Enhanced availability of serotonin increases activation of unfatigued muscle but exacerbates central fatigue during prolonged sustained contractions

Reduction and recovery of self-sustained muscle activity after fatiguing plantar flexor contractions

PURPOSE

Persistent inward calcium and sodium currents (PICs) are crucial for initiation and maintenance of motoneuron firing, and thus muscular force. However, there is a lack of data describing the effects of fatiguing exercise on PIC activity in humans. We simultaneously applied tendon vibration and neuromuscular electrical stimulation (VibStim) before and after fatiguing exercise. VibStim induces self-sustained muscle activity that is proposed to result from PIC activation.

METHODS

Twelve men performed 5-s maximal isometric plantar flexor contractions (MVC) with 5-s rests until joint torque was reduced to 70%MVC. VibStim trials consisted of five 2-s trains of neuromuscular electrical stimulation (20 Hz, evoking 10% MVC) of triceps surae with simultaneous Achilles tendon vibration (115 Hz) without voluntary muscle activation. VibStim was applied before (PRE), immediately (POST), 5-min (POST-5), and 10-min (POST-10) after exercise completion.

RESULTS

Sustained torque (Tsust) and soleus electromyogram amplitudes (EMG) measured 3 s after VibStim were reduced (Tsust: -59.0%, p < 0.001; soleus EMG: -38.4%, p < 0.001) but largely recovered by POST-5, and changes in MVC and Tsust were correlated across the four time points (r = 0.69; p < 0.001). After normalisation to values obtained at the end of the vibration phase to control for changes in fibre-specific force and EMG signal characteristics, decreases in Tsust (-42.9%) and soleus EMG (-22.6%) remained significant and were each correlated with loss and recovery of MVC (r = 0.41 and 0.46, respectively).

CONCLUSION

The parallel changes observed in evoked self-sustained muscle activity and force generation capacity provide motivation for future examinations on the potential influence of fatigue-induced PIC changes on motoneuron output.

Motor unit discharge rate modulation during isometric contractions to failure is intensity- and modality-dependent

The physiological mechanisms determining the progressive decline in the maximal muscle torque production capacity during isometric contractions to task failure are known to depend on task demands. Task-specificity of the associated adjustments in motor unit discharge rate (MUDR), however, remains unclear. This study examined MUDR adjustments during different submaximal isometric knee extension tasks to failure. Participants performed a sustained and an intermittent task at 20% and 50% of maximal voluntary torque (MVT), respectively (Experiment 1). High-density surface EMG signals were recorded from vastus lateralis (VL) and medialis (VM) and decomposed into individual MU discharge timings, with the identified MUs tracked from recruitment to task failure. MUDR was quantified and normalised to intervals of 10% of contraction time (CT). MUDR of both muscles exhibited distinct modulation patterns in each task. During the 20% MVT sustained task, MUDR decreased until ∼50% CT, after which it gradually returned to baseline. Conversely, during the 50% MVT intermittent task, MUDR remained stable until ∼40-50% CT, after which it started to continually increase until task failure. To explore the effect of contraction intensity on the observed patterns, VL and VM MUDR was quantified during sustained contractions at 30% and 50% MVT (Experiment 2). During the 30% MVT sustained task, MUDR remained stable until ∼80-90% CT in both muscles, after which it continually increased until task failure. During the 50% MVT sustained task the increase in MUDR occurred earlier, after ∼70-80% CT. Our results suggest that adjustments in MUDR during submaximal isometric contractions to failure are contraction modality- and intensity-dependent. KEY POINTS: During prolonged muscle contractions a constant motor output can be maintained by recruitment of additional motor units and adjustments in their discharge rate. Whilst contraction-induced decrements in neuromuscular function are known to depend on task demands, task-specificity of motor unit discharge behaviour adjustments is still unclear. In this study, we tracked and compared discharge activity of several concurrently active motor units in the vastii muscles during different submaximal isometric knee extension tasks to failure, including intermittent vs. sustained contraction modalities performed in the same intensity domain (Experiment 1), and two sustained contractions performed at different intensities (Experiment 2). During each task, motor units modulated their discharge rate in a distinct, biphasic manner, with the modulation pattern depending on contraction intensity and modality. These results provide insight into motoneuronal adjustments during contraction tasks posing different demands on the neuromuscular system.

Excitatory drive to spinal motoneurones is necessary for serotonin to modulate motoneurone excitability via 5-HT2 receptors in humans

Serotonin modulates corticospinal excitability, motoneurone firing rates and contractile strength via 5-HT2 receptors. However, the effects of these receptors on cortical and motoneurone excitability during voluntary contractions have not been explored in humans. Therefore, the purpose of this study was to investigate how 5-HT2 antagonism affects corticospinal and motoneuronal excitability with and without descending drive to motoneurones. Twelve individuals (aged 24 ± 4 years) participated in a double-blind, placebo-controlled, crossover study, whereby the 5-HT2 antagonist cyproheptadine was administered. Transcranial magnetic stimulation (TMS) was delivered to the motor cortex to produce motor evoked potentials (MEPs), and electrical stimulation at the cervicomedullary junction was used to generate cervicomedullary motor evoked potentials (CMEPs) in the biceps brachii at rest and during a range of submaximal elbow flexions. Evoked potentials were also obtained after a conditioning TMS pulse to produce conditioned MEPs and CMEPs (100 ms inter-stimulus interval). 5-HT2 antagonism reduced maximal torque (p < 0.001), and compared to placebo, reduced unconditioned MEP amplitude at rest (p = 0.003), conditioned MEP amplitude at rest (p = 0.033) and conditioned MEP amplitude during contractions (p = 0.020). 5-HT2 antagonism also increased unconditioned CMEP amplitude during voluntary contractions (p = 0.041) but not at rest. Although 5-HT2 antagonism increased long-interval intracortical inhibition, net corticospinal excitability was unaffected during voluntary contractions. Given that spinal motoneurone excitability was only affected when descending drive to motoneurones was present, the current study indicates that excitatory drive is necessary for 5-HT2 receptors to regulate motoneurone excitability but not intracortical circuits.

Serotonergic and noradrenergic contributions to motor cortical and spinal motoneuronal excitability in humans

Animal models indicate that motor behaviour is shaped by monoamine neuromodulators released diffusely throughout the brain and spinal cord. As an alternative to conducting a single study to explore the effects of neuromodulators on the human motor system, we have identified and collated human experiments investigating motor effects of well-characterised drugs that act on serotonergic and noradrenergic networks. In doing so, we present strong neuropharmacology evidence that human motor pathways are affected by neuromodulators across both healthy and clinical populations, insight that cannot be determined from a single reductionist experiment. We have focused our review on the effects that monoaminergic drugs have on muscle responses to non-invasive stimulation of the motor cortex and peripheral nerves, and other closely related tests of motoneuron excitability, and discuss how these measurement techniques elucidate the effects of neuromodulators at motor cortical and spinal motoneuronal levels. Although there is some heterogeneity in study methods, we find drugs acting to enhance extracellular concentrations of serotonin tend to reduce the excitability of the human motor cortex, and enhanced extracellular concentrations of noradrenaline increases motor cortical excitability by enhancing intracortical facilitation and reducing inhibition. Both monoamines tend to enhance the excitability of spinal motoneurons. Overall, this review details the importance of neuromodulators for the output of human motor pathways and suggests that commonly prescribed monoaminergic drugs target the motor system in addition to their typical psychiatric/neurological indications.

Caffeine does not influence persistent inward current contribution to motoneuron firing

The purpose of this study was to investigate whether caffeine consumption would change persistent inward current (PIC) contribution to motoneuron firing at increased contraction intensities and after repetitive sustained maximal contractions. Before and after the consumption of 6 mg·kg-1 of caffeine or placebo, 16 individuals performed isometric triangular-shaped ramp dorsiflexion contractions (to 20% and 40% of peak torque), followed by four maximal contractions sustained until torque production dropped to 60% of maximum, and consecutive 20% triangular-shaped contractions. Tibialis anterior motor unit firing frequencies were analyzed from high-density surface electromyograms. PIC contribution to motor unit firing was estimated by calculating the delta frequency (ΔF) using the paired motor unit technique. Motoneuron peak firing frequencies at 20% and 40% contractions and total torque-time integral during the repetitive sustained maximal contractions were also assessed. ΔF increased 0.69 peaks per second (pps) (95% CI = -0.98, -0.405; d = -0.87) from 20% to 40% contraction intensities and reduced 0.85 pps (95% CI = 0.66, 1.05; d = 0.99) after the repetitive sustained maximal contractions, regardless of caffeine consumption. Participants produced 337 Nm·s (95% CI = 49.9, 624; d = 0.63) more torque integral during the repetitive sustained maximal contractions after caffeine consumption. A strong repeated-measures correlation (r = 0.61; 95% CI = 0.49, 0.69) was observed between reductions of ΔF and peak firing frequencies after the repetitive sustained maximal contractions. PIC contribution to motoneuron firing increases from 20% to 40% contraction intensities, with no effect of caffeine (on rested tibialis anterior). Repetitive sustained maximal contractions reduced PIC contribution to motoneuron firing, regardless of caffeine or placebo consumption, evidencing that changes in intrinsic motoneuron properties contributed to performance loss. Caffeine-attenuated reduction of torque production capacity was unlikely mediated by PICs.NEW & NOTEWORTHY Persistent inward current (PIC) contribution to motoneuron firing increases with contraction intensities and is reduced after repetitive sustained maximal contractions, regardless of caffeine consumption. Reductions of PIC contribution to motoneuron firing and peak firing frequencies were largely associated, evidencing a novel mechanism underpinning decrements in maximal torque production capacity following repetitive sustained maximal contractions. Caffeine consumption attenuated neuromuscular performance reductions-allowing higher time-torque integral production during repetitive sustained maximal contractions. This was unlikely mediated by PIC.

