 11 steps identified in the chapter to solve formal problems.

Why do some golfers slice the ball? How can workers avoid developing low back pain? What cues can a physical education teacher provide to help students learn the underhand volleyball serve? Why do some elderly individuals tend to fall? We have all admired the fluid, graceful movements of highly skilled performers in various sports. We have also observed the awkward first steps of a young child, the slow progress of an injured person with a walking cast, and the hesitant, uneven gait of an elderly person using a cane. Virtually every activity class includes a student who seems to acquire new skills with utmost ease and a student who trips when executing a jump or misses the ball when attempting to catch, strike, or serve. What enables some individuals to execute complex movements so easily, while others appear to have difficulty with relatively simple movement skills?

Although the answers to these questions may be rooted in physiological, psychological, or sociological issues, the problems identified are all biomechanical in nature. This book will provide a foundation for identifying, analyzing, and solving problems related to the biomechanics of human movement.

During the early 1970s, the international community adopted the term biomechanics to describe the science involving the study of biological systems from a mechanical perspective (42). Biomechanists use the tools of mechanics, the branch of physics involving analysis of the actions of forces, to study the anatomical and functional aspects of living organisms (Figure 1-1). Statics and dynamics are two major subbranches of mechanics. Statics is the study of systems that are in a state of constant motion, that is, either at rest (with no motion) or moving with a constant velocity. Dynamics is the study of systems in which acceleration is present.

Kinematics and kinetics are further subdivisions of biomechanical study. Kinematics is the description of motion, including the pattern and speed of movement sequencing by the body segments that often translates to the degree of coordination an individual displays. Whereas kinematics describes the appearance of motion, kinetics is the study of the forces associated with motion. The study of human biomechanics may include questions such as whether the amount of force the muscles are producing is optimal for the intended purpose of the movement. Anthropometric factors, including the size, shape, and weight of the body segments, are other important considerations in a kinetic analysis.

Although biomechanics is relatively young as a recognized field of scientific inquiry, biomechanical considerations are of interest in several different scientific disciplines and professional fields. Biomechanists may have academic backgrounds in zoology; orthopedic, cardiac, or sports medicine; biomedical or biomechanical engineering; physical therapy; or kinesiology, with the commonality being an interest in the biomechanical aspects of the structure and function of living things.

The biomechanics of human movement is one of the subdisciplines of kinesiology, the study of human movement (Figure 1-2). Although some biomechanists study topics such as ostrich locomotion, blood flow through constricted arteries, or micromapping of dental cavities, this book focuses primarily on the biomechanics of human movement from the perspective of the movement analyst.

As shown in Figure 1-3, biomechanics is also a scientific branch of sports medicine. Sports medicine has been defined by Lamb as “an umbrella term that encompasses both clinical and scientific aspects of exercise and sport” (34). The American College of Sports Medicine is an example of an organization that promotes interaction between scientists and clinicians with interests in sports medicine–related topics.

What Problems Are Studied by Biomechanists?

As expected given the different scientific and professional fields represented, biomechanists study questions or problems that are topically diverse. For example, zoologists have examined the locomotion patterns of dozens of species of animals walking, running, trotting, and galloping at controlled speeds on a treadmill to determine why animals choose a particular stride length and stride rate at a given speed. They concluded that most vertebrates, including humans, select a gait that optimizes economy, or metabolic energy consumption, at a given speed (46). Research suggests that it is the cost of muscular force production that primarily governs the energy cost of running (50). Interestingly, this means that if a biped, such as a turkey, and a quadruped, such as a dog, are of similar body weights, they use about the same amount of energy when running, in spite of the apparent differences in body size and shape and running mechanics (51). This is true because although bipeds, compared to quadrupeds, tend to have the advantage of longer legs and the ability to take longer steps, they must recruit more muscle to support body weight. One of the challenges of this type of research is determining how to persuade a cat, a dog, or a turkey to run on a treadmill (Figure 1-4).

Among humans, although the energy cost of running increases with running speed, there are sizable differences in energy cost between individuals that become even larger as running speed increases (32). Although some individuals appear to run more smoothly and comfortably than others, no particular biomechanical factors have been associated with either good or poor running economy (32). Differences in muscle fiber type composition appear to translate into differences in energy utilization during running (see Chapter 6) (33). Strength training and high-altitude training have been shown to improve running economy by promoting better utilization of elastic energy and oxygen within the working muscles (55).

There are also changes in the energy cost of running and walking among growing children as their bodies undergo developmental changes in body proportions and motor skills. Between early childhood and young adulthood, there is a decrease in the amount of energy required for standing, walking, and running, with children expending 70% more energy to walk at a fast pace than adults (17).

