Fatigue and Muscle Activation
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Questions and Answers

Which statement accurately describes the effects of fatigue on spinal stretch reflexes compared to long-loop reflexes?

  • Both are similarly affected by maximal voluntary contraction.
  • Only spinal stretch reflexes are affected by maximal voluntary contraction.
  • Long-loop reflexes are significantly more affected than spinal stretch reflexes by maximal voluntary contraction.
  • Long-loop reflexes are unaffected while spinal stretch reflexes are affected by maximal voluntary contraction. (correct)

    • Which of the following factors is commonly associated with fatigue during exercise?

  • Increase in muscle temperature
  • Decreased facilitation of motoneurons (correct)
  • Improved CNS facilitation of motoneurons
  • Enhanced perfusion of oxygen to muscle tissues

    • What is the purpose of providing feedback during maximal voluntary contractions (MVC)?

  • It ensures all subjects experience the same performance decline.
  • It decreases the level of perceived exertion.
  • It allows subjects to reject efforts considered submaximal. (correct)
  • It eliminates the need for verbal encouragement.

    • Which type of proprioceptive afferents is particularly critical for signaling fatigue?

    <p>Type III/IV afferents</p> Signup and view all the answers

    Which implication relates to the measurement of maximal motor capacity during fatigue?

    <p>Real-time visual feedback should not reveal performance decline magnitude.</p> Signup and view all the answers

    What best defines central fatigue?

    <p>A progressive exercise-induced reduction in voluntary activation or neural drive to the muscle</p> Signup and view all the answers

    What does the term 'fatigability' refer to?

    <p>The rate of decline in an objective measure of motor performance</p> Signup and view all the answers

    What does research suggest about muscle activation at high stress situations?

    <p>Muscles have a reserve of force that is not utilized during normal conditions</p> Signup and view all the answers

    What is indicated by the behavior of the interpolated twitch in regards to muscle force utilization?

    <p>The reserve of muscle force is depleted swiftly at low force levels</p> Signup and view all the answers

    How does electrical stimulation contribute to understanding muscle capability post-fatigue?

    <p>It suggests voluntary muscle contractions exceed the maximum potential measured by MVC</p> Signup and view all the answers

    What can be inferred about the use of central drive during strenuous activities?

    <p>A strategic reserve of muscular force is preserved for higher intensity efforts</p> Signup and view all the answers

    Why might the central nervous system employ a strategy of maintaining a reserve of muscle force?

    <p>To prevent injury by managing fatigue levels during high exertion</p> Signup and view all the answers

    What demonstrates the distinction between central and peripheral fatigue during voluntary movements?

    <p>Peripheral nerve stimulation can still produce movement despite voluntary activation failure.</p> Signup and view all the answers

    What is a key indicator of central fatigue during high-intensity exercises?

    <p>A reduction in the ability to activate alpha motoneurons.</p> Signup and view all the answers

    In what way can central fatigue be assessed experimentally?

    <p>Using the interpolated twitch technique.</p> Signup and view all the answers

    What happens to motoneuron firing rates during a maximal voluntary contraction (MVC)?

    <p>They can diminish to approximately 50% of maximum.</p> Signup and view all the answers

    What effect does mental exertion, such as lecturing, have on physical performance?

    <p>It reduces the extent of loaded finger movements.</p> Signup and view all the answers

    How does muscle twitch force change following sustained maximal voluntary contraction?

    <p>Muscle twitch force diminishes as relaxation time increases.</p> Signup and view all the answers

    What physiological phenomenon can override central fatigue according to studies?

    <p>Asking subjects to exert a 'super' effort.</p> Signup and view all the answers

    What is the consequence of the reduced central drive to muscles during fatigue?

    <p>Decreased performance capacity due to inadequate motoneuron activation.</p> Signup and view all the answers

    What is the relationship between startling stimuli and muscle contractions during fatigue?

    <p>They can elicit greater contractions than maximal voluntary contractions.</p> Signup and view all the answers

    Study Notes

    Fatigue

    • Fatigability: The rate at which motor performance declines (e.g., MVC force, reaction time, movement accuracy).
    • Central fatigue: A decline in voluntary activation or neural drive to the muscle due to exercise.

    Do We Maximally Activate Muscles?

