Ricci Hannah

Human capacity for explosive force production: Neural and contractile determinants

This study assessed the integrative neural and contractile determinants of human knee extension explosive force production. Forty untrained participants performed voluntary and involuntary (supramaximally evoked twitches and octets – eight pulses at 300 Hz that elicit the maximum possible rate of force development) explosive isometric contractions of the knee extensors. Explosive force (F0–150 ms) and sequential rate of force development (RFD, 50‐ms epochs) were measured. Surface electromyography (EMG) amplitude was recorded (superficial quadriceps and hamstrings, 50‐ms epochs) and normalized (quadriceps to Mmax, hamstrings to EMGmax). Maximum voluntary force (MVF) was also assessed. Multiple linear regressions assessed the significant neural and contractile determinants of absolute and relative (%MVF) explosive force and sequential RFD. Explosive force production exhibited substantial interindividual variability, particularly during the early phase of contraction [F50, 13‐fold (absolute); 7.5‐fold (relative)]. Multiple regression explained 59–93% (absolute) and 35–60% (relative) of the variance in explosive force production. The primary determinants of explosive force changed during the contraction (F0–50, quadriceps EMG and Twitch F; RFD50–100, Octet RFD0–50; F100–150, MVF). In conclusion, explosive force production was largely explained by predictor neural and contractile variables, but the specific determinants changed during the phase of contraction.

RELIABILITY OF NEUROMUSCULAR MEASUREMENTS DURING EXPLOSIVE ISOMETRIC CONTRACTIONS, WITH SPECIAL REFERENCE TO ELECTROMYOGRAPHY NORMALIZATION TECHNIQUES

Introduction: This study determined the between‐session reliability of neuromuscular measurements during explosive isometric contractions, with special consideration of electromyography (EMG) normalization. Methods: Following familiarization, 13 men participated in 3 identical measurement sessions involving maximal and explosive voluntary contractions of the knee extensors, while force and surface EMG were recorded. Root mean square EMG amplitude was normalized to different reference measures: (evoked maximal M‐wave peak‐to‐peak amplitude and area, maximum and sub‐maximum voluntary contractions). Results: Explosive voluntary force measurements were reliable on a group level, whereas within‐subject reliability was low over the initial 50 ms and good from 100 ms onward. Normalization of EMG during explosive voluntary contractions, irrespective of the reference method, did not reduce the within‐subject variability, but it did reduce substantially the variability between‐subject. Conclusions: The high intra‐individual variability of EMG and early phase explosive voluntary force production may limit their use to measuring group as opposed to individual responses to an intervention. Muscle Nerve 46: 566–576, 2012

Explosive neuromuscular performance of males versus females

The purpose of the study was to investigate sex‐related differences in explosive muscular force production, as measured by electromechanical delay (EMD) and rate of force development (RFD), and to examine the physiological mechanisms responsible for any differences. The neuromuscular performance of untrained males (n= 20) and females (n= 20) was assessed during a series of isometric knee extension contractions; explosive and maximal voluntary efforts, as well as supramaximal evoked twitches and octets (eight pulses at 300 Hz). Evoked and voluntary EMD were determined from twitch and explosive contractions. The RFD was recorded over consecutive 50 ms time windows from force onset during evoked and explosive contractions, and normalized to maximal strength. Neuromuscular activity during explosive voluntary contractions was measured with EMG of the superficial knee extensors normalized to maximal M‐wave. Muscle size (thickness) and muscle–tendon unit (MTU) stiffness were assessed using ultrasonic images of the vastus lateralis at rest and during ramped contractions. Males and females had similar evoked and voluntary EMD. Males were 33% stronger (P < 0.001) and their absolute RFD was 26–56% greater (all time points P < 0.05) compared with females. Muscle size (P < 0.001) and absolute MTU stiffness were also greater for males (P < 0.05). However, normalized RFD was similar for both sexes during the first 150 ms of the explosive voluntary contractions (P > 0.05). This was consistent with the similar normalized twitch and octet RFD, MTU stiffness and agonist EMG (all P > 0.05). When differences in maximal strength were accounted for, the evoked capacity of the knee extensors for explosive force production and the ability to utilize that capacity during explosive voluntary contractions was similar for males and females.

