Muscle Fiber Types and Motor Units

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Questions and Answers

What is the primary factor that differentiates a Type I motor unit from a Type IIa motor unit?

  • The amount of myoglobin
  • The type of myosin ATPase (correct)
  • The number of capillaries
  • The speed of calcium handling

Which of the following muscle fiber types is primarily responsible for sustained, endurance activities like long-distance running?

  • Type I (correct)
  • Type IIx
  • Type IIa
  • All of the above

Which of the following statements accurately describes the difference between Type IIx and Type IIa muscle fibers?

  • Type IIx fibers are fatigue-resistant while Type IIa fibers fatigue quickly.
  • Type IIa fibers are larger and produce more force than Type IIx fibers.
  • Type IIx fibers have a higher oxidative capacity than Type IIa fibers.
  • Type IIa fibers are better suited for short bursts of intense activity compared to Type IIx fibers. (correct)

Which of the following is NOT a characteristic of motor units?

<p>They can contain multiple types of muscle fibers. (C)</p> Signup and view all the answers

What is the MOST likely explanation for the high proportion of Type I muscle fibers in the soleus muscle?

<p>The soleus muscle is responsible for maintaining posture and balance, requiring sustained contractions. (D)</p> Signup and view all the answers

What is the relationship between the speed of calcium handling and the speed of contraction in skeletal muscle fibers?

<p>Faster calcium handling results in faster contraction speeds. (B)</p> Signup and view all the answers

Which of the following statements is TRUE about muscle fiber variability?

<p>The proportion of muscle fiber types varies among individuals and among different muscles within the same individual. (B)</p> Signup and view all the answers

Which of the following is NOT considered a biochemical property of muscle fibers?

<p>Speed of contraction (A)</p> Signup and view all the answers

What is the primary energy system utilized by slow-oxidative fibers for ATP resynthesis?

<p>Aerobic system (D)</p> Signup and view all the answers

Which feature is NOT associated with slow-oxidative fibers?

<p>High myosin ATPase activity (D)</p> Signup and view all the answers

Which characteristic is exclusive to Type II fibers?

<p>High rate of crossbridge turnover (B)</p> Signup and view all the answers

What distinguishes Type IIx fibers from other muscle fiber types?

<p>Most rapid shortening velocity (D)</p> Signup and view all the answers

Which of the following describes the fatiguability of Type II fibers?

<p>Very fatigable (C)</p> Signup and view all the answers

Speed of shortening in Type II fibers is compared to which other type of fibers?

<p>Slow-twitch fibers (C)</p> Signup and view all the answers

In which type of activity do Type II fibers predominantly predominate?

<p>Anaerobic-type sprint activities (C)</p> Signup and view all the answers

What color are Type II fibers generally associated with?

<p>White (B)</p> Signup and view all the answers

What primarily determines which muscle fibers are innervated by specific motor neurons?

<p>Genetic factors (B)</p> Signup and view all the answers

What is the result of endurance versus strength training on muscle fiber type?

<p>Small changes (approximately 10%) can occur (C)</p> Signup and view all the answers

Which motor units are recruited first when a muscle contracts?

<p>Low-threshold motor units (A)</p> Signup and view all the answers

Selective recruitment refers to which scenario during muscle contractions?

<p>Selecting high-threshold motor units to achieve force and power quickly (A)</p> Signup and view all the answers

What effect does aging have on muscle fiber types?

<p>Decrease in type II motor units (B)</p> Signup and view all the answers

Which of the following factors does NOT predict success in athletic performance?

<p>Height of the athlete (C)</p> Signup and view all the answers

What describes the Size Principle in weightlifting?

<p>Low-threshold units are recruited before high-threshold units (D)</p> Signup and view all the answers

What is a characteristic feature of fast-twitch muscle fibers?

<p>Higher glycogen storage (D)</p> Signup and view all the answers

Flashcards

Motor Unit

A motor unit consists of one motor neuron and the muscle fibers it controls, all being of the same type.

Types of Muscle Fibers

Muscle fibers are classified into Type I (slow twitch), Type IIa (fast, moderate force), and Type IIx (fast, high force).