    • Electrical stimulation elicits more force than MVC after fatiguing exercise.
    • Suggests muscles possess more force production capacity than utilized during voluntary effort.
    • This indicates a reduced central drive.

    Interpolated Twitch

    • Introduced by Merton in 1954.
    • Interpolated twitch amplitude doesn't scale linearly with voluntary force.
    • More rapid decline in IT amplitude at low force levels compared to high force levels.
    • This suggests that muscle force reserves are depleted more quickly at lower force levels.

    Evidence for Central Fatigue

    • Peripheral nerve stimulation can produce movement even when voluntary activation fails.
    • Finger flexions against a 3 kg load at 24/s result in task failure.
    • Subsequent median nerve stimulation lifts the load, compensating for central fatigue.
    • Full rest restores both peripheral and central fatigue components.

    Early Evidence of Central Fatigue

    • Mosso (1904): Demonstrated a reduction in loaded finger movements following mental exertion (lecturing).

    Main Points on Central Fatigue

    • Central fatigue results from reduced capacity for activating alpha motoneurons.
    • The interpolated twitch technique assesses central fatigue.
    • Reduced central drive can be caused by physiological fatigue or the perception of fatigue.

    Changes in Motor System Function with Fatigue

    Changes in Motoneuron Function with Fatigue

    • CNS activation of motoneurons changes during fatigue.
    • Motoneuron firing rate drops to 50% of maximum during MVC.
    • Muscle relaxation time increases.

    Impact of Fatigue on Contractile Characteristics

    • Muscle twitch characteristics alter during fatigue.
    • After 60 seconds of MVC, twitch force diminishes and muscle relaxation time increases.
    • Reduction in "ripple" with 7 Hz tetanus.

    Voluntary Override of Central Fatigue

    • Telling individuals to exert a "super" effort slows down voluntary force decay.
    • Rate of force decay becomes similar to tetanic force decay.
    • Startling stimuli can elicit greater contractions than MVC.
    • This suggests non-corticospinal pathways can recruit additional motor neurons.

    Cortical & Subcortical Effects of Fatigue

    • Interpolated twitches elicited by stimulating the cortex and brainstem provide insights into CNS drive alterations during fatigue.
    • Fatigue affects cortical and subcortical motor systems differently.

    Implications for Measuring Maximal Motor Capacity

    • MVCs need instruction and practice.
    • Feedback of performance should be provided during MVC.
    • Vary visual feedback gain to avoid performance decline awareness.
    • Standardized verbal encouragement is crucial.
    • Subjects should be allowed to reject non-maximal efforts.
    • Rewards can be beneficial.

    Main Points on Fatigue and Motor Systems

    • Fatigue alters motor neuron firing characteristics, reducing force and coordination.
    • Non-corticospinal inputs to motoneurons can recruit motor neurons beyond cortical control.
    • Fatigue impacts cortical and subcortical motor systems differently.

    Fatigue Pathways

    A Neurophysiological View of Voluntary Force Preparation

    α-mn and γ-mn’s Receive Multiple Peripheral and Central Inputs

    Effects of Fatigue on Muscle Afferents

    • Fatigue alters the activity of afferents (sensory neurons).
    • Type III/IV afferents play a crucial role in signaling fatigue.

    Fatigue Alters Synergist Activity

    • Changes in non-stimulated muscle (MG) occur due to electrical stimulation of LG.
    • Mediated by agonist-to-synergist reflex pathways.

    Main Points on Fatigue and Afferents

    • All proprioceptive afferents are affected by fatigue.
    • Type III/IV afferents are particularly important for signaling fatigue.
    • Changes in proprioceptive afferents can reduce force capacity in non-fatigued muscles.

    Factors Causing Fatigue

    • No single factor is conclusively the ultimate "exercise-stopper."
    • Factors suggested include decreased CNS drive, decreased motoneuron facilitation, and muscle acidification.

    Factors Causing Fatigue

    • Potential contributing factors:
      • Decreased CNS drive
      • Decreased facilitation of motoneurons
      • Muscle acidification

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    Description

    Explore the concepts of fatigue, including fatigability, central fatigue, and muscle activation through electrical stimulation. This quiz delves into how muscles can produce more force than what is utilized during voluntary efforts, illustrating the complexities of muscular performance and activation. Test your understanding of these important physiological concepts.

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