Module 1
Body Systems, Biomechanics, and Bioenergetics
A. Musculoskeletal System
The musculoskeletal system of the human body consists of bones, joints, muscles,
and tendons configured to allow the great variety of movements characteristic of human
activity. This section describes the various components of the musculoskeletal system,
both individually and in the context of how they function together. The muscles of the
body do not act directly to exert force on the ground or other objects. Instead, they
function by pulling against bones that rotate about joints and transmit force to the
environment. Muscles can only pull, not push; but through the system of bony levers,
muscle pulling forces can be manifested as either pulling or pushing forces against
external objects.
here are approximately 206 bones in the body, though the number can vary. This
relatively light, strong structure provides leverage, support, and protection (figure 1.1).
The axial skeleton consists of the skull (cranium), vertebral column (vertebra C1 through
the coccyx), ribs, and sternum. The appendicular skeleton includes the shoulder (or
pectoral) girdle (left and right scapula and clavicle); bones of the arms, wrists, and hands
(left and right humerus, radius, ulna, carpals, metacarpals, and phalanges); the pelvic
girdle (left and right coxal or innominate bones); and the bones of the legs, ankles, and
feet (left and right femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges).
Junctions of bones are called joints. Fibrous joints (e.g., sutures of the skull) allow
virtually no movement; cartilaginous joints (e.g., intervertebral disks) allow limited
movement; and synovial joints (e.g., elbow and knee) allow considerable movement.
Sport and exercise movements occur mainly about the synovial joints, whose most
important features are low friction and large range of motion. Articulating bone ends are
covered with smooth hyaline cartilage, and the entire joint is enclosed in a capsule filled
with synovial fluid. There are usually additional supporting structures of ligament and
cartilage.
Virtually all joint movement consists of rotation about points or axes. Joints can
be categorized by the number of directions about which rotation can occur. Uniaxial
joints, such as the elbow, operate as hinges, essentially rotating about only one axis. The
knee is often referred to as a hinge joint, but its axis of rotation actually changes
throughout the joint range of motion. Biaxial joints, such as the ankle and wrist, allow
movement about two perpendicular axes. Multiaxial joints, including the shoulder and
hip ball-and-socket joints, allow movement about all three perpendicular axes that define
space.
Each skeletal muscle is an organ that contains muscle tissue, connective tissue,
nerves, and blood vessels. FiGure 1.1 (a) Front view and (b) rear view of an adult male
human skeleton. Fibrous connective tissue, or epimysium, covers the body’s more than
430 skeletal muscles. The epimysium is contiguous with the tendons at the ends of the
muscle (figure 1.3). The tendon is attached to bone periosteum, a specialized connective
tissue covering all bones; any contraction of the muscle pulls on the tendon and, in turn,
the bone. Limb muscles have two attachments to bone: proximal (closer to the trunk) and
distal (farther from the trunk). The two attachments of trunk muscles are termed superior
(closer to the head) and inferior (closer to the feet).
The junction between a motor neuron (nerve cell) and the muscle fibers it
innervates is called the motor end plate, or, more often, the neuromuscular junction
(figure 1.4). Each muscle cell has only one neuromuscular junction, although a single
motor neuron innervates many muscle fibers, sometimes hundreds or even thousands. A
motor neuron and the muscle fibers it innervates are called a motor unit. All the muscle
fibers of a motor unit contract together when they are stimulated by the motor neuron.
The interior structure of a muscle fiber is depicted in figure 1.5. The sarcoplasm, which is
the cytoplasm of a muscle fiber, contains contractile components consisting of protein
filaments, other proteins, stored glycogen and fat particles, enzymes, and specialized
organelles such as mitochondria and the sarcoplasmic reticulum.
Hundreds of myofibrils (each about 1 mm in diameter, 1/100 the diameter of a
hair) dominate the sarcoplasm. Myofibrils contain the apparatus that contracts the muscle
cell, which consists primarily of two types of myofilament: myosin and actin. The
myosin filaments (thick filaments about 16 nm in diameter, about 1/10,000 the diameter
of a hair) contain up to 200 myosin molecules. The myosin filament consists of a globular
head, a hinge point, and a fibrous tail. The globular heads protrude away from the myosin
filament at regular intervals, and a pair of myosin filaments forms a crossbridge, which
interacts with actin. The actin filaments (thin filaments about 6 nm in diameter) consist of
two strands arranged in a double helix. Myosin and actin filaments are organized
longitudinally in the smallest contractile unit of skeletal muscle, the sarcomere.
Sarcomeres average about 2.5 mm in length in a relaxed fiber (approximately 4,500 per
centimeter of muscle length) and are repeated the entire length of the muscle fiber.
In its simplest form, the sliding-filament theory states that the actin filaments at
each end of the sarcomere slide inward on myosin filaments, pulling the Z-lines toward
the center of the sarcomere and thus shortening the muscle fiber (figure 1.7). As actin
filaments slide over myosin filaments, both the H-zone and I-band shrink. The action of
myosin crossbridges pulling on the actin filaments is responsible for the movement of the
actin filament. Because only a very small displacement of the actin filament occurs with
each flexion of the myosin crossbridge, very rapid, repeated flexions must occur in many
crossbridges throughout the entire muscle for measurable movement to occur
Under normal resting conditions, little calcium is present in the myofibril (most of
it is stored in the sarcoplasmic reticulum), so very few of the myosin crossbridges are
bound to actin. Even with the actin binding site covered, myosin and actin still interact in
a weak bond, which becomes strong (and muscle tension is produced) when the actin
binding site is exposed after release of the stored calcium. Before myosin crossbridges
can flex, they must first attach to the actin filament. When the sarcoplasmic reticulum is
stimulated to release calcium ions, the calcium binds with troponin, a protein that is
situated at regular intervals along the actin filament (see figure 1.6) and has a high
affinity for calcium ions.
The energy for pulling action, or power stroke, comes from hydrolysis
(breakdown) of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and
phosphate, a reaction catalyzed by the enzyme myosin adenosine triphosphatase
(ATPase). Another molecule of ATP must replace the ADP on the myosin crossbridge
globular head in order for the head to detach from the active actin site and return to its
original position. This allows the contraction process to continue (if calcium is available
to bind to troponin) or relaxation to occur (if calcium is not available). It may be noted
that calcium plays a role in regulating a large number of events in skeletal muscle besides
contraction. These include glycolytic and oxidative energy metabolism, as well as protein
synthesis and degradation.
Muscle fibers are innervated by motor neurons that transmit impulses in the form
of electrochemical signals from the spinal cord to muscle. A motor neuron generally has
numerous terminal branches at the end of its axon and thus innervates many different
muscle fibers. The whole structure is what determines the muscle fiber type and its
characteristics, function, and involvement in exercise. When a motor neuron fires an
impulse or action potential, all of the fibers that it serves are simultaneously activated and
develop force. The extent of control of a muscle depends on the number of muscle fibers
within each motor unit. Muscles that must function with great precision, such as eye
muscles, may have motor units with as few as one muscle fiber per motor neuron.
