Females and males do not differ for fatigability, muscle damage and magnitude of the repeated bout effect following maximal eccentric contractions

Unaccustomed eccentric (ECC) exercise induces muscle fatigue as well as damage and initiates a protective response to minimize impairments from a subsequent bout (i.e., repeated bout effect; RBE). It is uncertain if the sexes differ for neuromuscular responses to ECC exercise and the ensuing RBE. Twenty-six young adults (13 females) performed 2 bouts (4 weeks apart) of 200 ECC maximal voluntary contractions (MVCs) of the dorsiflexors. Isometric (ISO) MVC torque and the ratio of ISO torque in response to low- versus high-frequency stimulation (10:100 Hz) were compared before and after (2–10 min and 2, 4, and 7 days) exercise. The decline in ECC and ISO MVC torque and the 10:100 Hz ratio following bout 1 did not differ between sexes (P > 0.05), with reductions from baseline of 31.5% ± 12.3%, 24.1% ± 15.4%, and 51.3% ± 12.2%, respectively. After bout 2, the 10:100 Hz ratio declined less (45.0% ± 12.4% from baseline) and ISO MVC torque recovered sooner compared with bout 1 but no differences between sexes were evident for the magnitude of the RBE (P > 0.05). These data suggest that fatigability with ECC exercise does not differ for the sexes and adaptations that mitigate impairments to calcium handling are independent of sex.

Novelty: One bout of 200 maximal eccentric dorsiflexor contractions caused equivalent muscle fatigue and damage for females and males. The repeated bout effect observed after a second bout 4 weeks later also had no sex-related differences. Prolonged low-frequency force depression is promoted as an indirect measure of muscle damage in humans.

The Sexes Do Not Differ for Neural Responses to Submaximal Elbow Extensor Fatigue

PURPOSE

To investigate possible sex-related differences in group III/IV muscle afferent feedback with isometric fatigue, we aimed to assess the effect of a sustained submaximal elbow extensor contraction on motoneuronal excitability (cervicomedullary motor evoked potential [CMEP]) and voluntary activation (VA).

METHODS

Twenty-four participants (12 females) performed a 15-min contraction at the level of EMG activity recorded at 15% of maximal torque. Each minute, CMEP were elicited by cervicomedullary stimulation with and without conditioning transcranial magnetic stimulation (TMS) delivered 100 ms earlier. Unconditioned and conditioned motor evoked potentials (MEP) in response to TMS were also recorded to assess motor cortical excitability. CMEP and MEP were normalized for changes in downstream excitability and expressed as percentage of their prefatigue (control) values. Postfatigue, VA was calculated from superimposed and resting tetani evoked by stimulation over triceps brachii.

RESULTS

Males were twice as strong as females, but the sexes did not differ for any variable during the fatigue protocol. On a 0-10 scale, RPE increased from ~2.5 to 9. The unconditioned CMEP did not change, whereas the conditioned CMEP was reduced by ~50%. By contrast, the unconditioned and conditioned MEP increased to ~200% and ~320% of the control values, respectively. At task termination, maximal torque was reduced ~40%, and VA was ~80%, down from a prefatigue value of ~96%.

CONCLUSIONS

Results support the scant published data on the elbow extensors and indicate no sex-related differences for isometric fatigue of this muscle group. The motoneuronal and VA data suggest that metabolite buildup and group III/IV muscle afferent activity were similar for females and males.

Low-frequency neural activity at rest is correlated with the movement-related cortical potentials elicited during both real and imagined movements

Ongoing low-frequency activity in the brain has been shown to indicate an inhibitory neural state; however, the effects of this low-frequency activity on event-related neural processes associated with movement preparation, including movement-related cortical potentials (MRCPs) or more specifically, the motor potential (MP), and event-related desynchronization (ERD) have not been assessed. Using data from 48 participants, the current study examined how ongoing mu and beta frequency activity at rest relates to the MP and mu and beta ERD during real or imagined movement of the fingers. Resting state EEG activity was collected for 1 min, prior to the real and imagined finger movement trials. 20 real and 20 imagined movement trials were collected for each hand. Resting beta activity correlated with MP amplitude during movement trials for both the right (r(47) = -0.304, p = 0.035) and left (r(47) = -0.468, p < 0.001) hands, whereas resting mu correlated with MP amplitude during motor imagery trials of both the right (r(47) = -0.289, p = 0.046) and left (r(47) = -0.330, p = 0.020) hands. Ongoing mu and beta activity was not significantly correlated with mu or beta ERD for both the movement and imagery trials. A connection between low-frequency activity and MP could inform biofeedback procedures that promote a reduction of this activity, ultimately allowing for easier identification of the intent to move.