The U.S. National Aeronautics and Space Administration (NASA) sponsors another multidisciplinary line of biomechanics research to promote understanding of the effects of microgravity on the human musculoskeletal system. Of concern is the fact that astronauts who have been out of the earth’s gravitational field for just a few days have returned with muscle atrophy, cardiovascular and immune system changes, and reduced bone density, mineralization, and strength, especially in the lower extremities (20). The issue of bone loss, in particular, is currently a limiting factor for long-term space flights, with bone lost at a rate of about 1% per month from the lumbar spine and 1.5% per month from the hips (39). Both increased bone resorption and decreased calcium absorption appear to be responsible (see Chapter 4) (56).

Since those early days of space flight, biomechanists have designed and built a number of exercise devices for use in space to take the place of normal bone-maintaining activities on earth. Some of this research has focused on the design of treadmills for use in space that load the bones of the lower extremity with deformations and strain rates that are optimal for stimulating new bone formation (14). Scientists have discovered that applying an anteriorly directed horizontal force to individuals running in low-gravity environments generates impactforces that are much more similar to those sustained when running on earth (11). This is an important finding, since lower-extremity bone strain, which is believed to be a critical link in the mechanical stimulation of bone growth and maintenance, is directly related to the magnitude of the ground reaction forces sustained (47). So far, however, no adequate substitute has been found for weight bearing for the prevention of bone loss in space (20).

Maintaining sufficient bone-mineral density is also a topic of concern here on earth. Osteoporosis is a condition in which bone mineral mass and strength are so severely compromised that daily activities can cause bone pain and fracturing (23). This condition is found in most elderly individuals, with earlier onset in women, and is becoming increasingly prevalent throughout the world with the increasing mean age of the population (31). Today, approximately 40% of women experience one or more osteoporotic fractures after age 50, and after age 60, about 90% of all fractures in both men and women are osteoporosis-related (35, 49). The most common fracture site is the vertebrae, with the presence of one fracture indicating increased risk for future vertebral and hip fractures (25).

Osteoporosis is neither a disease with acute onset nor an inevitable accompaniment of aging, but the result of a lifetime of habits that are erosive to the skeletal system (5). Among women, in particular, low levels of physical activity during adolescence have been positively correlated with increased risk for osteoporosis later in life (53). Other known risk factors for developing osteoporosis include physical inactivity over a lifetime; cigarette smoking; deficiencies in estrogen, calcium, and vitamin D; and excessive consumption of protein, caffeine, and alcohol (16). The two key strategies for preventing osteoporotic fractures are maximizing peak bone mass during adolescence and early adulthood and preventing bone loss at menopause (31). Importantly, studies show that a regular program of weight-bearing exercise, such as walking, among individuals with osteoporosis can increase bone health and strength (3).

Another problem area challenging biomechanists who study the elderly is mobility impairment. Age is associated with decreased ability to balance, and older adults both sway more and fall more than young adults, although the reasons for these changes are not well understood (45). Falls, and particularly fall-related hip fractures, are extremely serious, common, and costly medical problems among the elderly. Each year, falls cause large percentages of the wrist fractures, head injuries, vertebral fractures, and lacerations, as well as over 90% of the hip fractures, occurring in the United States (52). Biomechanical research teams are investigating the biomechanical factors that enable individuals to avoid falling, the characteristics of safe landings from falls, the forces sustained by different parts of the body during falls, and the ability of protective clothing and floors to prevent falling injuries (52). Promising work in the development of intervention strategies has shown that exercise walking can be effective in improving balance and reducing the likelihood of falling among sedentary older adults (6).

Research by clinical biomechanists has resulted in improved gait among children with cerebral palsy, a condition involving high levels of muscle tension and spasticity. The gait of the cerebral palsy individual is characterized by excessive knee flexion during stance. This problem is treated by surgical lengthening of the hamstring tendons to improve knee extension during stance. In some patients, however, the procedure also diminishes knee flexion during the swing phase of gait, resulting in dragging of the foot. After research showed that patients with this problem exhibited significant co-contraction of the rectus femoris with the hamstrings during the swing phase, orthopedists began treating the problem by surgically attaching the rectus femoris to the sartorius insertion (24). This creative, biomechanics research–based approach has enabled a major step toward gait normalization for children with cerebral palsy.

Research by biomedical engineers has also resulted in improved gait for children and adults with below-knee amputations. Ambulation with a prosthesis creates an added metabolic demand, which can be particularly significant for elderly amputees and for young active amputees who participate in sports requiring aerobic conditioning. In response to this problem, researchers have developed an array of lower-limb and foot prostheses that store and return mechanical energy during gait, thereby reducing the metabolic cost of locomotion (2, 22, 40, 58). Studies have shown that the more compliant prostheses are better suited for active and fast walkers, whereas prostheses that provide a more stable base of support are generally preferred for the elderly population (8). Microchip-controlled “Intelligent Prostheses” show promise for reducing the energy cost of walking at a range of speeds (13).