Changes in the number of active motor units in these small muscles can produce the
extremely fine gradations in force that are necessary for precise movements of the
eyeball. In contrast, the quadriceps muscle group, which moves the leg with much less
precision, may have several hundred fibers served by one motor neuron.
Each action potential traveling down a motor neuron results in a short period of
activation of the muscle fibers within the motor unit. The brief contraction that results is
referred to as a twitch. Activation of the sarcolemma results in the release of calcium
within the fiber, and contraction proceeds as previously described. Force develops if there
is resistance to the pulling interaction of actin and myosin filaments. Although calcium
release during a twitch is sufficient to allow optimal activation of actin and myosin, and
thereby maximal force of the fibers, calcium is removed before force reaches its
maximum, and the muscle relaxes (figure 1.8a). If a second twitch is elicited from the
motor nerve before the fibers completely relax, force from the two twitches summates,
and the resulting force is greater than that produced by a single twitch (figure 1.8b).
Decreasing the time interval between the twitches results in greater summation of
crossbridge binding and force.
Skeletal muscles are composed of fibers that have markedly different
morphological and physiological characteristics. These differences have led to several
different systems of classification, based on a variety of criteria. The most familiar
approach is to classify fibers according to twitch time, employing the terms slowtwitch
and fast-twitch fiber. Because a motor unit is composed of muscle fibers that are all of
the same type, it also can be designated using this classification system. A fast-twitch
motor unit is one that develops force and also relaxes rapidly and thus has a short twitch
time. Slow-twitch motor units, in contrast, develop force and relax slowly and have a
long twitch time.
Histochemical staining for myosin ATPase content is often used to classify fibers
as slow-twitch or fasttwitch. Although the techniques can stain for multiple fiber types,
the commonly identified fibers are Type I (slow-twitch), Type IIa (fast-twitch), and Type
IIx (fasttwitch). Another more specific method is to quantify the amount of myosin heavy
chain (MHC) protein; the nomenclature for this is similar to that with the myosin ATPase
methodology. The contrast in mechanical characteristics of Type I and Type II fibers is
accompanied by a distinct difference in the ability of the fibers to demand and supply
energy for contraction and thus to withstand fatigue. Type I fibers are generally efficient
and fatigue resistant and have a high capacity for aerobic energy supply, but they have
limited potential for rapid force development, as characterized by low myosin ATPase
activity and low anaerobic power.
Through everyday experiences, we are quite aware that a given muscle can vary
its level of force output according to the level required by a particular task. This ability to
vary or gradate force is essential for performance of smooth, coordinated patterns of
movement. Muscular force can be graded in two ways. One is through variation in the
frequency at which motor units are activated. If a motor unit is activated once, the twitch
that arises does not produce a great deal of force. However, if the frequency of activation
is increased so that the forces of the twitches begin to overlap or summate, the resulting
force developed by the motor unit is much greater. This method of varying force output is
especially important in small muscles, such as those of the hand. Even at low forces, most
of the motor units in these muscles are activated, albeit at a low frequency. Force output
of the whole muscle is intensified through increase in the frequency of firing of the
individual motor units. The other means of varying skeletal muscle force involves an
increase in force through varying the number of motor units activated, a process known
as recruitment. In large muscles, such as those in the thigh, motor units are activated at
near-tetanic frequency when called on. Increases in force output are achieved through
recruitment of additional motor units.
The type of motor unit recruited for a given activity is determined by its
physiological characteristics (table 1.2). For an activity such as distance running,
slowtwitch motor units are engaged to take advantage of their remarkable efficiency,
endurance capacity, and resistance to fatigue. If additional force is needed, as in a sprint
at the end of a race, the fast-twitch motor units are called into play to increase the pace;
unfortunately, exercise at such intensity cannot be maintained very long. If the activity
requires near-maximal performance, as in a power clean, most of the motor units are
called into play, with fast-twitch units making the more significant contribution to the
effort. Complete activation of the available motor neuron pool is probably not possible in
untrained people (4, 5, 6). Although the large fasttwitch units may be recruited if the
effort is substantial, under most circumstances it is probably not possible to activate them
at a high enough frequency for maximal force to be realized.
Proprioceptors are specialized sensory receptors located within joints, muscles,
and tendons. Because these receptors are sensitive to pressure and tension, they relay
information concerning muscle dynamics to the conscious and subconscious parts of the
central nervous system. The brain is thus provided with information concerning
kinesthetic sense, or conscious appreciation of the position of body parts with respect to
gravity. Most of this proprioceptive information, however, is processed at subconscious
levels so we do not have to dedicate conscious activity toward tasks such as maintaining
posture or position of body parts.
Muscle spindles are proprioceptors that consist of several modified muscle fibers
enclosed in a sheath of connective tissue (figure 1.9). These modified fibers, called
intrafusal fibers, run parallel to the normal, or extrafusal, fibers. Muscle spindles provide
information concerning muscle length and the rate of change in length. When the muscle
lengthens, spindles are stretched. This deformation activates the sensory neuron of the
spindle, which sends an impulse to the spinal cord, where it synapses (connects) with
motor neurons. This results in the activation of motor neurons that innervate the same
muscle. Spindles thus indicate the degree to which the muscle must be activated in order
to overcome a given resistance. As a load increases, the muscle is stretched to a greater
extent, and engagement of muscle spindles results in greater activation of the muscle.
Muscles that perform precise movements have many spindles per unit of mass to help
ensure exact control of their contractile activity. A simple example of muscle spindle
activity is the knee jerk reflex. Tapping on the tendon of the knee extensor muscle group
below the patella stretches the muscle spindle fibers. This causes activation of extrafusal
muscle fibers in the same muscle.
Golgi tendon organs (GTOs) are proprioceptors located in tendons near the
myotendinous junction and are in series, that is, attached end to end, with extrafusal
muscle fibers (figure 1.10). Golgi tendon organs are activated when the tendon attached
to an active muscle is stretched. As tension in the muscle increases, discharge of the
GTOs increases. The sensory neuron of the GTO synapses with an inhibitory interneuron
in the spinal cord, which in turn synapses with and inhibits a motor neuron that serves the
same muscle. The result is a reduction in tension within the muscle and tendon. Thus,
whereas spindles facilitate activation of the muscle, neural input from GTOs inhibits
muscle activation.
The primary roles of the cardiovascular system are to transport nutrients and
remove waste and by-products while assisting with maintaining the environment for all
the body’s functions. The cardiovascular system plays key roles in the regulation of the
body’s acid–base system, fluids, and temperature, as well as a variety of other
physiological functions. This section describes the anatomy and physiology of the heart
and the blood vessels.
The heart is a muscular organ composed of two interconnected but separate
pumps; the right side of the heart pumps blood through the lungs, and the left side pumps
blood through the rest of the body. Each pump has two chambers: an atrium and a
ventricle (figure 1.11). The right and left atria deliver blood into the right and left
ventricles. The right and left ventricles supply the main force for moving blood through
the pulmonary and peripheral circulations, respectively.