Electrically evoked force loss of the knee extensors is equivalent for young and old females and males

Data are scant on sex-related differences for electrically evoked contractions, which assess intrinsic contractile properties while limiting spinal and supraspinal adaptations to mitigate fatigue. Furthermore, the few studies that exist use stimulus frequencies considerably higher than the natural motor unit discharge rate for the target force. The purpose of this study was to compare force loss to electrically evoked contractions at a physiological stimulus frequency among young females (n = 12), young males (n = 12), old females (n = 11), and old males (n = 11). The quadriceps of the dominant leg were fatigued by 3 min of intermittent transcutaneous muscle belly stimulation (15 Hz stimulus train to initially evoke 25% of maximal voluntary force). Impairment of tetanic contractile impulse (area under the curve) did not differ between sexes for young or old adults or between age groups, with a pooled value of 55.2% ± 12.4% control at the end of fatigue. These data contrast with previous findings at 30 Hz, when the quadriceps of females had greater fatigue resistance than males for young and old adults. The present results highlight the impact stimulus frequency has on intrinsic fatigability of muscle; the findings have implications for future fatigue paradigms and treatment approaches when utilizing electrical stimulation for rehabilitation.

Novelty Fatigue was not different between sexes with a stimulation frequency comparable to discharge rates during voluntary contractions. These results highlight that stimulus frequency not only influences fatigue development but also between-group differences.

The impact of brain iron accumulation on cognition: A systematic review

Iron is involved in many processes in the brain including, myelin generation, mitochondrial function, synthesis of ATP and DNA and the cycling of neurotransmitters. Disruption of normal iron homeostasis can result in iron accumulation in the brain, which in turn can partake in interactions which amplify oxidative damage. The development of MRI techniques for quantifying brain iron has allowed for the characterisation of the impact that brain iron has on cognition and neurodegeneration. This review uses a systematic approach to collate and evaluate the current literature which explores the relationship between brain iron and cognition. The following databases were searched in keeping with a predetermined inclusion criterion: Embase Ovid, PubMed and PsychInfo (from inception to 31^st March 2020). The included studies were assessed for study characteristics and quality and their results were extracted and summarised. This review identified 41 human studies of varying design, which statistically assessed the relationship between brain iron and cognition. The most consistently reported interactions were in the Caudate nuclei, where increasing iron correlated poorer memory and general cognitive performance in adulthood. There were also consistent reports of a correlation between increased Hippocampal and Thalamic iron and poorer memory performance, as well as, between iron in the Putamen and Globus Pallidus and general cognition. We conclude that there is consistent evidence that brain iron is detrimental to cognitive health, however, more longitudinal studies will be required to fully understand this relationship and to determine whether iron occurs as a primary cause or secondary effect of cognitive decline.

The inclusion of interstimulus interval variability does not mitigate electrically-evoked fatigue of the knee extensors

PURPOSE

Transcutaneous electrical stimulation (TES) is used to activate muscles when volitional capacity is impaired but potential benefits are limited by rapid force loss (fatigue). Most TES fatigue protocols employ constant-frequency trains, with stimuli at a fixed interstimulus interval (ISI); however, a brief ISI between the first two pulses (variable-frequency train, VFT) to maximize the catchlike property of muscle can attenuate fatigue development. The purpose of this study was to investigate if a VFT that simulates intrinsic variability of voluntary motor unit discharge rates would also mitigate fatigue, owing to the sensitivity of muscle to acute activation history.