The severity of acute hypoxaemia determines distinct changes in intracortical and spinal neural circuits

The purpose of this study was to examine how two common methods of continuous hypoxaemia impact the activity of intracortical circuits responsible for inhibition and facilitation of motor output, and spinal excitability. Ten participants were exposed to 2 h of hypoxaemia at 0.13 fraction of inspired oxygen (F I O 2 ${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol) and 80% of peripheral capillary oxygen saturation (S p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol) using a simulating altitude device on two visits separated by a week. Using transcranial magnetic and peripheral nerve stimulation, unconditioned motor evoked potential (MEP) area, short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF), and F-wave persistence and area were assessed in the first dorsal interosseous (FDI) muscle before titration, after 1 and 2 h of hypoxic exposure, and at reoxygenation. The clamping protocols resulted in differing reductions inS p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ by 2 h (S p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol: 81.9 ± 1.3%,F I O 2 ${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol: 90.6 ± 2.5%). Although unconditioned MEP peak to peak amplitude and area did not differ between the protocols, SICI duringF I O 2 ${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping was significantly lower at 2 h compared toS p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping (P = 0.011) and baseline (P < 0.001), whereas ICF was higher throughout theF I O 2 ${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping compared toS p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping (P = 0.005). Furthermore, a negative correlation between SICI andS p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (rrm  = -0.56, P = 0.002) and a positive correlation between ICF andS p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (rrm  = 0.69, P = 0.001) were determined, where greater reductions inS p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ correlated with less inhibition and less facilitation of MEP responses. Although F-wave area progressively increased similarly throughout the protocols (P = 0.037), persistence of responses was reduced at 2 h and reoxygenation (P < 0.01) during theS p O 2 ${S_{{\mathrm{p}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol compared to theF I O 2 ${F_{{\mathrm{I}}{{\mathrm{O}}_{\mathrm{2}}}}}$ clamping protocol. After 2 h of hypoxic exposure, there is a reduction in the activity of intracortical circuits responsible for inhibiting motor output, as well as excitability of spinal motoneurones. However, these effects can be influenced by other physiological responses to hypoxia (i.e., hyperventilation and hypocapnia). NEW FINDINGS: What is the central question of this study? How do two common methods of acute hypoxic exposure influence the excitability of intracortical networks and spinal circuits responsible for motor output? What is the main finding and its importance? The excitability of spinal circuits and intracortical networks responsible for inhibition of motor output was reduced during severe acute exposure to hypoxia at 2 h, but this was not seen during less severe exposure. This provides insight into the potential cause of variance seen in motor evoked potential responses to transcranial magnetic stimulation (corticospinal excitability measures) when exposed to hypoxia.

Persons With Spinal Cord Injury Report Peripherally Dominant Serotonin-Like Syndrome After Use of Serotonergic Psychedelics

Psychedelic-assisted therapy (PAT) may treat various mental health conditions. Despite its promising therapeutic signal across mental health outcomes, less attention is paid on its potential to provide therapeutic benefits across complex medical situations within rehabilitation medicine. Persons with spinal cord injury (SCI) have a high prevalence of treatment-resistant mental health comorbidities that compound the extent of their physical disability. Reports from online discussion forums suggest that those living with SCI are using psychedelics, though the motivation for their use is unknown. These anecdotal reports describe a consistent phenomenon of neuromuscular and autonomic hypersensitivity to classical serotonergic psychedelics, such as psilocybin and lysergic acid diethylamide (LSD). Persons describe intense muscle spasms, sweating, and tremors, with an eventual return to baseline and no reports of worsening of their baseline neurological deficits. The discomfort experienced interferes with the subjective beneficial effects self-reported. This phenomenon has not been described previously in the academic literature. We aim to provide a descriptive review and explanatory theoretical framework hypothesizing this phenomenon as a peripherally dominant serotonin syndrome-like clinical picture-that should be considered as such when persons with SCI are exposed to classical psychedelics. Raising awareness of this syndrome may help our mechanistic understanding of serotonergic psychedelics and stimulate development of treatment protocols permitting persons with SCI to safely tolerate their adverse effects. As PAT transitions from research trials into accepted clinical and decriminalized use, efforts must be made from a harm reduction perspective to understand these adverse events, while also serving as an informed consent process aid if such therapeutic approaches are to be considered for use in persons living with SCI.

Relationship between disturbances of CO2 homeostasis and force output characteristics during isometric knee extension

To determine whether disturbances of CO₂ homeostasis alter force output characteristics of lower limb muscles, participants performed four isometric knee extension trials (MVC_30%, 10s each with 20-s rest intervals) in three CO₂ conditions (normocapnia [NORM], hypercapnia [HYPER], and hypocapnia [HYPO]). Respiratory frequency and tidal volume were matched between CO₂ conditions. In each MVC_30%, the participants exerted a constant force (30% of maximum voluntary contraction [MVC]). The force coefficient of variation (Fcv) during each MVC_30% and MVC before and after the four MVC_30% trials were measured. For the means of the four trials, Fcv was significantly lower in HYPER than in HYPO. However, within HYPER, a significant positive correlation was found between the increase in end-tidal CO₂ partial pressure and the increase in Fcv. MVCs in NORM and HYPO decreased significantly over the four trials, while no such reduction was observed in HYPER. These results suggest that perturbed CO₂ homeostasis influences the force output characteristics independently of breathing pattern variables.

Estimates of persistent inward currents in lower limb motoneurons are larger in females than in males

Noninvasive recordings of motor unit (MU) spike trains help us understand how the nervous system controls movement and how it adapts to various physiological conditions. The majority of participants in human and nonhuman animal physiology studies are male, and it is assumed that mechanisms uncovered in these studies are shared between males and females. However, sex differences in neurological impairment and physical performance warrant the study of sex as a biological variable in human physiology and performance. To begin addressing this gap in the study of biophysical properties of human motoneurons, we quantified MU discharge rates and estimates of persistent inward current (PIC) magnitude in both sexes. We decomposed MU spike trains from the tibialis anterior (TA), medial gastrocnemius (MG), and soleus (SOL) using high-density surface electromyography and blind source separation algorithms. Ten participants of each sex performed slow triangular (10 s up and down) isometric contractions to a peak of 30% of their maximum voluntary contraction. We then used linear mixed-effects models to determine if peak discharge rate and estimates of PICs were predicted by the fixed effects of sex, muscle, and their interaction. Despite a lack of sex-differences in peak discharge rates across all muscles, estimates of PICs were larger [χ2(1) = 6.26, P = 0.012] in females [4.73 ± 0.242 pulses per second (pps)] than in males (3.81 ± 0.240 pps). These findings suggest that neuromodulatory drive, inhibitory input, and/or biophysical properties of motoneurons differ between the sexes and may contribute to differences in MU discharge patterns.NEW & NOTEWORTHY Sex-related differences in motoneuron analyses have emerged with greater inclusion of female participants, however, mechanisms for these differences remain unclear. Estimates of persistent inward currents (i.e., ΔF) in motoneurons of the lower limb muscles were larger in females than in males. This suggests neuromodulatory drive, monoaminergic signaling, intrinsic motoneuron properties, and/or descending motor commands may differ between the sexes, which provides a potential mechanism underlying previously reported sex-related differences in motoneuron discharge patterns.

Blockade of 5-HT2 receptors suppresses motor unit firing and estimates of persistent inward currents during voluntary muscle contraction in humans

Serotonergic neuromodulation contributes to enhanced voluntary muscle activation. However, it is not known how the likely motoneurone receptor candidate (5-HT₂ ) influences the firing rate and activation threshold of motor units (MUs) in humans. The purpose of this study was to determine whether 5-HT₂ receptor activity contributes to human MU behaviour during voluntary ramped contractions of differing intensity. High-density surface EMG (HDsEMG) of the tibialis anterior was assessed during ramped isometric dorsiflexions at 10, 30, 50 and 70% of maximal voluntary contraction (MVC). MU characteristics were successfully extracted from HDsEMG of 11 young adults (four female) pre- and post-ingestion of 8 mg cyproheptadine or a placebo. Antagonism of 5-HT₂ receptors caused a reduction in MU discharge rate during steady-state muscle activation that was independent of the level of contraction intensity [P < 0.001; estimated mean difference (∆) = 1.06 pulses/s], in addition to an increase in MU derecruitment threshold (P < 0.013, ∆ = 1.23% MVC), without a change in force during MVC (P = 0.652). A reduction in estimates of persistent inward current amplitude was observed at 10% MVC (P < 0.001, ∆ = 0.99 Hz) and 30% MVC (P = 0.003, ∆ = 0.75 Hz) that aligned with 5-HT changes in MU firing behaviour attributable to 5-HT₂ antagonism. Overall, these findings indicate that 5-HT₂ receptor activity has a role in regulating the discharge rate in populations of spinal motoneurones when performing voluntary contractions. This study provides evidence of a direct link between MU discharge properties, persistent inward current activity and 5-HT₂ receptor activity in humans. KEY POINTS: Activation of 5-HT receptors on the soma and dendrites of motoneurones regulates their excitability. Previous work using chlorpromazine and cyproheptadine has demonstrated that the 5-HT₂ receptor regulates motoneurone activity in humans with chronic spinal cord injury and non-injured control subjects. It is not known how the 5-HT₂ receptor directly influences motor unit (MU) discharge and MU recruitment in larger populations of human motoneurones during voluntary contractions of differing intensity. Despite the absence of change in force during maximal voluntary dorsiflexions, 5-HT₂ receptor antagonism caused a reduction in MU discharge rate during submaximal steady-state muscle contraction, in addition to an increase in MU derecruitment threshold, irrespective of the submaximal contraction intensity. Reductions in estimates of persistent inward currents after 5-HT₂ receptor antagonism support the viewpoint that the 5-HT₂ receptor plays a crucial role in regulating motor activity, whereby a persistent inward current-based mechanism is involved in regulating the excitability of human motoneurones.

The effects of α-lactalbumin supplementation and handgrip contraction on soleus motoneuron excitability

INTRODUCTION

We tested two strategies that hypothetically increase serotonin availability (α-lactalbumin consumption and a remote submaximal handgrip contraction) on estimates of persistent inward currents (PICs) amplitude of soleus muscle in healthy participants.

METHODS

With a randomised, double-blind, and cross-over design, 13 healthy participants performed triangular-shaped ramp contractions with their plantar flexors (20% of maximal torque), followed by a 30-s handgrip sustained contraction (40% of maximal force) and consecutive repeated triangular-shaped contractions. This was performed before and after the consumption of either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (Corn-starch). Soleus motor units discharge rates were analysed from high-density surface electromyography signals. PICs were estimated by calculating the delta frequency (ΔF) of motor unit train spikes using the paired motor unit technique.

RESULTS

ΔF (0.19 pps; p = 0.001; d = 0.30) and peak discharge rate (0.20 pps; p < 0.001; d = 0.37) increased after the handgrip contraction, irrespective of the consumed supplement. No effects of α-lactalbumin were observed.

CONCLUSIONS

Our results indicate that 40 g of α-lactalbumin was unable to modify intrinsic motoneuron excitability. However, performing a submaximal handgrip contraction before the plantar flexion triangular contraction was capable of increasing ΔF and discharge rates on soleus motor units. These findings highlight the diffused effects of serotonergic input, its effects on motoneuron discharge behaviour, and suggest a cross-effector effect within human motoneurons.