The tricuspid valve and mitral valve (bicuspid valve) (collectively called
atrioventricular [AV] valves) prevent the flow of blood from the ventricles back into the
atria during ventricular contraction (systole). The aortic valve and pulmonary valve
(collectively, the semilunar valves) prevent backflow from the aorta and pulmonary
arteries into the ventricles during ventricular relaxation (diastole). Each valve opens and
closes passively; that is, each closes when a backward pressure gradient pushes blood
back against it, opening when a forward pressure gradient forces blood in the forward
direction.
The SA node is a small area of specialized muscle tissue located in the upper
lateral wall of the right atrium. The fibers of the node are contiguous with the muscle
fibers of the atrium, with the result that each electrical impulse that begins in the SA node
normally spreads immediately into the atria. The conductive system is organized so that
the impulse does not travel into the ventricles too rapidly, allowing time for the atria to
contract and empty blood into the ventricles before ventricular contraction begins. It is
primarily the AV node and its associated conductive fibers that delay each impulse
entering into the ventricles. The AV node is located in the posterior septal wall of the
right atrium.
The inherent rhythmicity and conduction properties of the myocardium (heart
muscle) are influenced by the cardiovascular center of the medulla, which transmits
signals to the heart through the sympathetic and parasympathetic nervous systems, both
of which are components of the autonomic nervous system. The atria are supplied with a
large number of both sympathetic and parasympathetic neurons, whereas the ventricles
receive sympathetic fibers almost exclusively. Stimulation of the sympathetic nerves
accelerates depolarization of the SA node (the chronotropic effect), which causes the
heart to beat faster. Stimulation of the parasympathetic nervous system slows the rate of
SA node discharge, which slows the heart rate. The resting heart rate normally ranges
from 60 to 100 beats/min; fewer than 60 beats/min is called bradycardia, and more than
100 beats/min is called tachycardia.
The electrical activity of the heart can be recorded at the surface of the body; a
graphic representation of this activity is called an electrocardiogram (ECG). The P-wave
is generated by the changes in the electrical potential of cardiac muscle cells that
depolarize the atria and result in atrial contraction. The QRS complex is generated by the
electrical potential that depolarizes the ventricles and results in ventricular contraction. In
contrast, the T-wave is caused by the electrical potential generated as the ventricles
recover from the state of depolarization; this process, called repolarization, occurs in
ventricular muscle shortly after depolarization. Although atrial repolarization occurs as
well, its wave formation usually occurs during the time of ventricular depolarization and
is thus masked by the QRS complex.
The function of arteries is to rapidly transport blood pumped from the heart.
Because blood pumped from the heart is under relatively high pressure, arteries have
strong, muscular walls. Small branches of arteries called arterioles act as control vessels
through which blood enters the capillaries. Arterioles play a major role in the regulation
of blood flow to the capillaries. Arterioles have strong, muscular walls that are capable of
closing the arteriole completely or allowing it to be dilated many times their size, thus
vastly altering blood flow to the capillaries in response to the needs of the tissues.
Two paramount functions of blood are the transport of oxygen from the lungs to
the tissues for use in cellular metabolism and the removal of carbon dioxide, the most
abundant by-product of metabolism, from the tissues to the lungs. The transport of
oxygen is accomplished by hemoglobin, the iron–protein molecule carried by the red
blood cells. Hemoglobin also has an additional important role as an acid–base buffer, a
regulator of hydrogen ion concentration, which is crucial to the rates of chemical
reactions in cells. Red blood cells, the major component of blood, have other functions as
well. For instance, they contain a large quantity of carbonic anhydrase, which catalyzes
the reaction between carbon dioxide and water to facilitate carbon dioxide removal.
The primary function of the respiratory system is the basic exchange of oxygen
and carbon dioxide. The anatomy of the human respiratory system is shown in figure
1.15. As air passes through the nose, the nasal cavities perform three distinct functions:
warming, humidifying, and purifying the air (13). Air is distributed to the lungs by way
of the trachea, bronchi, and bronchioles. The trachea is called the first-generation
respiratory passage, and the right and left main bronchi are the second-generation
passages; each division thereafter is an additional generation (bronchioles). There are
approximately 23 generations before the air finally reaches the alveoli, where gases are
exchanged in respiration.
The amount and movement of air and expired gases in and out of the lungs are
controlled by expansion and recoil of the lungs. The lungs do not actively expand and
recoil themselves but rather are acted upon to do so in two ways: by downward and
upward movement of the diaphragm to lengthen and shorten the chest cavity and by
elevation and depression of the ribs to increase and decrease the back-to-front diameter of
the chest cavity (13). Normal, quiet breathing is accomplished almost entirely by
movement of the diaphragm. During inspiration, contraction of the diaphragm creates a
negative pressure (vacuum) in the chest cavity, and air is drawn into the lungs. During
expiration, the diaphragm simply relaxes; the elastic recoil of the lungs, chest wall, and
abdominal structures compresses the lungs, and air is expelled. During heavy breathing,
the elastic forces alone are not powerful enough to provide the necessary respiratory
response. The extra required force is achieved mainly by contraction of the abdominal
muscles, which push the abdomen upward against the bottom of the diaphragm.
Pleural pressure is the pressure in the narrow space between the lung pleura and
the chest wall pleura (membranes enveloping the lungs and lining the chest walls). This
pressure is normally slightly negative. Because the lung is an elastic structure, during
normal inspiration the expansion of the chest cage is able to pull on the surface of the
lungs and creates a more negative pressure, thus enhancing inspiration. During
expiration, the events are essentially reversed.
During normal respiration at rest, only 3% to 5% of the total energy expended by
the body is required for pulmonary ventilation. During very heavy exercise, however, the
amount of energy required can increase to as much as 8% to 15% of total body energy
expenditure, especially if the person has any degree of increased airway resistance, as
occurs with exercise-induced asthma. Precautions, including physician evaluation of the
athlete, are often recommended, depending on the potential level of impairment.
With ventilation, oxygen diffuses from the alveoli into the pulmonary blood, and
carbon dioxide diffuses from the blood into the alveoli. The process of diffusion is a
simple random motion of molecules moving in opposite directions through the alveolar
capillary membrane. The energy for diffusion is provided by the kinetic motion of the
molecules themselves. Net diffusion of the gas occurs from the region of high
concentration to the region of low concentration. The rates of diffusion of the two gases
depend on their concentrations in the capillaries and alveoli and the partial pressure of
each gas (13). At rest, the partial pressure of oxygen in the alveoli is about 60 mmHg
greater than that in the pulmonary capillaries. Thus, oxygen diffuses into the pulmonary
capillary blood. Similarly, carbon dioxide diffuses in the opposite direction. This process
of gas exchange is so rapid as to be thought of as instantaneous.
B. Skeletal Musculature
To cause movement or to generate force against external objects, both ends of
each skeletal muscle must be attached to bone by connective tissue. Traditionally,
anatomists define the muscle’s origin as its proximal (toward the center of the body)
attachment, and its insertion as its distal (away from the center of the body) attachment.