Occupational biomechanics is a field that focuses on the prevention of work-related injuries and the improvement of working conditions and worker performance (10). Researchers in this field have learned that work-related low back pain can derive not only from the handling of heavy materials but from unnatural postures, sudden and unexpected motions, and the characteristics of the individual worker (63). Occupational biomechanists are also recognizing how important it is for workers to be both physically and mentally prepared for jobs in industry in order to prevent low back pain (63). Sophisticated biomechanical models of the trunk are now being used in the design of materials-handling tasks in industry to enable minimizing potentially injurious stresses to the low back (9).

In recent years, although the number of overall workplace injuries has decreased, carpal tunnel syndrome, a neurological impairment at the wrist often associated with occupational overuse, has steadily increased in frequency (7). Because carpal tunnel syndrome is particularly associated with repetitive keyboard use, studies are being conducted to explore novel keyboard designs that may be more biomechanically optimal than the traditional keyboard (41). Interesting new designs being tested include a keyboard split into left and right halves, with each half positioned directly in front of a shoulder, and split, vertically aligned keyboards that allow maintaining the wrists in neutral position (37).

Biomechanists have also contributed to performance improvements in selected sports through the design of innovative equipment. One excellent example of this is the Klapskate, the speed skate equipped with a hinge near the toes that allows the skater to plantar flex at the ankle during push-off, resulting in up to 5% higher skating velocities than were obtainable with traditional skates (27). The Klapskate was designed by van Ingen Schenau and de Groot, based on study of the gliding push-off technique in speed skating by van Ingen Schenau and Baker, as well as work on the intermuscular coordination of vertical jumping by Bobbert and van Ingen Schenau (15). When the Klapskate was used for the first time by competitors in the 1998 Winter Olympic Games, speed records were shattered in every event.

Numerous innovations in sport equipment and apparel have also resulted from findings of experiments conducted in experimental chambers called wind tunnels that involved controlled simulation of the air resistance actually encountered during particular sports. Examples include the aerodynamic helmets, clothing, and cycle designs used in competitive cycling, and the ultrasmooth suits worn in other competitive speed-related events, such as swimming, track, skating, and skiing. Wind tunnel experiments have also been conducted to identify optimal body configuration during events such as ski jumping (60).

Sport biomechanists have also directed efforts at improving the biomechanical, or technique, components of athletic performance. They have learned, for example, that factors contributing to superior performance in the long jump, high jump, and pole vault include high horizontal velocity going into takeoff and a shortened last step that facilitates continued elevation of the total-body center of mass (12, 26). Study of baseball pitchers has determined that high-velocity pitchers display greater external rotation at the shoulder, more forward trunk tilt at ball release, higher-extension angular velocity at the lead knee, and greater angular velocity of the pelvis and upper torso than lower-velocity pitchers (38, 57).

One rather dramatic example of performance improvement partially attributable to biomechanical analysis is the case of four-time Olympic discus champion Al Oerter. Mechanical analysis of the discus throw requires precise evaluation of the major mechanical factors affecting the flight of the discus. These factors are:

1. 1. The speed of the discus when it is released by the thrower

2. 2. The projection angle at which the discus is released

3. 3. The height above the ground at which the discus is released

4. 4. The angle of attack (the orientation of the discus relative to the prevailing air current)

By using computer simulation techniques, researchers can predict the needed combination of values for these four variables that will result in a throw of maximum distance for a given athlete (28). High-speed cameras can record performances in great detail, and when the film or video is analyzed, the actual projection height, velocity, and angle of attack can be compared to the computer-generated values required for optimal performance. At the age of 43, Oerter bettered his best Olympic performance by 8.2 m. Although it is difficult to determine the contributions of motivation and training to such an improvement, some part of Oerter’s success was a result of enhanced technique following biomechanical analysis (54). Most adjustments to skilled athletes’ techniques produce relatively modest results because their performances are already characterized by above-average technique.

Some of the research produced by sport biomechanists has been done in conjunction with the Sports Medicine Division of the United States Olympic Committee (USOC). The general goal of USOC research is to examine the ways in which mechanical factors limit the performances of elite American athletes training for Olympic and other international competition (19). Typically, this work is done in direct cooperation with the national coach of the sport to ensure the practicality of results. USOC-sponsored research has yielded much new information about the mechanical characteristics of elite performance in various sports. Because of continuing advances in scientific analysis equipment, the role of sport biomechanists in contributing to performance improvements is likely to be increasingly important in the future.

The influence of biomechanics is also being felt in sports popular with both nonathletes and athletes, such as golf. Computerized video analyses of golf swings designed by biomechanists are commonly available at golf courses and equipment shops. The science of biomechanics can play a role in optimizing the distance and accuracy of all golf shots, including putting, through analysis of body angles, joint forces, and muscle activity patterns (29). A common technique recommendation is to maintain a single fixed center of rotation to impart force to the ball (29).

