++
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.
+++
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. The speed of the discus when it is released by
the thrower
2. The projection angle at which the discus is released
3. The height above the ground at which the discus is
released
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.