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The Endurance Events
Myophysiology & Neurophysiology
for the Endurance Events
ATP and Energy

Energy Basics. All living organisms share the characteristic of requiring a constant supply of
usable energy in order to perform the functions needed for life. To perform their vital functions,
cells must use energy obtained by breaking down organic substances. To be useful, that energy
must be transferred from molecule to molecule or from one part of a cell to another.

Energy Transfer. The useful method of energy transfer involves the creation of high-energy
bonds by enzymes within cells. A high-energy bond is a covalent bond whose breakdown
releases energy the cell can harvest. In a human muscle cell, a high-energy bond generally
connects a phosphate group (PO43-) to an organic molecule. The resulting complex is called a
high-energy compound. Most high-energy compounds are derived from nucleotides, the
building blocks of nucleic acids in the cell.

Phosphorylation. The attachment of a phosphate group to another molecule is called
phosphorylation. This process on its own does not necessarily produce high-energy bonds. The
creation of a high energy compound requires a phosphate group, enzymes capable of catalyzing
the reactions involved, and suitable organic substrates to which the phosphate can be added.

Phosphorylation of AMP. The most important such substrate is the nucleotide adenosine
monophosphate (AMP). Attaching a second phosphate group produces adenosine diphosphate
(ADP). A significant energy input is required to convert AMP to ADP, and the second phosphate
is attached by a high-energy bond. Even more energy is required to add a third phosphate and
thereby create the high-energy compound adenosine triphosphate (ATP).
An ATP Molecule. Note the 3 Phosphates Located on the Terminal End

Energy Transfer. The conversion of ADP to ATP is the most important method of energy storage
in human cells. The reversion of ATP to ADP is the most important method of energy release.
The relationships can be shown as follows.
ADP + Phosphate Group + Energy  ATP + H2O

Enzymes. The conversion of ATP to ADP requires an enzyme known as adenosine
triphosphatase, or ATPase. Throughout life, cells continuously generate ATP from ADP and use
the energy provided by the ATP to perform vital functions, such as the synthesis of proteins or
the contraction of muscles.

Storage Capabilities. The human muscle cell is capable of storing very little ATP in its cytoplasm.
The conversion of organic molecules (carbohydrates, lipids, proteins) in the cell to ATP from ADP
is so rapid that there is little evolutionary need for storage. The amount of ATP in the human
muscle cell could be depleted in one to two seconds of activity unless recharged to pre-existing
levels. ATP supplies must be kept at peak concentrations and must not fall below 60% of resting
levels for muscular activity to continue.

Energy Pathways of the Cell. The three systems of metabolic pathways are available to replace
ATP concentrations in the cell. All three pathways are continually active in the cell, and as a
demand for energy increases, all three pathways will contribute to the reformation of ATP
molecules in varying degrees. The degree of involvement of each is primarily determined by the
rate of energy demand. These systems are as follows.
o
The Anaerobic Alactate (Creatine Phosphagen) Energy System
o
The Anaerobic Lactate (Glycolytic) Energy System
o
The Aerobic Energy System
Movement Basics

The Nervous System. The nervous system initiates movement by stimulating skeletal muscle
tissue, beginning the process of muscle contraction. The nervous system is then responsible for
initiating contraction. The effectiveness of the nervous system dictates much of an athlete’s
speed and power capabilities and certain characteristics of the muscle.

Skeletal Muscles. Skeletal muscles produce movement by contracting. These muscles are
attached to bones through connective tissues called tendons. The muscular contractions then
move the bones, producing movement. The muscular system is then responsible for producing
locomotive forces.

The Skeletal System. The skeletal system is composed primarily of bones and connective tissues
called ligaments, which hold them together. In addition to supplying structure and framework
to the body, the skeletal system accepts the forces produced by the muscles and transmits them
appropriately. The skeletal system is then responsible for force transmission.
Neurophysiology

The Nervous System. The nervous system is one of the body’s two control systems. The
nervous system recruits and activates muscle tissue to meet force production needs. The
nervous system’s effectiveness at recruiting muscle tissue is critical, and the characteristics of
the nervous system dictate to some degree the characteristics of skeletal muscle tissue.

