Remote contractions to mitigate reduced persistent inward current magnitudes in motoneurons of older adults

Motor units (MUs; i.e., the alpha motoneuron and all the muscle fibres it innervates) are the fundamental elements of the neuromuscular system that transform motor commands into mechanical actions. The transformation of the activation signal from the nervous system into single muscle fibre potentials is a non-linear function comprised of excitatory and inhibitory ionotropic synaptic inputs, which are subject to neuromodulation due to monoaminergic inputs (e.g., serotonin, noradrenaline). [...]</p

Elucidating the neurophysiology of local vibration: changes in neuromodulatory drive rather than presynaptic inhibition?

This Journal Club article highlights an article by Souron et al. To read this article, visit https://doi.org/10.1113/JP278469

Low‐threshold motor units can be a pain during experimental muscle pain

This Journal Club article highlights an article by Martinez‐Valdes et al. To read this article, visit https://doi.org/10.1113/JP279225

Factors underlying bench press performance in elite competitive powerlifters

Reya, M, Škarabot, J, Cvetičanin, B, and Šarabon, N. Factors underlying bench press performance in elite competitive powerlifters. J Strength Cond Res 35(8): 2179–2186, 2021—Previous investigations of 1 repetition maximum bench press (1RM BP) performance have been either descriptive or have explored a limited number of contributing variables. The purpose of this study was to investigate the interplay between structural, technical, and neuromuscular factors in relation to 1RM BP in competitive powerlifters. Thirteen national and international level male powerlifters (26 ± 9 years, 178 ± 6 cm, and 93.8 ± 9.9 kg) visited the laboratory twice. Anthropometric and ultrasound measures were taken on the first visit, whereas performance measures (voluntary activation level, isokinetic strength, and kinetic, kinematic, and electromyographic measurements during 1RM BP) were recorded on the second visit. Correlation and multiple regression were used to investigate the contribution of structural, technical, and neuromuscular variables to 1RM BP corrected for body mass using the Wilks coefficient. The highest degree of association was shown for structural (lean and bone mass, brachial index, arm circumference, and agonist cross-sectional area [CSA]; r = 0.58–0.74) followed by neuromuscular factors (elbow and shoulder flexion strength; r = 0.57–0.71), whereas technical factors did not correlate with 1RM BP performance (r ≤ 0.49). The multiple regression showed that lean body mass, brachial index, and isometric shoulder flexion torque predicted 59% of the common variance in 1RM BP. These data suggest that in a sample of elite competitive powerlifters, multiple factors contribute to 1RM BP with variables such as lean body mass, the agonist CSA, brachial index, and strength of the elbow and shoulder flexors being the greatest predictors of performance

Sex differences in neuromuscular aging: The role of sex hormones

Males and females experience different trajectories of neuromuscular function across the lifespan, with females demonstrating accelerated deconditioning in later life. We hypothesize that the menopause is a critical period in the female lifespan, during which the dramatic reduction in sex hormone concentrations negatively impacts synaptic input to the motoneuron pool, as well as motor unit discharge properties.</p

Corticospinal excitability during shortening and lengthening actions with incremental torque output

The modulation of motor evoked potentials (MEPs), an index of corticospinal excitability, has been shown to increase during isometric contractions with incremental torque output in accordance with the contribution between motor unit recruitment and firing rate of the muscle to increases in required torque output. However, the motor unit strategy of the muscle might not be the only factor influencing this behaviour, because differences in pre‐ and postsynaptic control have been reported between lengthening and shortening or isometric contractions. In 30 healthy adults, MEPs were elicited in tibialis anterior during shortening and lengthening contractions at 15, 25, 50 and 80% of contraction‐type‐specific maximal voluntary contraction torque. Background EMG activity increased progressively with greater torque output (P < 0.001) but was similar between contraction types (P = 0.162). When normalized to the maximal muscle response, MEPs were greater during shortening compared with lengthening contractions (P = 0.004) and increased stepwise with increased contraction intensities (P = 0.001). These data show an increase in corticospinal excitability with torque output from lower to higher contraction intensities, suggesting a greater contribution of motor unit recruitment to increased nervous system gain in the tibialis anterior. Despite differences in corticospinal control of shortening and lengthening contractions, the data suggest that the corticospinal responses to increases in torque output are not dependent on contraction type, because corticospinal excitability increased to a similar extent during shortening and lengthening actions. Thus, it is likely that the relationship between motor unit recruitment and firing rate of the muscle is the main determinant of corticospinal output with variations in nervous system gain

Myths and methodologies: how loud is the story told by the transcranial magnetic stimulation‐evoked silent period?

