A dynamic network involving M1-S1, SII-insular, medial insular, and cingulate cortices controls muscular activity during an isometric contraction reaction time task

Auditory and visual distraction improve muscle endurance: a randomised controlled trial

The main aim was to investigate the influence of various distracting stimuli on the endurance-strength and fatigue of the lumbar region in asymptomatic participants. Fifty-four healthy individuals were randomised to three groups: auditory distraction group (ADG), visual distraction group (VDG) and control group without distraction (CG). Lumbar muscle endurance and perceived fatigue were the outcome measures. Lumbar muscle endurance was assessed with the Biering-Sorensen test, and perceived fatigue was assessed with the modified Borg scale, once baseline and second with the distraction intervention. Lumbar muscle endurance showed significant changes over time, and there were intragroup differences for VDG and ADG. The direct comparison did show significant differences between both distraction groups with respect to the control group with a large effect size (ΔVDG-CG: p < 0.001, d = 1.55 and, ΔADG-CG: p = 0.008, d = 1.07) but not between the two distraction groups (ΔVDG-VDG: p = 0.56). Fatigue showed significant changes over time but not for group*time interaction, revealing intragroup differences for VDG and ADG. There were no intragroup differences in the CG for muscle resistance or fatigue, and there were no between-group differences. Auditory and visual distractors might produce a significant increase in muscle resistance during the Biering-Sorensen test. Both techniques are valid for increasing lumbar muscle endurance but also both stimuli produced a higher level of fatigue or perception of effort once the test was completed when compared with CG. Finally, we were unable to demonstrate that one type of stimulus produces superior results to the other.

The Effects of Performing Mental Exertion during Cycling Exercise on Fatigue Indices

This study investigated the effect of performing prolonged mental exertion during submaximal cycling exercise on exercise tolerance and fatigue. Participants performed 5 experimental sessions. Session 1: determination of cycling peak power output. Sessions 2 and 3: cycling to exhaustion at 65% peak power output with mental exertion or watching a movie. Sessions 4 and 5: cycling for 45 min at 65% peak power output with mental exertion or while watching a movie. During sessions 2–5, rate of perceived exertion and heart rate were recorded while cycling and cortisol and prolactin concentrations, psychomotor vigilance task performance, and maximal voluntary contraction were measured pre-and post-sessions. During sessions 2 and 3, time to exhaustion was reduced (p<0.01) and rate of perceived exertion was increased (p<0.01) in session 2 compared to 3. Cortisol, prolactin and heart rate increased and psychomotor vigilance task and maximal voluntary contraction decreased from pre-to post-sessions with no difference between sessions. Cortisol, prolactin and rate of perceived exertion were higher (p<0.03) in session 4 than 5. Heart rate increased and maximal voluntary contraction decreased from pre-to post-sessions with no difference between sessions. Prolonged mental exertion during cycling exercise reduces exercise tolerance, which appears to be mediated psychologically rather than physiologically.

The Nature of Self-Regulatory Fatigue and "Ego Depletion": Lessons From Physical Fatigue

Self-regulation requires overriding a dominant response and leads to temporary self-regulatory fatigue. Existing theories of the nature and causes of self-regulatory fatigue highlight physiological substrates such as glucose, or psychological processes such as motivation, but these explanations are incomplete on their own. Historically, theories of physical fatigue demonstrate a similar pattern of useful but incomplete explanations, as recent views of physical fatigue emphasize the roles of both physiological and psychological factors. In addition to accounting for multiple inputs, these newer views also explain how fatigue can occur even in the presence of sufficient resources. Examining these newer theories of physical fatigue can serve as a foundation on which to build a more comprehensive understanding of self-regulatory fatigue that integrates possible neurobiological underpinnings of physical and self-regulatory fatigue, and suggests the possible function of self-regulatory fatigue.

Neural mechanisms of mental fatigue

Fatigue is defined as a decline in the ability and efficiency of mental and/or physical activities that is caused by excessive mental and/or physical activities. Fatigue can be classified as physical or mental. Mental fatigue manifests as potentially impaired cognitive function and is one of the most significant causes of accidents in modern society. Recently, it has been shown that the neural mechanisms of mental fatigue related to cognitive task performance are more complex than previously thought and that mental fatigue is not caused only by impaired activity in task-related brain regions. There is accumulating evidence supporting the existence of mental facilitation and inhibition systems. These systems are involved in the neural mechanisms of mental fatigue, modulating the activity of task-related brain regions to regulate cognitive task performance. In this review, we propose a new conceptual model: the dual regulation system of mental fatigue. This model contributes to our understanding of the neural mechanisms of mental fatigue and the regulatory mechanisms of cognitive task performance in the presence of mental fatigue.