Nervous System Anatomy. The nervous system is composed of a central nervous system (CNS)
and a peripheral nervous system. The CNS consists of the brain and the spinal cord. The
peripheral nervous system consists of the branch nerves and the neuromuscular junctions.
These branch nerves can be afferent or efferent. Efferent nerves effect action, while afferent
nerves are sensory in nature.

The Neuron. A neuron is a nerve cell. Its purpose is to conduct a neural impulse to muscle
tissue, or to another neuron. The nerve cell is composed of a soma (cell body), and axon, and a
dendrite. The axon conducts the neural impulse toward the cell body, while the dendrite
conducts the impulse away from the cell body.

Motor Neurons. Motor neurons are neurons that, when activated, stimulate skeletal muscle
tissue and produce muscle contraction.

The Neural Impulse. The neural impulse is basically composed of electrical pulses. If a neuron
receives these pulses, and the pulses are of sufficient magnitude, the affected neuron is
stimulated to conduct the impulse. As the impulse travels quickly along the axon, a wave of
depolarization occurs causing voltage changes.

The All or None Principle. The all or none principle states that there are no varying degrees of
conduction, and neurons operate on an on or off basis. If the neural signal received is sufficient,
the neuron will conduct the received impulse. If a motor neuron is sufficiently stimulated, all
muscle fibers affected by that motor neuron will be stimulated into contraction. For these
reasons control of the force of muscular contraction is not possible at the cellular level.

Neurotransmitters. There are about 50 known neurotransmitters in humans. Acetylcholine and
norepinephrine are the two major neurotransmitters in regulating our physiological response to
exercise. Acetylcholine is the primary excitatory neurotransmitter for the neurons that innervate
skeletal muscle and for most parasympathetic neurons.

Control of Force. As indicated in our discussion of the all or none principle, the body cannot
control force production by altering the magnitude of signal from the nervous system. Force
must then be controlled in these other ways.
o
Recruitment. More motor units are called into play as force production needs rise. Each
unit has a unique threshold of response, so those with lower thresholds are recruited
first.
o
Rate Coding. Rate coding refers to the ability of the nervous system to increase the
frequency of the neural impulses. If these impulses occur sufficiently close together,
then the next impulse may begin before the previous subsides completely, allowing an
aggregate increase in the magnitude of the impulse. Tetanus results when these
individual impulses merge into one sustained impulse.
o
Temporal Patterning and Synchronization. Temporal Patterning and synchronization
refer to the ability of the body to call motor units into play in a particular sequence with
a particular timing. This unique pattern of timing of recruitment results in a greater total
force production.
Myophysiology

Muscle Anatomy
o
The Muscle Fiber. A single muscle cell is called a muscle fiber. These cells are unique
and resemble no other cell type. These fibers are individually capable of contraction.
Inside the muscle cell is a specialized cytoplasm called the sarcoplasm that fills most of
the volume of the cell.
o
Muscle Bundle Anatomy. Muscle fibers are packaged into bundles by several epithelial
coverings, linings, and connective tissues. These provide structure and hold the muscle
together. In order for oxygen, electrolytes, and nutrients carried in the blood vessels to
reach the sarcoplasm, all of these membranes have to be crossed. Delivery efficiency
can be improved through training.

Endomysium. The endomysium is the portion of epithelial tissue that covers
each muscle fiber or cell.

Sarcolemma. The muscle cell membrane or sarcolemma is just inside and
attached to the endomysium.

Epimysium. Large numbers of muscle fibers are held together by an epithelial
tissue called the perimysium, and the epimysium tissue encases the entire
muscle crating the muscle bundle.
Striated Skeletal Muscle Fiber Anatomy (Martini 2006)
o
Muscle Fiber Anatomy

Myofibrils. Striated skeletal muscle is composed of protein strands called
myofibrils. These myofibrils contain multiple units that are individually capable
of contraction called sarcomeres.

