Severe hypoxia affects exercise performance independently of afferent feedback and peripheral fatigue

Central and peripheral nervous system activity and muscle oxygenation in athletes during repeated-sprint exercise in normoxia and normobaric hypoxia

AIM

To investigate central and peripheral nervous system activity and muscle oxygenation in athletes during repeated-sprint exercise in normoxia and normobaric hypoxia.

METHODS

The effects on vastus lateralis muscle strength in a cross-over study were examined in 18 athletes (13 males, 5 females) completing 10 × 6-s cycle sprints. Immediately after and again 5 minutes post-exercise, electromyography (EMG), heart rate variability, maximal voluntary contraction (MVC), muscle oxygenation, peak power output, and arterial oxygen saturation were compared to 2 baseline sets named ("Baseline" and "Pre").

RESULTS

Post-exercise MVC was significantly lower (6.7 ± 10.0%) than Baseline, but root-mean-square amplitude during hypoxia (all-times) was significantly lower than normoxia (0.38 ± 0.19 vs 0.41 ± 0.17 mV). Comparative frequency analysis of the percentage change in pre- to post-exercise EMG area, at low 1-29 hz (type-1 fibre) and high 75-100 hz (type-2 fibre) areas, revealed a significant reduction in type-1 fibre activity relative to type-2, by 20-30% across time and by 10% in type-1 activity between conditions.

CONCLUSION

Exercise in hypoxia appeared to cause a temporary increase in central sympathetic nervous system activity and greater recruitment of type-2 muscle fibres, with accompanying reduction in type-1. Acute hypoxia may stimulate type-2 fibre conditioning.

Mechanomyography reflects the changes in oxygenated hemoglobin during electrically evoked cycling in individuals with spinal cord injury

BACKGROUND

Functional electrical stimulation (FES) cycling has been reported to enhance muscle strength and improve muscle fatigue resistance after spinal cord injury (SCI). Despite its proposed benefits, the quantification of muscle fatigue during FES cycling remains poorly documented. This study sought to quantify the relationship between the vibrational performance of electrically-evoked muscles measured through mechanomyography (MMG) and its oxidative metabolism through near-infrared spectroscopy (NIRS) characteristics during FES cycling in fatiguing paralyzed muscles in individuals with SCI.

METHODS

Six individuals with SCI participated in the study. They performed 30 min of FES cycling with MMG and NIRS sensors on their quadriceps throughout the cycling, and the signals were analyzed.

RESULTS

A moderate negative correlation was found between MMG root mean square (RMS) and oxyhaemoglobin (O2Hb) [r = -0.38, p = 0.003], and between MMG RMS and total hemoglobin (tHb) saturation [r = -0.31, p = 0.017]. Statistically significant differences in MMG RMS, O2Hb, and tHb saturation occurred during pre- and post-fatigue of FES cycling (p < 0.05).

CONCLUSIONS

MMG RMS was negatively associated with O2Hb and muscle oxygen derived from NIRS. MMG and NIRS sensors showed good inter-correlations, suggesting a promising use of MMG for characterizing metabolic fatigue at the muscle oxygenation level during FES cycling in individuals with SCI.

Effects of acute simulated altitude on the maximal lactate steady state in humans

We sought to determine the effects of acute simulated altitude on the maximal lactate steady state (MLSS) and physiological responses to cycling at and 10 W above the MLSS-associated power output (PO) (MLSSp and MLSSp+10, respectively). Eleven (4 females) participants (means [SD]; 28 [4] yr; V̇o2max: 54.3 [6.9] mL·kg-1·min-1) acclimatized to ∼1,100 m performed 30-min constant PO trials in simulated altitudes of 0 m sea level (SL), 1,111 m mild altitude (MILD), and 2,222 m moderate altitude (MOD). MLSSp, defined as the highest PO with stable (<1 mM change) blood lactate concentration ([BLa]) between 10 and 30 min, was significantly lower in MOD (209 [54] W) compared with SL (230 [56] W; P < 0.001) and MILD (225 [58] W; P = 0.001), but MILD and SL were not different (P = 0.12). V̇o2 and V̇co2 decreased at higher simulated altitudes due to lower POs (P < 0.05), but other end-exercise physiological responses (e.g., [BLa], ventilation [V̇e], heart rate [HR]) were not different between conditions at MLSSp or MLSSp + 10 (P > 0.05). At the same absolute intensity (MLSSp for MILD), [BLa], HR, and V̇E and all perceptual variables were exacerbated in MOD compared with SL and MILD (P < 0.05). Maximum voluntary contraction, voluntary activation, and potentiated twitch forces were exacerbated at MLSSp + 10 relative to MLSSp within conditions (P < 0.05); however, condition did not affect performance fatiguability at the same relative or absolute intensity (P > 0.05). As MLSSp decreased in hypoxia, adjustments in PO are needed to ensure the same relative intensity across altitudes, but common indices of exercise intensity may facilitate exercise prescription and monitoring in hypoxia.NEW & NOTEWORTHY This study demonstrates the power output and metabolic rate associated with the maximal lactate steady-state (MLSS) decline in response to simulated altitude; however, common indices of exercise intensity remained unchanged when cycling was performed at the work rate associated with MLSS at each simulated altitude. These results support previous studies that investigated the effects of hypoxia on alternative measures of the critical intensity of exercise and will inform exercise prescription/monitoring across altitudes.

Arterial desaturation rate does not influence self-selected knee extension force but alters ventilatory response to progressive hypoxia: A pilot study

The absolute magnitude and rate of arterial desaturation each independently impair whole-body aerobic exercise. This study examined potential mechanisms underlying the rate-dependent relationship. Utilizing an exercise protocol involving unilateral, intermittent, isometric knee extensions (UIIKE), we provided sufficient reperfusion time between contractions to reduce the accumulation of intramuscular metabolic by-products that typically stimulate muscle afferents. The objective was to create a milieu conducive to accentuating any influence of arterial desaturation rate on muscular fatigue. Eight participants completed four UIIKE sessions, performing one 3 s contraction every 30s at a perceived intensity of 50% MVC for 25 min. Participants voluntarily adjusted their force generation to maintain perceptual effort at 50% MVC without feedback. Reductions in inspired oxygen fraction (FI O2 ) decreased arterial saturation from >98% to 70% with varying rates in three trials: FAST (5.3 ± 1.3 min), MED (11.8 ± 2.7 min), and SLOW (19.9 ± 3.7 min). FI O2 remained at 0.21 during the control trial. Force generation and muscle activation remained at baseline levels throughout UIIKE trials, unaffected by the magnitude or rate of desaturation. Minute ventilation increased with hypoxia (p < 0.05), and faster desaturation rates magnified this response. These findings demonstrate that arterial desaturation magnitude and rate independently affect ventilation, but do not influence fatigue development during UIIKE.