METHODS

On two visits, 24 healthy adults (25.3 ± 3.7 years; 12 females) received 3 min of intermittent TES to the quadriceps of the dominant leg. Trains of eight pulses at 10 Hz were delivered with a constant (100 ms) or variable ISI (80-120 ms). Contractile impulse, rate of force development (RFD), and rate of relaxation (RFR) were determined for each tetanus RESULTS: During fatigue and recovery, contractile impulse did not differ between protocols (p ≥ 0.796) and sexes (p ≥ 0.493), with values of 77 ± 17% control at task end and 125 ± 19% control 2 min later. RFD and RFR also showed no effect of the protocol (p ≥ 0.310) or participant sex (p ≥ 0.119). Both measures slowed (38 ± 23% and 33 ± 22%, respectively) but dissociated during recovery as RFD remained 16 ± 18% below control at 5 min, whereas RFR recovered to control by 30 s (101 ± 22%).

CONCLUSION

Contrary to expectations, the VFT protocol did not attenuate fatigue development, which suggests no benefit to mimicking the inherent variability of motor unit discharge rates.

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.

An overview of prevalence, determinants and health outcomes of polypharmacy

A high rate of polypharmacy is, in part, a consequence of the increasing proportion of multimorbidity in the ageing population worldwide. Our understanding of the potential harm of taking multiple medications in an older, multi-morbid population, who are likely to be on a polypharmacy regime, is limited. This is a narrative literature review that aims to appraise and summarise recent studies published about polypharmacy. We searched MEDLINE using the search terms polypharmacy (and its variations, e.g. multiple prescriptions, inappropriate drug use, etc.) in titles. Systematic reviews and original studies in English published between 2003 and 2018 were included. In this review, we provide current definitions of polypharmacy. We identify the determinants and prevalence of polypharmacy reported in different studies. Finally, we summarise some of the findings regarding the association between polypharmacy and health outcomes in older adults, with a focus on frailty, hospitalisation and mortality. Polypharmacy was most often defined in terms of the number of medications that are being taken by an individual at any given time. Our review showed that the prevalence of polypharmacy varied between 10% to as high as around 90% in different populations. Chronic conditions, demographics, socioeconomics and self-assessed health factors were independent predictors of polypharmacy. Polypharmacy was reported to be associated with various adverse outcomes after adjusting for health conditions. Optimising care for polypharmacy with valid, reliable measures, relevant to all patients, will improve the health outcomes of older adult population.

Motor unit contributions to activation reduction and torque steadiness following active lengthening: a study of residual torque enhancement

Following active lengthening, steady-state isometric (ISO) torque is greater than a purely ISO contraction at the same muscle length, this is referred to as residual torque enhancement (rTE). A phenomenon of rTE is activation reduction, characterized by reduced electromyography (EMG) amplitude for a given torque output. We hypothesized that lower motor unit discharge rates would contribute to activation reduction and lessening torque steadiness. Ten young male subjects performed ISO dorsiflexion contractions at 10 and 20% of maximal voluntary contraction (MVC) torque. During rTE trials, the muscle was activated at 10° of plantar flexion, then the ankle was rotated to the ISO position at 40°. Fine wire electrodes recorded motor unit (MU)-discharge rates and variability from the tibialis anterior. Surface EMG quantified activation reduction, and steadiness was determined as the coefficient of variation of torque. The activation reduction was 44 and 24% at 10 and 20% MVC, respectively (P < 0.05). Fewer MUs were recorded in the rTE than ISO condition at 10% (~47%) and 20% (~36%) MVC (P < 0.05). Discharge rates were 19 and 26% lower in the rTE compared with the ISO condition for 10 and 20% MVC, respectively (P < 0.05), with no difference in variability between conditions (P > 0.05). Steadiness was ~22 and 18% lower for the rTE than ISO condition at 10 and 20% MVC (P < 0.05). Our findings indicate that activation reduction may be attributed to lower MU discharge rate and fewer detectable MUs and that this theoretically contributes to a reduction in steadiness in the rTE condition.NEW & NOTEWORTHY Our findings indicate that lower electromyographic activity during the torque enhanced condition following active lengthening compared with a purely isometric contraction arises from fewer active motor units and a lower discharge rate of those that are active. We used an acute condition of increased torque capacity to induce a decrease in net output of the motor neuron pool during a submaximal task to demonstrate, in humans, the impact of motor unit activity on torque steadiness.