Effects of α-lactalbumin on strength, fatigue and psychological parameters: a randomised double-blind cross-over study

PURPOSE

The neurotransmitter serotonin has a strong effect on behaviour and motor control. Regarding motor control, serotonin contributes to the development of fatigue and is also involved in the ability of motor neurones to operate across a large range of forces (gain control). The consumption of tryptophan-rich supplements (such as α-lactalbumin) is of interest because this amino acid is the only precursor for brain serotonin synthesis. Therefore, the purpose of this study was to determine the effects of α-lactalbumin supplementation on neuromuscular performance.

METHODS

Using a randomised double-blind cross-over design, 16 healthy participants performed plantar flexor and handgrip maximal voluntary contractions, a 30-s submaximal handgrip contraction, and a plantar flexor fatigue protocol before and 90 min after consuming either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (corn-starch). Sleepiness, mood, and cognition were assessed to evaluate any psychological effects.

RESULTS

α-Lactalbumin decreased force steadiness by 25% during the sustained submaximal handgrip contraction (p < 0.01) and induced greater fatigue (15% reduction in total torque-time integral, p = 0.01) during the fatigue protocol. These effects were not observed for the other control beverages. No effects were found for maximal or explosive strength, or psychological measurements.

CONCLUSIONS

40 g of α-lactalbumin increased handgrip force variability and reduced performance during fatiguing muscle contractions but did not influence brief maximal contractions or psychological parameters in healthy individuals. These findings support the hypothesis that the consumption of α-lactalbumin can increase motor neurone input-output gain and exacerbate central fatigue during sustained maximal exercise.

Effects of Pregnant Fatigue on the Development of Offspring in Rats

AIM

To explore the correlation between pregnant fatigue and intrauterine physical and neural development of offspring in rats.

METHODS

Sprague-Dawley pregnant rats were randomly divided into a normal control group, a mild fatigue group (stand in water for 6 hours/day), and a severe fatigue group (stand in water for 15 h/day). The levels of lactic acid, 5-Hydroxytryptamine and Interleukin-6 in cardiac serum of rats were used to evaluate the fatigue. The expression of S100β in the telencephalon, Insulin-like growth factor-1 (IGF-1) in the liver and Cyclooxygenage-2 (COX-2) in the small intestine tissues of fetal rats were examined. Frozen sections were taken from the telencephalon of rat pups to observe morphological changes in the hippocampal primordium.

RESULTS

Pregnant fatigue led to a decrease in food intake (F = 37.586, p = 0.000) and water intake (F = 23.608, p = 0.000) in rats. The IGF-1 mRNA expression of fetal rats in the severe fatigue group was lower than that in the control group (p = 0.0003). The expression of S100β mRNA (p = 0.000) and COX-2 mRNA (p = 0.0002) of fetal rats were higher in the severe fatigue group than in the control group. HE staining of the telencephalon of fetal rats in the pregnant fatigue group revealed sparse and irregular cell arrangement and increased gaps in the hippocampal primordial site.

CONCLUSION

Pregnant fatigue rats had both physical fatigue and mental fatigue. Fatigue during pregnancy affects physical development and neurodevelopment of offspring. Further research should elucidate the mechanisms of pregnant fatigue and its effects on offspring.

Critical considerations of the contribution of the corticomotoneuronal pathway to central fatigue

Neural drive originating in higher brain areas reaches exercising limb muscles through the corticospinal-motoneuronal pathway, which links the motor cortex and spinal motoneurones. The properties of this pathway have frequently been observed to change during fatiguing exercise in ways that could influence the development of central fatigue (i.e. the progressive reduction in voluntary muscle activation). However, based on differences in motor cortical and motoneuronal excitability between exercise modalities (e.g. single-joint vs. locomotor exercise), there is no characteristic response that allows for a categorical conclusion about the effect of these changes on functional impairments and performance limitations. Despite the lack of uniformity in findings during fatigue, there is strong evidence for marked 'inhibition' of motoneurones as a direct result of voluntary drive. Endogenous forms of neuromodulation, such as via serotonin released from neurones, can directly affect motoneuronal output and central fatigue. Exogenous forms of neuromodulation, such as brain stimulation, may achieve a similar effect, although the evidence is weak. Non-invasive transcranial direct current stimulation can cause transient or long-lasting changes in cortical excitability; however, variable results across studies cast doubt on its claimed capacity to enhance performance. Furthermore, with these studies, it is difficult to establish a cause-and-effect relationship between brain responsiveness and exercise performance. This review briefly summarizes changes in the corticomotoneuronal pathway during various types of exercise, and considers the relevance of these changes for the development of central fatigue, as well as the potential of non-invasive brain stimulation to enhance motor cortical excitability, motoneuronal output and, ultimately, exercise performance.

Enhanced availability of serotonin limits muscle activation during high-intensity, but not low-intensity, fatiguing contractions

Serotonin (5-HT) modulates motoneuron excitability during muscle contractions, where the release of 5-HT in the central nervous system (CNS) is linked to the intensity of physical activity. Although there is evidence that enhanced availability of 5-HT can exacerbate fatigue, these effects on the development of fatigue during different contraction intensities are largely unknown. The purpose of this study was to investigate how enhanced 5-HT availability affects voluntary muscle activation and corticospinal excitability during fatigue-inducing contractions. Two experiments were performed. In the first experiment (n = 11), twelve isometric elbow flexions at 20% maximal voluntary contractions (MVC) were performed for 2-min each with 40-s rest periods. In the second experiment (n = 14), twelve maximal isometric elbow flexions were held for 10-s each with 40-s rest periods. In both experiments, the selective serotonin reuptake inhibitor (20 mg paroxetine), or a placebo, was administered in a two-way crossover-design. Muscle responses to transcranial magnetic stimulation (TMS) of the motor cortex (both experiments 1 and 2), as well as motor point stimulation of the elbow flexors (experiment 2) were assessed. Paroxetine reduced both motor cortical (p = 0.018) and motor point voluntary activation (p = 0.036) during the maximal contraction protocol. Paroxetine also reduced exercise-induced lengthening of the TMS silent period during the submaximal (p = 0.037) and maximal (p = 0.002) contraction protocols. Activation of inhibitory 5-HT_1A receptors on motoneurons likely exacerbated exercise-induced reductions in voluntarily drive to the elbow flexors. However, 5-HT modulation of motor activity also appeared at the supraspinal level.

Voluntary activation of muscle in humans: does serotonergic neuromodulation matter?

Ionotropic inputs to motoneurones have the capacity to depolarise and hyperpolarise the motoneurone, whereas neuromodulatory inputs control the state of excitability of the motoneurone. Intracellular recordings of motoneurones from in vitro and in situ animal preparations have provided extraordinary insight into the mechanisms that underpin how neuromodulators regulate neuronal excitability. However, far fewer studies have attempted to translate the findings from cellular and molecular studies into a human model. In this review, we focus on the role that serotonin plays in muscle activation in humans. Serotonin (5-HT) is a potent regulator of neuronal firing rates which can influence the force that can be generated by muscles during voluntary contractions. We firstly outline structural and functional characteristics of the serotonergic system, and then describe how motoneurone discharge can be facilitated and suppressed depending on the 5-HT receptor subtype that is activated. We then provide a narrative on how 5-HT effects can influence voluntary activation during muscle contractions in humans, and detail how 5-HT may be a mediator of exercise-induced fatigue that arises from the central nervous system. Abstract figure legend Inputs to neuromodulatory receptors on motoneurones, such as those involved in the serotonergic system, modify the motoneuroneâ¿¿s responsiveness to ionotropic input. The release of serotonin (5-HT) into the spinal cord is linked to the level of motor activity being performed, where 5-HT can increase the discharge rate of motoneurones via excitatory 5-HT receptors on the soma and dendrites. This in turn can lead to increased voluntary muscle activation (VA) and maximal force generation. However, intense release of 5-HT onto motoneurones may lead to a spill over of 5-HT into extracellular compartments to activate inhibitory 5-HT receptors on the axon initial segment. This can cause a reduction in motoneurone discharge rate, thus decreasing VA and maximal force generation. To gain insight into the serotonergic contributions to muscle activation in humans, pharmacological interventions have been employed to enhance the concentration of 5-HT in the central nervous system or activate selective 5-HT receptors. This article is protected by copyright. All rights reserved.

Pharmacological effects of Eleutherococcus senticosus on the neurological disorders

Eleutherococcus senticosus is a medicinal plant widely used in traditional medicine and edible remedies with effects on anti-fatigue, sleep improvement, and memory enhancement. Recently, the application of E. senticosus to neurological disorders has been a focus. However, its overall pharmacological effect on neural diseases and relevant mechanisms are needed in an in-depth summary. In this review, the traditional uses and the therapeutic effect of E. senticosus on the treatment of fatigue, depression, Alzheimer's disease, Parkinson's disease, and cerebral ischemia were summarized. In addition, the underlying mechanisms involved in the anti-oxidative damage, anti-inflammation, neurotransmitter modulation, improvement of neuronal growth, and anti-apoptosis were discussed. This review will accelerate the understanding of the neuroprotective effects brought from the E. senticosus, and impetus its development as a phytotherapy agent against neurological disorders.

Post-fatigue ability to activate muscle is compromised across a wide range of torques during acute hypoxic exposure

The purpose of this study was to assess how severe acute hypoxia alters the neural mechanisms of muscle activation across a wide range of torque output in a fatigued muscle. Torque and electromyography responses to transcranial and motor nerve stimulation were collected from 10 participants (27 years ± 5 years, 1 female) following repeated performance of a sustained maximal voluntary contraction that reduced torque to 60% of the pre‐fatigue peak torque. Contractions were performed after 2 h of hypoxic exposure and during a sham intervention. For hypoxia, peripheral blood oxygen saturation was titrated to 80% over a 15‐min period and remained at 80% for 2 h. Maximal voluntary torque, electromyography root mean square, voluntary activation and corticospinal excitability (motor evoked potential area) and inhibition (silent period duration) were then assessed at 100%, 90%, 80%, 70%, 50% and 25% of the target force corresponding to the fatigued maximal voluntary contraction. No hypoxia‐related effects were identified for voluntary activation elicited during motor nerve stimulation. However, during measurements elicited at the level of the motor cortex, voluntary activation was reduced at each torque output considered (P = .002, η_p² = .829). Hypoxia did not impact the correlative linear relationship between cortical voluntary activation and contraction intensity or the correlative curvilinear relationship between motor nerve voluntary activation and contraction intensity. No other hypoxia‐related effects were identified for other neuromuscular variables. Acute severe hypoxia significantly impairs the ability of the motor cortex to voluntarily activate fatigued muscle across a wide range of torque output.