Cortical motor output decreases after neuromuscular fatigue induced by electrical stimulation of the plantar flexor muscles

AIM

Neuromuscular electrical stimulation (NMES) causes early onset of neuromuscular fatigue. Peripheral electrophysiological explorations suggest that supra-spinal alterations are involved through sensitive afferent pathways. As sensory input is projected over the primary somatosensory cortex (S1), S1 area involvement in inhibiting the central motor drive can be hypothesized. This study assessed cortical activity under a fatiguing NMES protocol at low frequency.

METHODS

Twenty healthy males performed five NMES sequences of 17 trains over the plantar flexors (30 Hz, 4 s on/6 s off). Before and after each sequence, neuromuscular tests composed of maximal voluntary contractions (MVCs) were carried out. Cortical activity was assessed during MVCs with functional near-infrared spectroscopy over S1 and primary motor (M1) areas, through oxy- [HbO] and deoxy-haemoglobin [HbR] variation. Electrophysiological data (H-reflex during MVC, EMG activity and level of voluntary activation) were also recorded.

RESULTS

MVC torque significantly decreased after the first 17 NMES trains (P < 0.001). The electrophysiological data were consistent with supra-spinal alterations. In addition, [HbO] declined significantly during the protocol over the S1 and M1 areas from the first 17 NMES trains (P < 0.01 and P < 0.001 respectively), while [HbR] increased (P < 0.05 and P < 0.01 respectively), indicating early decline in cortical activity over both primary cortical areas.

CONCLUSIONS

The declining cortical activity over the M1 area is highly consistent with the electrophysiological findings and supports motor cortex involvement in the loss of force after a fatiguing NMES protocol. In addition, the declining cortical activity over the S1 area indicates that the decreased motor output from M1 is not due to increased S1 inhibitory activity.

Neural mechanisms underlying chronic fatigue

Fatigue is defined as a condition or phenomenon of declined ability and efficiency of mental and/or physical activities, caused by excessive mental or physical activities, diseases, or syndromes. Acute fatigue is a normal condition that disappears after a period of rest; in contrast, chronic fatigue does not disappear after an ordinary rest. Chronic fatigue impairs daily activities and contributes to various medical conditions and death. In addition, many people complain of chronic fatigue. It would thus be of great value to clarify the mechanisms underlying chronic fatigue and to develop efficient treatment methods to overcome it. Here, we review data primarily from behavioral, neurophysiological, and neuroimaging experiments related to the neural mechanisms underlying chronic fatigue. We propose that repetitive and prolonged overwork and/or stress cause neural damage of a facilitation system, as well as central sensitization and classical conditioning of an inhibition system. We also propose a new treatment strategy for chronic fatigue on the basis of its underlying neural mechanisms.

Neural correlates of central inhibition during physical fatigue

Central inhibition plays a pivotal role in determining physical performance during physical fatigue. Classical conditioning of central inhibition is believed to be associated with the pathophysiology of chronic fatigue. We tried to determine whether classical conditioning of central inhibition can really occur and to clarify the neural mechanisms of central inhibition related to classical conditioning during physical fatigue using magnetoencephalography (MEG). Eight right-handed volunteers participated in this study. We used metronome sounds as conditioned stimuli and maximum handgrip trials as unconditioned stimuli to cause central inhibition. Participants underwent MEG recording during imagery of maximum grips of the right hand guided by metronome sounds for 10 min. Thereafter, fatigue-inducing maximum handgrip trials were performed for 10 min; the metronome sounds were started 5 min after the beginning of the handgrip trials. The next day, neural activities during imagery of maximum grips of the right hand guided by metronome sounds were measured for 10 min. Levels of fatigue sensation and sympathetic nerve activity on the second day were significantly higher relative to those of the first day. Equivalent current dipoles (ECDs) in the posterior cingulated cortex (PCC), with latencies of approximately 460 ms, were observed in all the participants on the second day, although ECDs were not identified in any of the participants on the first day. We demonstrated that classical conditioning of central inhibition can occur and that the PCC is involved in the neural substrates of central inhibition related to classical conditioning during physical fatigue.