Integrating physiological monitoring systems in military aviation: a brief narrative review of its importance, opportunities, and risks

Military pilots risk their lives during training and operations. Advancements in aerospace engineering, flight profiles, and mission demands may require the pilot to test the safe limits of their physiology. Monitoring pilot physiology (e.g. heart rate, oximetry, and respiration) inflight is in consideration by several nations to inform pilots of reduced performance capacity and guide future developments in aircraft and life-support system design. Numerous challenges, however, prevent the immediate operationalisation of physiological monitoring sensors, particularly their unreliability in the aerospace environment and incompatibility with pilot clothing and protective equipment. Human performance and behaviour are also highly variable and measuring these in controlled laboratory settings do not mirror the real-world conditions pilots must endure. Misleading or erroneous predictive models are unacceptable as these could compromise mission success and lose operator trust. This narrative review provides an overview of considerations for integrating physiological monitoring systems within the military aviation environment.Practitioner summary: Advancements in military technology can conflictingly enhance and compromise pilot safety and performance. We summarise some of the opportunities, limitations, and risks of integrating physiological monitoring systems within military aviation. Our intent is to catalyse further research and technological development.Abbreviations: AGS: anti-gravity suit; AGSM: anti-gravity straining manoeuvre; A-LOC: almost loss of consciousness; CBF: cerebral blood flow; ECG: electrocardiogram; EEG: electroencephalogram; fNIRS: functional near-infrared spectroscopy; G-forces: gravitational forces; G-LOC: gravity-induced loss of consciousness; HR: heart rate; HRV: heart rate variability; LSS: life-support system; NATO: North Atlantic Treaty Organisation; PE: Physiological Episode; PCO2: partial pressure of carbon dioxide; PO2: partial pressure of oxygen; OBOGS: on board oxygen generating systems; SpO2: peripheral blood haemoglobin-oxygen saturation; STANAG: North Atlantic Treaty Organisation Standardisation Agreement; UPE: Unexplained Physiological Episode; WBV: whole body vibration.

Beyond the Lungs: O 2 Supplementation Improves Cerebral Oxygenation and Fatigue during Exercise in Interstitial Lung Disease

PURPOSE

Cerebral hypoxia may exacerbate the perception of fatigue. We previously demonstrated that exercise-related hypoxemia, a hallmark of fibrotic interstitial lung disease ( f -ILD), dose dependently impairs cerebral oxygenation in these patients. It is unknown whether normalizing cerebral oxygenation with O 2 supplementation would be associated with positive changes in a relevant patient-centered outcome during exercise in f -ILD, such as improved perceived fatigue.

METHODS

Fourteen patients (12 males, 72 ± 8 yr, 8 with idiopathic pulmonary fibrosis, lung diffusing capacity for carbon monoxide = 44% ± 13% predicted) performed a constant-load (60% peak work rate) cycle test to symptom limitation (Tlim) breathing medical air. Fourteen controls cycled up to Tlim of an age- and sex-matched patient. Patients repeated the test on supplemental O 2 (fraction of inspired O 2 = 0.41 ± 0.08) for the same duration. Near-infrared spectroscopy and the rating-of-fatigue (ROF) scale assessed prefrontal cortex oxygenation and perceived fatigue, respectively.

RESULTS

Patients showed severe exertional hypoxemia (Tlim O 2 saturation by pulse oximetry = 80% ± 8%); they had poorer cerebral oxygenation (e.g., oxy-deoxyhemoglobin difference [HbDiff] = -3.5 ± 4.7 [range = -17.6 to +1.9] vs +1.9 ± 1.7 μmol from rest) and greater fatigue (ROF = 6.2 ± 2.0 vs 2.6 ± 2.3) versus controls under air ( P < 0.001). Reversal of exertional hypoxemia with supplemental O 2 led to improved HbDiff (+1.7 ± 2.4 μmol from rest; no longer differing from controls) and lower ROF scores (3.7 ± 1.2, P < 0.001 vs air) in patients. There was a significant correlation between O 2 -induced changes in HbDiff and ROF scores throughout exercise in f -ILD ( rrepeated-measures correlation = -0.51, P < 0.001).

CONCLUSIONS

Supplemental O 2 improved cerebral oxygenation during exercise in f -ILD, which was moderately associated with lower ratings of perceived fatigue. Reversing cerebral hypoxia with O 2 supplementation may thus have positive effects on patients' disablement beyond those expected from lower ventilation and dyspnea in this patient population.

Altitude-induced effects on neuromuscular, metabolic and perceptual responses before, during and after a high-intensity resistance training session

PURPOSE

We tested if an acute ascending to 2320 m above sea level (asl) affects corticospinal excitability (CSE) and intracortical inhibition (SICI) measured with transcranial magnetic stimulation (TMS) at rest, before, during and after a traditional hypertrophy-oriented resistance training (RT) session. We also explored whether blood lactate concentration (BLa), ratings of perceived exertion (RPE), perceived muscular pain and total training volume differed when the RT session was performed at hypoxia (H) or normoxia (N).

METHODS

Twelve resistance-trained men performed eight sets of 10 repetitions at 70% of one repetition maximum of a bar biceps curl at N (SpO2 = 98.0 ± 0.9%) and H (at 2320 asl, SpO2 = 94.0 ± 1.9%) in random order. Before each session, a subjective well-being questionnaire, the resting motor threshold (rMT) and a single pulse recruitment curve were measured. Before, during and after the RT session, BLa, RPE, muscle pain, CSE and SICI were measured.

RESULTS

Before the RT session only the rMT differed between H (- 5.3%) and N (ES = 0.38). RPE, muscle pain and BLa increased through the RT session and were greater at H than N (12%, 54% and 15%, respectively) despite a similar training volume (1618 ± 468 kg vs. 1638 ± 509 kg). CSE was reduced during the RT session (~ 27%) but recovered ten minutes after, regardless of the environmental condition. SICI did not change after any RT session.

CONCLUSIONS

The data suggest that acute exposure to moderate hypoxia slightly increased the excitability of the most excitable structures of the corticospinal tract but did not influence intracortical or corticospinal responses to a single RT session.

Repeated Short-Term Bouts of Hyperoxia Improve Aerobic Performance in Acute Hypoxia

ABSTRACT

Faulhaber, M, Schneider, S, Rausch, LK, Dünnwald, T, Menz, V, Gatterer, H, Kennedy, MD, and Schobersberger, W. Repeated short-term bouts of hyperoxia improve aerobic performance in acute hypoxia. J Strength Cond Res 37(10): 2016-2022, 2023-This study aimed to test the effects of repeated short-term bouts of hyperoxia on maximal 5-minute cycling performance under acute hypoxic conditions (3,200 m). Seventeen healthy and recreationally trained individuals (7 women and 10 men) participated in this randomized placebo-controlled cross-over trial. The procedures included a maximal cycle ergometer test and 3 maximal 5-minute cycling time trials (TTs). TT1 took place in normoxia and served for habituation and reference. TT2 and TT3 were conducted in normobaric hypoxia (15.0% inspiratory fraction of oxygen). During TT2 and TT3, the subjects were breathing through a face mask during five 15-second periods. The face mask was connected through a nonrebreathing T valve to a 300-L bag filled with 100% oxygen (intermittent hyperoxia) or ambient hypoxic air (placebo). The main outcome was the mean power output during the TT. Statistical significance level was set at p < 0.05. The mean power output was higher in the intermittent hyperoxia compared with the placebo condition (255.5 ± 49.6 W vs. 247.4 ± 48.2 W, p = 0.001). Blood lactate concentration and ratings of perceived exertion were significantly lower by about 9.7 and 7.3%, respectively, in the intermittent hyperoxia compared with the placebo condition, whereas heart rate values were unchanged. IH application increased arterial oxygen saturation (82.9 ± 2.6% to 92.4 ± 3.3%, p < 0.001). Repeated 15-second bouts of hyperoxia, applied during high-intensity exercise in hypoxia, are sufficient to increase power output. Future studies should focus on potential dose-response effects and the involved mechanisms.