Power Loading-Induced Fatigue Is Influenced by Menstrual Cycle Phase

PURPOSE

This study aimed to examine the effects of fatiguing power loading on neuromuscular properties, force production, and metabolic capacities during four phases of the menstrual cycle (MC): menstruation (M), midfollicular (mid FOL), ovulation (OV), and midluteal (mid LUT).

METHODS

Sixteen eumenorrheic women performed sessions of maximal explosive leg press (2 × 10 at 60% one-repetition maximum load with 2-min recovery between sets). Serum hormones and neuromuscular responses were measured.

RESULTS

The loading protocol significantly decreased power (between -14.2% and -12.5%; P < 0.001) and maximal force production (between maximum voluntary force (MVC); -15.0% and -7.8%; P < 0.001-0.05), while decreasing activation level (between AL; -6.9% and -2.2%; P < 0.001-0.05) in all MC phases. The decreases in AL were greater during mid LUT (P < 0.01) compared with OV. Changes in MVC and AL were associated (r2 = 0.53; P < 0.01) at all MC phases. The decrease in EMG during MVC did not differ between the MC phases; however, mean power frequency was higher during M (+7.7%; P < 0.05) and mid LUT (+3.1%; P < 0.05) compared with OV (-7.5%). Resting twitch force decreased during mid FOL (-6.9%; P < 0.05) and mid LUT (-16.2%; P < 0.001), and these values were significantly decreased (P < 0.05) compared with OV. In addition, resting twitch force at mid LUT was lower (P < 0.01) compared with M. Blood lactate levels increased more (P < 0.05) during M compared with mid LUT. Some serum hormone concentrations were associated with fatigue-induced changes in neuromuscular properties and force production, but these correlations behaved differently between the MC phases.

CONCLUSIONS

OV may offer a more favorable hormonal milieu for acute neural responses, whereas mid FOL and mid LUT seem to be superior for acute muscular responses.

5-HT2 receptor antagonism reduces human motoneuron output to antidromic activation but not to stimulation of corticospinal axons

The intrinsic electrical properties of motoneurons strongly affect motoneuron excitability to fast-acting excitatory ionotropic inputs. Serotonin (5-HT) is a neurochemical that alters the intrinsic properties of motoneurons, whereby animal models and in vitro experiments indicate that 5-HT increases motoneuron excitability by activating 5-HT₂ receptors on the somato-dendritic compartment. In the current study, we examined how antagonism of the 5-HT₂ receptor affects motoneuron excitability in humans. We hypothesised that motoneuron excitability would be reduced. The 5-HT₂ antagonist cyproheptadine was administered to ten healthy participants in a double-blinded, placebo-controlled, crossover trial. Electrical cervicomedullary stimulation was used to deliver a synchronised excitatory volley to motoneurons to elicit cervicomedullary motor evoked potentials (CMEPs) in the surface electromyography (EMG) signal of the resting biceps brachii. Likewise, electrical peripheral nerve stimulation was used to generate antidromic spikes in motoneurons and cause recurrent discharges, which were recorded with surface EMG as F-waves in a resting hand muscle. Compared to placebo, we found that 5-HT₂ antagonism reduced the amplitude and persistence of F-waves but did not affect CMEP amplitude. 5-HT₂ antagonism also reduced maximal contraction strength. The reduced recurrent discharge of motoneurons with 5-HT₂ antagonism suggests that 5-HT₂ receptors modulate the electrical properties of the initial segment or soma to promote excitability. Conversely, as cyproheptadine did not affect motoneuron excitability to brief synaptic input, but affected maximal contractions requiring sustained input, it seems likely that the 5-HT₂ mediated amplification of synaptic input at motoneuron dendrites is functionally significant only when excitatory input activates persistent inward currents.

From chronic stress and anxiety to neurodegeneration: Focus on neuromodulation of the axon initial segment

To adapt to the sustained demands of chronic stress, discrete brain circuits undergo structural and functional changes often resulting in anxiety disorders. In some individuals, anxiety disorders precede the development of motor symptoms of Parkinson's disease (PD) caused by degeneration of neurons in the substantia nigra (SN). Here, we present a circuit framework for probing a causal link between chronic stress, anxiety, and PD, which postulates a central role of abnormal neuromodulation of the SN's axon initial segment by brainstem inputs. It is grounded in findings demonstrating that the earliest PD pathologies occur in the stress-responsive, emotion regulation network of the brainstem, which provides the SN with dense aminergic and cholinergic innervation. SN's axon initial segment (AIS) has unique features that support the sustained and bidirectional propagation of activity in response to synaptic inputs. It is therefore, especially sensitive to circuit-mediated stress-induced imbalance of neuromodulation, and thus a plausible initiating site of neurodegeneration. This could explain why, although secondary to pathophysiologies in other brainstem nuclei, SN degeneration is the most extensive. Consequently, the cardinal symptom of PD, severe motor deficits, arise from degeneration of the nigrostriatal pathway rather than other brainstem nuclei. Understanding when and how circuit dysfunctions underlying anxiety can progress to neurodegeneration, raises the prospect of timed interventions for reversing, or at least impeding, the early pathophysiologies that lead to PD and possibly other neurodegenerative disorders.

Polymorphisms of the serotonin receptors genes in patients with bruxism: a systematic review

This study aimed to investigate if SNP rs6313, SNP rs2770304, SNP rs4941573, and SNP rs1923884 of the 5-HT2A receptor gene and SNP rs6295 of the 5-HT1A receptor gene are associated with bruxism etiology.

METHODOLOGY

This systematic review was registered in PROSPERO (CRD42018094561). A search was conducted for articles published in or before May 2021. To qualify for eligibility in this review, the studies had to be case-controls, cohort or cross-sectional. The inclusion criteria were the articles with a group of patients with bruxism and a control group in which the presence of these SNPs was evaluated. The exclusion criteria were the investigations of other polymorphisms, the studies that did not consider a control group for comparison, case reports, and reviews. The NOS and JBI were used to evaluate the methodological quality of studies.

RESULTS

We conducted this study with databases, such as Web of Science, Scopus, Embase, PubMed/MEDLINE, and ProQuest. We considered four studies eligible. A total of 672 participants were included,187 with sleep bruxism, 105 with awake bruxism, 89 with sleep and awake bruxism, and 291 controls. One study found a strong association between the SNPs rs6313, rs2770304 and rs4941573 of the 5-HT2A receptor gene and sleep bruxism. In one study, we considered the C allele of the SNP rs2770304 a risk factor for sleep bruxism. We found no significant results of other SNPs in sleep bruxers compared to controls. We found no positive association concerning the SNPs and groups of awake bruxism and sleep and awake bruxism.

CONCLUSION

The different results regarding the SNPs in sleep bruxers could be explained by the genetic distinction between Chilean, Mexican, Japanese, and Polish population. More clinical trials and prospective studies must be conducted with larger sample size and in different ethnicities to confirm the results of this review.

Psychiatric medication and physical performance parameters - Are there implications for treatment?

INTRODUCTION

The impact of psychiatric medications and their enhancing or impairing effects on physical performance remains inconclusive. Therefore, with this systematic review we provide a comprehensive overview of frequently used psychotropic drugs and their effects on physical performance for the purpose of providing empirical information and deriving prescription and therapy recommendations for clinical practice.

METHODS

We systematically searched PubMed, PsycInfo, and Cochrane databases and extracted human studies investigating the effect of psychotropic drugs on parameters associated with the level of physical performance, such as exercise time, oxygen consumption, heart rate, muscle contraction or blood lactate concentration in physically healthy participants. 36 studies - comprising a broad range of psychotropic agents, such as antidepressants, antipsychotics, sedatives, and stimulants - were selected for final analyses.

RESULTS

Most studies (N = 32) were randomized controlled trials (RCT) with a double-blind crossover design. Antidepressants (N = 21) were the most frequently studied drug class, with contradictory results e.g., performance enhancement in warm environment but not in temperate conditions for bupropion or inconsistent findings between studies for other antidepressants. Antipsychotics (N = 3) mainly showed impairing effects on physical performance, while stimulants (N = 4) were often performance-enhancing. Sedatives (N = 9) may cause a hangover effect.

CONCLUSION

The examined studies with heterogeneous design showed different effects of psychiatric medications on physical performance. Antipsychotics seemed to be performance impairing, while the findings for antidepressants and sedatives were more inconsistent. Stimulants were the only group with consistent performance-enhancing effects. However, most studies were conducted with a small sample size (N < 10), mostly in well-trained subjects rather than in patients with psychiatric disorders, and most studies used single-dose designs. These issues impede the formulation of generalized conclusions for treatment regimes and should therefore be considered in further longitudinal studies for clinically reliable statements. Nevertheless, answering our research question is quite relevant for clinical practice and therapeutic prescription and should be further investigated especially considering the high drop-out rates in drug treatment.

SYSTEMATIC REVIEW REGISTRATION

[https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=276103], identifier [CRD42021276103].

Physiological tremor is suppressed and force steadiness is enhanced with increased availability of serotonin regardless of muscle fatigue

Although there is evidence that 5-HT acts as an excitatory neuromodulator to enhance maximal force generation, it is largely unknown how 5-HT activity influences the ability to sustain a constant force during steady-state contractions. A total of 22 healthy individuals participated in the study, where elbow flexion force was assessed during brief isometric contractions at 10% maximal voluntary contraction (MVC), 60% MVC, MVC, and during a sustained MVC. The selective serotonin reuptake inhibitor, paroxetine, suppressed physiological tremor and increased force steadiness when performing the isometric contractions. In particular, a main effect of drug was detected for peak power of force within the 8-12 Hz range (P = 0.004) and the coefficient of variation (CV) of force (P < 0.001). A second experiment was performed where intermittent isometric elbow flexions (20% MVC sustained for 2 min) were repeatedly performed so that serotonergic effects on physiological tremor and force steadiness could be assessed during the development of fatigue. Main effects of drug were once again detected for peak power of force in the 8-12 Hz range (P = 0.002) and CV of force (P = 0.003), where paroxetine suppressed physiological tremor and increased force steadiness when the elbow flexors were fatigued. The findings of this study suggest that enhanced availability of 5-HT in humans has a profound influence of maintaining constant force during steady-state contractions. The action of 5-HT appears to suppress fluctuations in force regardless of the fatigue state of the muscle.NEW & NOTEWORTHY Converging lines of research indicate that enhanced serotonin availability increases maximal force generation. However, it is largely unknown how serotonin influences the ability to sustain a constant force. We performed two experiments to assess physiological tremor and force steadiness in unfatigued and fatigued muscle when serotonin availability was enhanced in the central nervous system. Enhanced availability of serotonin reduced physiological tremor amplitude and improved steadiness regardless of muscle fatigue.

