Neuromuscular function (motor unit) in exercise

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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
• 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
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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
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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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Neuromuscular junction (NMJ)
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Muscle
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Process of muscle contraction

• ?? Video

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Types of muscle contraction
Concentric

• Concentric
• Eccentric
• Isometric
• Isokinetic

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 ofStrength Gains

• Recruitment of additional motor units for 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 MuscleSoreness (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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Armstrongs Sequence ofEvents 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 athletes needs.
• Low-repetition, high-intensity training improves
  muscle strength while high-repetition,
  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
  training.

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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