The myth of the 'unaffected' side after unilateral stroke: is reorganisation of the non-infarcted corticospinal system to re-establish balance the price for recovery?

Background

Bilateral changes in the hemispheric reorganisation have been observed chronically after unilateral stroke. Our hypotheses were that activity dependent competition between the lesioned and non-lesioned corticospinal systems would result in persisting asymmetry and be associated with poor recovery.

Methods

Eleven subjects (medium 6.5 years after stroke) were compared to 9 age-matched controls. The power spectral density (PSD) of the sensorimotor electroencephalogram (SM1-EEG) and electromyogram (EMG) and corticomuscular coherence (CMC) were studied during rest and isometric contraction of right or left opponens pollicis (OP). Global recovery was assessed using NIH score.

Findings

There was bilateral loss of beta frequency activity in the SM1-EEGs and OP-EMGs in strokes compared to controls. There was no difference between strokes and controls in symmetry indices estimated between the two corticospinal systems for SM1-EEG, OP-EMG and CMC. Performance correlated with preservation of beta frequency power in OP-EMG in both hands. Symmetry indices for the SM1-EEG, OP-EMG and CMC correlated with recovery.

Interpretation

Significant changes occurred at both cortical and spinomuscular levels after stroke but to the same degree and in the same direction in both the lesioned and non-lesioned corticospinal systems. Global recovery correlated with the degree of symmetry between corticospinal systems at all three levels — cortical and spinomuscular levels and their connectivity (CMC), but not with the absolute degree of abnormality. Re‐establishing balance between the corticospinal systems may be important for overall motor function, even if it is achieved at the expense of the non-lesioned system.

Neural substrates activated by viewing others expressing fatigue: a magnetoencephalography study

The neural substrates of the fatigue sensation have not been totally identified. Several lines of evidence demonstrate that seeing emotional changes in others activates brain regions involved in experiencing similar emotions. We hypothesized that there exists a mirror system regarding the fatigue sensation and that brain regions associated with the fatigue sensation may be activated by viewing other individuals expressing fatigue. In this study, we attempted to identify the neural substrates activated by viewing other fatigued individuals using magnetoencephalography (MEG). Twelve healthy participants were enrolled in our study after providing written informed consent. During MEG recordings, they viewed a set of pictures projected on a screen. The pictures, which were presented in a randomized order, were of a person with a fatigued or neutral facial expression. When participants viewed pictures of people with fatigued expressions, we were able to estimate equivalent current dipoles (ECDs) in the posterior cingulate cortex (PCC) in 9 of 12 participants approximately 300 ms after the onset of each picture presentation. When they viewed pictures of people with neutral expressions, we were not able to estimate corresponding ECDs for any participant. The PCC is the brain region activated by viewing others expressing fatigue, suggesting existence of the shared neural substrates of felt and observed fatigue.

Limitation of physical performance in a muscle fatiguing handgrip exercise is mediated by thalamo-insular activity

In this study, we investigated central/supraspinal processes mediating cessation of a muscle fatiguing exercise. Fifteen male subjects performed 39 intermittent, isometric handgrip contractions (13 s on, 5-6 s off) with the dominant right hand while brain activation was assessed by means of functional magnetic resonance imaging (fMRI). An adaptive, partly stochastic protocol was designed such that in approximately 50% of the contraction trials the required force could not be held until the end of the trial (task failure trial). Trials performed in compliance with the force requirements (succeeded trial) were compared with task failure trials concerning neural activity during a small time window before task failure occurred. The data revealed significantly increased activation contralaterally in both the mid/anterior insular cortex and the thalamus during the investigated time window in the case of subsequent task failure. In accordance with other studies investigating sensations that alert the organism to urgent homeostatic imbalance such as air hunger, hunger for food, and pain, we assume that an increased thalamo-insular activation in the context of a fatigue-induced handgrip exercise could reflect increased homeostatic disturbance in the exercising muscle and may be of essential importance by mediating task failure to maintain the integrity of the organism.