The effects of forearm position and contraction intensity on cortical and spinal excitability during a submaximal force steadiness task of the elbow flexors

Elbow flexor force steadiness is less with the forearm pronated (PRO) compared with neutral (NEU) or supinated (SUP) and may relate to neural excitability. Although not tested in a force steadiness paradigm, lower spinal and cortical excitability was observed separately for biceps brachii in PRO, possibly dependent on contractile status at the time of assessment. This study aimed to investigate position-dependent changes in force steadiness as well as spinal and cortical excitability at a variety of contraction intensities. Thirteen males (26 ± 7 yr; means ± SD) performed three blocks (PRO, NEU, and SUP) of 24 brief (~6 s) isometric elbow flexor contractions (5, 10, 25 or 50% of maximal force). During each contraction, transcranial magnetic stimulation or transmastoid stimulation was delivered to elicit a motor-evoked potential (MEP) or cervicomedullary motor-evoked potential (CMEP), respectively. Force steadiness was lower in PRO compared with NEU and SUP (P ≤ 0.001), with no difference between NEU and SUP. Similarly, spinal excitability (CMEP/maximal M wave) was lower in PRO than NEU (25 and 50% maximal force; P ≤ 0.010) and SUP (all force levels; P ≤ 0.004), with no difference between NEU and SUP. Cortical excitability (MEP/CMEP) did not change with forearm position (P = 0.055); however, a priori post hoc testing for position showed excitability was 39.8 ± 38.3% lower for PRO than NEU at 25% maximal force (P = 0.006). The data suggest that contraction intensity influences the effect of forearm position on neural excitability and that reduced spinal and, to a lesser extent, cortical excitability could contribute to lower force steadiness in PRO compared with NEU and SUP.NEW & NOTEWORTHY To address conflicting reports about the effect of forearm position on spinal and cortical excitability of the elbow flexors, we examine the influence of contraction intensity. For the first time, excitability data are considered in a force steadiness context. Motoneuronal excitability is lowest in pronation and this disparity increases with contraction intensity. Cortical excitability exhibits a similar pattern from 5 to 25% of maximal force. Lower corticospinal excitability likely contributes to relatively poor force steadiness in pronation.

Effect of blood flow occlusion on corticospinal excitability during sustained low-intensity isometric elbow flexion

Blood flow occlusion (BFO) has been used to study the influence of group III/IV muscle afferents after fatiguing exercise, but it is unknown how BFO-induced activity of these afferents affects motor cortical and motoneuronal excitability during low-intensity exercise. Therefore, the purpose of this study was to assess the acute effect of BFO on peripheral [maximal M wave (M_max)], spinal [cervicomedullary motor evoked potential (CMEP) normalized to M_max], and motor cortical [motor evoked potential (MEP) normalized to CMEP] excitability. Nine healthy men completed a sustained isometric contraction of the elbow flexors at 20% of maximal force under three conditions: 1) contractile failure with BFO, 2) a time-matched trial without restriction [free flow (FFiso)], and 3) contractile failure with free flow (FFfail). Time to failure for BFO (and FFiso) were ~80% shorter than that for FFfail (P < 0.05). For FFfail and FFiso, M_max area decreased ~17% and ~7%, respectively (P < 0.05), with no change during BFO. CMEP/M_max area increased ~226% and ~80% during BFO and FFfail, respectively (P < 0.05), with no change during FFiso (P > 0.05). The increase in normalized CMEP area was greater for BFO and FFfail compared with FFiso and for BFO compared with FFfail. MEP/CMEP area was not different among the protocols (P > 0.05) and increased ~64% with time (P < 0.05). It is likely that group III/IV muscle afferent feedback to the spinal cord modulates the large increase in motoneuronal excitability for the BFO compared with FFfail and FFiso protocols.NEW & NOTEWORTHY We have observed how blood flow occlusion modulates motor cortical, spinal, and peripheral excitability during and immediately after a sustained low-intensity isometric elbow flexion contraction to failure. We conclude that blood flow occlusion causes a greater and more rapid increase in motoneuronal excitability.

Validation and comparison of two automated methods for quantifying brain white matter hyperintensities of presumed vascular origin

Objectives

White matter hyperintensities (WMH) are a common imaging finding indicative of cerebral small vessel disease. Lesion segmentation algorithms have been developed to overcome issues arising from visual rating scales. In this study, we evaluated two automated methods and compared them to visual and manual segmentation to determine the most robust algorithm provided by the open-source Lesion Segmentation Toolbox (LST).