Sometimes the origin is defined as the more stationary structure to which the muscle is
attached and the insertion as the more mobile structure. This definition can lead to a
confusing reversal of the origin and insertion. For example, during a straight-leg sit-up,
the origin of the iliacus muscle is the femur, because of its relative immobility. The
pelvis, being more mobile, is the insertion. However, during the leg raise exercise, the
pelvis is relatively immobile and would therefore become the origin, while the more
mobile femur would become the insertion. The traditional definition therefore provides
the most consistency.
Muscles are attached to bone in various ways. In fleshy attachments, which are
most often found at the proximal end of a muscle, muscle fibers are directly affixed to the
bone, usually over a wide area so that force is distributed rather than localized. Fibrous
attachments, such as tendons, blend into and are continuous with both the muscle sheaths
and the connective tissue surrounding the bone. They have additional fibers that extend
into the bone itself, making for a very strong union.
A muscle is called a synergist when it assists indirectly in a movement. For
example, the muscles that stabilize the scapula act as synergists during upper arm
movement. Without these synergists, the muscles that move the upper arm (many of
which originate on the scapula) would not be effective in bringing about this movement.
Synergists are also required to control body motion when the agonist is a muscle that
crosses two joints. For example, the rectus femoris muscle crosses the hip and knee,
acting to flex the hip and extend the knee when contracting. Rising from a low squat
involves both hip and knee extension. If the rectus femoris is to act to extend the knee as
a person rises without inclining the trunk forward, then hip extensor muscles such as the
gluteus maximus must act synergistically to counteract the hip flexion that would
otherwise result from tension in the rectus femoris.
Most human muscles that rotate the limbs about body joints operate at a
mechanical advantage of less than 1.0 (that is, at a mechanical disadvantage). This is why
internal muscle forces are much greater than the forces exerted by the body on external
objects. For example, in figure 2.2, because the resistance moment arm is eight times
longer than the muscle moment arm, muscle force must be eight times the resistive force.
The extremely high internal forces experienced by muscles and tendons account in large
part for injury to these tissues. During actual movement, the categorization of a lever as
first, second, or third class often depends on the somewhat arbitrary decision of where the
fulcrum lies. Therefore, understanding the principle of mechanical advantage is of much
greater importance than being able to classify levers.
Considerable variation in human anatomical structure exists, including the points
at which tendons are attached to bone. A person whose tendons are inserted on the bone
farther from the joint center should be able to lift heavier weights because muscle force
acts through a longer moment arm and thus can produce greater torque around the joint.
(In figure 2.6, for example, consider how the moment arm [M] would change if the
tendon insertion were farther to the right.) It is important, however, to recognize the
trade-off involved in tendon insertion. The mechanical advantage gained by having
tendons insert farther from the joint center is accompanied by a loss of maximum speed
because, with the tendon inserted farther from the joint center, the muscle has to contract
more to make the joint move through a given range of motion. In other words, a given
amount of muscle shortening results in less rotation of body segments about a joint,
which translates into a loss in movement speed.
C. Anatomical Planes and Major Body Movements
The body is erect, the arms are down at the sides, and the palms face forward.
Anatomical views of the body, as in magnetic resonance imaging, are generally shown in
the sagittal, frontal, and transverse planes, which slice the body into left–right, front–
back, and upper–lower sections, respectively, not necessarily at the midpoint. The
anatomical planes are also useful for describing the major body movements. Examples of
exercise movements that take place in these planes include standing barbell curl (sagittal
plane), standing lateral dumbbell raise (frontal plane), and dumbbell fly (transverse
plane).
Biomechanical analysis of human movement can be used to quantitatively analyze
the target activity. In the absence of the requisite equipment and expertise, however,
simple visual observation is adequate for identifying the basic features of a sport
movement. Exercises can then be selected that involve similar movement around the
same joints, thereby incorporating specificity of training. Slow-motion videotape can
facilitate the observation. Also, commercially available software enables more detailed
analysis of sport movements captured in digital video.
Although a program providing resistance exercise for all the movements in figure
2.10 would be both comprehensive and balanced, some of the movements are commonly
omitted from standard exercise programs whereas others receive particular emphasis.
Important sport movements not usually incorporated into standard resistance training
programs include shoulder internal and external rotation (throwing, tennis), knee flexion
(sprinting), hip flexion (kicking, sprinting), ankle dorsiflexion (running), hip internal and
external rotation (pivoting), hip adduction and abduction (lateral cutting), torso rotation
(throwing, batting), and the various neck movements (boxing, wrestling).
D. Human Strength and Power
The terms strength and power are widely used to describe some important abilities
that contribute to maximal human efforts in sport and other physical activities.
Unfortunately, there is often little consistency in the way the terms are used. This section
provides a scientific basis for understanding human strength and power and shows how
various factors contribute to their manifestation. Though it is widely accepted that
strength is the ability to exert force, there is considerable disagreement as to how strength
should be measured. The weight that a person can lift is probably the oldest quantitative
measure of strength. Technological developments have popularized the use of isometric
strength testing and also isokinetic strength testing. All sports involve acceleration
(change in velocity per unit time) of the body and, for some sports, of an implement as
well (e.g., baseball bat, javelin, tennis racket).
Because of individual differences in the ability to exert force at different
velocities (43), strength scores obtained from isometric and low-speed resistance tests
may vary in predictive ability when the force is required with concomitant high velocity.
Thus, testing an athlete’s force capabilities at various loads may provide more insight into
the person’s sport-specific capabilities and weaknesses (6). Although controlling and
monitoring velocity during strength testing require sophisticated equipment, the resulting
strength scores may be more meaningfully related to sport ability than are static strength
measures or maximum loads lifted.
The curiosity about force capacity at particular velocities of movement or at high
velocity has led to heightened interest in power as a measurement of the ability to exert
force at higher speeds. Outside of the scientific realm, power is loosely defined as
“explosive strength” (42). However, in physics, power is precisely defined as the time
rate of doing work, where work is the product of the force exerted on an object and the
distance the object moves in the direction in which the force is exerted.
Because power equals the product of force and velocity, when force is exerted on
a weight in the direction opposite to the one in which the weight is moving (as when a
weight is lowered in a controlled manner), calculated power has a negative sign, as does
calculated work. All such “negative” power and work occur during eccentric muscle
actions, such as lowering a weight or decelerating at the end of a rapid movement.
Strictly speaking, there is no such thing as negative work or power. The term negative
work really refers to work performed on, rather than by, a muscle. When a weight is
lifted, muscles perform work on the weight, increasing the weight’s potential energy.
When the weight is lowered, its potential energy is used to perform an equal amount of
work on the athlete. Thus, while repetitions are performed, the athlete and weight
alternately perform work on each other, rather than the athlete’s alternately performing
positive and negative work. The rate at which the repetitions are performed determines
the power output. The rate at which the bar would accelerate downward in free fall is 9.8
m/s2 . If the net force applied was 980 N (F1 ), the acceleration rate would be 0 m/s2 . If
we remove 200 N of force (200 N divided by the bar mass of 100 kg, a = F/m) the
acceleration rate of the bar would be 2 m/s2 downward (in other words, controlling the
bar’s rate of acceleration by decreasing the force applied).
