Neurophysiological Basis of Movement World V: Evolving and Changing Behaviors

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

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

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).

Fatigue: Changes in Twitch Contraction

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

Fatigue-Induced Changes in Muscle Activity

Fatigue-Induced Changes in the Level of Muscle Activation

Changes in Muscle Response to Stretch

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)

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

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)

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).

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.