Electromyographic signature of isometric squat in the highest refuge in Europe

Reports of electromyography during hypoxic exercise are contrasting, due to protocol and muscle diversity. This work aimed to investigate alterations in muscle activation and myoelectrical fatigue during exercise at high-altitude in those muscles primarily involved in trekking. Twelve young adults balanced by gender and age were tested at low (1,667 m) and high (4,554 m, "Capanna Margherita", Italy) altitude, during an isometric squat lasting 60 seconds. High-density surface electromyography was performed from the quadriceps of right limb. The root mean square (RMS), median frequency with its slope, and muscle fiber conduction velocity (MFCV) were computed. Neither males nor females showed changes in median frequency (Med: 36.13 vs 35.63 Hz) and its slope (Med: -9 vs -12 degree) in response to high-altitude trekking, despite a great inter-individual heterogeneity, nor differences were found for MFCV. RMS was not significantly equivalent, with greater values at low altitude (0.385 ± 0.104 mV) than high altitude (0.346 ± 0.090 mV). Unexpected results can be due either to a postural compensation of the whole body compensating for a relatively greater effort or to the inability to support muscle activation after repeated physical efforts.  Interesting results may emerge by measuring simultaneously electromyography, muscle oxygenation and kinematics comparing trekking at normoxia vs hypoxia.

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.

Moderate and Severe Acute Normobaric Hypoxia and the 3-Repetition Deadlift, Hand-Release Push-Up, and Leg Tuck Events From the Army Combat Fitness Test

INTRODUCTION

The newly implemented Army Combat Fitness Test (ACFT) of the U.S. Army seeks to revolutionize the Army's fitness culture and reduce the rate of preventable injuries among soldiers. The initial rollout of the ACFT has been met with several challenges, including a gender-neutral scoring system. The ACFT has undergone several revisions to adapt to the present state of U.S. Army physical fitness; however, the test faces several more obstacles as more data become available. The ACFT was designed to measure combat readiness, a useful tool for units facing deployment or a change in duty station to a high-altitude environment. Reduced oxygen availability (hypoxia) at high altitude influences many physiological functions associated with physical fitness, such that there is an increased demand for oxygen in exercising muscle. Therefore, the purpose was to investigate the effects of normoxic and two levels of hypoxia exposure (moderate and severe; fraction of inspired oxygen [FiO2]: 16.0% and 14.3%) during the 3-repetition deadlift (MDL), hand-release push-up (HRP), and leg tuck (LTK) events of the ACFT.

MATERIALS AND METHODS

Fourteen recreationally active men (n = 10) and women (n = 4) soldier analogs (27.36 ± 1.12 years, height 1.71 ± 2.79 m, weight 80.60 ± 4.24 kg) completed the MDL, HRP, and LTK at normoxia and acute normobaric moderate (MH; FiO2 16%) and severe (SH; FiO2 14.3%) hypoxic exposure. Scores and performance were recorded for each event, and heart rate (HR) and total body oxygen saturation (SpO2) were monitored throughout. Repeated-measures analysis of variance (ANOVA) was used to assess differences in modified ACFT scores, performance, HR, and SpO2 among hypoxic conditions, with follow-up one-way ANOVA and paired t-test when appropriate.

RESULTS

Total body oxygen saturation was decreased at MH and SH conditions compared to normoxia but did not vary between ACFT events. Heart rate was not influenced by altitude but did increase in response to exercise. Scores of the modified total and individual ACFT events were not different between normoxia, MH, and SH. There was also no difference in performance based on the amount of weight lifted during the MDL and number of repetitions of the HRP and LTK events in response to hypoxic exposure.

CONCLUSIONS

Performance and scores of the modified ACFT were not influenced by acute normobaric MH and SH exposure compared to normoxia. Further investigations should examine the full testing battery of the ACFT to provide a comprehensive analysis and potential evidence for such differences.

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.

Influence of an Acute Exposure to a Moderate Real Altitude on Motoneuron Pool Excitability and Jumping Performance

The aim of the study was to test whether ascending to a moderate real altitude affects motoneuron pool excitability at rest, as expressed by a change in the H-reflex amplitude, and also to elucidate whether a possible alteration in the motoneuron pool excitability could be reflected in the execution of lower-body concentric explosive (squat jump; SJ) and fast eccentric-concentric (drop jump; DJ) muscle actions. Fifteen participants performed four experimental sessions that consisted of the combination of two real altitude conditions [low altitude (low altitude, 690 m), high altitude (higher altitude, 2,320 m)] and two testing procedures (H-reflex and vertical jumps). Participants were tested on each testing day at 8, 11, 14 and 17 h. The only significant difference (p < 0.05) detected for the H-reflex was the higher H-reflex response (25.6%) obtained 15 min after arrival at altitude compared to baseline measurement. In terms of motor behavior, DJ height was the only variable that showed a significant interaction between altitude conditions (LA and HA) and time of measurement (8, 11, 14 and 17 h) as DJ height increased more during successive measurements at HA compared to LA. The only significant difference between the LA and HA conditions was observed for DJ height at 17 h which was higher for the HA condition (p = 0.04, ES = 0.41). Although an increased H-reflex response was detected after a brief (15–20 min) exposure to real altitude, the effect on motorneuron pool excitability could not be confirmed since no significant changes in the H-reflex were detected when comparing LA and HA. On the other hand, the positive effect of altitude on DJ performance was accentuated after 6 h of exposure.

Neuromuscular fatigability at high altitude: Lowlanders with acute and chronic exposure, and native highlanders

Ascent to high altitude is accompanied by a reduction in partial pressure of inspired oxygen, which leads to interconnected adjustments within the neuromuscular system. This review describes the unique challenge that such an environment poses to neuromuscular fatigability (peripheral, central and supraspinal) for individuals who normally reside near to sea level (SL) (<1000 m; ie, lowlanders) and for native highlanders, who represent the manifestation of high altitude-related heritable adaptations across millennia. Firstly, the effect of acute exposure to high altitude-related hypoxia on neuromuscular fatigability will be examined. Under these conditions, both supraspinal and peripheral fatigability are increased compared with SL. The specific mechanisms contributing to impaired performance are dependent on the exercise paradigm and amount of muscle mass involved. Next, the effect of chronic exposure to high altitude (ie, acclimatization of ~7-28 days) will be considered. With acclimatization, supraspinal fatigability is restored to SL values, regardless of the amount of muscle mass involved, whereas peripheral fatigability remains greater than SL except when exercise involves a small amount of muscle mass (eg, knee extensors). Indeed, when whole-body exercise is involved, peripheral fatigability is not different to acute high-altitude exposure, due to competing positive (haematological and muscle metabolic) and negative (respiratory-mediated) effects of acclimatization on neuromuscular performance. In the final section, we consider evolutionary adaptations of native highlanders (primarily Himalayans of Tibet and Nepal) that may account for their superior performance at altitude and lesser degree of neuromuscular fatigability compared with acclimatized lowlanders, for both single-joint and whole-body exercise.