Blockade of 5-HT2 receptors suppresses rate of torque development and motor unit discharge rate during rapid contractions

Serotonin (5-HT) is a neuromodulator that is critical for regulating the excitability of spinal motoneurons and the generation of muscle torque. However, the role of 5-HT in modulating human motor unit activity during rapid contractions has yet to be assessed. Nine healthy participants (23.7 ± 2.2 yr) ingested 8 mg of the competitive 5-HT₂ antagonist cyproheptadine in a double-blinded, placebo-controlled, repeated-measures experiment. Rapid dorsiflexion contractions were performed at 30%, 50% and 70% of maximal voluntary contraction (MVC), where motor unit activity was assessed by high-density surface electromyographic decomposition. A second protocol was performed where a sustained, fatigue-inducing dorsiflexion contraction was completed prior to undertaking the same 30%, 50% and 70% MVC rapid contractions and motor unit analysis. Motor unit discharge rate (p < 0.001) and rate of torque development (RTD; p = 0.019) for the unfatigued muscle were both significantly lower for the cyproheptadine condition. Following the fatigue inducing contraction, cyproheptadine reduced motor unit discharge rate (p < 0.001) and RTD (p = 0.024), where the effects of cyproheptadine on motor unit discharge rate and RTD increased with increasing contraction intensity. Overall, these results support the viewpoint that serotonergic effects in the central nervous system occur fast enough to regulate motor unit discharge rate during rapid powerful contractions.

Severe acute hypoxia impairs recovery of voluntary muscle activation after sustained submaximal elbow flexion

The purpose of this study was to determine how severe acute hypoxia alters neural mechanisms during, and following, a sustained fatiguing contraction. Fifteen participants (25 ± 3.2 years, six female) were exposed to a sham condition and a hypoxia condition where they performed a 10 min elbow flexor contraction at 20% of maximal torque. For hypoxia, peripheral blood oxygen saturation ( S p O 2 ) was titrated to 80% over a 15 min period and maintained for 2 h. Maximal voluntary contraction torque, EMG root mean square, voluntary activation, rating of perceived muscle fatigue, and corticospinal excitability (motor-evoked potential) and inhibition (silent period duration) were then assessed before, during and for 6 min after the fatiguing contraction. No hypoxia-related effects were identified for neuromuscular variables during the fatigue task. However, for recovery, voluntary activation assessed by motor point stimulation of biceps brachii was lower for hypoxia than sham at 4 min (sham: 89% ± 7%; hypoxia: 80% ± 12%; P = 0.023) and 6 min (sham: 90% ± 7%; hypoxia: 78% ± 11%; P = 0.040). Similarly, voluntary activation (P = 0.01) and motor-evoked potential area (P = 0.002) in response to transcranial magnetic stimulation of the motor cortex were 10% and 11% lower during recovery for hypoxia compared to sham, respectively. Although an S p O 2 of 80% did not affect neural activity during the fatiguing task, motor cortical output and corticospinal excitability were reduced during recovery in the hypoxic environment. This was probably due to hypoxia-related mechanisms involving supraspinal motor circuits. KEY POINTS: Acute hypoxia has been shown to impair voluntary activation of muscle and alter the excitability of the corticospinal motor pathway during exercise. However, little is known about how hypoxia alters the recovery of the motor system after performing fatiguing exercise. Here we assessed hypoxia-related responses of motor pathways both during active contractions and during recovery from active contractions, with transcranial magnetic stimulation and motor point stimulation of the biceps brachii. Fatiguing exercise caused reductions in voluntary activation, which was exacerbated during recovery from a 10 min sustained elbow flexion in a hypoxic environment. These results suggest that reductions in blood oxygen concentration impair the ability of motor pathways in the CNS to recover from fatiguing exercise, which is probably due to hypoxia-induced mechanisms that reduce output from the motor cortex.

Construction and Evaluation of a Rat Model of Postpartum Fatigue

OBJECTIVES

The aim of the study was to construct and evaluate a rat model of postpartum fatigue.

DESIGN

This is an article about animal model building.

METHODS

Sprague-Dawley rats on the 1st day after delivery were randomized into control group and fatigue group. The deep sleep of rats was interfered with by forcing them to stand in water, to make the rats experience mental and physical fatigue. To maintain galactosis and lactation, rats and pups were caged for 90 min after every 3 h of separation. The control group was separated routinely without any stimulus. The model was evaluated from mental and physical fatigue on the 8th day and 15th day. The mental fatigue was evaluated by a water maze test and the rat's 5-hydroxytryptamine (5-HT) level in hippocampus, while the physical fatigue was evaluated using lactic acid level in serum and duration of weight-loaded forced swimming.

RESULTS

Among the 7-day and 14-day modeling groups, compared with the control group, the success rate of water maze landing was significantly decreased, the time for water maze landing was significantly prolonged and 5-HT level in hippocampus significantly decreased in the fatigue group. With respect to physical fatigue, among the 7-day and 14-day modeling groups, the lactic acid level in serum in the fatigue group was significantly increased, and the duration of exhaustive swimming of rats was significantly shortened.

LIMITATIONS

A small sample size was the main limitation of this study.

CONCLUSIONS

We have successfully constructed a rat model of postpartum fatigue by forcing postpartum rats to stand in water, which was similar to a level of stress that contributes to the development of postpartum fatigue. Our model opens the door for future studies evaluating the effectiveness of pharmacological and behavioral therapies.

Tai Chi postural training for dyskinesia rehabilitation: a study protocol for a randomised controlled trial in convalescent ischaemic stroke patients

INTRODUCTION

Acute ischaemic stroke (AIS) is not only seriously damaging to the physical and mental health of patients, but also has become a major social public health problem. Effective dyskinesia rehabilitation treatment in convalescence is of great significance for AIS patients' prognosis and quality of life. Tai Chi (TC) shows great potential in improving motor function. This trial aims to evaluate the clinical efficacy of modified TC postural training (TPT), and to explore the related central-peripheral neurotransmitter mechanisms.

METHODS/DESIGN

The proposed study will be a multicentre randomised controlled trial. The trial will randomise 120 eligible AIS patients in a 1:1 ratio to receive TPT or Bobath rehabilitation training. Each training session will last 40 min and will be implemented once a day and five times per week (from Monday to Friday) in a duration of 4 weeks. After finishing the 4-week treatment, another 3-month follow-up period will be seen. Root mean square generated from the surface electromyogram (sEMG) will be the primary outcome. Other sEMG time-domain parameters and frequency-domain parameters and clinical scales assessment will be the secondary outcomes. Peripheral blood samples will be collected at baseline and at the end of 4-week treatment, which will be used to explore the related therapeutic mechanisms. Intention-to-treat analysis and per-protocol analysis will both be implemented in this trial.

ETHICS AND DISSEMINATION

The study has been approved by Ethics Committee of Dongzhimen Hospital Affiliated to Beijing University of Chinese Medicine, being granted approval numbers DZMEC-KY-2020-22. The research results will be disseminated through (open access) peer-reviewed publications and presentations at conferences.

TRIAL REGISTRATION NUMBER

ChiCTR2000032999.

Estimates of persistent inward currents increase with the level of voluntary drive in low-threshold motor units of plantar flexor muscles

This study tested whether estimates of persistent inward currents (PICs) in the human plantar flexors would increase with the level of voluntary drive. High-density surface electromyograms were collected from soleus and gastrocnemius medialis of 21 participants (29.2 ± 2.6 yr) during ramp-shaped isometric contractions to 10%, 20%, and 30% (torque rise and decline of 2%/s and 30-s duration) of each participant's maximal torque. Motor units identified in all the contraction intensities were included in the paired-motor unit analysis to calculate delta frequency (ΔF) and estimate the PICs. ΔF is the difference in discharge rate of the control unit at the time of recruitment and derecruitment of the test unit. Increases in PICs were observed from 10% to 20% [Δ = 0.6 pulse per second (pps); P < 0.001] and from 20% to 30% (Δ = 0.5 pps; P < 0.001) in soleus and from 10% to 20% (Δ = 1.2 pps; P < 0.001) but not from 20% to 30% (Δ = 0.09 pps; P = 0.724) in gastrocnemius medialis. Maximal discharge rate increased for soleus and gastrocnemius medialis from 10% to 20% [Δ = 1.75 pps (P < 0.001) and Δ = 2.43 pps (P < 0.001), respectively] and from 20% to 30% [Δ = 0.80 pps (P < 0.017) and Δ = 0.92 pps (P = 0.002), respectively]. The repeated-measures correlation identified associations between ΔF and increases in maximal discharge rate for soleus (r = 0.64; P < 0.001) and gastrocnemius medialis (r = 0.77; P < 0.001). An increase in voluntary drive tends to increase PIC strength, which has key implications for the control of force but also for comparisons between muscles or studies when relative force levels might be different. Increases in voluntary descending drive amplify PICs in humans and provide an important spinal mechanism for motor unit discharging, and thus force output modulation.NEW & NOTEWORTHY Animal experiments and computational models have shown that motor neurons can amplify the synaptic input they receive via persistent inward currents. Here we show in humans that this amplification varies proportionally to the magnitude of the voluntary drive to the muscle.

Reductions in motoneuron excitability during sustained isometric contractions are dependent on stimulus and contraction intensity

Cervicomedullary stimulation provides a means of assessing motoneuron excitability. Previous studies demonstrated that during low-intensity sustained contractions, small cervicomedullary evoked potentials (CMEPs) conditioned using transcranial magnetic stimulation (TMS-CMEPs) are reduced, whereas large TMS-CMEPs are less affected. As small TMS-CMEPs recruit motoneurons most active during low-intensity contractions whereas large TMS-CMEPs recruit a high proportion of motoneurons inactive during the task, these results suggest that reductions in motoneuron excitability could be dependent on repetitive activation. To further test this hypothesis, this study assessed changes in small and large TMS-CMEPs across low- and high-intensity contractions. Twelve participants performed a sustained isometric contraction of the elbow flexor for 4.5 min at the electromyography (EMG) level associated with 20% maximal voluntary contraction force (MVC; low intensity) and 70% MVC (high intensity). Small and large TMS-CMEPs with amplitudes of ∼15% and ∼50% M_max at baseline, respectively, were delivered every minute throughout the tasks. Recovery measures were taken at 1-, 2.5- and 4-min postexercise. During the low-intensity trial, small TMS-CMEPs were reduced at 2-4 min (P ≤ 0.049) by up to -10% M_max, whereas large TMS-CMEPs remained unchanged (P ≥ 0.16). During the high-intensity trial, small and large TMS-CMEPs were reduced at all time points (P < 0.01) by up to -14% and -33% M_max, respectively, and remained below baseline during all recovery measures (P ≤ 0.02). TMS-CMEPs were unchanged relative to baseline during recovery following the low-intensity trial (P ≥ 0.24). These results provide novel insight into motoneuron excitability during and following sustained contractions at different intensities and suggest that contraction-induced reductions in motoneuron excitability depend on repetitive activation.NEW & NOTEWORTHY This study measured motoneuron excitability using cervicomedullary evoked potentials conditioned using transcranial magnetic stimulation (TMS-CMEPs) of both small and large amplitudes during sustained low- and high-intensity contractions of the elbow flexors. During the low-intensity task, only the small TMS-CMEP was reduced. During the high-intensity task, both small and large TMS-CMEPs were substantially reduced. These results indicate that repetitively active motoneurons are specifically reduced in excitability compared with less active motoneurons in the same pool.