Other concerns of sport biomechanists relate to minimizing sport injuries through both identifying dangerous practices and designing safe equipment and apparel. In recreational runners, for example, research shows that the most serious risk factors for overuse injuries are training errors such as a sudden increase in running distance or intensity, excess cumulative mileage, running on cambered surfaces, and improper footwear (43). The complexity of safety-related issues increases when the sport is equipment-intensive. Evaluation of protective helmets involves ensuring not only that the impact characteristics offer reliable protection but also that the helmet does not overly restrict wearers’ peripheral vision.

An added complication is that equipment designed to protect one part of the body may actually contribute to the likelihood of injury in another part of the musculoskeletal system. Modern ski boots and bindings, while effective in protecting the ankle and lower leg against injury, unfortunately contribute to severe bending moments at the knee when the skier loses balance. This factor has contributed to the nearly threefold increase in the incidence of knee injuries in skiing since 1972 (30). Injuries in snowboarding are also more frequent with rigid, as compared to pliable, boots, although more than half of all snowboarding injuries are to the upper extremity (36, 48).

Another challenging area of research for biomechanists in the realm of sport safety is investigation of the efficacy of prophylactic knee braces designed to protect the knees from valgus stresses that could damage the medial collateral ligaments. The wearing of such braces by healthy individuals has been a contentious issue since the American Academy of Orthopaedics issued a position statement against their use in 1987 (35). Research shows that knee braces can contribute 20–30% added resistance against lateral blows to the knee, with custom-fitted braces providing the best protection (1). A possible concern, however, is that knee braces act to change the pattern of lower-extremity muscle activity during gait, with less work performed at the knee and more at the hip (18). Other documented problems that appear to affect some athletes more than others and may be brace-specific include reduced sprinting speed and earlier onset of fatigue (1).

An area of biomechanics research with implications for both safety and performance is sport shoe design. Today sport shoes are designed both to prevent excessive loading and related injuries and to enhance performance. Because the ground or playing surface, the shoe, and the human body compose an interactive system, athletic shoes are specifically designed for particular sports, surfaces, and anatomical considerations. Aerobic dance shoes are constructed to cushion the metatarsal arch. Football shoes to be used on artificial turf are designed to minimize the risk of knee injury. Running shoes are available for training and racing on snow and ice. In fact, sport shoes today are so specifically designed for designated activities that wearing an inappropriate shoe can contribute to the likelihood of injury (59, 62).

These examples illustrate the diversity of topics addressed in biomechanics research, including some examples of success and some areas of continuing challenge. Clearly, biomechanists are contributing to the knowledge base on the full gamut of human movement, from the gait of the physically challenged child to the technique of the elite athlete. Although varied, all of the research described is based on applications of mechanical principles in solving specific problems in living organisms. This book is designed to provide an introduction to many of those principles and to focus on some of the ways in which biomechanical principles may be applied in the analysis of human movement.

Why Study Biomechanics?

As is evident from the preceding section, biomechanical principles are applied by scientists and professionals in a number of fields to problems related to human health and performance. Knowledge of basic biomechanical concepts is also essential for the competent physical education teacher, physical therapist, physician, coach, personal trainer, or exercise instructor.

An introductory course in biomechanics provides foundational understanding of mechanical principles and their applications in analyzing movements of the human body. The knowledgeable human movement analyst should be able to answer the following types of questions related to biomechanics: Why is swimming not the best form of exercise for individuals with osteoporosis? What is the biomechanical principle behind variableresistance exercise machines? What is the safest way to lift a heavy object? Is it possible to judge what movements are more/less economical from visual observation? At what angle should a ball be thrown for maximum distance? From what distance and angle is it best to observe a patient walk down a ramp or a volleyball player execute a serve? What strategies can an elderly person or a football lineman employ to maximize stability? Why are some individuals unable to float?

Perusing the objectives at the beginning of each chapter of this book is a good way to highlight the scope of biomechanical topics to be covered at the introductory level. For those planning careers that involve visual observation and analysis of human movement, knowledge of these topics will be invaluable.

Scientific research is usually aimed at providing a solution for a particular problem or answering a specific question. Even for the nonresearcher, however, the ability to solve problems is a practical necessity for functioning in modern society. The use of specific problems is also an effective approach for illustrating basic biomechanical concepts.

Quantitative versus Qualitative Problems

Analysis of human movement may be either quantitative or qualitative. Quantitative implies that numbers are involved, and qualitative refers to a description of quality without the use of numbers. After watching the performance of a standing long jump, an observer might qualitatively state, “That was a very good jump.” Another observer might quantitatively announce that the same jump was 2.1 m in length. Other examples of qualitative and quantitative descriptors are displayed in Figures 1-5 and 1-6.