The work and power equations just presented apply to an object moving from one
location to another in a straight line. Work and power are also required to start an object
rotating about an axis or to change the velocity at which it rotates, even if the object as a
whole does not move through space at all. The angle through which an object rotates is
called its angular displacement, the SI unit for which is the radian (rad); 1 rad = 180° ÷ p
= 57.3°, where p = 3.14. Angular velocity is the object’s rotational speed, measured in
radians per second (rad/s). Torque is expressed in newton-meters (N·m), but should not
be confused with work, which is also expressed in newton-meters. The difference is that
the distance component of the torque unit refers to the length of the moment arm (which
is perpendicular to the line of action of the force), while the distance component of the
work unit refers to the distance moved along the line of action of the force. Just as for
movement through space, the work done in rotating an object is measured in joules (J),
and power in watts (W).
The discrepancy between the common and scientific definitions of power has led
to misunderstandings. For example, in the sport of powerlifting, which involves high
forces but relatively low movement speeds, less mechanical power is produced than in
several other sports, including Olympic lifting (6). Despite the discrepancy, the sport of
powerlifting is unlikely to be renamed. In all other contexts, the strength and conditioning
professional should use the word power only in its scientific sense to avoid ambiguity.
Furthermore, although the word strength is often associated with slow velocities and the
word power with high velocities of movement, both variables reflect the ability to exert
force at a given velocity. Power is a direct mathematical function of force and velocity.
Therefore, if at any instant, any two of the variables force, velocity, and power are
known, the third can be calculated. If an individual can generate high force or high power
at a particular velocity of movement, precisely the same ability is being described—that
is, the ability to accelerate a mass at that particular velocity. Therefore, it is not correct to
associate strength with low speed and power with high speed. Strength is the capacity to
exert force at any given velocity, and power is the mathematical product of force and
velocity at whatever speed. What is critical is the ability to exert force at velocities
characteristic of a given sport to overcome gravity and accelerate the body or an
implement. For a sport movement made relatively slow by high resistance, low-velocity
strength is critical, whereas for a movement that is very fast due to low resistance, high-
velocity strength is important. For example, when offensive and defensive American
football linemen push against each other, their velocity of movement is slowed by the
muscular force exerted by the opposing player as well as the inertia of the opposing
player’s body mass. Because the muscles are prevented from contracting at high velocity,
the ability to exert force and power at low velocity is an important component of
performance. In contrast, a badminton player’s muscles quickly reach high velocity as a
result of the minimal inertial resistance of the lightweight racket and the player’s arm.
Therefore, the ability to exert force and power at high velocity is critical to making rapid
adjustments in a stroke.
Several biomechanical factors are involved in the manifestation of human
strength, including neural control, muscle cross-sectional area, muscle fiber arrangement,
muscle length, joint angle, muscle contraction velocity, joint angular velocity, and body
size. These factors are discussed next, as are the three-dimensional strength relationship
and the strength-to-mass ratio. All else being equal, the force a muscle can exert is related
to its cross-sectional area rather than to its volume (11, 31). For example, if two athletes
of similar percent body fat but different height have the same biceps circumference, their
upper arm muscle cross-sectional areas are about the same. Although the taller (and
therefore heavier) athlete’s longer muscle makes for greater muscle volume, the strength
of the two athletes’ biceps should be about the same. With the same strength but greater
body weight, the taller athlete has less ability to lift and accelerate his or her own body—
for example, when performing calisthenics or gymnastics. This is why most elite
gymnasts are not very tall. As described in chapter 1, resistance training increases both
the strength and cross-sectional area of muscle.
Maximally contracting muscles have been found capable of generating forces of
23 to 145 psi (16-100 N/cm2 ) of muscle cross-sectional area (21). This wide range can
be partially accounted for by the variation in the arrangement and alignment of
sarcomeres in relation to the long axis of the muscle (figure 2.11) (21). A pennate muscle
has fibers that align obliquely with the tendon, creating a featherlike arrangement. The
angle of pennation is defined as the angle between the muscle fibers and an imaginary
line between the muscle’s origin and insertion; 0° corresponds to no pennation.
Many human muscles are pennated (20, 39), but few have angles of pennation in
excess of 15°. Actually, the angle of pennation does not remain constant for a given
muscle, but increases as the muscle shortens. Any factor that affects angle of pennation
would thus affect strength and velocity of shortening as long as the cross-sectional area
remains the same. Muscles with greater pennation have more sarcomeres in parallel and
fewer sarcomeres in series; they are therefore better able to generate force but have a
lower maximal shortening velocity than nonpennate muscles. In comparison, lesser
amounts of pennation can be advantageous for producing high velocities due to the
greater number of sarcomeres in a row, at the expense of number of sarcomeres in
parallel. The amount of pennation, however, has an effect on the muscles’ ability to
generate eccentric, isometric, or low speed concentric force (40). Most importantly,
although angle of pennation may vary depending on hereditary factors, it is modifiable
through training, which could help account for some of the differences in strength and
speed seen in individuals who seem to have muscles of the same size.
Muscle torque varies with joint angular velocity according to the type of muscular
action (figure 2.13). Tests have shown that during isokinetic (constant-speed) concentric
exercise by human subjects, torque capability declines as angular velocity increases. In
contrast, during eccentric exercise, as joint angular velocity increases, maximal torque
capability increases until about 90°/s (1.57 rad/s), after which it declines gradually (4).
That means that the greatest muscle force can be obtained during eccentric muscle action.
This is exemplified by athletes who employ “cheating” movements when a weight cannot
be lifted using strict form. For example, an individual who reaches a “sticking point” in
the biceps curl exercise due to the limit of concentric elbow flexor strength usually leans
the torso back, allowing the elbow flexors to exert increased force by operating
isometrically or eccentrically and thereby enabling continued movement of the bar.
In sport activities such as sprinting and jumping, the ratio of the strength of the
muscles involved in the movement to the mass of the body parts being accelerated is
critical. Thus, the strength-to-mass ratio directly reflects an athlete’s ability to accelerate
his or her body. If, after training, an athlete increases body mass by 15% but increases
force capability by only 10%, the strength-to-mass ratio, and thus the athlete’s ability to
accelerate, is reduced. A sprinter or jumper may benefit by experimenting with muscle
mass to determine the highest strength-to-mass ratio, which would result in the best
possible performance.
There has always been interest in comparing the performances of athletes in
different weight categories. The most obvious method for doing so is to divide the weight
lifted by the athlete’s body weight. However, such an adjustment is biased against larger
athletes because it does not take into account the expected drop in the strength-to-mass
ratio with increasing body size. Various formulas have been derived to more equitably
compare loads lifted. In the classic formula, the load lifted is divided by body weight to
the two-thirds power, thus accounting for the relationship of cross-sectional area versus
volume. Other formulas have since been developed because the classic formula seemed
to favor athletes of middle body weight over lighter and heavier athletes (5). However,
the determination by the classic formula that the performances of medium-weight athletes
are usually the best may indeed be unbiased. Because of the bell-shaped curve describing
the normal distribution of anthropometric characteristics among the population, the body
weights of a majority of people are clustered close to the mean.
