Motor Units and Force Generation
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Questions and Answers

Which of the following is true about concentric muscle action?

  • Speed of movement does not change
  • Sarcomere shortens, filaments slide towards center (correct)
  • Joint angle does not change
  • Myosin cross-bridges break and do not re-form
  • Muscle lengthens while producing force

At what speed of concentric contraction would you be able to generate the most force?

  • Very slow speeds (correct)
  • It's impossible to determine based on the information provided.
  • Very fast speeds
  • Force output is the same at all speeds.
  • Moderate speeds

Which of the following is an example of an eccentric muscle action?

  • Jumping up and down
  • Lifting a weight
  • Running
  • Holding a weight in a static position
  • Lowering a weight slowly (correct)

Which phrase best describes the relationship between frequency of stimuli and muscle force production?

<p>Increasing frequency results in more force production. (A)</p> Signup and view all the answers

Which of the following is NOT a way to acutely increase muscle force output?

<p>Increasing the length of the sarcomere (E)</p> Signup and view all the answers

Which of the following statements is TRUE about isometric muscle action (also known as static contraction)?

<p>The muscle produces force but does not change length. (C)</p> Signup and view all the answers

What is the relationship between the number of muscle fibers innervated by a motor neuron and the precision of a muscle's movement?

<p>Muscles with greater precision have fewer muscle fibers per motor neuron. (A)</p> Signup and view all the answers

When a motor neuron is stimulated, what happens to the muscle fibers it innervates?

<p>All muscle fibers in the motor unit contract simultaneously. (C)</p> Signup and view all the answers

Which of the following factors does NOT influence the force generation of a group of muscles?

<p>The length of the muscle fibers. (A)</p> Signup and view all the answers

What is the effect of increasing the frequency of motor unit discharge on muscle tension?

<p>Increases the total tension. (A)</p> Signup and view all the answers

Which type of motor unit generates greater force?

<p>Larger, Type II motor units. (C)</p> Signup and view all the answers

How does preloading a muscle affect force generation?

<p>It increases the force generated by the muscle. (B)</p> Signup and view all the answers

What is the primary factor determining the force generated by a single muscle fiber?

<p>The number of crossbridges formed. (B)</p> Signup and view all the answers

Which of the following statements is TRUE about the relationship between a motor unit and muscle fiber activation?

<p>A motor unit represents a single motor neuron and all the muscle fibers it innervates. (D)</p> Signup and view all the answers

Flashcards

Motor Unit

The motor unit consists of a motor neuron and muscle fibers it innervates.

Muscle Fiber Innervation

Generally, each muscle fiber receives input from one neuron, but one neuron can serve many fibers.

Precision vs. Muscle Fibers

Muscles needing precision have fewer fibers per motor neuron, while those needing less precision have more.

Action Potential in Motor Neurons

A strong enough stimulus in a motor neuron activates all muscle fibers in its motor unit to contract synchronously.

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Force Gradation

A motor unit does not exert force gradation; it either triggers action or does not.

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Factors Affecting Force Generation

Force generation in muscles depends on number of motor units, frequency of discharge, type of motor unit, preloading, and speed of contraction.

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Motor Unit Recruitment

More motor units recruited leads to greater force generation in muscles.

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Crossbridges and Force

The amount of force generated in a muscle fiber is dependent on the number of crossbridges formed.

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Sustained Contractions

Normal body movements consist of sustained muscle contractions, not just simple twitches.

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Summation

Increased frequency of stimuli leads to muscle fibers not relaxing fully, resulting in greater force production.

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Isometric Contraction

A muscle action where the muscle produces force without changing length, and the joint angle remains the same.

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Isotonic Contraction

A muscle action where the muscle changes length while generating force at a constant load.

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Isokinetic Contraction

A muscle action where the speed of movement remains constant throughout the contraction.

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Concentric Contraction

A type of dynamic muscle action where the muscle shortens while producing force.

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Eccentric Contraction

A type of muscle action where the muscle lengthens while producing force, often while resisting a load.

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Force-Velocity Relationship

Describes how the ability to generate force differs at various contraction speeds.

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Study Notes

Motor Units

  • A motor unit consists of the anterior motor neuron and the specific muscle fibers it innervates
  • Each muscle fiber typically receives input from only one neuron
  • A neuron can innervate many muscle fibers
  • The number of muscle fibers per motor neuron is related to the muscle's function (e.g., muscles requiring less precision may have more fibers per neuron)
  • Muscles requiring fine motor control have fewer fibers per neuron, while those needing more power have more fibers per neuron

All-or-None Principle

  • A strong enough stimulus to trigger an action potential in a motor neuron activates all muscle fibers in that unit simultaneously
  • A motor neuron stimulus does not cause some fibers to contract
  • Each motor unit either contracts fully or not at all; no partial contractions
  • A stronger action potential does not result in a stronger contraction within a unit

Gradation of Force (Acute)

  • The force generated by a muscle depends on several factors within a group of muscles
    • Number of motor units recruited (more units = more force)
    • Frequency of motor unit discharge (more frequent = greater tension)
    • Type of motor units recruited (larger/Type II = more force)
    • Preloading the muscle (activating stretch reflex = greater force)
    • Speed of contraction (affects force generation)

Summation of Force

  • Normal movements are sustained contractions built from simple twitches
  • Increasing stimulus frequency in fibers limits time to relax allowing summation of force production
  • This faster frequency leads to stronger contractions (tetanus)

Muscle Actions

  • Static (isometric): Muscle produces force without changing length; joint angle unchanged
  • Isotonic: Muscle produces force with change in length; concentric (shortening) or eccentric (lengthening)
  • Isokinetic: Muscle movement at constant speed

Force-Velocity Curve

  • Illustrates the relationship between force and velocity of muscle contraction (force production decreasing with increased shortening velocity)
  • Maximal isometric force is greatest at zero velocity (no shortening or lengthening)
  • Force decreases as shortening velocity increases, whereas force decreases more slowly as lengthening velocity increases

Review Questions

  • Five ways muscles increase force output
  • Types of muscle actions (examples)
  • Relationship between force and velocity of contraction
  • Force/power generation at specific concentric speeds
  • Comparison of force generation between concentric, eccentric, and isometric muscle actions

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Motor Units PDF

Description

This quiz explores the concepts of motor units and their role in muscle function, including the all-or-none principle and gradation of force. Test your understanding of how the structure of motor units affects muscle contraction and the relation between motor neurons and muscle fibers.

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