Methods

We compared WMH data from visual ratings (Scheltens’ scale) with those derived from algorithms provided within LST. We then compared spatial and volumetric WMH data derived from manually-delineated lesion maps with WMH data and lesion maps provided by the LST algorithms.

Results

We identified optimal initial thresholds for algorithms provided by LST compared with visual ratings (Lesion Growth Algorithm (LGA): initial κ and lesion probability thresholds, 0.5; Lesion Probability Algorithm (LPA) lesion probability threshold, 0.65). LGA was found to perform better then LPA compared with manual segmentation.

Conclusion

LGA appeared to be the most suitable algorithm for quantifying WMH in relation to cerebral small vessel disease, compared with Scheltens’ score and manual segmentation. LGA offers a user-friendly, effective WMH segmentation method in the research environment.

High-Altitude Acclimatization Improves Recovery from Muscle Fatigue

PURPOSE

We investigated the effect of high-altitude acclimatization on peripheral fatigue compared with sea level and acute hypoxia.

METHODS

At sea level (350 m), acute hypoxia (environmental chamber), and chronic hypoxia (5050 m, 5-9 d) (partial pressure of inspired oxygen = 140, 74 and 76 mm Hg, respectively), 12 participants (11 in chronic hypoxia) had the quadriceps of their dominant leg fatigued by three bouts of 75 intermittent electrically evoked contractions (12 pulses at 15 Hz, 1.6 s between train onsets, and 15 s between bouts). The initial peak force was ~30% of maximal voluntary force. Recovery was assessed by single trains at 1, 2, and 3 min postprotocol. Tissue oxygenation of rectus femoris was recorded by near-infrared spectroscopy.

RESULTS

At the end of the fatigue protocol, the impairments of peak force and peak rates of force development and relaxation were greater (all P < 0.05) in acute hypoxia (~51%, 53%, and 64%, respectively) than sea level (~43%, 43%, and 52%) and chronic hypoxia (~38%, 35%, and 48%). Peak force and rate of force development recovered faster (P < 0.05) in chronic hypoxia (pooled data for 1-3 min: ~84% and 74% baseline, respectively) compared with sea level (~73% and 63% baseline) and acute hypoxia (~70% and 55% baseline). Tissue oxygenation did not differ among conditions for fatigue or recovery (P > 0.05).

CONCLUSIONS

Muscle adaptations occurring with chronic hypoxia, independent of other adaptations, positively influence muscle contractility during and after repeated contractions at high altitude.

The Time Course of Motoneuronal Excitability during the Preparation of Complex Movements

For a simple RT task, movement complexity increases RT and also corticospinal excitability, as measured by the motor evoked potential (MEP) elicited by TMS of the motor cortex. However, it is unknown if complexity-related increases in corticospinal excitability during the preparation of movement are mediated at the cortical or spinal level. The purposes of this study were to establish a time course of motoneuronal excitability before prime mover activation and to assess task-dependent effects of complex movements on motoneuronal and cortical excitability in a simple RT paradigm. It was hypothesized that motoneuronal and cortical excitability would increase before prime mover activation and in response to movement complexity. In a seated position, participants completed ballistic elbow extension/flexion movements with their dominant arm to one, two, or three targets. TMS and transmastoid stimulation (TS) were delivered at 0%, 70%, 80% or 90% of mean premotor RT for each complexity level. Stimulus intensities were set to elicit MEPs and cervicomedullary MEPs (CMEPs) of ∼10% of the maximal M-wave in the triceps brachii. Compared with 0% RT, motoneuronal excitability (CMEP amplitude) was already 10% greater at 70% RT. CMEP amplitude also increased with movement complexity as both the two- and three-movement conditions had greater motoneuronal excitability than the one-movement condition (p < .038). Importantly, when normalized to the CMEP, there was no increase in MEP amplitude. This suggests that complexity-related increases in corticospinal excitability are likely to be mediated more by increased excitability at a motoneuronal than cortical level.

