Transcript Neuromuscular function (motor unit) in exercise

Neuromuscular function
(motor unit) in exercise
By Lect. Thawatchai Lukseng
M.Sc. (Physiology)
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Contents
• Basic structures and functions of the
neuromuscular system
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Motor unit and muscle fiber types
Characteristics of muscle fiber types
Motor unit recruitment
Neurotransmitters
Muscle spindle
Golgi tendon organ
Neuromuscular junction
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Contents (cont.)
• Muscle
– Process of muscle contraction (See
video)
– Types of muscle contraction (isotonic,
isometric, isokinetic)
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Basic structures and
functions of the
neuromuscular system
Motor units and muscle fiber types
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Muscle fiber types
• Historically divided into "fast-twitch" and
"slow-twitch"
• Sometimes called "white" and "red" or "light"
and "dark"
• Actually 3 fiber types with "new" names:
– Type I or "slow oxidative (SO)" or "slow-twitch"
– Type IIa or "fast oxidative glycolytic (FOG)" or
"intermediate"
– Type IIb or "fast glycolytic (FG) or "fast-twitch"
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• Most muscles of body are about 50-50
Type I and Type II
• although some muscle groups in certain
individuals are heavily one type or the
other
• Genetics and training affect proportion
of fiber types
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Characteristics of fiber types
• Type I (SO):
– Lots of mitochondria
– Lots of myoglobin (thus their darker, redder color)
– Lots of oxidative enzymes
– Large capacity for aerobic metabolism, good
endurance, slow to fatigue
• Type IIa (FOG):
– Intermediate fiber type
• In between SO and FG, but not particularly
good at either endurance or speed
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Characteristics of fiber types
• Good news: highly adaptable and may
become Type I (SO) with endurance
training (but can't become Type IIb)
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Characteristics of fiber types
(cont.)
• Type IIb (FG):
– Rich in glycolytic enzymes
– Large anaerobic capacity
– Fast speed of contraction
– Few mitochondria
– Little myoglobin
– Little endurance capacity
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Motor unit recruitment
• SO motor units are recruited first during
exercise.
• As exercise intensity increases, there is
a progressive and additive increase in
FOG and FG motor unit recruitment.
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• Electromyography
– Electromyography is the study of
muscle function from the detection of
the electrical activity emanating from
the depolarization of nerves and
muscle membranes that accompany
contraction.
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Neurotransmitters
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Muscle spindle
ขนาน
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Muscle spindle
• Send CNS info regarding length
• Static – how long the muscle is
• Dynamic – changes in length
• Fine motor control muscles have lots
• Gross motor control muscles have fewer
• Look like short muscle fiber wrapped in
nerves
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Golgi tendon organ (GTO)
• Monitor tension of muscle
• Have high threshold
• Important in protecting muscle
• Threshold can be altered through
training
• Important in strength activities
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อนุกรม
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Neuromuscular junction (NMJ)
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Muscle
Process of muscle contraction
• ดู Video
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Types of muscle contraction
Concentric
• Concentric
• Eccentric
• Isometric
• Isokinetic
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Flexion
Extension
Eccentric
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Neuromuscular responses
to exercise and DOMS
Lect. Thawatchai Lukseng
M.Sc. (Physiology)
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Contents
• Terminology
– Muscular strength
– Muscular power
– Muscular endurance
• Possible neural factors of strength gains
• Muscular size
– Atrophy
– Hypertrophy
– Hyperplasia
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Contents (cont.)
• Neural activation and fiber hypertrophy
• Results of resistance training
• Effects of muscular inactivity
• Muscle soreness
– Acute muscle soreness
– Delayed-onset muscle soreness and injury
– Reducing the negative effects of DOMS
• Resistance training programs
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Terminology
• Strength is the maximal force a
muscle or muscle group can generate.
• Power is the product of strength and
the speed of movement.
• Muscular endurance is the capacity
to sustain repeated muscle actions.
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Did You Know . . . ?
• Resistance training
programs can produce a
25% to 100% improvement
in strength within 3 to 6
months.
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Possible Neural Factors of
Strength Gains
• Recruitment of additional motor units for
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greater force production
Counteraction of autogenic inhibition,
allowing greater force production
Reduction of coactivation of agonist and
antagonist muscles
Changes in the discharge rates of motor units
Changes in the neuromuscular junction
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VARIABLE-RESISTANCE
TRAINING
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Muscle Size
• Hypertrophy refers to increases in
muscle size.