Effect of coil orientation on strength-duration time constant and I-wave activation with controllable pulse parameter transcranial magnetic stimulation

Highlights • S–D time constants are longer for anterior–posterior than posterior–anterior induced currents.• Brief (30 μs) anterior-posterior currents evoke the longest latency MEP.• Selective stimulation of neural elements may be achieved by manipulating pulse width and orientation.

Pulse Duration as Well as Current Direction Determines the Specificity of Transcranial Magnetic Stimulation of Motor Cortex during Contraction

Highlights • Selective stimulation of inputs to corticospinal neurons may be achieved by manipulating current direction and pulse duration.• Neural populations recruited by brief (30 μs) anterior–posterior currents exhibited the greatest sensitivity to somatosensory input.• Pulse duration is an important determinant of what is activated with TMS in human motor cortex.

Longer Electromechanical Delay Impairs Hamstrings Explosive Force versus Quadriceps.

INTRODUCTION Explosive neuromuscular performance refers to the ability to rapidly increase force in response to neuromuscular activation. The lower explosive force production of the hamstrings relative to the quadriceps could compromise knee joint stability and increase the risk of anterior cruciate ligament injury. However, the time course of the rise in explosive force of the hamstrings and quadriceps from their initial activation, and thus the explosive hamstrings-to-quadriceps (H/Q) force ratio, has not been documented. METHODS The neuromuscular performance of 20 untrained males was assessed during a series of isometric knee flexion and extension contractions, with force and surface EMG of the hamstrings and quadriceps recorded during explosive and maximum voluntary contractions. Hamstrings force was expressed relative to quadriceps force to produce hamstring-to-quadriceps ratios of explosive H/Q force and H/Q maximum voluntary force. For the explosive contractions, agonist electromechanical delay (EMD), agonist and antagonist neural activation were assessed. RESULTS The quadriceps was 79% stronger than the hamstrings, but quadriceps explosive force was up to 480% greater than the hamstrings from 25 to 50 ms after first activation. Consequently, the explosive H/Q force ratio was very low at 25 and 50 ms (0%-17%) and significantly different from H/Q maximum voluntary force ratio (56%). Hamstrings EMD was 95% greater than quadriceps EMD (44.0 vs 22.6 ms), resulting in a 21-ms later onset of force in the hamstrings that appeared to explain the low explosive H/Q force ratio in the early phase of activation. CONCLUSIONS Prolonged hamstrings EMD appears to impair early phase (0-50 ms) explosive force production relative to the quadriceps and may render the knee unstable and prone to anterior cruciate ligament injury during this period.

Locomotor muscle fatigue is not critically regulated after prior upper body exercise

This study examined the effects of prior upper body exercise on subsequent high-intensity cycling exercise tolerance and associated changes in neuromuscular function and perceptual responses. Eight men performed three fixed work-rate (85% peak power) cycling tests: 1) to the limit of tolerance (CYC); 2) to the limit of tolerance after prior high-intensity arm-cranking exercise (ARM-CYC); and 3) without prior exercise and for an equal duration as ARM-CYC (ISOTIME). Peripheral fatigue was assessed via changes in potentiated quadriceps twitch force during supramaximal electrical femoral nerve stimulation. Voluntary activation was assessed using twitch interpolation during maximal voluntary contractions. Cycling time during ARM-CYC and ISOTIME (4.33 ± 1.10 min) was 38% shorter than during CYC (7.46 ± 2.79 min) (P < 0.001). Twitch force decreased more after CYC (-38 ± 13%) than ARM-CYC (-26 ± 10%) (P = 0.004) and ISOTIME (-24 ± 10%) (P = 0.003). Voluntary activation was 94 ± 5% at rest and decreased after CYC (89 ± 9%, P = 0.012) and ARM-CYC (91 ± 8%, P = 0.047). Rating of perceived exertion for limb discomfort increased more quickly during cycling in ARM-CYC [1.83 ± 0.46 arbitrary units (AU)/min] than CYC (1.10 ± 0.38 AU/min, P = 0.003) and ISOTIME (1.05 ± 0.43 AU/min, P = 0.002), and this was correlated with the reduced cycling time in ARM-CYC (r = -0.72, P = 0.045). In conclusion, cycling exercise tolerance after prior upper body exercise is potentially mediated by central fatigue and intolerable levels of sensory perception rather than a critical peripheral fatigue limit.