Prolonged low-frequency force depression is underestimated when assessed with doublets compared with tetani in the dorsiflexors

Prolonged low-frequency force depression (PLFFD) after damaging eccentric exercise may last for several days. Historically, PLFFD has been calculated from the tetanic force responses to trains of supramaximal stimuli. More recently, for methodological reasons, stimulation has been reduced to two pulses. However, it is unknown whether doublet responses provide a valid measure of PLFFD in the days after eccentric exercise. In 12 participants, doublets and tetani were elicited at 10 and 100 Hz before and after (2, 3, 5 min, 48 and 96 h) 200 eccentric maximal voluntary contractions of the dorsiflexors. Doublet and tetanic torque responses at 10 Hz were similarly depressed throughout recovery (P > 0.05; e.g., 2 min: 58.9 ± 12.8% vs. 57.1 ± 14.5% baseline; 96 h: 85.6 ± 11.04% vs. 85.1 ± 10.8% baseline). At 100 Hz, doublet torque was impaired more than tetanic torque at all time points (P < 0.05; e.g., 2 min: 70.5 ± 14.2% vs. 88.1 ± 11.7% baseline; 96 h: 83.0 ± 14.2% vs. 98.7 ± 9.5% baseline). As a result, the postfatigue reduction of the 10 Hz-to-100 Hz ratio (PLFFD) was markedly greater for tetani than for doublets (P < 0.05; e.g., 2 min: 64.3 ± 15.1% vs. 83.0 ± 5.8% baseline). In addition, the doublet ratio recovered by 48 h (99.2 ± 5.0% baseline), whereas the tetanic ratio was still impaired at 96 h (88.2 ± 9.7% baseline). Our results indicate that doublets are not a valid measure of PLFFD in the minutes and days after eccentric exercise. If study design favors the use of paired stimuli, it should be acknowledged that the true magnitude and duration of PLFFD are likely underestimated. NEW & NOTEWORTHY Prolonged low-frequency force depression (PLFFD) will result from damaging exercise and may last for several days. After 200 eccentric maximal dorsiflexor contractions, we compared the gold-standard measure of PLFFD (calculated using trains of supramaximal stimulation) to the value obtained from an alternative technique that is becoming increasingly common (paired supramaximal stimuli). Doublets underestimated the magnitude and duration of PLFFD compared with tetani, so caution must be used when reporting PLFFD derived from paired stimuli.

The Effects of Sex and Motoneuron Pool on Central Fatigue

PURPOSE

It is uncertain if sex influences central fatigue because the reduction in voluntary activation (VA) has been reported as not different between the sexes for elbow flexors (EF) but greater in males compared with females for knee extensors. This disparity could result from the facilitatory and inhibitory effects of group III/IV muscle afferents on flexor versus extensor motoneurons, respectively. The purpose of this study was to examine central fatigue and motoneuron responsiveness of EF and elbow extensors (EE) in males and females.

METHODS

Twenty-two participants (11 females) performed a 2-min isometric maximal voluntary contraction of EF and EE (on separate days) followed by 2 min of recovery. EMG potentials were recorded from biceps or triceps brachii in response to the stimulation of the brachial plexus (Mmax), corticospinal tract (cervicomedullary motor evoked potential [CMEP]), and motor cortex (motor evoked potential [MEP]). Superimposed and resting doublets (for determining VA) were evoked via muscle belly stimulation of biceps or triceps brachii. Only CMEP and superimposed doublets were recorded during fatigue.

RESULTS

There was no effect of sex on CMEP area for either muscle group during fatigue or recovery. During the 2 min after EE fatigue, mean normalized CMEP and MEP area were ∼85% and ∼141% of control, indicating inhibition and facilitation of the motoneurons and motor cortex, respectively. VA during recovery was significantly reduced in males but not females for the EF, and unchanged in either sex for the EE.

CONCLUSION

The findings do not support the concept that equivocal findings regarding sex differences in central fatigue are related to augmented effects of group III/IV afferent feedback in males compared with females.

Supraspinal Fatigue and Neural-evoked Responses in Lowlanders and Sherpa at 5050 m

PURPOSE

At high altitude, Lowlanders exhibit exacerbated fatigue and impaired performance. Conversely, Sherpa (native Highlanders) are known for their outstanding performance at altitude. Presently, there are no reports comparing neuromuscular fatigue and its etiology between Lowlanders and Sherpa at altitude.