It is important to recognize that qualitative does not mean general. Qualitative descriptions may be general, but they may also be extremely detailed. It can be stated qualitatively and generally, for example, that a man is walking down the street. It might also be stated that the same man is walking very slowly, appears to be leaning to the left, and is bearing weight on his right leg for as short a time as possible. The second description is entirely qualitative but provides a more detailed picture of the movement.

Both qualitative and quantitative descriptions play important roles in the biomechanical analysis of human movement. Biomechanical researchers rely heavily on quantitative techniques in attempting to answer specific questions related to the mechanics of living organisms. Clinicians, coaches, and teachers of physical activities regularly employ qualitative observations of their patients, athletes, or students to formulate opinions or give advice.

Solving Qualitative Problems

Qualitative problems commonly arise during daily activities. Questions such as what clothes to wear, whether to major in botany or English, and whether to study or watch television are all problems in the sense that they are uncertainties that may require resolution. Thus, a large portion of our daily lives is devoted to the solution of problems.

Analyzing human movement, whether to identify a gait anomaly or to refine a technique, is essentially a process of problem solving. Whether the analysis is qualitative or quantitative, this involves identifying, then studying or analyzing, and finally answering a question or problem of interest.

To effectively analyze a movement, it is essential first to formulate one or more questions regarding the movement. Depending on the specific purpose of the analysis, the questions to be framed may be general or specific. General questions, for example, might include the following:

1. 1. Is the movement being performed with adequate (or optimal) force?

2. 2. Is the movement being performed through an appropriate range of motion?

3. 3. Is the sequencing of body movements appropriate (or optimal) for execution of the skill?

4. 4. Why does this elderly woman have a tendency to fall?

5. 5. Why is this shot putter not getting more distance?

More specific questions might include these:

1. 1. Is there excessive pronation taking place during the stance phase of gait?

2. 2. Is release of the ball taking place at the instant of full elbow extension?

3. 3. Does selective strengthening of the vastus medialis obliquus alleviate mistracking of the patella for this person?

Once one or more questions have been identified, the next step in analyzing a human movement is to collect data. The form of data most commonly collected by teachers, therapists, and coaches is qualitative visual observation data. That is, the movement analyst carefully observes the movement being performed and makes either written or mental notes. To acquire the best observational data possible, it is useful to plan ahead as to the optimal distance(s) and perspective(s) from which to make the observations. These and other important considerations for qualitatively analyzing human movement are discussed in detail in Chapter 2.

Formal versus Informal Problems

When confronted with a stated problem taken from an area of mathematics or science, many individuals believe they are not capable of finding a solution. Clearly, a stated math problem is different from a problem such as what to wear to a particular social gathering. In some ways, however, the informal type of problem is the more difficult one to solve. According to Wickelgren (61), a formal problem (such as a stated math problem) is characterized by three discrete components:

1. 1. A set of given information

2. 2. A particular goal, answer, or desired finding

3. 3. A set of operations or processes that can be used to arrive at the answer from the given information

In dealing with informal problems, however, individuals may find the given information, the processes to be used, and even the goal itself to be unclear or not readily identifiable.

Solving Formal Quantitative Problems

Formal problems are effective vehicles for translating nebulous concepts into well-defined, specific principles that can be readily understood and applied in the analysis of human motion. People who believe themselves incapable of solving formal stated problems do not recognize that, to a large extent, problem-solving skills can be learned. Entire books on problem-solving approaches and techniques are available. However, most students are not exposed to coursework involving general strategies of the problem-solving process. A simple procedure for approaching and solving problems involves 11 sequential steps:

1. 1. Read the problem carefully. It may be necessary to read the problem several times before proceeding to the next step. Only when you clearly understand the information given and the question(s) to be answered should you undertake step 2.

2. 2. Write down the given information in list form. It is acceptable to use symbols (such as v for velocity) to represent physical quantities if the symbols are meaningful.

3. 3. Write down what is wanted or what is to be determined, using list form if more than one quantity is to be solved for.

4. 4. Draw a diagram representing the problem situation, clearly indicating all known quantities and representing those to be identified with question marks. (Although certain types of problems may not easily be represented diagrammatically, it is critically important to carry out this step whenever possible to accurately visualize the problem situation.)

5. 5. Identify and write down the relationships or formulas that might be useful in solving the problem. (More than one formula may be useful and/or necessary.)

6. 6. From the formulas that you wrote down in step 5, select the formula(s) containing both given variables (from step 2) and the variables that are desired unknowns (from step 3). If a formula contains only one unknown variable that is the variable to be determined, skip step 7 and proceed directly to step 8.

7. 7. If you cannot identify a workable formula (in more difficult problems), certain essential information was probably not specifically stated but can be determined by inference and by further thought on and analysis of the given information. If this occurs, it may be necessary to repeat step 1 and review the pertinent information relating to the problem presented in the text.

8. 8. Once you have identified the appropriate formula(s), write the formula(s) and carefully substitute the known quantities given in the problem for the variable symbols.