Sarcomeres. The sarcomere is the smallest anatomical unit of muscle tissue
that is individually capable of displaying contractile characteristics. The
sarcomere consists of noncontractile proteins (proteins not involved in the
contraction process) that provide structure to the muscle tissue, and contractile
proteins (proteins involved in the contraction process), as well as other parts
essential to the contraction process.

Thick Filaments. Thick filaments are composed of the protein myosin,
and are found in the middle of the sarcomere and lie between the thin
filaments. Each thick fiber possesses many crossbridges, which extend
toward the thin filaments. There is a head at the end of each
crossbridge, capable of carrying an ATP molecule.

Thin Filaments. The thin filaments are composed of the proteins actin,
troponin, and tropomyosin. They are attached to Z disks, which are
noncontractile protein ends of the sarcomere, and extend toward the
middle of the sarcomere.

The Sarcoplasmic Recticulum. The sarcoplasmic reticulum is a net-like
system of tubules and vesicles (T-system) surrounding each myofibril
that serve as the pathway for integrated neural impulses. The
longitudinal tubules run parallel to the myofibrils and terminate at the
end of vesicles, while the perpendicular transverse tubules interconnect
forming the net. The outer vesicles the reticulum contain large amounts
of calcium ions (Ca ++). When the neural impulse travels over the Tsystem and between the outer vesicles, calcium is released.

Striations. The overlapping of the thick and thin filaments give muscle
fibers a striated appearance. These striations correspond to zones
where only thin filaments are present, only thick filaments are present,
or both are present in overlapping arrangements.
Zones within a Human Skeletal Muscle Sarcomere (Martini 2006)
Actin and Myosin Filaments

The Sliding Filament Theory. The Sliding Filament Theory of muscular contraction was
proposed by Huxley and was based on his analysis of the photograph below. He proposed that
the overall length of the muscle decreases as one filament (actin) slides over the filament
(myosin).
Huxley’s Original Micrograph and Hypothesis
o
ATP. The role and importance of ATP as an energy carrier now becomes apparent in
understanding the process of muscle contraction. In order for contraction to occur in a
muscle, ATP must be present and readily replaced if the contraction - relaxation contraction chain of events is going to continue.
o
Stages of the Sliding Filament Theory

Rest. ATP molecules present in the muscle cell bind to the end of the myosin
cross-bridge. In the absence of free calcium, the troponin of the actin filaments
inhibits the myosin cross-bridge from binding with actin.

Excitation-Coupling. When an impulse from a motor nerve reaches the motor
end-plate, the neurotransmitter acetylcholine is released, causing
depolarization and triggering the release of calcium from the vesicles of the
sarcoplasmic reticulum. Calcium is immediately bound by the troponin
molecules on the actin filaments. This results in the turning on of active sites on
the actin filament triggering conformational changes of both troponin and
tropomyosin.

Contraction. For contraction to occur, the crossbridge head must be loaded
with an ATP molecule. The turning on of active sites exposes the myosin binding
site that contains the enzyme myosin ATPase. This enzyme causes ATP to be
reduced to ADP and a loose inorganic phosphate molecule. Since any chemical
reduction releases energy, some energy will become available for each ATP
reduced. This released energy allows the cross-bridge to swivel, creating
movement.

Recharging. The first stage of recharging is the breaking of the old bond
between the actin and myosin crossbridge. Once a new ATP is reloaded, the
bond between the myosin crossbridge and the active site on the actin filament
is broken and the ATP cross-bridge is freed from the actin.

Relaxation. When the flow of nervous impulses over the motor nerve
innervating the skeletal muscle ceases, calcium is released from troponin and
returned to storage in the outer vesicles of the sarcoplasmic reticulum.