METHODS

At 5050 m, nine age-matched Lowlanders and Sherpa (31 ± 10 vs 30 ± 12 yr, respectively) completed a 4-min sustained isometric elbow flexion at 25% maximal voluntary contraction (MVC) torque. Mid-minute, stimuli were applied to the motor cortex and brachial plexus to elicit a motor-evoked potential and maximal compound muscle action potential (Mmax), respectively. Supraspinal fatigue was assessed as the reduction in cortical voluntary activation (cVA) from prefatigue to postfatigue. Cerebral hemoglobin concentrations and tissue oxygenation index (TOI) were measured over the prefrontal cortex by near-infrared spectroscopy.

RESULTS

Prefatigue, MVC torque, and cVA were significantly greater for Lowlanders than Sherpa (79.5 ± 3.6 vs 50.1 ± 11.3 N·m, and 95.4% ± 2.7% vs 88.2% ± 6.6%, respectively). With fatigue, MVC torque and cVA declined similarly for both groups (~24%-26% and ~5%-7%, respectively). During the task, motor-evoked potential area increased more and sooner for Lowlanders (1.5 min) than Sherpa (3.5 min). The Mmax area was lower than baseline throughout fatigue for Lowlanders but unchanged for Sherpa. TOI increased earlier for Lowlanders (2 min) than Sherpa (4 min). Total hemoglobin increased only for Lowlanders (2 min). Mmax was lower, whereas TOI and total hemoglobin were higher for Lowlanders than Sherpa during the second half of the protocol.

CONCLUSIONS

Although neither MVC torque loss nor development of supraspinal fatigue was different between groups, neural-evoked responses and cerebral oxygenation indices were less perturbed in Sherpa. This represents an advantage for maintenance of homeostasis, presumably due to bequeathed genotype and long-term altitude adaptations.

Central contributions to torque depression: an antagonist perspective

Torque depression (TD) is the reduction in steady-state isometric torque following active muscle shortening when compared to an isometric reference contraction at the same muscle length and activation level. Central nervous system excitability differs in the TD state. While torque production about a joint is influenced by both agonist and antagonist muscle activation, investigations of corticospinal excitability have focused on agonist muscle groups. Hence, it is unknown how the TD state affects spinal and supraspinal excitability of an antagonist muscle. Eight participants (~ 24y, three female) performed 14 submaximal dorsiflexion contractions at the intensity needed to maintain a level of integrated electromyographic activity in the soleus equivalent to 15% of that recorded during a maximum plantar flexion contraction. The seven contractions of the TD protocol included a 2 s isometric phase at an ankle angle of 140°, a 1 s shortening phase at 40°/s, and a 7 s isometric phase at an angle of 100°. The seven isometric reference contractions were performed at an ankle angle of 100° for 10 s. Motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and maximal M-waves (Mmax) were recorded from the soleus in both conditions. In the TD compared to isometric reference state, a 13% reduction in dorsiflexor torque was accompanied by 10% lower spinal excitability (normalized CMEP amplitude; CMEP/Mmax), and 17% greater supraspinal excitability (normalized MEP amplitude; MEP/CMEP) for the soleus muscle. These findings demonstrate a neuromechanical coupling following active muscle shortening and indicate that the underlying mechanisms of TD influence antagonist activation during voluntary force production.

Neuromuscular adaptations to healthy aging

Even in the absence of disease or disability, aging is associated with marked physiological adaptations within the neuromuscular system. An ability to perform activities of daily living and maintain independence with advanced age is reliant on the health of the neuromuscular system. Hence, it is critical to elucidate the age-related adaptations that occur within the central nervous system and the associated muscles to design interventions to maintain or improve neuromuscular function in the elderly. This brief review focuses on the neural alterations observed at both spinal and supraspinal levels in healthy humans in their seventh decade and beyond. The topics addressed are motor unit loss and remodelling, neural drive, and responses to transcranial magnetic stimulation of the motor cortex.