9. 9. Using the simple algebraic techniques reviewed in Appendix A, solve for the unknown variable by (a) rewriting the equation so that the unknown variable is isolated on one side of the equals sign and (b) reducing the numbers on the other side of the equation to a single quantity.

10. 10. Do a commonsense check of the answer derived. Does it seem too small or too large? If so, recheck the calculations. Also check to ensure that all questions originally posed in the statement of the problem have been answered.

11. 11. Clearly box in the answer and include the correct units of measurement.

Figure 1-7 provides a summary of this procedure for solving formal quantitative problems. These steps should be carefully studied, referred to, and applied in working the quantitative problems included at the end of each chapter. Sample Problems 1.1 and 1.2 illustrate the use of this procedure.

Units of Measurement

Providing the correct units of measurement associated with the answer to a quantitative problem is important. Clearly, an answer of 2 cm is quite different from an answer of 2 km. It is also important to recognize the units of measurement associated with particular physical quantities. Ordering 10 km of gasoline for a car when traveling in a foreign country would clearly not be appropriate.

The predominant system of measurement still used in the United States is the English system. The English system of weights and measures arose over the course of several centuries primarily for purposes of commerce and land parceling. Specific units came largely from royal decrees. For example, a yard was originally defined as the distance from the end of the nose of King Henry I to the thumb of his extended arm. The English system of measurement displays little logic. There are 12 inches to the foot, 3 feet to the yard, 5280 feet to the mile, 16 ounces to the pound, and 2000 pounds to the ton.

The system of measurement that is presently used by every major country in the world except the United States is Le Système International d’Unites (the International System of Units), which is commonly known as the S.I. or the metric system. The metric system originated as the result of a request of King Louis XVI to the French Academy of Sciences in the 1790s. Although the system fell briefly from favor in France, it was readopted in 1837. In 1875, the Treaty of the Meter was signed by 17 countries agreeing to adopt the metric system.

Since that time the metric system has enjoyed worldwide popularity for several reasons. First, it entails only four base units—the meter, of length; the kilogram, of mass; the second, of time; and the degree Kelvin, of temperature. Second, the base units are precisely defined, reproducible quantities that are independent of factors such as gravitational force. Third, all units excepting those for time relate by factors of 10, in contrast to the numerous conversion factors necessary in converting English units of measurement. Last, the system is used internationally.

For these reasons, as well as the fact that the metric system is used almost exclusively by the scientific community, it is the system used in this book. For those who are not familiar with the metric system, it is useful to be able to recognize the approximate English system equivalents of metric quantities. Two conversion factors that are particularly valuable are 2.54 cm for every inch and approximately 4.45 N for every pound. All of the relevant units of measurement in both systems and common English-metric conversion factors are presented in Appendix C.

Biomechanics is a multidisciplinary science involving the application of mechanical principles in the study of the structure and function of living organisms. Because biomechanists come from different academic backgrounds and professional fields, biomechanical research addresses a spectrum of problems and questions.

Basic knowledge of biomechanics is essential for competent professional analysts of human movement, including physical education teachers, physical therapists, physicians, coaches, personal trainers, and exercise instructors. The structured approach presented in this book is designed to facilitate the identification, analysis, and solution of problems or questions related to human movement.

A baseball player hits a triple to deep center field. As he is approaching third base, he notices that the incoming throw to the catcher is wild, and he decides to break for home plate. The catcher retrieves the ball 10 m from the plate and runs back toward the plate at a speed of 5 m/s. As the catcher starts running, the base runner, who is traveling at a speed of 9 m/s, is 15 m from the plate. Given that time = distance/speed, who will reach the plate first?

Solution

• Step 1 Read the problem carefully.
• Step 2 Write down the given information:
  • base runner’s speed = 9 m/s
  • catcher’s speed = 5 m/s
  • distance of base runner from plate = 15 m
  • distance of catcher from plate = 10 m
• Step 3 Write down the variable to be identified: Find which player reaches home plate in the shortest time.
• Step 4 Draw a diagram of the problem situation.
• Step 5 Write down formulas of use:
  • time = distance/speed
• Step 6 Identify the formula to be used: It may be assumed that the formula provided is appropriate because no other information relevant to the solution has been presented.
• Step 7 Reread the problem if all necessary information is not available: It may be determined that all information appears to be available.
• Step 8 Substitute the given information into the formula:
• 
• Step 9 Solve the equations:
• 
• Step 10 Check that the answer is both reasonable and complete.
• Step 11 Box in the answer:
  • The base runner arrives at home plate first, by 0.33 s.

A man sits in a 20 kg wheelchair at the top of a short ramp. When the brakes on the wheelchair are suddenly released, the wheelchair begins to roll down the ramp, accelerating at a rate of 0.5 m/s2. If the net force causing the wheelchair to roll down the ramp is 45 Newtons (N), what is the mass of the man in the wheelchair? (Hint: The relationship to be used in solving this problem is force = mass × acceleration. For more information on the metric units of kilograms, meters, and Newtons, refer to Appendix C.) This time, work through the 11 steps on your own.