A bibliometric analysis study of blood flow restriction using CiteSpace

[Purpose] To assess the current state-of-the-art and the prevailing trends regarding the global use of blood flow restriction (BFR) in the past 20 years. [Participants and Methods] We retrieved literature relating to BFR from 1999 to 2020 using Web of Science. We conducted a bibliometric analysis of countries/institutions, cited journals, authors/cited authors, cited references, and keywords using CiteSpace. An analysis of counts and centrality was used to examine publication output, countries/institutions, core journals, active authors, foundation references, hot topics, and frontiers. [Results] Seven hundred seventy five references were included and the total number of publications has been continually increasing over the investigated period. Representatives of important academic groups are the Japanese scholars from the University of Tokyo as represented by Takashi Abe. Jeremy Paul Loenneke’s article (centrality: 0.15) was the most representative and symbolic reference with the highest centrality. The three topics identified were intervention (intensity resistance exercise, IRE), physiology (ischemia and muscular function) and behavior (adaptation and increase). The four frontier topics were phosphorylation, reduction, low intensity and arterial occlusion. [Conclusion] This study provides an insight into BFR and offers valuable information for BFR researchers to identify new perspectives for potential cooperation with collaborators and their related cooperative institutions.

Self-Selected Motivational Music Enhances Physical Performance in Normoxia and Hypoxia in Young Healthy Males

Humans exposed to hypoxia are susceptible to physiological and psychological impairment. Music has ergogenic effects through enhancing psychological factors such as mood, emotion, and cognition. This study aimed to investigate music as a tool for mitigating the performance decrements observed in hypoxia. Thirteen males (mean ± SD; 24 ± 4 years) completed one familiarization session and four experimental trials; (1) normoxia (sea level, 0.209 FiO₂) and no music; (2) normoxia (0.209 FiO₂) with music; (3) normobaric hypoxia (∼3800 m, 0.13 FiO₂) and no music; and (4) normobaric hypoxia (0.13 FiO₂) with music. Experimental trials were completed at 21°C with 50% relative humidity. Music was self-selected prior to the familiarization session. Each experimental trial included a 15-min time trial on an arm bike, followed by a 60-s isometric maximal voluntary contraction (MVC) of the biceps brachii. Supramaximal nerve stimulation quantified central and peripheral fatigue with voluntary activation (VA%) calculated using the doublet interpolation method. Average power output (W) was reduced with a main effect of hypoxia (p = 0.02) and significantly increased with a main effect of music (p = 0.001). When combined the interaction was additive (p = 0.87). Average MVC force (N) was reduced in hypoxia (p = 0.03) but VA% of the biceps brachii was increased with music (p = 0.02). Music reduced subjective scores of mental effort, breathing discomfort, and arm discomfort in hypoxia (p < 0.001). Music increased maximal physical exertion through enhancing neural drive and diminishing detrimental mental processes, enhancing performance in normoxia (6.3%) and hypoxia (6.4%).

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.

Effect of moderate and Severe Hypoxic exposure coupled with fatigue on psychomotor vigilance testing, muscle tissue oxygenation, and muscular performance

PURPOSE

The purpose of this study is to examine the effects of fatigue on muscular performance, oxygenation saturation, and cognition following acute hypoxic exposure at Normoxia, Moderate Hypoxia (MH), and Severe Hypoxia (SH).

METHODS

Twelve males performed 3 sets of leg extensions to failure under Normoxia (FiO2: 21%), MH (Fi02: 15.4%), and SH (Fi02: 12.9%). Heart rate, peripheral oxygenation saturation, total saturation index, psychomotor vigilance testing reaction time, psychomotor vigilance error rate, maximum strength, and repetitions to failure were measured throughout each visit.

RESULTS

The primary findings indicated that MH and SH resulted in significant decreases in psychomotor vigilance test performance (MH: 388.25-427.17 ms, 0.41-0.33 error rate; SH: 398.17-445.42 ms reaction time, 0.25-1.00 error rate), absolute muscle tissue oxygen saturation (Abs-StO2) (MH:67.22% compared to SH:57.56%), but similar muscular strength, heart rate, and patterns of muscle tissue oxygen saturation responses (StO2%) during fatigue when compared to Normoxia. There was an acute decrease in the ability to remain vigilant and/or respond correctly to visual stimuli as indicated by the worsened reaction time (PVTRT) during MH (FiO2: 15.4%) and increased PVTRT and error rate (PVTE) during SH (FiO2: 12.9%) conditions.

CONCLUSIONS

Acute hypoxic exposure in the current study was not a sufficient stimuli to elicit hypoxic-related changes in HR, muscular strength (1-RM), or repetitions to failure. The SpO2 responses were hypoxic-level dependent with increasing levels of hypoxia resulting in greater and more sustained reductions in SpO2. The combined SpO2 and StO2 responses at MH and SH suggested a balance between the muscles metabolic demand remaining lower than the muscle oxygen diffusion capacity. During the SH condition, Abs-StO2 suggested greater metabolic stress than Normoxia and MH conditions during the fatiguing leg extensions. The patterns of responses for StO2% during the three sets of leg press to failure indicated that exercise is a more potent influencer to muscle oxygenation status than hypoxic conditions (FiO2: 15.4 and 12.9%).

The influence of carbohydrate ingestion on peripheral and central fatigue during exercise in hypoxia: A narrative review

Hypoxia impairs aerobic performance by accelerating fatiguing processes. These processes may originate from sites either distal (peripheral) or proximal (central) to the neuromuscular junction, though these are not mutually exclusive. Peripheral mechanisms include decrements in muscle glycogen or fluctuations in intramuscular metabolites, whereas central responses commonly refer to reductions in central motor drive elicited by alterations in blood glucose and neurotransmitter concentrations as well as arterial hypoxemia. Hypoxia may accelerate both peripheral and central pathways of fatigue, with the level of hypoxia strongly dictating the degree and primary locus of impairment. As more people journey to hypoxic settings for work and recreation, developing strategies to improve work capacity in these environments becomes increasingly relevant. Given that sea level performance improves with nutritional interventions such as carbohydrate (CHO) ingestion, a similar strategy may prove effective in delaying fatigue in hypoxia, particularly considering how the metabolic pathways enhanced with CHO supplementation overlap the fatiguing pathways upregulated in hypoxia. Many questions regarding the relationship between CHO, hypoxia, and fatigue remain unanswered, including specifics on when to ingest, what to ingest, and how varying altitudes influence supplementation effectiveness. Therefore, the purpose of this narrative review is to examine the peripheral and central mechanisms contributing to fatigue during aerobic exercise at varying degrees of hypoxia and to assess the role of CHO ingestion in attenuating fatigue onset.