Muscarinic receptor blockade causes postcontraction enhancement in corticospinal excitability following maximal contractions

Although synaptic transmission in motor pathways can be regulated by neuromodulators, such as acetylcholine, few studies have examined how cholinergic activity affects cortical and spinal motor circuits following muscle contractions of varying intensities. This was a human, double-blinded, placebo-controlled, crossover study. Participants attended two sessions where they were administered either a placebo or 25 mg of promethazine. Electromyography of the abductor digiti minimi (ADM) was measured for all conditions. Motor evoked potentials (MEPs) were obtained via motor cortical transcranial magnetic stimulation (TMS), and F waves were obtained via ulnar nerve electrical stimulation. MEPs and F waves were examined: 1) when the muscle was at rest; 2) after the muscle had been active; and 3) after the muscle had been fatigued. MEPs were unaffected by muscarinic receptor blockade when measurements were recorded from resting muscle or following a 50% isometric maximal voluntary contraction (MVC). However, muscarinic receptor blockade increased MEP area following a 10-s MVC (P = 0.019) and following a fatiguing 60-s MVC (P = 0.040). F wave area and persistence were not affected by promethazine for any muscle contraction condition. Corticospinal excitability was influenced by cholinergic effects when voluntary drive to the muscle was high. Given that spinal motoneurone excitability remained unaffected, it is likely that cholinergic effects are influential within the motor cortex during strong muscle contractions. Future research should evaluate how cholinergic effects alter the relationship between subcortical structures and the motor cortex, as well as brainstem neuromodulatory pathways and spinal motoneurons.NEW & NOTEWORTHY The relationship between motor function and cholinergic circuitry in the central nervous system is complex. Although many studies have approached this issue at the cellular level, few studies have examined cholinergic mechanisms in humans performing muscle contractions. This study demonstrates that blockade of muscarinic acetylcholine receptors enhances motor evoked potentials (elicited with transcranial magnetic stimulation) following strong muscle contractions, but not weak muscle contractions.

Human corticospinal-motoneuronal output is reduced with 5-HT2 receptor antagonism

Animal models indicate that serotonin (5-HT) release onto motoneurons facilitates motor output, particularly during strong motor activities. However, evidence for 5-HT effects during human movement are limited. This study examined how antagonism of the 5-HT₂ receptor, which is a 5-HT receptor that promotes motoneuron excitability, affects human movement. Ten healthy participants (24.2 ± 1.9 yr) ingested 8 mg of cyproheptadine (competitive 5-HT₂ antagonist) in a double-blinded, placebo-controlled, repeated-measures design. Transcranial magnetic stimulation (TMS) of the motor cortex was used to elicit motor evoked potentials (MEPs) from biceps brachii. First, stimulus-response curves (90%-160% active motor threshold) were obtained during very weak elbow flexions (10% of maximal). Second, to determine if 5-HT effects are scaled to the intensity of muscle contraction, TMS at a fixed intensity was applied during elbow flexions of 20%, 40%, 60%, 80%, and 100% of maximal. Cyproheptadine reduced the size of MEPs across the stimulus-response curves (P = 0.045). Notably, MEP amplitude was 22.3% smaller for the cyproheptadine condition for the strongest TMS intensity. In addition, cyproheptadine reduced maximal torque (P = 0.045), lengthened the biceps silent period during maximal elbow flexions (P = 0.037), and reduced superimposed twitch amplitude during moderate-intensity elbow flexions (P = 0.035). This study presents novel evidence that 5-HT₂ receptors influence corticospinal-motoneuronal output, which was particularly evident when a large number of descending inputs to motoneurons were active. Although it is likely that antagonism of 5-HT₂ receptors reduces motoneuron gain to ionotropic inputs, supraspinal mechanisms may have also contributed to the study findings.NEW & NOTEWORTHY Voluntary contractions and responses to magnetic stimulation of the motor cortex are dependent on serotonin activity in the central nervous system. 5-HT₂ antagonism decreased evoked potential size to high-intensity stimulation, and reduced torque and lengthened inhibitory silent periods during maximal contractions. We provide novel evidence that 5-HT₂ receptors are involved in muscle activation, where 5-HT effects are strongest when a large number of descending inputs activate motoneurons.

Inhibition and facilitation of nociceptively evoked muscular activity by fentanyl or dexmedetomidine in isoflurane-anaesthetized pigs

OBJECTIVE

To investigate motor and cardiovascular responses to dexmedetomidine or fentanyl in isoflurane-anaesthetized pigs.

STUDY DESIGN

Experimental, balanced, block randomized, two-group design.

ANIMALS

A group of 16 crossbred pigs, 55 ± 8 days (mean ± standard deviation) old.

METHODS

Deltoid electromyography (EMG) was recorded during isoflurane anaesthesia. Electrical stimulation using 5, 10, 20 and 40 mA of the distal right thoracic limb elicited a nociceptive withdrawal reflex (NWR), quantified by the area under the curve (AUC) for the simulation intensity versus EMG amplitude response curve. Latency to movement evoked by clamping a claw for maximum 60 seconds was noted. Arterial blood pressure and pulse rate were recorded. Data were sampled at baseline and during dexmedetomidine 0.25, 0.5, 1.0, 2.0, 4.0 and 8.0 μg kg^-1 hour^-1 or fentanyl 5, 10, 20, 40, 80 and 160 μg kg^-1 hour^-1 infusions. The influence of infusion rate on NWR AUC and spontaneous EMG was analysed using a mixed model, with p < 5%.

RESULTS

NWR AUC increased at fentanyl 5 μg kg^-1 hour^-1 but decreased at fentanyl 40, 80 and 160 μg kg^-1 hour^-1 and dexmedetomidine 4.0 and 8.0 μg kg^-1 hour^-1. All pigs at fentanyl 80 μg kg^-1 hour^-1, and three pigs at dexmedetomidine 8.0 μg kg^-1 hour^-1 had mechanical latencies greater than 60 seconds. Spontaneous EMG activity increased accompanied by visually evident 'shivering' at fentanyl 5, 10 and 20 μg kg^-1 hour^-1 but decreased at dexmedetomidine 2, 4 and 8 μg kg^-1 hour^-1. Clinically relevant effects of increasing infusion rates on blood pressure or pulse rate were not observed.

CONCLUSION AND CLINICAL RELEVANCE

If anaesthetic plane or antinociception is evaluated in pigs, response to claw clamping and NWR will not necessarily give uniform results when comparing drugs. If only one method is used, results should be interpreted cautiously.

Graded doses of grape seed methanol extract attenuated ataxic movement, cerebellocortical toxicity, and impairment following chronic carbamazepine treatment in male Wistar rats

This study investigated the effects of chronic cotreatment of carbamazepine (CBZ) with grape seed methanolic extract (GSME) on the markers of neurotoxicity and motor coordination in male rats. Thirty male Wistar rats were randomized into 5 groups (n = 6) and treated orally with propylene glycol (PG 0.1 ml/day), CBZ (25 mg/kg), CBZ (25 mg/kg) + GSME (200 mg/kg), CBZ (25 mg/kg) + GSME (100 mg/kg), or CBZ (25 mg/kg) + GSME (50 mg/kg) for 28 days. Thereafter, the animals were subjected to motor-coordination tests and, eventually, sacrificed by cervical dislocation. The cranium was opened and the brain excised. The prefrontal cortex (PFC) and cerebellum were homogenized for the biochemical assessment, while representative brain was fixed in 10% neutral-buffered formalin for the histomorphological investigation. The results were presented as mean ± SEM, analyzed using one-way analysis of variance (ANOVA) and Student-Newman-Keuls post hoc analysis where appropriate, while p < 0.05 was considered statistically significant. Indices of motor coordination were significantly (p = 0.0014) impaired with a significant (p = 0.0001) increase in the concentration of malondialdehyde (MDA) in the PFC and cerebellar tissue. In addition, the activities of glutathione increased (p = 0.0001) significantly in the CBZ + GSME-treated rats. All these anomalies were attenuated in the CBZ + GSME treated rats. Coadministration of GSME with CBZ may ameliorate CBZ-induced neurotoxicity, histoarchitectural disorganization of PFC and cerebellum with resultant effect on fine motor actions.

Neurophysiological relationship of neuromuscular fatigue and stress disorder in PTSD patients

INTRODUCTION

Apart from mental disorders, other complications that have been reported in some patients with Post-Traumatic Stress Disorder (PTSD), include physical pain and being quick to fatigue, which can severely affect the patients' daily life. Therefore, this study aims to evaluate the relationship between PTSD and physical fatigue in people with PTSD.

METHOD

18 military men with (n = 9) and without PTSD (n = 9), with an age range of 45-60 years, volunteered to participate. They were randomly assigned into two groups: PTSD and non-PTSD groups. Recording of the surface electromyography (EMG) in a specific muscle was conducted twice in both groups, once at baseline and then again after a single session of fatiguing exercise. Data were analyzed by ANOVA with repeated measure (2✕2) at the significance level of 0.05.

RESULTS

Results showed that there was a significant main effect of intervention on electrical activity and neural conduction variables in the PTSD group (p = 0.04, p = 0.02). There was also an effect of time for the both variables (P < 0.001).

CONCLUSION

Stress disorders may affect the time to fatigue in PTSD patients and subsequently cause some difficulties in their daily life.