Non-invasive brain stimulation as a tool to study cerebellar-M1 interactions in humans

The recent development of non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) has allowed the non-invasive assessment of cerebellar function in humans. Early studies showed that cerebellar activity, as reflected in the excitability of the dentate-thalamo-cortical pathway, can be assessed with paired stimulation of the cerebellum and the primary motor cortex (M1) (cerebellar inhibition of motor cortex, CBI). Following this, many attempts have been made, using techniques such as repetitive TMS and transcranial electrical stimulation (TES), to modulate the activity of the cerebellum and the dentate-thalamo-cortical output, and measure their impact on M1 activity. The present article reviews literature concerned with the impact of non-invasive stimulation of cerebellum on M1 measures of excitability and “plasticity” in both healthy and clinical populations. The main conclusion from the 27 reviewed articles is that the effects of cerebellar “plasticity” protocols on M1 activity are generally inconsistent. Nevertheless, two measurements showed relatively reproducible effects in healthy individuals: reduced response of M1 to sensorimotor “plasticity” (paired-associative stimulation, PAS) and reduced CBI following repetitive TMS and TES. We discuss current challenges, such as the low power of reviewed studies, variability in stimulation parameters employed and lack of understanding of physiological mechanisms underlying CBI.

Effects of Quadripulse Stimulation on Human Motor Cortex Excitability: A Replication Study

Affiliations. a Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, London, UK b Neurology Unit, University-Hospital S. Maria della Misericordia, Udine, Italy c Department of Health and Sports, Institute for Human Movement and Medical Sciences, Niigata University of Health and Welfare, Niigata City, Japan d Department of Rehabilitation Medicine, Keio University School of Medicine, Shinjuku-ku, Tokyo, Japan † These authors contributed equally to this work.

Controllable Pulse Parameter TMS and TMS-EEG As Novel Approaches to Improve Neural Targeting with rTMS in Human Cerebral Cortex

Repetitive transcranial magnetic stimulation (rTMS) can produce after-effects on the excitability and function of the stimulated cortical site that outlasts the period of stimulation for several minutes or hours (Hamada et al., 2008; Huang et al., 2005; Ridding and Ziemann, 2010; Sommer et al., 2013). These are thought to involve early phases of long term potentiation/depression at cortical synapses. Depending on the area stimulated, the after-effects can influence performance of a variety of cognitive and motor tasks, as well as learning (Parkin et al., 2015; Censor and Cohen, 2011). Reports of beneficial effects on behaviour in healthy populations have led to widespread interest in applying rTMS therapeutically, for example in patients with neuropsychiatric and neurological disorders (George et al., 2013; Lefaucheur et al., 2014; Ridding and Rothwell, 2007). A major issue with rTMS protocols is that the effects vary considerably within and between individuals (Hamada et al., 2013; Lopez-Alonso et al., 2014; Simeoni et al., 2016; Hinder et al., 2014; Vallence et al., 2015; Vernet et al., 2013; Goldsworthy et al., 2014; Maeda et al., 2000), which causes problems in replication of results in a research setting (Heroux et al., 2015), and is an obstacle to using rTMS in a therapeutic setting. A separate, but related, issue is that rTMS over a given cortical area is often assumed to affect all neuronal populations equally and thus affect all behaviours involving that area similarly, but this may not be true. Here we argue that advanced technologies and methodologies, such as controllable pulse parameter TMS (cTMS; (Peterchev et al., 2014)) and combining TMS with electroencephalography (EEG) (Ilmoniemi and Kicic, 2010; Peterchev et al., 2014), might facilitate the development of more selective forms of stimulation targeting particular neuronal populations or brain states, and ultimately improve the reliability and behavioural specificity of rTMS protocols.