Fiber Types
o
Contraction Speeds. The time required to accomplish these stages determined the
contraction speed (twitch) of a fiber. Some muscle fibers twitch faster than others, and
some people have a greater ratio of faster to slower twitching muscle fibers than other
people do. Enzyme availability is very specific to the different fibers, and ultimately is
the reason for most of a fibers speed characteristics.

Slow Twitch Fibers. Slower twitch fibers (ST) are called slow twitch, oxidative,
type 1, or red, depending upon the taxonomy scheme used. The fibers show
higher myoglobin content resulting in a more efficient usage of the cell’s limited
oxygen supply. This reflects a greater dependence on the aerobic system
(oxidative metabolism).

Fast Twitch Fibers. Faster twitch fibers are classified into hundreds of types. For
simplicity, physiologists classify these fast twitch varieties as FTA or FOG
(oxidative-glycolytic), FTB (glycolytic). Fast twitch fibers contain more protein
contractile filaments which can produce higher forces than slow twitch fibers.
This larger size is accompanied by the development of a more extensive
sarcoplasmic reticulum, which facilitates a much greater release of calcium and
is responsible for a more robust contraction.
o

Functional Differences in Fiber Types
Motor Units
o
Motor Unit Anatomy. A single motor neuron and the muscle fibers it acts upon is called
a motor unit. A motor neuron can innervate from 5 to 150 or more muscle fibers. A high
fiber to nerve ratio is associated with gross movements requiring considerable force,
whereas a low fiber to nerve ratio exists when very precise, low force demands, are
required of muscle.
Motor Units (Eisenberg and Lee 2004)
o
Motor Unit Function. Motor units may function in these two ways.

Volitional Function. In this type of operation, cognitive activity originates the
transmission of impulses to the motor neuron, activating it and producing
contraction in its pool of fibers. Movement is consciously originated.

The Reflex Arc. In this type of operation, a signal generated by some sensory
organ or proprioceptor is transmitted to the motor neuron, activating it and
producing contraction in its pool of fibers. The brain is left out of the loop, and
movement is not consciously originated. This type of movement is used in all
reflex actions.
Neuromuscular Adaptations to Training

Improved Recruitment. Neuromuscular training produces strength gains resulting from
heightened and improved recruitment of muscle fibers by the motorneurons.

Improved Rate Coding. Neuromuscular training produces strength gains resulting from,
improved rate coding capabilities.

Improved Synchronization. Neuromuscular training produces strength gains resulting from
improved synchronization of contraction.

Improved Speeds of Contraction. Although these gains are limited, neuromuscular training may
result in increased speeds of contraction due to shifts in myosin isoforms and enzymes.

Other Strength Gains. Strength gains also result in response to the tension placed on muscle
tissue during training. These increased tension levels are present in concentric activity, and are
even greater in eccentric work.

Improved Speed and Coordination. Neuromuscular training results in increased speed
characteristics, improved coordination, and associated increases in running economy and
efficiency.
Causes of Neuromuscular Fatigue


Fatigue at the Neuromuscular Junction. A limitation may occur at the neuromuscular junction,
preventing nerve impulse transmission to the muscle fiber membrane. A number of fatiguing
factors may be responsible. They include:
o
Acetycholine. A reduction in the release capability of acetylcholine may occur.
o
Cholinesterase. Hyperactivity in the enzyme cholinesterase which will inhibit
acetylcholine function. Hypoactivity in cholinesterase which will cause acetylcholine to
accumulate, paralyzing the fiber.
o
Threshold Changes. the muscle fiber membrane may develop a higher threshold
o
Receptor Inavailability. Other substances compete with acetylcholine for receptor
sites.
Acidity and Contractile Mechanism Fatigue. Inability of muscle filaments to contract properly
may result because of an acidic condition caused by accumulation of H+ ions. The hydrogen ions
concentration hinders the amount of calcium released from the sarcoplasmic reticulum and
interferes with the calcium troponin binding capacity. Hydrogen out competes calcium for
biding spaces.
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