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ACE’s Essentials of Exercise
Science
for Fitness Professionals
NEUROMUSCULAR ANATOMY &
PHYSIOLOGY (Ch 1, 2 & 5)
Overview of Neuromuscular Concepts
– Structure & function of nerve, muscle, motor
unit and associated connective tissue.
– Acute responses to exercise
– Chronic adaptations to exercise
– Flexibility training
Nervous System
 The nervous system connects the muscles to the brain and spinal
cord through a network of nerve circuits.
 Structurally, it is divided into the central nervous system (CNS) and
peripheral nervous system (PNS).
– The CNS consists of the brain and spinal cord, while the PNS consists of all the
nerve structures outside the brain and spinal cord.
 Nerves are made up of multiple nerve cells
called neurons.
 Sensory nerves carry impulses to the
CNS, while motor nerves carry impulses
from the CNS to the PNS.
Muscular System
 Three types of muscle:
– Skeletal
• Attaches to the skeleton via tendons, contracts to move bones
• Voluntary
• Striated appearance
– Smooth
• Found on the walls of hollow organs and tubes (e.g., stomach and blood
vessels)
• Involuntary
• Smooth appearance
– Cardiac
• Forms the walls of the heart
• Involuntary
• Smooth appearance
Skeletal Muscle Fiber Types
 Skeletal fibers can be divided into two general categories based on
how quickly they contract.
– Slow-twitch muscle fibers contain relatively large amounts of mitochondria and
are surrounded by more capillaries than fast-twitch fibers.
• Slow-twitch fibers contract more slowly than fast-twitch fibers.
• They have lower force outputs, but are more efficient and fatigue-resistant than fasttwitch fibers.
– Fast-twitch muscle fibers are further subdivided into fast-glycolytic and fastoxidative glycolytic fibers.
• Type IIx muscle fibers contain a relatively small amount of mitochondria, have a limited
capacity for aerobic metabolism, and fatigue more easily than slow-twitch fibers.
• Type IIx have considerable anaerobic capacity, and are the largest and fastest, and are
capable of producing the most force, of all the skeletal muscle fibers.
• Type IIa muscle fibers possess speed, fatigue, and force-production capabilities
somewhere between type I and type IIx fibers.
• Type IIa fibers are also called intermediate fibers.
Comparison of Muscle Fiber Types
 The following table compares the three types of muscle fiber
using the relative terms low, medium, and high.
Type I
Type IIa
Type IIx
Speed of contraction
Low
Medium
High
Force capacity
Low
Medium
High
Fatigue resistance
High
Medium
Low
Mitochondrial content
High
Medium
Low
Size
Low
Medium
High
Efficiency
High
Medium
Low
Aerobic capacity
High
Medium
Low
Anaerobic capacity
Low
Medium
High
Muscle-fiber Microanatomy
 Skeletal muscles are made up
of many muscle fibers
(muscle cell) held in place by
connective tissue (fascia).
 Muscle fibers are made up of
myofibrils (protein filaments)
composed of a series of
repeating segments called
sarcomeres .
 Sarcomeres, made up of
thick (myosin) and thin
(actin) myofilaments, are the
functional contracting unit of
skeletal muscle.
Muscle Contraction
 Sliding filament model
– When acetylcholine is
released from the CNS
and detected, calcium
is released.
– Calcium exposes
binding sites along the
actin for the myosin to
attach.
– If sufficient ATP is
present, cross-bridges
are formed and the
myosin pulls the actin
toward the center,
thereby shortening the
sarcomere.
Neuromuscular Physiology
 Nerves are made up of neurons (nerve cells), of which there are two
types:
– Sensory neurons
– Motor neurons
 Motor neurons connect (synapse) with the muscle at a
neuromuscular junction (motor end plate).
 A motor unit is made up of one motor neuron
and all of the muscle cells it innervates.
 The number of muscle cells a motor neuron
innervates depends on the precision and
accuracy required of that muscle.
Neuromuscular anatomy
 Motor unit
– A motor nerve and all its associated muscle fibers
– All fibers comprising a motor unit are homogeneous (they are
either all fast-twitch or all slow-twitch).
– Motor units made up of 5–10 fibers are responsible for fine,
delicate movements such as blinking the eye.
– Motor units made up of thousands of fibers are responsible for
forceful movements such as jumping.
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Muscle-fiber Types: Fast Twitch
 Fast-twitch (FT) muscle fibers
– Utilize primarily the phosphagen and anaerobic glycolysis energy
systems
– Specialized for anaerobic metabolism
– FT motor units innervate more muscle fibers, allowing greater force
production.
Muscle-fiber Types: Slow Twitch
 Slow-twitch (ST) muscle fibers
– Well equipped for oxygen delivery
– High number of oxidative enzymes
– High number of mitochondria; aerobic glycolysis and fatty-acid oxidation
– Used for low-intensity, longer-duration activities (e.g., walking, jogging,
and swimming)
– Usually more abundant in fatigue-resistant muscles (e.g., postural
muscles)
Muscle-fiber Distribution
 Muscle-fiber distribution is largely determined by genetics.
– Most people have about equal percentages of FT and ST
fibers.
– Persons better at low-intensity endurance activities may
have a larger percentage of ST fibers.
– Persons better at high-intensity, sudden bursts of activity
probably have a larger percentage of FT fibers.
– “Intermediate” fiber types have a high capacity for both fast
anaerobic and slow aerobic movements, and are adaptable
based upon the training stimulus.
Muscle-fiber Response to Training
 All three muscle-fiber types are highly trainable.
– Adapt to the specific demand placed on them
– Muscle-fiber types are recruited sequentially in response to force
generation: ST then FT
– FT muscle fibers are more closely related to the hypertrophy
(increase in size) of fibers in response to a strength program.
– Muscular endurance training is specific to both ST and FT fibers
and motor units.
Muscle Contractility

