1 / 14

Neurophysiological Basis of Movement: Fatigue and its Effects on Muscle Function

This lecture explores the neurophysiological basis of muscle fatigue, including the decrease in muscle force generation, changes in neuromuscular synapses, alterations in peripheral receptors, and psychological factors. It also discusses the changes in muscle activity, muscle reflexes, and electromyogram spectrum induced by fatigue. Additionally, the lecture examines the test of central versus peripheral fatigue and the effects of fatigue in individuals with multiple sclerosis.

darrelllopez
Download Presentation

Neurophysiological Basis of Movement: Fatigue and its Effects on Muscle Function

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Neurophysiological Basis of Movement World V: Evolving and Changing Behaviors

  2. Lecture 27: Fatigue Potential contributors: • A decrease in the ability of muscle fibers to generate force • A decrease in the efficacy of neuromuscular synapses • Changes in the activity of certain peripheral receptors leading to changes in their reflex effects • Changes in the patterns of firing (recruitment patterns) of alpha-motoneurons • Changes at any level of the hypothetical process of generation of a motor command • Psychological factors, including (in particular) motivation

  3. A V L B V L Fatigue An action potential recorded by surface electrodes has a smaller amplitude, longer duration, and slower conduction speed in a fatigued muscle (A) than in a nonfatigued muscle (B).

  4. A Force Time Stim B Force Time Stim Fatigue A twitch contraction of a fatigued muscle (A) is characterized by a longer relaxation phase as compared to a nonfatigued muscle (B).

  5. Fatigue: Changes in Twitch Contraction

  6. Fatigue-Induced Changes Within a Muscle • Slowing of conduction velocity of muscle action potentials (possibly related to an increase in extracellular K+) • Alteration of the excitation threshold to external stimulation • Slowing of the relaxation phase of a twitch contraction • Posttetanic depression of twitch force

  7. Fatigue-Induced Changes in Muscle Activity

  8. Fatigue-Induced Changes in the Level of Muscle Activation

  9. Changes in Muscle Response to Stretch

  10. Fatigue-Induced Changes of Muscle Reflexes • Depression of short-latency reflexes (M1), possibly due to the presynaptic action of small free endings sensitive to products of muscle metabolism • Less pronounced depression of medium-latency reflexes (M2) • No changes or even facilitation of long-latency reflexes (M3)

  11. A Power B 0 120 160 Frequency 40 80 (Hz) Fatigue-Induced Changes in EMG Spectrum The spectrum of an electromyogram recorded by surface electrodes in a fatigued muscle (A) is shifted toward low frequencies as compared to the spectrum of an electromyogram recorded in a nonfatigued muscle (B).

  12. Fatigue-Induced Changes in “Neural Commands” • Changes in the firing of individual MUs (spectral changes, synchronization?) • Most changes are at a peripheral level • Making use of motor redundancy (rotation among agonists) • Changes in “synergies” (weak-link hypothesis)

  13. Ratio 3 R-4/1 R-4/1 (malingerer) 2.5 2 1.5 1 0.5 0 0 25 50 75 100 125 Target Torque (% of MVC) Test of Central vs. Peripheral Fatigue R-4/1: the ratio between the magnitudes of the responses to the fourth (fatigued muscle) and first (nonfatigued muscle) stimuli. Muscle response to an electrical stimulus (T) decreases with muscle background force. It decreases with fatigue (R<1) but increases with malingering (R>1).

  14. R-4/1 MS 2.5 2 1.5 Control 1 0.5 0 Background force (% max) 0 25 50 75 100 125 Fatigue in Multiple Sclerosis R-4/1: the ratio between the magnitudes of the responses to the fourth (fatigued muscle) and first (nonfatigued muscle) stimuli. Patients with multiple sclerosis show abnormal fatigue. Their response to electrical stimulation (red) suggests a central cause for their inability to maintain force.

More Related