5-HT antagonism indirectly increases motor unit discharge rate and cervicomedullary motor evoked potential amplitude during submaximal elbow flexions.

Spinal motoneurone excitability is heavily regulated by serotonin via somatodendritic 5-HT receptors. However, the effects of these receptors on the excitability of motoneurones in the upper limb are not clearly understood. Therefore, the purpose of this study was to assess the effects of 5-HT antagonism on motor unit discharge characteristics of the biceps brachii and evoked responses to cervicomedullary stimulation.

Differential effects of stimulation waveform and intensity on the neural structures activated by lumbar transcutaneous spinal cord stimulation.

Lumbar transcutaneous spinal cord stimulation (TSS) evokes synchronized muscle responses, termed spinally evoked motor response (sEMR). Whether the structures TSS activates to evoke sEMRs differ when TSS intensity and waveform are varied is unknown. In 15 participants (9 F, 6 M), sEMRs were evoked by TSS over L1-L3 (at sEMR threshold and suprathreshold intensities) with conventional (one 400-µs biphasic pulse) or high-frequency burst (ten 40-µs biphasic pulses at 10 kHz) stimulus waveforms in vastus medialis (VM), tibialis anterior (TA), and medial gastrocnemius (MG) muscles.

Neuromuscular characteristics of eccentric, concentric and isometric contractions of the knee extensors.

Purpose: We compared voluntary drive and corticospinal responses during eccentric (ECC), isometric (ISOM) and concentric (CON) muscle contractions to shed light on neurophysiological mechanisms underpinning the lower voluntary drive in a greater force production in ECC than other contractions.

Methods: Sixteen participants (20-33 years) performed ISOM and isokinetic (30°/s) CON and ECC knee extensor contractions (110°-40° knee flexion) in which electromyographic activity (EMG) was recorded from vastus lateralis. Voluntary activation (VA) was measured during ISOM, CON and ECC maximal voluntary contractions (MVCs).

Persistent inward currents in human motoneurons: emerging evidence and future directions.

The manner in which motoneurons respond to excitatory and inhibitory inputs depends strongly on how their intrinsic properties are influenced by the neuromodulators serotonin and noradrenaline. These neuromodulators enhance the activation of voltage-gated channels that generate persistent (long-lasting) inward sodium and calcium currents (PICs) into the motoneurons. PICs are crucial for initiating, accelerating, and maintaining motoneuron firing.

Can the cross-education of strength attenuate the impact of detraining after a period of strength training? A quasi-randomized trial.

Purpose: Unilateral strength training may attenuate the decline in muscle strength and size in homologous, contralateral muscles. This study aimed to determine whether the cross-education of strength could specifically attenuate the effects of detraining immediately after a short (prehabilitation-type) period of strength training.

Methods: Twenty-six strength-trained participants were assigned to either four weeks of unilateral strength training of the stronger arm (UNI) or detraining (Detrain).

Does lower-limb osteoarthritis alter motor cortex descending drive and voluntary activation? A systematic review and meta-analysis.

Purpose: The aim of the study was to quantify motor cortex descending drive and voluntary activation (VA) in people with lower-limb OA compared to controls.

Methods: A systematic review and meta-analysis according to the PRISMA guidelines was carried out. Seven databases were searched until 30 December 2022.

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

Serotonin modulates corticospinal excitability, motoneurone firing rates and contractile strength via 5-HT 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-HT antagonism affects corticospinal and motoneuronal excitability with and without descending drive to motoneurones.

Effects of jaw clenching and mental stress on persistent inward currents estimated by two different methods.

Spinal motoneuron firing depends greatly on persistent inward currents (PICs), which in turn are facilitated by the neuromodulators serotonin and noradrenaline. The aim of this study was to determine whether jaw clenching (JC) and mental stress (MS), which may increase neuromodulator release, facilitate PICs in human motoneurons. The paired motor unit (MU) technique was used to estimate PIC contribution to motoneuron firing.

Differences in Trapezius Muscle H-Reflex between Asymptomatic Subjects and Symptomatic Shoulder Pain Subjects.

In chronic shoulder pain, adaptations in the nervous system such as in motoneuron excitability, could contribute to impairments in scapular muscles, perpetuation and recurrence of pain and reduced improvements during rehabilitation. The present cross-sectional study aims to compare trapezius neural excitability between symptomatic and asymptomatic subjects. In 12 participants with chronic shoulder pain (symptomatic group) and 12 without shoulder pain (asymptomatic group), the H reflex was evoked in all trapezius muscle parts, through C3/4 nerve stimulation, and the M-wave through accessory nerve stimulation.

Contraction Velocity of the Elbow Flexors Assessed by Tensiomyography: A Comparison Between Formulas.

Mesquita, RNO, Latella, C, Ruas, CV, Nosaka, K, and Taylor, JL. Contraction velocity of the elbow flexors assessed by tensiomyography: A comparison between formulas. J Strength Cond Res 37(10): 1969-1977, 2023-Muscle contraction velocity ( Vc ) assessed by tensiomyography is a promising measure for athlete profiling.

Sprint cycling rate of torque development associates with strength measurement in trained cyclists.

Purpose: A cyclist's rate of force/torque development (RFD/RTD) and peak force/torque can be measured during single-joint or whole-body isometric tests, or during cycling. However, there is limited understanding of the relationship between these measures, and of the mechanisms that contribute to each measure. Therefore, we examined the: (i) relationship between quadriceps central and peripheral neuromuscular function with RFD/RTD in isometric knee extension, isometric mid-thigh pull (IMTP), and sprint cycling; and (ii) relationship among RFD/RTD and peak force/torque between protocols.