• Step 2 net force = 45 N
  • wheelchair mass = 20 kg
  • acceleration = 0.5 m/s2
• Step 3 Find: mass of man
• Step 4
• 
• 
• Steps 5 and 6
• 
• Steps 8 and 9
• 
• Step 11 70 kg

1. 1. Locate and read three articles from the scientific literature that report the results of biomechanical investigations. (The Journal of Biomechanics, the Journal of Applied Biomechanics, and Medicine and Science in Sports and Exercise are possible sources.) Write a one-page summary of each article, and identify whether the investigation involved statics or dynamics and kinetics or kinematics.

2. 2. List 8–10 websites that are related to biomechanics, and write a paragraph describing each site.

3. 3. Write a brief discussion about how knowledge of biomechanics may be useful in your intended profession or career.

4. 4. Choose three jobs or professions, and write a discussion about the ways in which each involves quantitative and qualitative work.

5. 5. Write a summary list of the problem-solving steps identified in the chapter, using your own words.

6. 6. Write a description of one informal problem and one formal problem.

7. 7. Step by step, show how to arrive at a solution to one of the problems you described in Problem 6.

8. 8. Solve for x in each of the equations below. Refer to Appendix A for help if necessary.

9. • a.

10. • b.

11. • c.

12. • d.

13. • e.

14. • f.

15. • g.

16. • h.

17. • i.

18. • j.

19. (Answers: a. 125; b. 45; c. 4.5; d. −6; e. 7.9; f. 8.9; g. 3.2; h. 75; i. 54; j. −6.5)

20. 9. Two schoolchildren race across a playground for a ball. Tim starts running at a distance of 15 m from the ball, and Jan starts running at a distance of 12 m from the ball. If Tim’s average speed is 4.2 m/s and Jan’s average speed is 4.0 m/s, which child will reach the ball first? Show how you arrived at your answer. (See Sample Problem 1.1.) (Answer: Jan reaches the ball first.)

21. 10. A 0.5 kg ball is kicked with a force of 40 N. What is the resulting acceleration of the ball? (See Sample Problem 1.2.) (Answer: 80 m/s2)

1. 1. Select a specific movement or sport skill of interest, and read two or three articles from the scientific literature that report the results of biomechanical investigations related to the topic. Write a short paper that integrates the information from your sources into a scientifically based description of your chosen movement.

2. 2. When attempting to balance your checkbook, you discover that your figures show a different balance in your account than was calculated by the bank. List an ordered, logical set of procedures that you may use to discover the error. You may use list, outline, or block diagram format.

3. 3. Sarah goes to the grocery store and spends half of her money. On the way home, she stops for an ice cream cone that costs $0.78. Then she stops and spends one-fourth of her remaining money to settle a $5.50 bill at the dry cleaners. How much money did Sarah have originally? (Answer: $45.56)

4. 4. Wendell invests $10,000 in a stock portfolio made up of Petroleum Special at $30 per share, Newshoe at $12 per share, and Beans & Sprouts at $2.50 per share. He places 60% of the money in P.S., 30% in N, and 10% in B & S. With market values changing (P.S. down $3.12, N up 80%, and B & S up $0.20), what is his portfolio worth six months later? (Answer: $11,856)

5. 5. The hypotenuse of right triangle ABC (shown here) is 4 cm long. What are the lengths of the other two sides? (Answer: A = 2 cm; B = 3.5 cm)

   

6. 6. In triangle DEF, side E is 4 cm long and side F is 7 cm long. If the angle between sides E and F is 50 degrees, how long is side D? (Answer: 5.4 cm)

7. 7. An orienteer runs 300 m north and then 400 m to the southeast (at a 45˚ angle to north). If he has run at a constant speed, how far away is he from the starting position? (Answer: 283.4 m)

8. 8. John is out for his daily noontime run. He runs 2 km west, then 2 km south, and then runs on a path that takes him directly back to the place he started at.

9. • a. How far did John run?

10. • b. If he has run at an average speed of 4 m/s, how long did the entire run take?

11. (Answers: a. 6.83 km; b. 28.5 min)

12. 9. John and Al are in a 15 km race. John averages 4.4 m/s during the first half of the race and then runs at a speed of 4.2 m/s until the last 200 m, which he covers at 4.5 m/s. At what average speed must Al run to beat John? (Answer: > 4.3 m/s)

13. 10. A sailboat heads north at 3 m/s for 1 hour and then tacks back to the southeast (at 45˚ to north) at 2 m/s for 45 minutes.

14. • a. How far has the boat sailed?

15. • b. How far is it from its starting location?

