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

2. Potential contributors:
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. 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). A B V L

4. Fatigue A
Force
Time
Stim B
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. 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). A B
Power
Frequency
(Hz) 40 80
120
160

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. 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.
0.5 1
1.5 2
2.5 3 25 50 75
100
125
R-4/1
R-4/1 (malingerer)
Ratio
Target Torque (% of MVC)
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. 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.
0.5 1
1.5 2
2.5 25 50 75
100
125
R-4/1
Background force (% max)
Control MS
Patients with multiple sclerosis show abnormal fatigue. Their response to electrical stimulation (red) suggests a central cause for their inability to maintain force.