Corticospinal and intracortical responses from both motor cortices following unilateral concentric versus eccentric contractions.

Cross-education is the phenomenon where training of one limb can cause neuromuscular adaptations in the opposite untrained limb. This effect has been reported to be greater after eccentric (ECC) than concentric (CON) strength training; however, the underpinning neurophysiological mechanisms remain unclear. Thus, we compared responses to transcranial magnetic stimulation (TMS) in both motor cortices following single sessions of unilateral ECC and CON exercise of the elbow flexors.

Motor cortical excitability and pre-supplementary motor area neurochemistry in healthy adults with substantia nigra hyperechogenicity.

Substantia nigra (SN) hyperechogenicity, viewed with transcranial ultrasound, is a risk marker for Parkinson's disease. We hypothesized that SN hyperechogenicity in healthy adults aged 50-70 years is associated with reduced short-interval intracortical inhibition in primary motor cortex, and that the reduced intracortical inhibition is associated with neurochemical markers of activity in the pre-supplementary motor area (pre-SMA). Short-interval intracortical inhibition and intracortical facilitation in primary motor cortex was assessed with paired-pulse transcranial magnetic stimulation in 23 healthy adults with normal (n = 14; 61 ± 7 yrs) or abnormally enlarged (hyperechogenic; n = 9; 60 ± 6 yrs) area of SN echogenicity.

Comparison between Eccentric-Only and Coupled Concentric-Eccentric Contractions for Neuromuscular Fatigue and Muscle Damage.

Purpose: Eccentric contractions induce muscle damage, but less is known about the effects of preceding concentric contractions to eccentric contractions on muscle damage. We compared eccentric-only (ECC) and coupled concentric and eccentric contractions (CON-ECC) of the knee extensors for parameters of neuromuscular fatigue and muscle damage.

Methods: Twenty participants (age, 19-36 yr) were randomly placed into an ECC or a CON-ECC group (n = 10 per group), without significant (P > 0.

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.

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.

Location-specific cutaneous electrical stimulation of the footsole modulates corticospinal excitability to the plantarflexors and dorsiflexors during standing.

Non-noxious electrical stimulation to distinct locations of the foot sole evokes location-specific cutaneous reflex responses in lower limb muscles. These reflexes occur at latencies that may enable them to be mediated via a transcortical pathway. Corticospinal excitability to the plantarflexors and dorsiflexors was measured in 16 participants using motor evoked potentials (MEPs).

5-HT 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-HT receptors on the somato-dendritic compartment. In the current study, we examined how antagonism of the 5-HT receptor affects motoneuron excitability in humans.

Effects of reciprocal inhibition and whole-body relaxation on persistent inward currents estimated by two different methods.

Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole-body relaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well-established paired motor unit (MU) technique.

The effect of acute intermittent hypoxia on human limb motoneurone output.

New Findings: What is the central question of this study? Does a single session of repeated bouts of acute intermittent hypoxic breathing enhance the motoneuronal output of the limb muscles of healthy able-bodied participants? What is the main finding and its importance? Compared to breathing room air, there were some increases in motoneuronal output following acute intermittent hypoxia, but the increases were variable across participants and in time after the intervention and depended on which neurophysiological measure was checked.

Abstract: Acute intermittent hypoxia (AIH) induces persistent increases in output from rat phrenic motoneurones. Studies in people with spinal cord injury (SCI) suggest that AIH improves limb performance, perhaps via postsynaptic changes at cortico-motoneuronal synapses.

Quadriceps Muscle Fatigue Reduces Extension and Flexion Power During Maximal Cycling.

To investigate how quadriceps muscle fatigue affects power production over the extension and flexion phases and muscle activation during maximal cycling. Ten participants performed 10-s maximal cycling efforts without fatigue and after 120 bilateral maximal concentric contractions of the quadriceps muscles. Extension power, flexion power and electromyographic (EMG) activity were compared between maximal cycling trials.

Early Detection of Prolonged Decreases in Maximal Voluntary Contraction Force after Eccentric Exercise of the Knee Extensors.

Purpose: We examined whether the magnitude of muscle damage indicated by changes in maximal voluntary isometric contraction (MVIC) strength 1 to 3 d after unaccustomed eccentric exercise (ECC) was correlated with changes in central and peripheral neuromuscular parameters immediately post-ECC.

Methods: Twenty participants (19-36 yr) performed six sets of eight eccentric contractions of the knee extensors. Rate of force development (RFD) during knee extensor MVIC, twitch force, rate of force development (RFDRT) and rate of relaxation (RRRT) of the resting twitch, maximal M-wave (MMAX), voluntary activation, silent period duration, motor-evoked potentials (MEP) and short-interval intracortical inhibition were assessed before, immediately after, and 1 to 3 d post-ECC.

Human corticospinal-motoneuronal output is reduced with 5-HT 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-HT receptor, which is a 5-HT receptor that promotes motoneuron excitability, affects human movement.

Effects of postexercise blood flow occlusion on quadriceps responses to transcranial magnetic stimulation.

For a fatigued hand muscle, group III/IV afferent firing maintains intracortical facilitation (ICF) without influencing corticospinal excitability. Exercise of larger muscles produces greater afferent firing. Thus, this study investigated if fatigue-related firing of group III/IV afferents from a large muscle group (quadriceps) modulates intracortical and corticospinal networks.