E. Sources of Resistance to Muscle Contraction
The most common sources of resistance for strength training exercises are gravity,
inertia, friction, fluid resistance, and elasticity. This section provides information on the
force and power required to overcome these forms of resistance. An understanding of the
principles behind exercise devices using the various forms of resistance can provide
insight into their effectiveness and applicability. Popular terminology for weight and
mass is often incorrect. For example, some barbell and stack-machine plates are labeled
in pounds. The pound is a unit of force, not mass. In actuality, only the mass of a barbell
plate stays constant, while its weight varies according to the local acceleration due to
gravity. The kilogram designation on a weight plate refers to its mass. It is not correct to
say that an object weighs a certain number of kilograms, since weight refers to force, not
mass. Instead, one should say “The mass of the barbell is 85 kg.” The amount of mass an
individual can lift will be slightly affected by terrestrial location because of variations in
the acceleration due to gravity around the globe (see table 2.1). That same 85 kg barbell
would feel like approximately 14 kg if it were on the moon, even though it did not
physically change.
The gravitational force on an object always acts downward. Since, by definition,
the moment arm by which a force produces torque is perpendicular to the line of action of
the force, the moment arm of a weight is always horizontal. Thus, torque due to an
object’s weight is the product of the weight and the horizontal distance from the weight
to the pivot point (joint). During an exercise, although the weight does not change, its
horizontal distance from a given joint axis changes constantly. When the weight is
horizontally closer to the joint, it exerts less resistive torque; when it is horizontally
farther from a joint, it exerts more resistive torque. For example, in an arm curl, the
horizontal distance from the elbow to the barbell is greatest when the forearm is
horizontal. Thus, in that position the athlete must exert the greatest muscle torque to
support the weight. The moment arm decreases as the forearm rotates either upward or
downward away from the horizontal, decreasing the resistive torque arising from the
weight (see figure 2.7). When the weight is directly above or below the elbow pivot
point, there is no resistive torque from the weight.
Nautilus Sports/Medical Industries popularized the concept of tailoring resistive
torque through the range of joint motion by creating an exercise machine that uses a cam
of variable radius; this changes the length of the moment arm through which the weight
stack acts (figure 2.14). The rationale was to provide more resistance at points in the
range of motion where the muscles could exert greater torque, and less resistance where
the muscles could apply less torque. For the system to work as planned, however, the
athlete has to move at a constant, slow angular velocity, which is difficult to do
consistently. Also, cam-based machines frequently fail to match normal human torque
capability patterns.
In addition to gravitational force, a barbell or weight stack, when accelerated,
exerts inertial force on the athlete. Though the force of gravity acts only downward,
inertial force can act in any direction. The upward force an athlete exerts equals the
weight lifted plus any inertial force, which is the mass times the upward acceleration of
the bar. Horizontal bar acceleration occurs if the athlete exerts net force on the bar
directed to the front, back, left, or right. All exercises involve some acceleration at the
beginning to bring the bar from a zero to an upward velocity, as well as some
deceleration near the top of the exercise to bring the bar’s velocity back to zero so that it
does not continue its trajectory and fly out of the lifter’s hands. With this acceleration
pattern, the agonist muscles receive resistance in excess of bar weight early in the range
of motion, but resistance less than bar weight toward the end of the range of motion (27).
The athlete decelerates the bar by either (a) reducing upward force on the bar to less than
bar weight to let some or all of the bar’s weight decelerate it or (b) pushing down against
the bar using the antagonist muscles. In either case, the deceleration has the effect of
providing less resistance to the agonist muscles late in the range of motion.
Acceleration and deceleration are characteristic of virtually all natural
movements. For example, sprinting requires the athlete’s arms and legs to go through
repeated cycles of acceleration and deceleration. Throwing a baseball, discus, shot, or
javelin all involve sequences of body movements that accelerate the objects to high
release speeds. Because acceleration is a particular kind of movement pattern, training
with accelerative movements can provide specificity of training. That is why explosive
exercises, such as the power clean and high pull, are used in training for many different
sports in which the leg and hip muscles provide force for accelerating the body. The
bracketing technique, in which the athlete performs the sport movement with less than
normal and greater than normal resistance, is another form of acceleration training.
According to the force–velocity relationship of muscle, a shot-putter who trains with an
extra-heavy shot develops greater forces during the accelerative movement than when
using the normal shot because the inertia of the heavier implement forces the muscle to
contract at relatively low speed. When a relatively light shot is used, the lower inertia of
the shot enables the putter to accelerate the shot more rapidly and to reach a higher speed
of release, thereby training the neuromuscular system to operate within desired
acceleration and speed ranges. Although the principle of increasing or decreasing the load
during a movement as described has the theoretical basis for increasing acceleration
capacity through the aforementioned methods, one should also consider the influence that
such changes in loading have during highly specific or technique-oriented activities such
as throwing or sprinting. For example, changing implement loading could have some
negative consequences on technique since the body needs time to adjust the motor pattern
for that particular movement with the new load.
The resistive force encountered by an object moving through a fluid (liquid or
gas), or by a fluid moving past or around an object or through an opening, is called fluid
resistance. Fluid resistance is a significant factor in such sport activities as swimming,
rowing, golf, sprinting, discus throwing, and baseball pitching. (Except for swimming
and rowing, in which the fluid is water, all these involve air resistance.) The phenomenon
has become important in resistance training with the advent of hydraulic (liquid) and
pneumatic (gas) exercise machines and with the increasing popularity of swimming pool
exercise routines, particularly among older people and pregnant women. The two sources
of fluid resistance are surface drag, which results from the friction of a fluid passing
along the surface of an object, and form drag, which results from the way in which a fluid
presses against the front or rear of an object passing through it. Cross-sectional (frontal)
area has a major effect on form drag.
F. Joint Biomechanics
In contrast to quadrupeds, whose vertebral columns hang like the cables on a
suspension bridge, humans normally stand upright, with the vertebral bones stacked one
on top of another, separated by rubbery disks. The advantage we gain from our upright
posture and free use of the arms and hands is accompanied by the disadvantage of having
our intervertebral disks under compressive force even when we are merely standing,
sitting, walking, or running—and under even more compressive force when we are lifting
and carrying (14). When we are in a standing position, any force we exert with the upper
body must be transmitted through the back to the legs and ground. In addition, the back
muscles act at a great mechanical disadvantage and must generate forces much greater
than the weight of an object lifted. It is for these reasons that the back is particularly
vulnerable to injury. It should be noted, however, that spinal internal loads are quite
variable with varying postures during the lift (24) and that deep squatting positions with
load are not necessarily associated with back injury.