Serotonergic Modulation of Locomotor Activity From Basal Vertebrates to Mammals

During the last 50 years, the serotonergic (5-HT) system was reported to exert a complex modulation of locomotor activity. Here, we focus on two key factors that likely contribute to such complexity. First, locomotion is modulated directly and indirectly by 5-HT neurons. The locomotor circuitry is directly innervated by 5-HT neurons in the caudal brainstem and spinal cord. Also, indirect control of locomotor activity results from ascending projections of 5-HT cells in the rostral brainstem that innervate multiple brain centers involved in motor action planning. Second, each approach used to manipulate the 5-HT system likely engages different 5-HT-dependent mechanisms. This includes the recruitment of different 5-HT receptors, which can have excitatory or inhibitory effects on cell activity. These receptors can be located far or close to the 5-HT release sites, making their activation dependent on the level of 5-HT released. Here we review the activity of different 5-HT nuclei during locomotor activity, and the locomotor effects of 5-HT precursors, exogenous 5-HT, selective 5-HT reuptake inhibitors (SSRI), electrical or chemical stimulation of 5-HT neurons, genetic deletions, optogenetic and chemogenetic manipulations. We highlight both the coherent and controversial aspects of 5-HT modulation of locomotor activity from basal vertebrates to mammals. This mini review may hopefully inspire future studies aiming at dissecting the complex effects of 5-HT on locomotor function.

Current anti-depressant use is associated with cortical bone deficits and reduced physical function in elderly women

BACKGROUND

Anti-depressants, particularly selective serotonin reuptake inhibitors (SSRIs), are associated with an increased risk of fracture. The mechanism is unclear and may be due to effects on bone metabolism, muscle strength, falls or other factors. It is unknown if serotonin norepinephrine reuptake inhibitors (SNRIs) have similar effects.

METHODS

We compared musculoskeletal health in current female anti-depressant users and non-users from a population-based multiethnic (35.6% black, 22.3% white and 42.1% mixed) cohort study of adults ≥65 years old in New York (N = 195) using dual x-ray absorptiometry (DXA), trabecular bone score (TBS), vertebral fracture assessment (VFA), high resolution peripheral quantitative computed tomography (HR-pQCT), body composition, and grip strength.

RESULTS

Current anti-depressant users were more likely to be white than non-white (OR 1.9, 95% CI 1.2-2.9) and were shorter than non-users, but there were no differences in age, weight, BMI, physical activity, calcium/vitamin D intake, falls or self-rated health. There were more pelvic fractures in current vs. non-users (7.1% vs. 0%, p = 0.04). Age- and weight-adjusted T-score by DXA was lower in current users at the 1/3-radius (-1.6 ± 1.1 vs. -1.0 ± 1.4, p = 0.04) site only. There was no difference in TBS, vertebral fractures or fat/lean mass by DXA. Age- and weight-adjusted grip strength was 13.3% lower in current users vs. non-users (p = 0.04). By HR-pQCT, age- and weight-adjusted cortical volumetric BMD (Ct. vBMD) was 4.8% lower in users vs. non-users at the 4% radius site (p = 0.007). A similar cortical pattern was seen at the proximal (30%) tibia. When assessed by anti-depressant class, deteriorated cortical microstructure was present only in SSRI users at the radius and only in SNRI users at the proximal tibia.

CONCLUSIONS

Anti-depressant use is associated with cortical deterioration and reduced physical function, but effects may be class-specific. These findings provide insight into the mechanism by which anti-depressants may contribute to the increased fracture risk in older women.

Physiological sex differences affect the integrative response to exercise: acute and chronic implications

NEW FINDINGS

What is the topic of this review? We review sex differences within physiological systems implicated in exercise performance; specifically, how they integrate to determine metabolic thresholds and fatigability. Thereafter, we discuss the implications that these sex differences might have for long-term adaptation to exercise. What advances does it highlight? The review collates evidence from recent physiological studies that have investigated sex as a biological variable, demonstrating that the physiological response to equivalent 'dosages' of exercise is not the same in males and females; thus, highlighting the need to research diversity in physiological responses to interventions.

ABSTRACT

The anatomical and physiological differences between males and females are thought to determine differences in the limits of human performance. The notion of studying sex as a biological variable has recently been emphasized in the biosciences as a vital step in enhancing human health. In this review, we contend that the effects of biological sex on acute and chronic responses must be studied and accounted for when prescribing aerobic exercise, much like any intervention targeting the optimization of physiological function. Emerging evidence suggests that the response of physiological systems to exercise differs between males and females, potentially mediating the beneficial effects in healthy and clinical populations. We highlight evidence that integrative metabolic thresholds during exercise are influenced by phenotypical sex differences throughout many physiological systems. Furthermore, we discuss evidence that female skeletal muscle is more resistant to fatigue elicited by equivalent dosages of high-intensity exercise. How the different acute responses affect the long-term trainability of males and females is considered, with discussion about tailoring exercise to the characteristics of the individual presented within the context of biological sex. Finally, we highlight the influence of endogenous and exogenous sex hormones on physiological responses to exercise in females. Sex is one of many mediating influences on the outcomes of exercise, and with careful experimental designs, physiologists can advance the collective understanding of diversity in physiology and optimize outcomes for both sexes.

Inability to increase the neural drive to muscle is associated with task failure during submaximal contractions

Motor unit firing and contractile properties during a submaximal contraction until failure were assessed with a new tracking technique. Two distinct phases in firing behavior were observed, which compensated for changes in twitch area and predicted time to failure. However, the late increase in firing rate was below the rates attained in the absence of fatigue, which points to an inability of the central nervous system to sufficiently increase the neural drive to muscle with fatigue.

Early life fluoxetine treatment causes long-term lean phenotype in skeletal muscle of rats exposed to maternal lard-based high-fat diet

There is a concern about early life exposure to Selective Serotonin Reuptake Inhibitors (SSRI) in child development and motor system maturation. Little is known, however, about the interaction of environmental factors, such as maternal nutrition, associated with early exposure to SSRI. The increased maternal consumption of high-fat diets is worrisome and affects serotonin system development with repercussions in body phenotype. This study aimed to assess the short- and long-term effects of neonatal fluoxetine treatment on the body and skeletal muscle phenotype of rats exposed to a maternal lard-based high-fat (H) diet during the perinatal period. A maternal lard-based high-fat diet causes reduced birth weight, a short-term reduction in type IIA fibers in the soleus muscle, and in type IIB fibers in the Extensor Digitorum Longus (EDL) muscle, reducing Lactate Dehydrogenase (LDH) activity in both muscles. In the long-term, the soleus showed reduced muscle weight, smaller area and perimeter of muscle fibers, while the EDL muscle showed reduced Citrate Synthase (CS) activity in offspring from the rats on the maternal lard-based high-fat diet. Early-life exposure to fluoxetine reduced body weight and growth and reduced soleus weight and enzymatic activity in young rats. Exposure to neonatal fluoxetine in adult rats caused a decreased body mass index, less food intake, and reduced muscle weight with reduced CS and LDH activity. Neonatal fluoxetine in young rats exposed to a maternal lard-based high-fat diet caused reduced body weight and growth, reduced soleus weight as well as area and perimeter of type I muscle fibers. In adulthood, there was a reduction in food intake, increased proportion of IIA type fibers, reduced area and perimeter of type IIB, and reduction in levels of CS activity in EDL muscle. Neonatal fluoxetine treatment in rats exposed to a maternal lard-based, high-fat diet induces a reduction in muscle weight, an increase in the proportion of oxidative fibers and greater oxidative enzymatic activity in adulthood.

Time course of neuromuscular responses to acute hypoxia during voluntary contractions

NEW FINDINGS

What is the central question of this study? How does acute hypoxia alter central and peripheral fatigue during brief and sustained maximal voluntary muscle contractions? What is the main finding and its importance? Perception of fatigue during muscle contractions was increased progressively for 2 h after hypoxic exposure. However, an increase in motor cortex excitability and a decrease in voluntary activation of skeletal muscle were observed across the entire protocol when performing brief (3 s) maximal contractions. These adaptations were abolished if the brief contraction was held for a duration of 20 s, which was presumably attributable to a successful redistribution of blood to overcome the reduced oxygen content.

ABSTRACT

Few studies have examined the time course of changes in the motor system after acute exposure to hypoxia. Thus, the purpose of this study was to examine how acute hypoxia affects corticospinal excitability, voluntary activation (VA) and the perception of fatigue during brief (3 s) and sustained (20 s) maximal voluntary contractions (MVCs). Fourteen healthy individuals (23 ± 2.2 years of age; four female) were exposed to hypoxia and sham conditions. During hypoxia, peripheral blood oxygen saturation was titrated over a 15 min period and remained at 80% during testing. Corticospinal excitability and VA were assessed before titration (Pre), 0, 1 and 2 h after. At each time point, the brief and sustained elbow flexion MVCs were performed. Motor evoked potentials (MEPs) were obtained using transcranial magnetic stimulation. Superimposed and resting twitches were obtained from motor point stimulation of biceps brachii to calculate the level of VA, and ratings of perceived fatigue were obtained with a modified CR-10 Borg scale. A condition-by-time interaction was detected for the CR-10 Borg scale, whereby perception of fatigue increased progressively throughout the hypoxia protocol. However, main effects of MEP area and VA indicated that corticospinal excitability increased, and VA of the biceps brachii decreased, throughout the hypoxia protocol. Given that these changes in MEP area and VA were seen only when performing the brief MVCs (and not during the sustained MVCs), performing longer contractions might overcome reduced oxygen content by redirecting blood flow to active areas of the motor system.

The serotonin reuptake blocker citalopram destabilizes fictive locomotor activity in salamander axial circuits through 5-HT1A receptors

Serotoninergic (5-HT) neurons are powerful modulators of spinal locomotor circuits. Most studies on 5-HT modulation focused on the effect of exogenous 5-HT and these studies provided key information about the cellular mechanisms involved. Less is known about the effects of increased release of endogenous 5-HT with selective serotonin reuptake inhibitors. In mammals, such molecules were shown to destabilize the fictive locomotor output of spinal limb networks through 5-HT_1A receptors. However, in tetrapods little is known about the effects of increased 5-HT release on the locomotor output of axial networks, which are coordinated with limb circuits during locomotion from basal vertebrates to mammals. Here, we examined the effect of citalopram on fictive locomotion generated in axial segments of isolated spinal cords in salamanders, a tetrapod where raphe 5-HT reticulospinal neurons and intraspinal 5-HT neurons are present as in other vertebrates. Using electrophysiological recordings of ventral roots, we show that fictive locomotion generated by bath-applied glutamatergic agonists is destabilized by citalopram. Citalopram-induced destabilization was prevented by a 5-HT_1A receptor antagonist, whereas a 5-HT_1A receptor agonist destabilized fictive locomotion. Using immunofluorescence experiments, we found 5-HT-positive fibers and varicosities in proximity with motoneurons and glutamatergic interneurons that are likely involved in rhythmogenesis. Our results show that increasing 5-HT release has a deleterious effect on axial locomotor activity through 5-HT_1A receptors. This is consistent with studies in limb networks of turtle and mouse, suggesting that this part of the complex 5-HT modulation of spinal locomotor circuits is common to limb and axial networks in limbed vertebrates.NEW & NOTEWORTHY Little is known about the modulation exerted by endogenous serotonin on axial locomotor circuits in tetrapods. Using axial ventral root recordings in salamanders, we found that a serotonin reuptake blocker destabilized fictive locomotor activity through 5-HT_1A receptors. Our anatomical results suggest that serotonin is released on motoneurons and glutamatergic interneurons possibly involved in rhythmogenesis. Our study suggests that common serotoninergic mechanisms modulate axial motor circuits in amphibians and limb motor circuits in reptiles and mammals.

