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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: nonathletes2: 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 SFRFF 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

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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