• Atrophy refers to decreases in muscle
size.
• Muscular strength involves more than
just muscle size.
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Muscle Hypertrophy
• Transient hypertrophy is the pumping up
of muscle during a single exercise bout due to
fluid accumulation from the blood plasma into
the interstitial spaces of the muscle.
• Chronic hypertrophy is the increase of
muscle size after long-term resistance
training due to changes in number of muscle
fibers (fiber hyperplasia) or size of muscle
fibers (fiber hypertrophy).
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Microscopic Views of Muscle Cross
Sections Before and After Training
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Fiber Hypertrophy
• The numbers of myofibrils and actin and
myosin filaments increase, resulting in more
cross-bridges.
• Muscle protein synthesis increases during the
postexercise period.
• Testosterone plays a role in promoting muscle
growth.
• Training at higher intensities appears to cause
greater fiber hypertrophy than training at
lower intensities.
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Fiber Hyperplasia
• Muscle fibers split in half with intense weight
training.
• Each half then increases to the size of the
parent fiber.
• Satellite cells may also be involved in the
generation of skeletal muscle fiber.
• It has been clearly shown to occur in animal
models; only a few studies show this occurs
in humans too.
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Heavy Resistance Training in
Cats
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Neural Activation and Fiber
Hypertrophy
• Early gains in strength appear to be
more influenced by neural factors.
• Long-term strength increases are
largely the result of muscle fiber
hypertrophy.
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Results of Resistance
Training
• It increases muscle size (hypertrophy).
• It alters neural control of trained
muscles.
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Effects of Muscular Inactivity
• Muscular atrophy (decrease in muscle
size)
• Decrease in muscle protein synthesis
• Rapid strength loss
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Acute Muscle Soreness
• Results from an accumulation of the
end products of exercise in the
muscles
– Accumulation of H+
– Lactate
– Tissue edema
• Usually disappears within minutes or
hours after exercise
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Delayed-Onset Muscle
Soreness (DOMS)
• Results primarily from eccentric action
• Is associated with damage or injury
within muscle
• May be caused by inflammatory reaction
inside damaged muscles
• May be due to edema (accumulation of
fluid) inside muscle compartment
• Is felt 12 to 48 hours after a strenuous
bout of exercise
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Armstrong’s Sequence of
Events in DOMS
1. Structural damage
2. Impaired calcium availability
3. Accumulation of irritants
4. Increased microphage activity
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DOMS and Performance
• DOMS causes a reduction in the
force-generating capacity of
muscles.
• Maximal force-generating capacity
returns after days or weeks.
• Muscle glycogen synthesis is
impaired with DOMS.
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MUSCLE FIBERS AFTER A MARATHON
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MUSCLE BEFORE AND AFTER
A MARATHON
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Key Points: Muscle Soreness
• Acute muscle soreness occurs
late during or immediately after
an exercise bout.
• Delayed-onset muscle soreness
(DOMS) occurs 12 to 48 hours
after exercise (especially
eccentric exercise).
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• DOMS may include structural
damage to muscle cells or
inflammatory reactions within the
muscles.
• Muscle soreness may be an
important part of maximizing the
resistance training response.
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Reducing Muscle Soreness
• Reduce eccentric component of muscle
action during early training.
• Start training at a low intensity,
increasing gradually.
• Begin with a high-intensity, exhaustive
bout of eccentric-action exercise to
cause much soreness initially but
decrease future pain.
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Did You Know . . . ?
Resistance training can
benefit almost everyone,
regardless of sex, age, level of
athletic involvement, or sport.
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Resistance Training Programs
• Resistance training can use static or
dynamic actions.
• A needs analysis is necessary for
designing a program for a specific
athlete’s needs.
• Low-repetition, high-intensity training
improves muscle strength while highrepetition, low-intensity training
improves endurance training.
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• Periodization prevents
overtraining by varying the
volume and intensity of training.
• Typically volume is gradually
decreased while intensity is
gradually increased.
• Strength gains are specific to the
speed of training and the
movement patterns used in
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training.
Lab DOMS
• Biceps brachii muscle  80% of 1 RM
– Eccentric contraction 20 times, 4 rounds
– Pause duration = 5 mins
• Quadriceps muscle (1)  80% of 1 RM
– Eccentric contraction 20 times, 4 rounds
– Pause duration = 5 mins
• Quadriceps muscle (2)  steps at 96
bpm, 5 mins
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