Unlike voluntary contractions, stimulated contractions of a hand muscle do not reduce voluntary activation or motoneuronal excitability

Voluntary force declines during sustained, maximal voluntary contractions (MVC) due to changes in muscle and central nervous system properties. Central fatigue, an exercise-induced reduction in voluntary activation, is influenced by multiple processes. Some may occur independently of descending voluntary drive. To differentiate the effects associated with voluntary drive from other central and peripheral influences, we measured voluntary activation and motoneuron excitability following fatiguing contractions produced voluntarily or by electrical stimulation. On two separate days, participants performed either a 2-min MVC of adductor pollicis muscle or received 2-min continuous supramaximal electrical stimulation of the ulnar nerve. In study 1 (n = 14), the superimposed twitch elicited by ulnar nerve stimulation during brief MVCs was increased, and, hence, voluntary activation was reduced, up to 240 s after the 2-min MVC [-20 ± 12% (SD), P = 0.002] but not the 2-min stimulated contraction (-4 ± 7%), despite large reductions in MVC force (voluntary, -54 ± 18%; stimulated, -46 ± 16%). In study 2 (n = 12), F-waves recorded from the adductor pollicis were reduced in area for 150 s following the 2-min MVC (-21 ± 16%, P = 0.007) but not after the stimulated contraction (5 ± 27%). Therefore, voluntary activation and motoneuron excitability decreased only when descending voluntary drive was present during the fatiguing task. The findings do not exclude a cortical or brain stem contribution to the reduced voluntary activation but suggest that neither sensory feedback from the fatigued muscle nor repetitive activation of motoneurons underlie the changes, whereas they are consistent with motoneuronal inhibition by released factors linked to voluntary drive.NEW & NOTEWORTHY We demonstrate that reductions in voluntary activation and motoneuron excitability following 2-min isometric maximal contractions in humans occur only when fatigue is produced through voluntary contractions and not through electrically stimulated contractions. This is contrary to studies that suggest that changes in the superimposed twitch and therefore voluntary activation are explained by changes in peripheral factors alone. Thus, the interpolated twitch technique remains a viable tool to assess voluntary activation and central fatigue.

Sex differences in muscle activity emerge during sustained low-intensity contractions but not during intermittent low-intensity contractions

Sex differences in motor performance may arise depending on the mode of contraction being performed. In particular, contractions that are held for long durations, rather than contractions that are interspersed with rest periods, may induce greater levels of fatigue in men compared to women. The purpose of this study was to examine fatigue responses in a cohort of healthy men (n = 7, age [mean] = 21.6 ± [SD] 1.1 year) and women (n = 7, age: 22.0 ± 2.0 year) during sustained isometric and intermittent isometric contractions. Two contraction protocols were matched for intensity (20% MVC) and total contraction time (600-s). Biceps brachii EMG and elbow flexion torque steadiness were examined throughout each protocol, and motor nerve stimulation was used to quantify central and peripheral fatigue. Overall, there were few sex-related differences in the fatigue responses during intermittent contractions. However, men exhibited progressively lower maximal torque generation (39% versus 27% decrease), progressively greater muscle activity (220% versus 144% increase), progressively greater declines in elbow flexion steadiness (354% versus 285% decrease), and progressively greater self-perception of fatigue (Borg scale: 8.8 ± 1.2 versus 6.3 ± 1.1) throughout the sustained contractions. The mechanism underlying fatigue responses had a muscle component, as voluntary activation of the biceps brachii did not differ between sexes, but the amplitude of resting twitches decreased throughout the sustained contractions (m: 32%, w: 10% decrease). As generating large sustained forces causes a progressive increase in intramuscular pressure and mechanical occlusion-which has the effect of enhancing metabolite accumulation and peripheral fatigue-it is likely that the greater maximal strength of men contributed to their exacerbated levels of fatigue.

Effects of a commercially available branched-chain amino acid-alanine-carbohydrate-based sports supplement on perceived exertion and performance in high intensity endurance cycling tests

Background

Sports nutritional supplements containing branched-chain amino acids (BCAA) have been widely reported to improve psychological and biological aspects connected to central fatigue and performance in endurance exercise, although the topic is still open to debate. The aim of the present study was to determine whether the intake of a commercially available BCAA-based supplement, taken according to the manufacturer’s recommendations, could affect the rating of perceived exertion (RPE) and performance indexes at the beginning (1d) and end of a 9-week (9w) scheduled high intensity interval training program, with an experimental approach integrating the determination of psychometric, performance, metabolic and blood biochemical parameters.

Methods

This was a randomized double-blind placebo-controlled study. Thirty-two untrained, healthy young adults (20 males and 12 female) were enrolled. A high-intensity endurance cycling (HIEC) test was used to induce fatigue in the participants: HIEC consisted in ten 90 s sprints interspersed by ten 3 min recovery phases and followed by a final step time to exhaustion was used. In parallel with RPE, haematological values (creatine kinase, alanine, BCAA, tryptophan, ammonia and glucose levels), and performance indexes (maximal oxygen consumption - VO_2max, power associated with lactate thresholds - W_LT1, W_LT2 and time to exhaustion - TTE) were assessed. All subject took the supplement (13.2 g of carbohydrates; 3.2 g of BCAA and 1.6 g of L-alanine per dose) or placebo before each test and training session. Dietary habits and training load were monitored during the entire training period.

Results

The administration of the supplement (SU) at 1d reduced RPE by 9% during the recovery phase, as compared to the placebo (PL); at 9w the RPE scores were reduced by 13 and 21% during the sprint and recovery phase, respectively; at 9w, prolonged supplement intake also improved TTE and TRIMP. SU intake invariably promoted a rapid increase (within 1 h) of BCAA serum blood levels and prevented the post-HIEC tryptophan: BCAA ratio increase found in the PL group, at both 1d and 9w. There was no difference in dietary habits between groups and those habits did not change over time; no difference in glycemia was found between SU and PL. VO_2max, W_LT1 and W_LT2 values improved over time, but were unaffected by supplement intake.

Conclusions

On the whole, these results suggest that i) the intake of the BCAA-based commercially available supplement used in this study reduces RPE as a likely consequence of an improvement in the serum tryptophan: BCAA ratio; ii) over time, reduced RPE allows subjects to sustain higher workloads, leading to increased TRIMP and TTE.

Mutual Interactions among Exercise, Sport Supplements and Microbiota

The adult gut microbiota contains trillions of microorganisms of thousands of different species. Only one third of gut microbiota are common to most people; the rest are specific and contribute to enhancing genetic variation. Gut microorganisms significantly affect host nutrition, metabolic function, immune system, and redox levels, and may be modulated by several environmental conditions, including physical activity and exercise. Microbiota also act like an endocrine organ and is sensitive to the homeostatic and physiological changes associated with training; in turn, exercise has been demonstrated to increase microbiota diversity, consequently improving the metabolic profile and immunological responses. On the other side, adaptation to exercise might be influenced by the individual gut microbiota that regulates the energetic balance and participates to the control of inflammatory, redox, and hydration status. Intense endurance exercise causes physiological and biochemical demands, and requires adequate measures to counteract oxidative stress, intestinal permeability, electrolyte imbalance, glycogen depletion, frequent upper respiratory tract infections, systemic inflammation and immune responses. Microbiota could be an important tool to improve overall general health, performance, and energy availability while controlling inflammation and redox levels in endurance athletes. The relationship among gut microbiota, general health, training adaptation and performance, along with a focus on sport supplements which are known to exert some influence on the microbiota, will be discussed.

Effect of fatigue-related group III/IV afferent firing on intracortical inhibition and facilitation in hand muscles

Fatiguing exercise causes a reduction in motor drive to the muscle. Group III/IV muscle afferent firing is thought to contribute to this process; however, the effect on corticospinal and intracortical networks is poorly understood. In two experiments, participants performed sustained maximal isometric finger abductions of the first dorsal interosseous (FDI) muscle, with postexercise blood flow occlusion (OCC) to maintain the firing of group III/IV afferents or without occlusion (control; CON). Before and after exercise, single- and paired-pulse transcranial magnetic stimulation (TMS) tested motor evoked potentials (MEPs), intracortical facilitation [ICF (12 ms)], and short-interval intracortical inhibition [SICI₂ (2 ms), SICI₃ (3 ms)]. Ulnar nerve stimulation elicited maximal M waves (M_MAX). For experiment 1 (n = 16 participants), TMS intensities were 70% and 120% of resting motor threshold (RMT) for the conditioning and MEP stimuli, respectively. For experiment 2 (n = 16 participants), the MEP was maintained at 1 mV before and after exercise and the conditioning stimulus individualized. In experiment 1, MEP/M_MAX was reduced after exercise (~48%, P = 0.007) but was not different between conditions. No changes occurred in ICF or SICI. In experiment 2, MEP/M_MAX increased (~27%, P = 0.027) and less inhibition (SICI₂: ~21%, P = 0.021) occurred after exercise for both conditions, whereas ICF decreased for CON only (~28%, P = 0.006). MEPs and SICI₂ were modulated by fatiguing contractions but not by group III/IV afferent firing, whereas sustained afferent firing appeared to counteract postexercise reductions in ICF in FDI. The findings do not support the idea that actions of group III/IV afferents on motor cortical networks contribute to the reduction in voluntary activation observed in other studies.NEW & NOTEWORTHY This is the first study to investigate, in human hand muscles, the action of fatigue-related group III/IV muscle afferent firing on intracortical facilitation and inhibition. In fatigued and nonexercised hand muscles, intracortical inhibition is reduced after exercise but is not modulated differently by the firing of group III/IV afferents. However, facilitation is maintained for the fatigued muscle when group III/IV afferents fire, but these results are unlikely to explain the reduction in voluntary activation observed in other studies.