Effects of graded hypoxia during exhaustive intermittent cycling on subsequent exercise performance and neuromuscular responses

PURPOSE

This study examined the effect of graded hypoxia during exhaustive intermittent cycling on subsequent exercise performance and neuromuscular fatigue characteristics in normoxia.

METHODS

Fifteen well-trained cyclists performed an exhaustive intermittent cycling exercise (EICE 1; 15 s at 30% of anaerobic power reserve interspersed with 45 s of passive recovery) at sea level (SL; FiO₂ ~ 0.21), moderate (MH; FiO₂ ~ 0.16) and severe hypoxia (SH; FiO₂ ~ 0.12). This was followed, after 30 min of passive recovery in normoxia, by an identical exercise bout in normoxia (EICE 2). Neuromuscular function of the knee extensors was assessed at baseline, after EICE 1 (post-EICE 1), and EICE 2 (post-EICE 2).

RESULTS

The number of efforts completed decreased with increasing hypoxic severity during EICE 1 (SL: 39 ± 30, MH: 22 ± 13, SH: 13 ± 6; p ≤ 0.02), whereas there was no difference between conditions during EICE 2 (SL: 16 ± 9, MH: 20 ± 14, SH: 24 ± 17; p ≥ 0.09). Maximal torque (p = 0.007), peripheral (p = 0.02) and cortical voluntary activation (p < 0.001), and twitch torque (p < 0.001) decreased from baseline to post-EICE 1. Overall, there were no significant difference in any neuromuscular parameters from post-EICE 1 to post-EICE 2 (p ≥ 0.08).

CONCLUSION

Increasing hypoxia severity during exhaustive intermittent cycling hampered exercise capacity, but did not influence performance and associated neuromuscular responses during a subsequent bout of exercise in normoxia performed after 30 min of rest.

Differential Brain and Muscle Tissue Oxygenation Responses to Exercise in Tibetans Compared to Han Chinese

The Tibetans’ better aerobic exercise capacity at altitude remains ill-understood. We tested the hypothesis that Tibetans display better muscle and brain tissue oxygenation during exercise in hypoxia. Using near-infrared spectrometry (NIRS) to provide indices of tissue oxygenation, we measured oxy- and deoxy-hemoglobin ([O₂Hb] and [HHb], respectively) responses of the vastus lateralis muscle and the right prefrontal cortex in ten Han Chinese and ten Tibetans during incremental cycling to exhaustion in a pressure-regulated chamber at simulated sea-level (air at 1 atm: normobaric normoxia) and 5,000 m (air at 0.5 atm: hypobaric hypoxia). Hypoxia reduced aerobic capacity by ∼22% in both groups (d = 0.8, p < 0.001 vs. normoxia), while Tibetans consistently outperformed their Han Chinese counterpart by ∼32% in normoxia and hypoxia (d = 1.0, p = 0.008). We found cerebral [O₂Hb] was higher in Tibetans at normoxic maximal effort compared Han (p = 0.001), while muscle [O₂Hb] was not different (p = 0.240). Hypoxic exercise lowered muscle [O₂Hb] in Tibetans by a greater extent than in Han (interaction effect: p < 0.001 vs. normoxic exercise). Muscle [O₂Hb] was lower in Tibetans when compared to Han during hypoxic exercise (d = 0.9, p = 0.003), but not during normoxic exercise (d = 0.4, p = 0.240). Muscle [HHb] was not different between the two groups during normoxic and hypoxic exercise (p = 0.778). Compared to Han, our findings revealed a higher brain tissue oxygenation in Tibetans during maximal exercise in normoxia, but lower muscle tissue oxygenation during exercise in hypoxia. This would suggest that the Tibetans privileged oxygenation of the brain at the expense of that of the muscle.

Linear and nonlinear analyses of normal and fatigue heart rate variability signals for miners in high-altitude and cold areas

BACKGROUND AND OBJECTIVE

Fatigue is an important cause of operational errors, and human errors are the main cause of accidents. This study is an exploratory study in China. Field tests were conducted on heart rate variability (HRV) parameters and physiological indicators of fatigue among miners in high-altitude, cold and low-oxygen areas. This paper studies heart activity patterns during work fatigue in miners.

METHODS

Fatigue affects both the sympathetic and parasympathetic nervous systems, and it is expressed as an abnormal pattern of HRV parameters. Thirty miners were selected as subjects for a field test, and HRV was extracted from 60 groups of electrocardiography (ECG) datasets as basic signals for fatigue analysis. Then, we analyzed the HRV signals of the miners using linear (time domain and frequency domain) and nonlinear dynamics (Poincaré plot and sample entropy (SampEn)), and a Pearson's correlation coefficient analysis and t-tests were performed on the measured indices.

RESULTS

The results showed that the time-domain indices (SDNN, RMSSD, SDSD, pNN50, RRn, heart rate (HR), R-wave humps (RH)) and the coefficient of variation (CV)) and the frequency-domain indices (low frequency/high frequency (LF/HF), LFnorm and HFnorm) clearly changed after fatigue. These features were selected using a Poincaré plot, sample entropy, Pearson's correlation coefficient and a t-test for further analysis. The fatigue characteristics and sensitivity parameters of miners in a high-altitude, cold and hypoxic environment were obtained.

CONCLUSIONS

This study provides deep insight into the use of linear and nonlinear fatigue characteristics to effectively and reliably identify miner fatigue. Furthermore, the study provides a reference for clinical studies of acute mountain sickness in high-altitude, cold and hypoxic environments.

Neuromuscular and perceptual responses during repeated cycling sprints-usefulness of a "hypoxic to normoxic" recovery approach

PURPOSE

We investigated the consequence of varying hypoxia severity during an initial set of repeated cycling sprints on performance, neuromuscular fatigability, and exercise-related sensations during a subsequent set of repeated sprints in normoxia.

METHODS

Nine active males performed ten 4-s sprints (recovery = 30 s) at sea level (SL; FiO₂ ~ 0.21), moderate (MH; FiO₂ ~ 0.17) or severe normobaric hypoxia (SH; FiO₂ ~ 0.13). This was followed, after 8 min of passive recovery, by five 4-s sprints (recovery = 30 s) in normoxia.

RESULTS

Mean power decrement during Sprint 10 was exacerbated in SH compared to SL and MH (- 34 ± 12%, - 22 ± 13%, - 25 ± 14%, respectively, p < 0.05). Sprint performance during Sprint 11 recovered to that of Sprint 1 in all conditions (p = 0.267). All exercise-related sensations at Sprint 11 recovered significantly compared to Sprint 1, with no difference for Set 2 (p > 0.05). Ratings of overall perceived discomfort, difficulty breathing, and limb discomfort were exacerbated during Set 1 in SH versus SL (p < 0.05). Compared to SL, the averaged MPO value for Set 2 was 5.5 ± 3.0% (p = 0.003) lower in SH. Maximal voluntary force and twitch torque decreased similarly in all conditions immediately after Set 1 (p < 0.05), without further alterations after Set 2. Peripheral and cortical voluntary activation values did not change (p > 0.05).