Muscle contractility depends on maximal force production, speed of
contraction, and muscle fiber efficiency.

Fast-twitch muscle fibers
– Contain more myosin cross-bridges per cross-sectional area of fiber and
produce 10–20% more force than slow-twitch muscle fibers
– Have a higher concentration of myosin ATPase, allowing them to
contract at a higher speed

Slow-twitch muscle fibers
– Are more efficient at using oxygen
to generate ATP to fuel continuous
muscle contractions due to their
higher concentrations of myoglobin,
larger number of capillaries, and
higher mitochondrial enzyme
activity.
Muscle Fatigue
 Muscle fatigue is associated with an acute bout of prolonged
exercise in which muscular performance declines and sensations of
muscle pain occur.
– When muscle glycogen is depleted, an increase
in the use of fat for energy occurs.
– Fat mobilization and oxidation are much
slower, resulting in a reduction of power
output of the muscle.
– Drinking a glucose and water solution near the
point of fatigue may help for a short time, but
glycogen will remain depleted.
– High-carbohydrate diets (>60% of calories
from carbohydrates) and carbohydrate loading
can extend performance before “hitting the wall.”
Connective Tissue anatomy
 Connective tissue—serves to connect,
support, and anchor various body
parts
– Fascia—a sheet or band of fibrous tissue that
lies deep to the skin or forms an attachment for
muscles and organs
– Tendon—when a muscle contracts to produce
movement, it pulls on a tendon, which attaches
the muscle to the bone
– Ligaments—supportive structures found at
joints that connect bones to other bones
– Cartilage—serves as padding between the
bones at a joint and functions to provide
cushioning and the smooth gliding of joint
movement
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Muscle contraction
 Muscle contraction
– A contraction occurs when an
electronic impulse is transmitted
from the brain to the muscle.
– Muscle contraction is the result of
the interaction of the actin and
myosin filaments, which causes a
shortening of the individual
sarcomeres, and therefore, a
shortening of their associated
muscle fibers.
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Reflexes: Stretch Reflex
 Muscle spindles
– Sensory receptors
that lie parallel to the
muscle fibers
– Respond to muscle
fibers being overstretched by causing
the muscle to
contract
– Component of the
stretch reflex
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Reflexes: GTO
 Golgi tendon
organs
– Sensory receptors
located in the muscletendon junction
– Respond to increased
muscle tension by
causing the muscle to
relax
– Component of
inhibition
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Muscle Tension: All or none principle
 All-or-none principle
– When a single muscle fiber contracts (or shortens), it
generates its maximum force capability.
– When a motor unit is stimulated, all the muscle fibers
it innervates contract with maximum force.
– The amount of force generated during a muscle
group’s contraction depends on the following:
• The size of the individual muscle fiber (the larger the fiber, the
greater the force during contraction)
• The number of muscle fibers recruited (more fibers equal more
force)
• The length of the muscle fiber prior to contraction (a muscle
generates maximum force when it begins its contraction at 1.2
times its resting length)
• The speed of contraction (the slower the movement, the more
force that is produced)
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Muscle Tension: Optimal force production

