Chapter 1. muscle fibers, motor
units, and motoneurons
PF. Gardiner, Advanced
neuromuscular exercise physiology
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需要複習的基本知識
• Basic structure of skeletal muscles and muscle
fibers
• Sliding filament theory 肌絲滑動學說
• Excitation-contraction, neuromuscular junction
神經肌肉接合
• Motor units 動作單位: A motoneuron and the
muscle fibers that innervate
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Importance of muscle heterogeneity
• Contractile performance of whole muscle is
correlated with muscle fiber type composition
– Gardiner Table 1.1
Subjects:
1: nonathletes
2: athletes
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Orderly motor unit recruitment
• Controlled by motoneurons, from brain
• Henneman’s size principle
– According to size of motoneurons
– As the force increased, the larger motoneurons are
recruited
– Based on muscle unti size, motoneuron size
• Intrinsic excitability of a motoneuron determines
the probability of its being recruited during
excitation of a motor pool
– Smaller motoneurons more excitable
– Excitability: S > FR > FF (I > IIa > IIb, IIx), motor
units recruited in SFRFF as force demand and
voluntary effort increase
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Estimate and actual motoneuron size
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Late adaptation of motoneuron
• EMG shows a gradual increase in motor unit excitation
under constant-load, maintained contraction
– Compensate for late adaptation and to increase recruitment
of motor units to maintain the force
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Chapter 2. motor unit recruitment
during different types of
movements
PF. Gardiner, Advanced
neuromuscular exercise physiology
7
Measure motor unit recruitment
• Fine-wire electrodes inserting into muscle
– Record muscle fiber activities in immediate
vicinity
• Estimate motor unit recruitment patter by
spectral properties
• Biochemical index by muscle biopsy
– change in PCr/Cr ratio
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Motor units are recruited from weakest
to strongest, slowest to fastest
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Motor units are recruited from weakest
to strongest, slowest to fastest
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Motor units are recruited from weakest
to strongest, slowest to fastest
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Motor units are recruited from weakest
to strongest, slowest to fastest
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Motor unit rotation
• A recruited unit stops firing while another unit
starting firing, during sustained contraction
– Could happen in many muscles
• Theory: motor units drop out due to ↑
thresholds for motoneuron firing resulting from
inactivation of Na and Ca channels
• Motoneuron ion channels may influence
performance during long-duration exercise
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Motor unit rotation during sustained
contraction
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Isometric contractions vs movements
• Motor units are recruited differently during
contraction with movement vs isometric
contraction
– Even at the same relative force
– Functional magnetic resonance imaging (fMRI)
signal from sensorimotor cortex is different
– fMRI measure changes in cerebral metabolism
relative to oxygen supply
• Force threshold for motor units: slow isotonic
contraction < isometric contraction
– Controlled by brain, not peripheral nerve
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Isometric contractions vs movements
• The behavior of the same motor units are
different in isometric vs isotonic contractions
• Different motor units show different changes in
firing rates in isometric vs isotonic contractions
• No change in order of recruitment of motor
units
– More motor units activated during slow isotonic
contraction (vs isometric)
– Each generated less force because of lower firing
frequency
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Different motor units responses
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Lengthening contractions
• Motor unit recruitment different in shortening
vs lengthening contractions
• Motor units could be classified as S (active
during shortening), L (active during
lengthening), or S + L, in humans
– L unit: 15% units in soleus, 50% gastrocnemius
– L unit often were not recruited at all during
shortening movements, only recruited in relatively
high forces or very rapid contractions
• NOT all motor units behave the same in
shortening vs lengthening
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Shortening vs lengthening contractions
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Lengthening contraction summary
•
•
•
•
lengthening contractions vs shortening
Recruitment of motor units is different
motor unit pool is less activated
Cerebral cortex uses a different strategy of
motor unit recruitment
• order of recruitment of motor unit types is
generally preserved
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Unilateral vs bilateral contractions
• slight ↓ in the maximal voluntary force of a
muscle or muscle group that occurs when the
contralateral muscle or muscle group contracts at
the same time
– Knee extension, hand grip, wrist/elbow flexion…
– Isometric and dynamic movements
– NOT in all subjects, especially pianoists
• NOT occur with other than homologous muscles
on opposite side of the body
– NOT due to amount of muscle tissues that brain can
activate
• Modulated by Interlimb signals
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Rhythmic complex contractions
• Running, cycling, from glycogen depletion studies
• Muscle glycogen in prolonged cycling at 75%
VO2max
– 0-20 min: Type I, IIa: ↓↓glycogen; IIab, IIb: no loss
– Afterwards: : I, IIa ↓↓↓glycogen; IIab, IIb: ↓glycogen
– Exhaustion: I, IIa depleted; ; IIab, IIb: ↓↓glycogen but
not depleted
• Highest-threshold units require the most effort to be
recruited and can NOT be recruited continuously due
to late adaptation
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Rhythmic complex contractions
• ALL muscle fibers are recruited at
supramaximal intensity (cycling at 200%
VO2max)
• Type I fibers CAN use significant amounts of
glycogen anaerobically for high-intensity tasks
• NO significant derecruitment of Type I during
high-intensity, anaerobic tasks
– Still follow size principle
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