The vertebral column is naturally S-shaped, being slightly rounded (kyphotic) in
the thoracic spine and lordotic in the lumbar spine. The wedged shape of the vertebrae
gives the spine its natural curve. However, the intervertebral disks are flat when the back
is in its S shape. When the lower back is rounded, the ventral (toward the anterior) edges
of the vertebral bodies squeeze the front portions of the intervertebral disks. In contrast,
extreme arching of the back results in squeezing the dorsal (toward the posterior) portions
of the disks. Such uneven squeezing of the intervertebral disks likely increases the risk of
disk rupture (3). Thus, resistance training exercises should generally be performed with
the lower back in a moderately arched position to reduce risk of damage to the disks.
The shoulder is particularly prone to injury during resistance training, due to both
its structure and the forces to which it is subjected during a training session. Like the hip,
the shoulder is capable of rotating in any direction. The hip is a stable ball-and-socket
joint, but the glenoid cavity of the shoulder, which holds the head of the humerus, is not a
true socket and is significantly less stable. The shoulder joint has the greatest range of
motion of all the joints in the human body; but the joint’s excessive mobility contributes
to its vulnerability, as does the proximity of the bones, muscles, tendons, ligaments, and
bursae in the shoulder.
The knee is prone to injury because of its location between two long levers (the
upper and lower leg). Flexion and extension about the knee occur almost exclusively in
the sagittal plane. Rotation in the frontal plane and transverse plane is prevented mainly
by ligamentous and cartilaginous stabilizing structures. Frontal plane torque on the knee
occurs, for example, when a football player is hit at midleg from the side while the foot is
planted firmly on the ground. Fortunately, in training, resistive torques occur almost
exclusively within the knee’s normal plane of rotation
The primary concerns with elbow and wrist injury involve overhead lifts (8).
However, the risk with overhead lifting is quite small in comparison to the common
source of injury of these joints, which includes participation in overhead sports such as
throwing events or the tennis serve (8). Other examples of possible injury are elbow
dislocation, sometimes observed in gymnastics (29), and overuse-related injuries such as
traction apophysitis, sometimes observed in diving, wrestling, and hockey (29). One of
the primary concerns is epiphyseal growth plate damage or overuse either in the posterior
aspect of the elbow or in the distal radius in young athletes (29). The prevalence of elbow
or wrist injury with weightlifting is very sporadic and often referred to in the literature
only through case studies. One study indicated a tricep tendon tear in a middle-aged
competitive weightlifter (35) and another a bilateral distal bicep tendon rupture in a
recreational weight trainer (38). A study examining 245 competitive powerlifters found
an extremely low incidence of elbow and wrist injury (41). Only very limited data have
been presented to suggest possible distal radial epiphysis fracture in adolescent
weightlifters (22). According to a recent study, which surveyed 500 experts in the field of
sports medicine, most respondents indicated that avoiding resistance training before
physeal closure was not necessary.
G. Essential Terminology
Bioenergetics, or the flow of energy in a biological system, concerns primarily the
conversion of macronutrients—carbohydrate, protein, and fats, which contain chemical
energy—into biologically usable forms of energy. It is the breakdown of the chemical
bonds in these macronutrients that provides the energy necessary to perform biological
work.
The breakdown of large molecules into smaller molecules, associated with the
release of energy, is termed catabolism. The synthesis of larger molecules from smaller
molecules can be accomplished using the energy released from catabolic reactions; this
building-up process is termed anabolism. The breakdown of protein into amino acids is
an example of catabolism, while the formation of protein from amino acids is an anabolic
process. Exergonic reactions are energy-releasing reactions and are generally catabolic.
Endergonic reactions require energy and include anabolic processes and the contraction
of muscle. Metabolism is the total of all the catabolic or exergonic and anabolic or
endergonic reactions in a biological system. Energy derived from catabolic or exergonic
reactions is used to drive anabolic or endergonic reactions through an intermediate
molecule, adenosine triphosphate (ATP). Adenosine triphosphate allows the transfer of
energy from exergonic to endergonic reactions. Without an adequate supply of ATP,
muscular activity and muscle growth would not be possible. Thus, it is apparent that
when designing training programs, strength and conditioning professionals need to have a
basic understanding of how exercise affects ATP hydrolysis and resynthesis.
In discussion of exercise-related bioenergetics, the terms anaerobic and aerobic
metabolism are often used. Anaerobic processes do not require the presence of oxygen,
whereas aerobic mechanisms depend on oxygen. The phosphagen and glycolytic systems
are anaerobic mechanisms that occur in the sarcoplasm of a muscle cell. The Krebs cycle,
electron transport, and the rest of the oxidative system are aerobic mechanisms that occur
in the mitochondria of muscle cells and require oxygen as the terminal electron acceptor.
The body stores approximately 80 to 100 g (about 3 ounces) of ATP at any given
time, which does not represent a significant energy reserve for exercise (107). In addition,
ATP stores cannot be completely depleted due to the necessity for basic cellular function.
In fact, ATP concentrations may decrease by up to 50% to 60% (34, 71, 100, 143) of the
preexercise levels during experimentally induced muscle fatigue. Therefore, the
phosphagen system uses the creatine kinase reaction (equation 3.2) to maintain the
concentration of ATP. Under normal circumstances, skeletal muscle concentrations of CP
are four to six times higher than ATP concentrations (107). Therefore, the phosphagen
system, through CP and the creatine kinase reaction, serves as an energy reserve for
rapidly replenishing ATP. In addition, Type II (fast-twitch) muscle fibers contain higher
concentrations of CP than Type I (slow-twitch) fibers (95, 132); thus, individuals with
higher percentages of Type II fibers may be able to replenish ATP faster through the
phosphagen system during anaerobic, explosive exercise.
Glycolysis is the breakdown of carbohydrate—either glycogen stored in the
muscle or glucose delivered in the blood—to resynthesize ATP (22, 143). The process of
glycolysis involves multiple enzymatically catalyzed reactions (figure 3.2). As a result,
the ATP resynthesis rate during glycolysis is not as rapid as with the single-step
phosphagen system; however, the capacity to produce ATP is much higher due to a larger
supply of glycogen and glucose compared to CP. As with the phosphagen system,
glycolysis occurs in the sarcoplasm.
Recent evidence suggests that there are specific break points in the lactate
accumulation curve (figure 3.5) as exercise intensity increases (39, 98). The exercise
intensity or relative intensity at which blood lactate begins an abrupt increase above the
baseline concentration has been termed the lactate threshold (LT) (161). The LT
represents a significantly increased reliance on anaerobic mechanisms for energy
production to meet demand. The LT corresponds well with the ventilatory threshold
(breaking point in the relationship between ventilation and V . O2 ) and is often used as a
marker of the anaerobic threshold.
The oxidative system, the primary source of ATP at rest and during low-intensity
activities, uses primarily carbohydrate and fats as substrates (62). Protein does not
provide a significant contribution to total energy; however, the use of protein does
significantly increase during long-term starvation and long bouts (>90 minutes) of
exercise (41, 102). At rest, approximately 70% of the ATP produced is derived from fats
and 30% from carbohydrate. Following the onset of activity, as the intensity of the
exercise increases, there is a shift in substrate preference from fats to carbohydrate.