Length-tension relationship
–
The amount of force that a muscle can exert
is related to its length.
–
Peak force production is usually seen at
resting length or slightly greater (1.2 times
resting length).
–
At approximate resting length, more of the
myosin cross-bridge heads can align with
active actin receptor sites.
–
Therefore, participants with poor posture that
have chronically shortened or lengthened
muscle groups are not able to produce
optimal force at the misaligned joints.
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Muscle Tension: Optimal force production
 Force-velocity relationship
– A maximal contraction is dependent on
the number of actin and myosin crossbridges formed.
– The higher the speed of contraction, the
lower the number of connected myosin
and actin cross-bridges.
– An optimal speed of contraction while
lifting weights appears to be a 1- to 2second concentric action, followed by a
2- to 4-second eccentric action.
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Muscular training
 Flexibility = Range of Motion (ROM)
– Stretching should be performed after warm-up when muscles are
more warmer and more elastic.
– Stretching should be done to the point of mild discomfort only, to
avoid DOMS
 Types of Stretching
– Static—holding 15-30 sec.
– Ballistic—characterized by bobbing or bouncing (to be discouraged)
– Dynamic—often seen as the same as ballistic, but involves moving
through a range of motion.
– PNF (Proprioceptive Neuromuscular Facilitation)--characterized by
contract-relax. Generally requires a partner.
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Chronic Adaptations: Resistance training
 Neural adaptations
– Improved motor unit recruitment patterns
– Improved motor learning
– Neural adaptations are responsible for gains in
strength with little or no change in muscle crosssectional area after as few as 6 weeks of training.
 Hypertrophy of fast-twitch fibers
 Increased size and number of actin and myosin
filaments
 Increased lean body mass
 Increased connective tissue strength
 Decreased risk of joint injury
 Increased bone mineral density
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Chronic Adaptations: Neuromuscular
 More significant as a result
of resistance training than
aerobic training
– Occur in the early part of a strengthtraining program
(1–3 weeks) before muscle
hypertrophy occurs
 Motor-unit recruitment and
synchronization
– All-or-none principle: When
activated, all muscle fibers in a
motor unit contract maximally.
Chronic Adaptations: Neuromuscular Changes
 Rate coding
– A motor unit produces varying levels of force depending on the
frequency at which it is stimulated.
• Twitch
• Summation
• Tetanus
– May increase with resistance training
 Diminished co-contraction
– In opposing muscles, when
maximizing force generated
by the agonist, the
activation of the antagonist
must be diminished.
Chronic Adapations: Muscle Growth

After a prolonged period of resistance training, chronic hypertrophy is
responsible for strength gains.

Fiber hypertrophy
– Results from one or more of the following: increased number of myofibrils,
increased number of actin and myosin filaments, more sarcoplasm, and more
connective tissue
– Increased protein synthesis
– Eccentric actions combined with high-velocity training promote greater increases.