UBC-Nepal Expedition: An experimental overview of the 2016 University of British Columbia Scientific Expedition to Nepal Himalaya

The University of British Columbia Nepal Expedition took place over several months in the fall of 2016 and was comprised of an international team of 37 researchers. This paper describes the objectives, study characteristics, organization and management of this expedition, and presents novel blood gas data during acclimatization in both lowlanders and Sherpa. An overview and framework for the forthcoming publications is provided. The expedition conducted 17 major studies with two principal goals—to identify physiological differences in: 1) acclimatization; and 2) responses to sustained high-altitude exposure between lowland natives and people of Tibetan descent. We performed observational cohort studies of human responses to progressive hypobaric hypoxia (during ascent), and to sustained exposure to 5050 m over 3 weeks comparing lowlander adults (n = 30) with Sherpa adults (n = 24). Sherpa were tested both with (n = 12) and without (n = 12) descent to Kathmandu. Data collected from lowlander children (n = 30) in Canada were compared with those collected from Sherpa children (n = 57; 3400–3900m). Studies were conducted in Canada (344m) and the following locations in Nepal: Kathmandu (1400m), Namche Bazaar (3440m), Kunde Hospital (3480m), Pheriche (4371m) and the Ev-K2-CNR Research Pyramid Laboratory (5050m). The core studies focused on the mechanisms of cerebral blood flow regulation, the role of iron in cardiopulmonary regulation, pulmonary pressures, intra-ocular pressures, cardiac function, neuromuscular fatigue and function, blood volume regulation, autonomic control, and micro and macro vascular function. A total of 335 study sessions were conducted over three weeks at 5050m. In addition to an overview of this expedition and arterial blood gas data from Sherpa, suggestions for scientists aiming to perform field-based altitude research are also presented. Together, these findings will contribute to our understanding of human acclimatization and adaptation to the stress of residence at high-altitude.

UBC-Nepal expedition: peripheral fatigue recovers faster in Sherpa than lowlanders at high altitude

KEY POINTS

The reduced oxygen tension of high altitude compromises performance in lowlanders. In this environment, Sherpa display superior performance, but little is known on this issue. Sherpa present unique genotypic and phenotypic characteristics at the muscular level, which may enhance resistance to peripheral fatigue at high altitude compared to lowlanders. We studied the impact of gradual ascent and exposure to high altitude (5050 m) on peripheral fatigue in age-matched lowlanders and Sherpa, using intermittent electrically-evoked contractions of the knee extensors. Peripheral fatigue (force loss) was lower in Sherpa during the first part of the protocol. Post-protocol, the rate of force development and contractile impulse recovered faster in Sherpa than in lowlanders. At any time, indices of muscle oxygenation were not different between groups. Muscle contractile properties in Sherpa, independent of muscle oxygenation, were less perturbed by non-volitional fatigue. Hence, elements within the contractile machinery contribute to the superior physical performance of Sherpa at high altitude.

ABSTRACT

Altitude-related acclimatisation is characterised by marked muscular adaptations. Lowlanders and Sherpa differ in their muscular genotypic and phenotypic characteristics, which may influence peripheral fatigability at altitude. After gradual ascent to 5050 m, 12 lowlanders and 10 age-matched Sherpa (32 ± 10 vs. 31 ± 11 years, respectively) underwent three bouts (separated by 15 s rest) of 75 intermittent electrically-evoked contractions (12 pulses at 15 Hz, 1.6 s between train onsets) of the dominant leg quadriceps, at the intensity which initially evoked 30% of maximal voluntary force. Trains were also delivered at minutes 1, 2 and 3 after the protocol to measure recovery. Tissue oxygenation index (TOI) and total haemoglobin (tHb) were quantified by a near-infrared spectroscopy probe secured over rectus femoris. Superficial femoral artery blood flow was recorded using ultrasonography, and delivery of oxygen was estimated (eDO₂ ). At the end of bout 1, peak force was greater in Sherpa than in lowlanders (91.5% vs. 84.5% baseline, respectively; P < 0.05). Peak rate of force development (pRFD), the first 200 ms of the contractile impulse (CI₂₀₀ ), and half-relaxation time (HRT) recovered faster in Sherpa than in lowlanders (percentage of baseline at 1 min: pRFD: 89% vs. 74%; CI₂₀₀ : 91% vs. 80%; HRT: 113% vs. 123%, respectively; P < 0.05). Vascular measures were pooled for lowlanders and Sherpa as they did not differ during fatigue or recovery (P < 0.05). Mid bout 3, TOI was decreased (90% baseline) whereas tHb was increased (109% baseline). After bout 3, eDO₂ was markedly increased (1266% baseline). The skeletal muscle of Sherpa seemingly favours repeated force production at altitude for similar oxygen delivery compared to lowlanders.