Transcranial magnetic stimulation (TMS) of the motor cortex evokes a response in the muscle that can be recorded via electromyography (EMG). One component of this response, when elicited during a voluntary contraction, is a period of EMG silence, termed the silent period (SP), which follows a motor evoked potential (MEP). Modulation of SP duration was long thought to reflect the degree of intracortical inhibition. However, the evidence presented in this review suggests that both cortical and spinal mechanisms contribute to generation of the SP, which makes prefacing SP with ‘cortical’ misleading. Further investigations with multi‐methodological approaches, such as TMS–EEG coupling or interaction of TMS with neuroactive drugs, are needed to make such inferences with greater confidence. A multitude of methodological factors can influence the SP and thus confound the interpretation of this measure; namely, background muscle activity, instructions given to the participant, stimulus intensity and the size of the MEP preceding the SP, and the approach to analysis. A systematic understanding of how the confounding factors influence the interpretation of SP is lacking, which makes standardization of the methodology difficult to conceptualize. Instead, the methodology should be guided through the lens of the research question and the population studied, ensuring greater reproducibility, repeatability and comparability of data sets. Recommendations are provided for the best practice within a given context of the experimental design

Corticospinal responses during passive shortening and lengthening of tibialis anterior and soleus in older compared to younger adults

Corticospinal responses have been shown to increase and decrease with passive muscle shortening and lengthening, respectively, as a result of changes in muscle spindle afferent feedback. The ageing sensory system is accompanied by a number of alterations that might influence the processing and integration of sensory information. Consequently, corticospinal excitability might be modulated differently whilst changing muscle length. In 10 older adults (66 ± 4 years), corticospinal responses (MEP/Mmax) were evoked in a static position, and during passive shortening and lengthening of soleus (SOL) and tibialis anterior (TA), and these data were compared to the re‐analysed data pool of 18 younger adults (25 ± 4 years) published previously. Resting motor threshold was greater in SOL compared to TA (P max between the static position, passive shortening or lengthening in SOL (young: all 0.02 ± 0.01; older: 0.05 ± 0.04, 0.03 ± 0.02 and 0.04 ± 0.01, respectively; P = 0.298), and responses were not dependent on age (P = 0.090). Conversely, corticospinal responses in TA were modulated differently between the age groups (P = 0.002), with greater MEP/Mmax during passive shortening (0.22 ± 0.12) compared to passive lengthening (0.13 ± 0.10) and static position (0.10 ± 0.05) in young (P < 0.001), but unchanged in older adults (0.19 ± 0.11, 0.22 ± 0.11 and 0.18 ± 0.07, respectively; P ≥ 0.867). The present experiment shows that length‐dependent changes in corticospinal excitability in TA of the young are not evident in older adults. This suggests impaired sensorimotor response during muscle length changes in older age that might only be present in ankle flexors, but not extensors

The knowns and unknowns of neural adaptations to resistance training

The initial increases in force production with resistance training are thought to be primarily underpinned by neural adaptations. This notion is irmly supported by evidence displaying motor unit adaptations following resistance training; however, the precise locus of neural adaptation remains elusive. The purpose of this review is to clarify and critically discuss the literature concerning the site(s) of putative neural adaptations to short-term resistance training. The proliferation of studies employing non-invasive stimulation techniques to investigate evoked responses have yielded variable results, but generally support the notion that resistance training alters intracortical inhibition. Nevertheless, methodological inconsistencies and the limitations of techniques, e.g., limited relation to behavioural outcomes and the inability to measure volitional muscle activity, preclude irm conclusions. Much of the literature has focused on the corticospinal tract; however, preliminary research in non-human primates suggests reticulospinal tract is a potential substrate for neural adaptations to resistance training, though human data is lacking due to methodological constraints. Recent advances in technology have provided substantial evidence of adaptations within a large motor unit population following resistance training. However, their activity represents the transformation of aferent and eferent inputs, making it challenging to establish the source of adaptation. Whilst much has been learned about the nature of neural adaptations to resistance training, the puzzle remains to be solved. Additional analyses of motoneuron iring during diferent training regimes or coupling with other methodologies (e.g., electroencephalography) may facilitate the estimation of the site(s) of neural adaptations to resistance training in the future

Startling stimuli increase maximal motor unit discharge rate and rate of force development in humans

Maximal rate of force development in adult humans is determined by the maximal motor unit discharge rate, however the origin of the underlying synaptic inputs remains unclear. Here, we tested a hypothesis that the maximal motor unit discharge rate will increase in response to a startling cue, a stimulus that purportedly activates the pontomedullary reticular formation neurons that make mono- and disynaptic connections to motoneurons via fast-conducting axons. Twenty-two men were required to produce isometric knee extensor forces “as fast and as hard” as possible from rest to 75% of maximal voluntary force, in response to visual (VC), visual auditory (VAC; 80 dB), or visual-startling cue (VSC; 110 dB). Motoneuron activity was estimated via decomposition of high-density surface electromyogram recordings over the vastus lateralis and medialis muscles. Reaction time was significantly shorter in response to VSC compared to VAC and VC. The VSC further elicited faster neuromechanical responses including a greater number of discharges per motor unit per second and greater maximal rate of force development, with no differences between VAC and VC. We provide evidence, for the first time, that the synaptic input to motoneurons increases in response to a startling cue, suggesting a contribution of subcortical pathways to maximal motoneuron output in humans.</p