Twitch Characteristics

The time it takes for a muscle fiber to contract and relax after stimulation.

Fatigability

The ability of muscle fibers to sustain contraction over time before exhaustion.

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Oxidative Capacity

The ability of muscle fibers to utilize oxygen for energy production.

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Glycolytic Capacity

The amount of energy a muscle fiber can produce through anaerobic metabolism.

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Myosin ATPase

An enzyme that influences the speed of muscle contraction by hydrolyzing ATP.

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Proportions of Muscle Fiber Types

Different muscles within the body have varying proportions of slow and fast twitch fibers, averaging 50:50.

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Aerobic System

Energy production system relying on oxygen for ATP resynthesis.

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Slow-Oxidative Fibers

Muscle fibers that contract slowly and rely on oxidative metabolism.

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Characteristics of Slow Fibers

Features include low myosin ATPase, large mitochondria, and high capillary density.

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Fast-Twitch Fibers

Muscle fibers that contract quickly and fatigue easily.

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IIx Fibers

Fast-twitch fibers with the highest anaerobic potential and fastest shortening velocities.

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IIa Fibers

Fast-oxidative-glycolytic fibers that can use both aerobic and anaerobic metabolism.

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Energy Transfer in Fast Fibers

Fast-twitch fibers rely on a short-term glycolytic system for energy.

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Capillary Density in Muscles

Refers to the number of capillaries surrounding muscle fibers, indicating oxygen supply.

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Determinants of Fiber Type

Factors that influence muscle fiber type include genetic factors and training.

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Genetic Factors

Innate characteristics that determine which motor neurons control muscle fibers.

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Training Factors

Endurance and strength training can induce changes in muscle fiber types.

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Motor Unit Recruitment

The body recruits low-threshold motor units before high-threshold ones for muscle activity.

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Size Principle

Describes the recruitment order of muscle fibers, starting from the smallest to the largest.

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Selective Recruitment

The ability to recruit high-threshold motor units directly for explosive movements.

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Comparison of Fiber Types

Fast twitch fibers are for speed and strength, while slow twitch fibers are for endurance.

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Effects of Aging on Muscle

As muscles age, there's a loss of type II motor units, affecting strength.

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Study Notes

Muscle Fiber Types

  • Fast twitch muscle fibers use anaerobic metabolism for fuel, providing short bursts of speed. They fatigue quickly and are great for sprinters.
  • Slow twitch muscle fibers use oxygen for fuel, providing continuous energy and extended muscle contraction. They are slow to fatigue and ideal for marathoners.

Motor Unit Characteristics

  • A motor unit contains one specific muscle fiber type (Type I or Type II) or a subdivision of Type II.
  • Motor units are classified based on twitch, tension, and fatigability characteristics of the muscle fibers.

Twitch Characteristics

  • Type I fibers are slow twitch, low force, and fatigue-resistant.
  • Type IIx fibers are fast twitch, high force, and fatigue quickly.
  • Type IIa fibers are fast twitch, moderate force, and fatigue resistant.
  • Motor neurons stimulate muscle fibers, modulating their properties and adaptive response to stimuli.

Muscle Fiber Type Properties

  • The proportion of muscle fiber types varies between individuals and muscles.
  • Skeletal muscle fibers differ in oxidative and glycolytic capacity, number of capillaries, mitochondria, myoglobin content, sarcoplasmic enzymes, and muscle fiber efficiency.
  • For example, soleus is primarily Type I (80%).

Motor Unit Size Principle

  • Low-threshold motor units are recruited first, followed by high-threshold motor units.
  • This is crucial for coordinated muscle actions.
  • Lower-threshold motor units have lower force capabilities.
  • This gradual recruitment allows for smooth, controlled movements or fine-tuning of muscle actions.
  • Recruitment of motor units is orderly.

Review Questions

  • Compare and contrast fast and slow twitch fibers based on metabolic and mechanical properties.
  • Explain the structural and physiological properties that influence fiber performance.
  • Apply the size principle to weightlifting, and explain how different fiber types are recruited during activity.
  • Explain the impact of selective recruitment on fiber type recruitment.

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