CONCLUSION

Exercise-related sensations, rather than neuromuscular function integrity, may play a pivotal role in influencing performance of repeated sprints and its recovery.

Assessment of fatigue-related biochemical alterations in a rat swimming model under hypoxia

It is well known that exercise-induced fatigue is exacerbated following hypoxia exposure and may arise from central and/or peripheral mechanisms. To assess the relative contribution of peripheral and central factors to exercise-induced fatigue under hypoxia, a rat model of fatigue by a bout of exhaustive swimming was established and fatigue-related biochemical changes in normoxic and severe hypoxic conditions were compared. Rats were randomly divided into four groups: normoxia resting (NR), exhaustive swimming (NE), hypoxia resting (HR) and exhaustive swimming (HE). The swimming time to exhaustion with a weight equal to 2.5% of their body weight reduced under hypoxia. There were lower blood lactate levels, lower gastrocnemius pAMPK/AMPK ratios and higher gastrocnemius glycogen contents in the HE than in the NE groups, which all suggested a lower degree of peripheral fatigue in the HE group than in the NE group. Meanwhile, there was a significant increase in striatal 3,4-dihydroxyphenylacetic acid (DOPAC) caused by exhaustive swimming under normoxia, whereas this increase was almost blunted under severe hypoxia, indicating that hypoxia might exacerbate exercise-induced central fatigue. These biochemical changes suggest that from normoxia to severe hypoxia, the relative contribution of peripheral and central factors to exercise-induced fatigue alters, and central fatigue may play a predominant role in the decline in exercise performance under hypoxia.

Liver CEBPβ Modulates the Kynurenine Metabolism and Mediates the Motility for Hypoxia-Induced Central Fatigue in Mice

Central fatigue is defined as a failure of the central nervous system to adequately drive the muscle, manifesting limited development, and maintenance of locomotor activity. A plateau in hypoxia leads to central fatigue and followed by maximal motility recession. However, the underlying mechanism is still unclear. The present study describes a mechanism by which liver CEBPβ (CCAAT/enhancer-binding protein beta) induced by hypoxic environment alters the kynurenine (KYN) metabolism and causes the suppression of motility function recession. The activation of CEBPβ under hypoxia increases the liver expression of tryptophan dioxygenase, thereby enhancing the conversion of tryptophan into KYN; the KYN metabolite can traverse the blood-brain barrier and result in the suppression of motility function. However, the knockdown of CEBPβ by injecting pAAV-shRNA-CEBPβ via the hepatic portal vein reduces the KYN production and improves the motility function. KYN is a neurochemical that which restricts the exercise capacity after injection in the basal ganglia in mice. Reducing the plasma KYN protects the brain from hypoxia-induced changes associated with fatigue, and the knockdown liver of CEBPβ in mice renders resistance to fatigue post-acute hypoxia or tryptophan treatment. This study reveals resistance to central fatigue as a strategy for acclimatization to hypoxia mediated by transcription factor CEBPβ in the liver.

The effect of severe and moderate hypoxia on exercise at a fixed level of perceived exertion

Purpose

The purpose of this study was to determine the primary cues regulating perceived effort and exercise performance using a fixed-RPE protocol in severe and moderate hypoxia.

Methods

Eight male participants (26 ± 6 years, 76.3 ± 8.6 kg, 178.5 ± 3.6 cm, 51.4 ± 8.0 mL kg^− 1 min^− 1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot {V}$$\end{document}V˙O_2max) completed three exercise trials in environmental conditions of severe hypoxia (F_IO₂ 0.114), moderate hypoxia (F_IO₂ 0.152), and normoxia (F_IO₂ 0.202). They were instructed to continually adjust their power output to maintain a perceived effort (RPE) of 16, exercising until power output declined to 80% of the peak 30-s power output achieved.

Results

Exercise time was reduced (severe hypoxia 428 ± 210 s; moderate hypoxia 1044 ± 384 s; normoxia 1550 ± 590 s) according to a reduction in F_IO₂ (P < 0.05). The rate of oxygen desaturation during the first 3 min of exercise was accelerated in severe hypoxia (− 5.3 ± 2.8% min^− 1) relative to moderate hypoxia (− 2.5 ± 1.0% min^− 1) and normoxia (− 0.7 ± 0.3% min^− 1). Muscle tissue oxygenation did not differ between conditions (P > 0.05). Minute ventilation increased at a faster rate according to a decrease in F_IO₂ (severe hypoxia 27.6 ± 6.6; moderate hypoxia 21.8 ± 3.9; normoxia 17.3 ± 3.9 L min^− 1). Moderate-to-strong correlations were identified between breathing frequency (r = − 0.718, P < 0.001), blood oxygen saturation (r = 0.611, P = 0.002), and exercise performance.

Conclusions

The primary cues for determining perceived effort relate to progressive arterial hypoxemia and increases in ventilation.

A Systematic Review of Integrated Functional Near-Infrared Spectroscopy (fNIRS) and Transcranial Magnetic Stimulation (TMS) Studies

Background: The capacity for TMS to elicit neural activity and manipulate cortical excitability has created significant expectation regarding its use in both cognitive and clinical neuroscience. However, the absence of an ability to quantify stimulation effects, particularly outside of the motor cortex, has led clinicians and researchers to pair noninvasive brain stimulation with noninvasive neuroimaging techniques. fNIRS, as an optical and wearable neuroimaging technique, is an ideal candidate for integrated use with TMS. Together, TMS+fNIRS may offer a hybrid alternative to "blind" stimulation to assess NIBS in therapy and research. Objective: In this systematic review, the current body of research into the transient and prolonged effects of TMS on fNIRS-based cortical hemodynamic measures while at rest and during tasks are discussed. Additionally, studies investigating the relation of fNIRS to measures of cortical excitability as produced by TMS-evoked Motor-Evoked-Potential (MEP) are evaluated. The aim of this review is to outline the integrated use of TMS+fNIRS and consolidate findings related to use of fNIRS to monitor changes attributed to TMS and the relationship of fNIRS to cortical excitability itself. Methods: Key terms were searched in PubMed and Web-of-Science to identify studies investigating the use of both fNIRS and TMS. Works from Google-Scholar and referenced works in identified papers were also assessed for relevance. All published experimental studies using both fNIRS and TMS techniques in the study methodology were included. Results: A combined literature search of neuroimaging and neurostimulation studies identified 53 papers detailing the joint use of fNIRS and TMS. 22/53 investigated the immediate effects of TMS at rest in the DLPFC and M1 as measured by fNIRS. 21/22 studies reported a significant effect in [HbO] for 40/54 stimulation conditions with 14 resulting an increase and 26 in a decrease. While 15/22 studies also reported [HbR], only 5/37 conditions were significant. Task effects of fNIRS+TMS were detailed in 16 studies, including 10 with clinical populations. Most studies only reported significant changes in [HbO] related measures. Studies comparing fNIRS to changes in MEP-measured cortical excitability suggest that fNIRS measures may be spatially more diffuse but share similar traits. Conclusion: This review summarizes the progress in the development of this emerging hybrid neuroimaging & neurostimulation methodology and its applications. Despite encouraging progress and novel applications, a lack of replicated works, along with highly disparate methodological approaches, highlight the need for further controlled studies. Interpretation of current research directions, technical challenges of TMS+fNIRS, and recommendations regarding future works are discussed.