16. (Answers: a. 16.2 km; b. 8.0 km)

• Courses in anatomy, physiology, mathematics, physics, and engineering provide background knowledge for biomechanists.

• In research, each new study, investigation, or experiment is usually designed to address a particular question or problem.

• The USOC began funding sports medicine research in 1981. Other countries began sponsoring research to boost the performance of elite athletes in the early 1970s.

• Impact testing of protective sport helmets is carried out scientifically in engineering laboratories.

Bartlett RM: Current issues in the mechanics of athletic activities. A position paper, J Biomech 30:477, 1997.

Reviews important or contentious current issues in the study of the biomechanics of athletic activities.

Chaffin DB and Andersson GBJ: Occupational biomechanics (3rd ed), New York, 1999, John Wiley & Sons.

Serves as a comprehensive text on occupational biomechanics.

Elliott B: Biomechanics: an integral part of sport science and sport medicine, J Sci Med Sport 2:299, 1999.

Provides an overview of clinical biomechanics and occupational biomechanics, as well as the role of biomechanics in sports science and sports medicine, in a review format.

Zeitz, P: The art and craft of problem solving, New York, 1999, John Wiley & Sons.

Provides general strategies, as well as specific tools and techniques, for solving quantitative problems.

American College of Sports Medicine—Biomechanics Interest Group

• www.acsm.org
• Provides a link to the American College of Sports Medicine Member Service Center, which links to the ACSM Interest Groups, including the Biomechanics Interest Group.

American Society of Biomechanics

• http://asb-biomech.org/
• Home page of the American Society of Biomechanics; provides information about the organization, conference abstracts, and graduate programs in biomechanics.

Biomch-L Newsgroup

• http://www.biomch-l.org/
• Provides information about an e-mail discussion group for biomechanics and human/animal movement science.

Biomechanics Classes on the Web

• www.uoregon.edu/~karduna/biomechanics/
• Contains links to over 100 biomechanics classes with web-based instructional components.

Biomechanics World Wide

• http://www.uni-due.de/~qpd800/WSITECOPY.html
• A comprehensive site with links to other websites for a wide spectrum of topics related to biomechanics.

Biomechanics Yellow Pages

• http://isbweb.org/o/content/view/208/140/
• Provides information on technology used in biomechanics-related work and includes a number of downloadable video clips.

International Society of Biomechanics

• www.isbweb.org/
• Home page of the International Society of Biomechanics (ISB); provides information on ISB, biomechanical software and data, and pointers to other sources of biomechanics-related information.

What do 1996 U.S. figure skating champion Rudy Galindo and 1998 Olympic gold medal winner Tara Lipinski have in common besides figure skating success? They have both had double hip replacements, Galindo at age 32 and Lipinski at age 18.

Overuse injuries among figure skaters are on the rise at an alarming rate, with most involving the lower extremities and lower back (4, 21). With skaters performing more and more technically demanding programs including multirotation jumps, on-ice training time for elite skaters now typically includes over 100 jumps per day, six days per week, year after year.

Yet unlike most modern sports equipment, the figure skate has undergone only very minor modifications since 1900. The soft-leather, calf-high boots of the nineteenth century are now made of stiffer leather to promote ankle stability and are not quite as high to allow a small amount of ankle motion. However, the basic design of the rigid boot with a screwed-on steel blade has not changed.

The problem with the traditional figure skate is that when a skater lands after a jump, the rigid boot severely restricts motion at the ankle, forcing the skater to land nearly flat-footed and preventing motion at the ankle that could help attenuate the landing shock that gets translated upwards through the musculoskeletal system. Not surprisingly, the incidence of overuse injuries in figure skating is mushrooming due to the increased emphasis on performing jumps, the increase in training time, and the continued use of outdated equipment.

To address this problem, biomechanist Jim Richards and graduate student Dustin Bruening, working at the University of Delaware’s Human Performance Lab, have designed and tested a new figure skating boot. Following the design of modern-day Alpine skiing and in-line skating boots, the new boot incorporates an articulation at the ankle that permits flexion movement but restricts potentially injurious sideways movement.

The boot enables skaters to land toe-first, with the rest of the foot hitting the ice more slowly. This extends the landing time, thereby spreading the impactforce over a longer time and dramatically diminishing the peak force translated up through the body. As shown in the graph, the new boot attenuates the peak landing force on the order of 30%.

Although the new figure skating boot design was motivated by a desire to reduce the incidence of stress injuries in skating, it may also promote performance. The ability to move through a larger range of motion at the ankle may well enable higher jump heights and concomitantly more rotations while the skater is in the air.

Skaters who adopt the new boot are finding that using it effectively requires a period of acclimatization. Those who have been skating in the traditional boot for many years tend to have reduced strength in the musculature surrounding the ankle. Improving ankle strength is likely to be necessary for optimal use of a boot that now allows ankle motion.