During high-intensity aerobic exercise, almost 100% of the energy is derived from
carbohydrate if an adequate supply is available, with only minimal contributions from
fats and protein. However, during prolonged, submaximal, steady-state work, there is a
gradual shift from carbohydrate back to fats, and to a very small extent protein, as energy
substrates.
The oxidative metabolism of blood glucose and muscle glycogen begins with
glycolysis. If oxygen is present in sufficient quantities, the end product of glycolysis,
pyruvate, is not converted to lactate but is transported to the mitochondria, where it is
converted to acetyl-CoA (a two-carbon molecule), which enters the Krebs cycle, also
known as the citric acid cycle or tricarboxylic acid cycle (7, 61). The Krebs cycle is a
series of reactions that continues the oxidation of the substrate from glycolysis and
produces two ATP indirectly from guanine triphosphate (GTP), via substrate-level
phosphorylation, for each molecule of glucose.
Fats can also be used by the oxidative energy system. Triglycerides stored in fat
cells can be broken down by an enzyme, hormone-sensitive lipase, to produce free fatty
acids and glycerol. This releases a portion of the total free fatty acids from the fat cells
into the blood, where they can circulate and enter muscle fibers and undergo oxidation
(88, 121). Additionally, limited quantities of triglycerides are stored within the muscle
along with a form of hormone-sensitive lipase to produce an intramuscular source of free
fatty acids (22, 47). Free fatty acids enter the mitochondria, where they undergo beta
oxidation, a series of reactions in which the free fatty acids are broken down, resulting in
the formation of acetyl-CoA and hydrogen protons (figure 3.6). The acetyl-CoA enters
the Krebs cycle directly, and the hydrogen atoms are carried by NADH and FADH2 to
the ETC (22). The result is hundreds of ATP molecules supplied by beta oxidation. For
example, the breakdown of a single triglyceride molecule containing three 16-carbon
chain free fatty acids (palmitic acid) can be metabolized by beta oxidation to yield over
300 ATP molecules (>100 ATP per palmitic acid). The overarching concept is that fat
oxidation is capable of a tremendous capacity for ATP synthesis compared to
carbohydrate and protein oxidation.
Although not a significant source of energy for most activities, protein can be
broken down into its constituent amino acids by various metabolic processes. Most of
these amino acids can then be converted into glucose (in a process known as
gluconeogenesis), pyruvate, or various Krebs cycle intermediates to produce ATP (figure
3.6). The contribution of amino acids to the production of ATP has been estimated to be
minimal during short-term exercise but may contribute 3% to 18% of the energy
requirements during prolonged activity (20, 138). The major amino acids that are
oxidized in skeletal muscle are believed to be the branched-chain amino acids (leucine,
isoleucine, and valine), although alanine, aspartate, and glutamate may also be used (69).
The nitrogenous waste products of amino acid degradation are eliminated through the
formation of urea and small amounts of ammonia (22). The elimination through
formation of ammonia is significant because ammonia is toxic and is associated with
fatigue.
H. Substrate Depletion and repletion
Energy substrates—molecules that provide starting materials for bioenergetic
reactions, including phosphagens (ATP and CP), glucose, glycogen, lactate, free fatty
acids, and amino acids—can be selectively depleted during the performance of activities
of various intensities and durations. Subsequently, the energy that can be produced by the
bioenergetic systems is reduced. Fatigue experienced during many activities is frequently
associated with the depletion of phosphagens (66, 87) and glycogen (21, 78, 90, 131); the
depletion of substrates such as free fatty acids, lactate, and amino acids typically does not
occur to the extent that performance is limited. Consequently, the depletion and repletion
pattern of phosphagens and glycogen following physical activity is important in exercise
and sport bioenergetics.
Fatigue during exercise appears to be at least partially related to the decrease in
phosphagens (i.e., ATP and CP). Phosphagen concentrations in muscle are more rapidly
depleted as a result of high-intensity anaerobic exercise compared to aerobic exercise (66,
87). Creatine phosphate can decrease markedly (50-70%) during the first stage of high-
intensity exercise of short and moderate duration (5-30 seconds) and can be almost
completely depleted as a result of very intense exercise to exhaustion (84, 91, 96, 108).
Muscle ATP concentrations may decrease only slightly (34) or may decrease up to 50%
to 60% (143) of the preexercise levels during experimentally induced fatigue. It should
also be noted that dynamic muscle actions that produce external work use more metabolic
energy and typically deplete phosphagens to a greater extent than do isometric muscle
actions.
Limited stores of glycogen are available for exercise. Approximately 300 to 400 g
of glycogen are stored in the body’s total muscle and about 70 to 100 g in the liver (135).
Resting concentrations of liver and muscle glycogen can be influenced by training and
dietary manipulations (56, 135). Research suggests that both anaerobic training, including
sprinting and resistance training (16, 104), and stereotypical aerobic endurance training
(64, 65) can increase resting muscle glycogen concentration with concomitantly
appropriate nutrition.
Glycogen depletion can be a limiting factor both for long-duration, low-intensity
exercise supported primarily by aerobic metabolism and for repeated, high-intensity
exercise supported primarily by anaerobic mechanisms. Of importance to resistance
training, sprinting, and other primarily anaerobic activities is the effect of metabolic
acidosis on limiting contractile force (53, 78, 114, 115, 123). Several other factors have
been implicated in the development of muscle fatigue and may limit exercise
performance, including increased intracellular inorganic phosphate, ammonia
accumulation, increased ADP, and impaired calcium release from the sarcoplasmic
reticulum (4, 5, 129, 154, 158). Further research is needed to delineate the causes of
muscular fatigue and the limiting factors in exercise performance.
I. Oxygen uptake and the Aerobic and Anaerobic Contributions to Exercise
Oxygen uptake (or consumption) is a measure of a person’s ability to take in
oxygen via the respiratory system and deliver it to the working tissues via the
cardiovascular system, and the ability of working tissues (predominantly skeletal muscle)
to use oxygen. During low-intensity exercise with a constant power output, oxygen
uptake increases for the first few minutes until a steady state of uptake (oxygen demand
equals oxygen consumption) is reached.
Inappropriate exercise intensities and rest intervals can permit the “selection” of
specific primary energy systems during training for specific athletic events (22, 107,
155). Few sports or physical activities require maximal sustained effort to exhaustion or
near exhaustion, such as competitive middle-distance sprints (400 m to 1,600 m). Most
sports and training activities produce metabolic profiles that are very similar to those of a
series of high-intensity, constant- or near-constant-effort exercise bouts interspersed with
rest periods, such as American football, basketball, and hockey.
In this type of exercise, the required exercise intensity (power output) that must be
met during each exercise bout is much greater than the maximal power output that can be
sustained using aerobic energy sources alone. Increasing aerobic power through primarily
aerobic endurance training while simultaneously compromising or neglecting anaerobic
power and anaerobic capacity training is of little benefit to athletes in these sports (82,
109). For example, it would be of little benefit for a baseball player to run miles during
training rather than focusing on exercises that improve anaerobic power and capacity.