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.

Effect of carbohydrate ingestion on central fatigue during prolonged running exercise in moderate hypoxia

To determine whether acute exposure to moderate hypoxia alters central and peripheral fatigue and to test whether carbohydrate ingestion impacts fatigue characteristics, 12 trained runners completed three running trials lasting 1 h each at 65% of normoxic maximum oxygen uptake. The first trial was performed in normoxia [inspired O₂ fraction ( FiO2 ) = 0.21], and the last two trials were completed in hypoxia ( FiO2  = 0.15). Participants ingested a placebo drink in normoxia (NORM-PLA), a placebo drink in hypoxia (HYP-PLA), or a carbohydrate solution in hypoxia (HYP-CHO). HYP conditions were randomized. Peripheral [change in potentiated quadriceps twitch force (ΔQ_tw,pot)] and central [change in voluntary activation (ΔVA)] fatigue were assessed via preexercise-to-postexercise changes in magnetically evoked quadriceps twitch. In HYP, blood was drawn to determine the ratio of free-tryptophan (f-TRP) to branched-chain amino acids (BCAA). After exercise, peripheral fatigue was reduced to a similar degree in normoxia and hypoxia (ΔQ_tw,pot = -4.5 ± 1.3% and -4.0 ± 1.5% in NORM-PLA and HYP-PLA, respectively; P = 0.61). Central fatigue was present after normoxic and hypoxic exercise but to a greater degree in HYP-PLA compared with NORM-PLA (ΔVA: -4.7 ± 0.9% vs. -1.9 ± 0.7%; P < 0.01). Carbohydrate ingestion did not influence central fatigue (ΔVA in HYP-CHO: -5.7 ± 1.2%; P = 0.51 vs. HYP-PLA). After exercise, no differences were observed in the ratio of f-TRP to BCAA between HYP-PLA and HYP-CHO ( P = 0.67). Central fatigue increased during prolonged running exercise in moderate hypoxia although the ratio of f-TRP to BCAA remained unchanged. Ingesting carbohydrates while running in hypoxia did not influence fatigue development. NEW & NOTEWORTHY Hypoxic exposure influences the origin of exercise-induced fatigue and the rate of fatigue development depending on the severity of hypoxia. Our data suggest that moderate hypoxia increases central, but not peripheral, fatigue in trained runners exercising at 65% of normoxic maximum oxygen uptake. The increase in central fatigue was unaffected by carbohydrate intake and occurred although the ratio of free tryptophan to branched-chain amino acids remained unchanged.

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.

Oxygenation time course and neuromuscular fatigue during repeated cycling sprints with bilateral blood flow restriction

The aim was to evaluate changes in peripheral and cerebral oxygenation, cardiorespiratory, and performance differences, as well as neuromuscular fatigue across multiple levels of blood flow restriction (BFR) during a repeated cycling sprint test to exhaustion (RST). Participants performed three RST (10-sec maximal sprints with 20-sec recovery until exhaustion) with measurements of power output and V̇O2peak as well as oxygenation (near-infrared spectroscopy) of the vastus lateralis and prefrontal cortex. Neuromuscular fatigue was assessed by femoral nerve stimulation to evoke the vastus lateralis. Tests were conducted with proximal lower limb bilateral vascular occlusion at 0%, 45%, and 60% of resting pulse elimination pressure. Total work decreased with BFR (52.5 ± 22.9% at 45%, 68.6 ± 32.6% at 60%, P < 0.01 compared with 0%) as V̇O2peak (12.6 ± 9.3% at 45%, 18.2 ± 7.2% at 60%, compared with 0%, P < 0.01). Decreased changes in muscle deoxyhemoglobin (∆[HHb]) during sprints were demonstrated at 60% compared to 0% (P < 0.001). Changes in total hemoglobin concentrations (∆[tHb]) increased at both 45% and 60% compared with 0% (P < 0.001). Cerebral ∆[tHb] increased toward exhaustion (P < 0.05). Maximal voluntary contraction (MVC), voluntary activation level (VAL), and root mean square (RMS)/M-wave ratio decreased at 60% compared with 0% (P < 0.001, all). MVC and VAL decreased between 45% and 60% (P < 0.05, both). The application of BFR during RST induced greater changes in tissue perfusion (via blood volume, ∆[tHb]) suggesting a possible stimulus for vascular blood flow regulation. Additionally, high-intensity sprint exercise with partial ischemia may challenge cerebral blood flow regulation and influence local fatigue development due to protection of cerebral function.

UBC-Nepal expedition: acclimatization to high-altitude increases spinal motoneurone excitability during fatigue in humans

KEY POINTS

Acute exposure and acclimatization to hypoxia are associated with an impairment and partial recovery, respectively, of the capability of the central nervous system to drive muscles during prolonged efforts. Motoneurones play a vital role in muscle contraction and in fatigue, although the effect of hypoxia on motoneurone excitability during exercise has not been assessed in humans. We studied the impact of fatigue on motoneurone excitability in normoxia, acute and chronic exposure (5050 m) to hypoxia. Performance was worse in acute hypoxia but recovered to the normoxic standard in chronic hypoxia, in parallel with an increased excitability of the motoneurones compared to acute exposure to hypoxia. These findings reveal that prolonged hypoxia causes a heightened motoneurone responsiveness during fatiguing exercise; such an adaptation might favour the restoration of performance where low pressures of oxygen are chronically present.

ABSTRACT

The fatigue-induced failure of the motor cortex to drive muscles maximally increases in acute hypoxia (AH) compared to normoxia (N) but improves with acclimatization (chronic hypoxia; CH). Despite their importance to muscle output, it is unknown how locomotor motoneurones in humans are affected by hypoxia and acclimatization. Eleven participants performed 16 min of submaximal [25% maximal torque (maximal voluntary contraction, MVC)] intermittent isometric elbow flexions in N, AH (environmental chamber) and CH (7-14 days at 5050 m) (P_I O₂  = 140, 74 and 76 mmHg, respectively). For each minute of the fatigue protocol, motoneurone responsiveness was measured with cervicomedullary stimulation delivered 100 ms after transcranial magnetic stimulation (TMS) used to transiently silence voluntary drive. Every 2 min, cortical voluntary activation (cVA) was measured with TMS. After the task, MVC torque declined more in AH (∼20%) than N and CH (∼11% and 14%, respectively, P < 0.05), with no differences between N and CH. cVA was lower in AH than N and CH at baseline (∼92%, 95% and 95%, respectively) and at the end of the protocol (∼82%, 90% and 90%, P < 0.05). During the fatiguing task, motoneurone excitability in N and AH declined to ∼65% and 40% of the baseline value (P < 0.05). In CH, motoneurone excitability did not decline and, late in the protocol, was ∼40% higher compared to AH (P < 0.05). These novel data reveal that acclimatization to hypoxia leads to a heightened motoneurone responsiveness during fatiguing exercise. Positive spinal and supraspinal adaptations during extended periods at altitude might therefore play a vital role for the restoration of performance after acclimatization to hypoxia.