Fiber hyperplasia
– Stress to the muscle stimulates the migration of satellite cells to the damaged
region to fuel existing muscle fibers and/or produce new ones.
– In humans, most evidence points to muscle-fiber hypertrophy as the primary
cause of increased muscle size associated with resistance training.
Enhancing Muscle Growth Through Exercise
 A resistance-training program that stimulates protein
synthesis (and muscle growth) increases levels of
testosterone and growth hormone.
– Growth hormone increases the availability of amino acids for
protein synthesis and stimulates the release of IGF-1, which
works with GH to stimulate muscle growth.
– Testosterone promotes the release of GH and interacts with
neuromuscular system to stimulate muscle growth.
– These responses are brought about by performing large-musclegroup, multijoint exercises at high intensities with short rest
intervals (30 to 60 seconds).
DOMS
 Delayed onset muscle soreness
– Peaks 24-48 hours after exercising for the first time in awhile or
doing something new.
– Exacerbated by eccentric contractions and overstretching.
– It does go away in a few days.
– Attempt to reduce DOMS by starting at a low intensity and
progressing slowly through the first few weeks while minimizing
eccentric actions.
Chapter 11 - Injury Prevention &
Emergency Procedures
30
Flexibility: Tissue Properties
 Tissue elasticity
– Mechanical property that allows a tissue to return to its original
shape or size when an applied force is removed (“temporary
deformation”)
– A tissue reaches its “elastic limit” when it is stretched beyond the
point where it cannot return to its normal length when tensile
(stretching) force is removed.
– The difference between the original resting length and new
resting length is called “permanent deformation.”
– The new state of permanent elongation is called “plastic stretch.”
– Static stretching elongates the tissue to a point where
deformation remains after the tension is removed.
Flexibility Training (cont.)
 Tissue plasticity
– Allows the tissue to deform when it is loaded past its elastic limit
– Once a tissue is set past its yield point, tissue failure resulting in
additional deformation may occur with small increases in force.
 Tissue viscoelasticity
– Viscosity allows tissues to resist loads and is dependent on time
and temperature.
– With exposure to low loads, most tissues exhibit elastic behavior;
exposure to higher loads causes tissue to exhibit a plastic
response
Neurological Properties of Stretching
 Autogenic inhibition: the activation of a Golgi tendon organ (GTO)
inhibits muscle spindle response
– Initially, a low-force, long-duration (static) stretch stimulates low-grade
muscle spindle activity and temporary increases in muscle tension.
– Muscle spindles become desensitized as the stretch continues (referred
to as stress-relaxation).
– After 7 to 10 seconds, the increase in muscle tension activates the GTO
response, inhibiting muscle spindle activity and allowing further muscle
stretching.
– Holding the stretch beyond 10 seconds stresses the collagen fibers,
causing plastic deformation and lengthening the tissue (creep).
– When the stretch ends, muscle spindles reestablish their threshold.
– Repeating the stretch a finite number of times produces a gradual
increase in muscle extensibility.
Neurological Properties of Stretching (cont.)
 Reciprocal inhibition
– Activating the muscle on one side of a joint (i.e., the agonist)
coincides with neural inhibition of the opposing muscle on the
other side of the joint (i.e., the antagonist) to facilitate movement.
– Example
• While performing a supine hamstring stretch, contraction of the hip
flexor muscles on the leg being stretched will produce more active
hip flexion, resulting in reciprocal inhibition of the hamstring muscle
group, allowing them to be stretched further.
Static Stretching and Permanent Tissue Elongation
 Static stretching
– Low-force, long-duration stretching at elevated tissue
temperatures is more likely to result in plastic lengthening
(compared to high-force, short duration).
– Dynamic stretching can be used to warm up the muscles, and be
followed by static stretching.
– Static stretching is most effective when performed during the
cool-down at the end of a training session.
– Holding the stretch for 15 to 30 seconds appears to be the most
effective means of increasing range of motion.
– Each stretch should be repeated up to three to four times.
Static Stretching
 Static stretching involves moving the joints to place
the targeted muscle group in an end-range position
and holding that position for up to 30 seconds.
–
Does not elicit the stretch reflex, reducing the risk of
injury with proper technique
–
Active
–
Passive
Active Isolated Stretching (AIS)
 AIS follows a design similar to a traditional strengthtraining program.
– Stretches, which never last longer than 2 seconds, are
performed in sets, with each movement exceeding the
resistance point of the prior stretch by a few degrees.
– Each set isolates an individual muscle.
Proprioceptive Neuromuscular Facilitation (PNF)
 PNF incorporates the principles of autogenic inhibition and
reciprocal inhibition.
 There are three types of PNF, all of which begin with a passive
(partner) 10-second pre-stretch.
– Hold-relax
– Contract-relax
– Hold-relax with agonist contraction
Dynamic and Ballistic Stretching
 Dynamic stretching
– Mimics the movement pattern to be used in an upcoming
workout.
– Dynamic stretching is an effective component of a warm-up.
 Ballistic stretching
– Incorporates bouncing-type movements
– Typically triggers the stretch-reflex, which increases the risk of
injury
– Has a role in conditioning and training if done correctly
Myofascial Release


Myofascial release applies pressure to tight, restricted areas of fascia and
underlying muscle in an attempt to relieve tension and improve flexibility
–
It is thought that sustained pressure to a tight area can inhibit tension in a
muscle by stimulating the GTO to bring about autogenic inhibition.
–
Trigger points can be diminished through the application of pressure followed by
static stretching of the tight area.
In the fitness setting, a foam roller is used, allowing the exerciser to control
his or her own intensity and duration of pressure.
Summary
– Structure & function of nerve, muscle, motor
unit and associated connective tissue.
– Acute responses to exercise
– Chronic adaptations to exercise
– Flexibility training