Neuromuscular Fatigue during Prolonged Exercise in Hypoxia

PURPOSE

Prolonged cycling exercise performance in normoxia is limited because of both peripheral and central neuromuscular impairments. It has been reported that cerebral perturbations are greater during short-duration exercise in hypoxia compared with normoxia. The purpose of this study was to test the hypothesis that central deficits are accentuated in hypoxia compared with normoxia during prolonged (three bouts of 80 min separated by 25 min) whole-body exercise at the same relative intensity.

METHODS

Ten subjects performed two sessions consisting of three 80-min cycling bouts at 45% of their relative maximal aerobic power in normoxia and hypoxia (FiO2 = 0.12). Before exercise and after each bout, maximal voluntary force, voluntary activation assessed with nerve stimulation and transcranial magnetic stimulation, corticospinal excitability (motor evoked potential), intracortical inhibition (cortical silent period), and electrical (M-wave) and contractile (twitch and doublet peak forces) properties of the knee extensors were measured. Prefrontal and motor cortical oxygenation was also recorded during each cycling bout in both conditions.

RESULTS

A significant but similar force reduction (≈-22%) was observed at the end of exercise in normoxia and hypoxia. The modifications of voluntary activation assessed with transcranial magnetic stimulation and nerve stimulation, motor evoked potential, cortical silent period, and M-wave were also similar in both conditions. However, cerebral oxygenation was reduced in hypoxia compared with normoxia.

CONCLUSION

These findings show that when performed at the same relative low intensity, prolonged exercise does not induce greater supraspinal fatigue in hypoxia compared with normoxia. Despite lower absolute exercise intensities in hypoxia, reduced brain O2 availability might contribute to similar amounts of central fatigue compared with normoxia.

Effects of high-altitude exposure on supraspinal fatigue and corticospinal excitability and inhibition

PURPOSE

While acute hypoxic exposure enhances exercise-induced central fatigue and can alter corticospinal excitability and inhibition, the effect of prolonged hypoxic exposure on these parameters remains to be clarified. We hypothesized that 5 days of altitude exposure would (i) normalize exercise-induced supraspinal fatigue during isolated muscle exercise to sea level (SL) values and (ii) increase corticospinal excitability and inhibition.

METHODS

Eleven male subjects performed intermittent isometric elbow flexions at 50% of maximal voluntary contraction to task failure at SL and after 1 (D1) and 5 (D5) days at 4350 m. Transcranial magnetic stimulation and peripheral electrical stimulation were used to assess supraspinal and peripheral fatigues. Pre-frontal cortex and biceps brachii oxygenation was monitored by near-infrared spectroscopy.

RESULTS

Exercise duration was not statistically different between SL (1095 ± 562 s), D1 (1132 ± 516 s), and D5 (1440 ± 689 s). No significant differences were found between the three experimental conditions in maximal voluntary activation declines at task failure (SL -16.8 ± 9.5%; D1 -25.5 ± 11.2%; D5 -21.8 ± 7.0%; p > 0.05). Exercise-induced peripheral fatigue was larger at D5 versus SL (100 Hz doublet at task failure: -58.8 ± 16.6 versus -41.8 ± 20.1%; p < 0.05). Corticospinal excitability at 50% maximal voluntary contraction was lower at D5 versus SL (brachioradialis p < 0.05, biceps brachii p = 0.055). Cortical silent periods were shorter at SL versus D1 and D5 (p < 0.05).

CONCLUSIONS

The present results show similar patterns of supraspinal fatigue development during isometric elbow flexions at SL and after 1 and 5 days at high altitude, despite larger amount of peripheral fatigue at D5, lowered corticospinal excitability and enhanced corticospinal inhibition at altitude.

Recovery of central and peripheral neuromuscular fatigue after exercise

Sustained physical exercise leads to a reduced capacity to produce voluntary force that typically outlasts the exercise bout. This “fatigue” can be due both to impaired muscle function, termed “peripheral fatigue,” and a reduction in the capacity of the central nervous system to activate muscles, termed “central fatigue.” In this review we consider the factors that determine the recovery of voluntary force generating capacity after various types of exercise. After brief, high-intensity exercise there is typically a rapid restitution of force that is due to recovery of central fatigue (typically within 2 min) and aspects of peripheral fatigue associated with excitation-contraction coupling and reperfusion of muscles (typically within 3–5 min). Complete recovery of muscle function may be incomplete for some hours, however, due to prolonged impairment in intracellular Ca2+ release or sensitivity. After low-intensity exercise of long duration, voluntary force typically shows rapid, partial, recovery within the first few minutes, due largely to recovery of the central, neural component. However, the ability to voluntarily activate muscles may not recover completely within 30 min after exercise. Recovery of peripheral fatigue contributes comparatively little to the fast initial force restitution and is typically incomplete for at least 20–30 min. Work remains to identify what factors underlie the prolonged central fatigue that usually accompanies long-duration single joint and locomotor exercise and to document how the time course of neuromuscular recovery is affected by exercise intensity and duration in locomotor exercise. Such information could be useful to enhance rehabilitation and sports performance.

Prediction of Critical Power and W' in Hypoxia: Application to Work-Balance Modelling

Purpose: Develop a prediction equation for critical power (CP) and work above CP (W′) in hypoxia for use in the work-balance (WBAL′) model.

Methods: Nine trained male cyclists completed cycling time trials (TT; 12, 7, and 3 min) to determine CP and W′ at five altitudes (250, 1,250, 2,250, 3,250, and 4,250 m). Least squares regression was used to predict CP and W′ at altitude. A high-intensity intermittent test (HIIT) was performed at 250 and 2,250 m. Actual and predicted CP and W′ were used to compute W′ during HIIT using differential (WBALdiff′) and integral (WBALint′) forms of the WBAL′ model.

Results: CP decreased at altitude (P < 0.001) as described by 3rd order polynomial function (R² = 0.99). W′ decreased at 4,250 m only (P < 0.001). A double-linear function characterized the effect of altitude on W′ (R² = 0.99). There was no significant effect of parameter input (actual vs. predicted CP and W′) on modelled WBAL′ at 2,250 m (P = 0.24). WBALdiff′ returned higher values than WBALint′ throughout HIIT (P < 0.001). During HIIT, WBALdiff′ was not different to 0 kJ at completion, at 250 m (0.7 ± 2.0 kJ; P = 0.33) and 2,250 m (−1.3 ± 3.5 kJ; P = 0.30). However, WBALint′ was lower than 0 kJ at 250 m (−0.9 ± 1.3 kJ; P = 0.058) and 2,250 m (−2.8 ± 2.8 kJ; P = 0.02).

Conclusion: The altitude prediction equations for CP and W′ developed in this study are suitable for use with the WBAL′ model in acute hypoxia. This enables the application of WBAL′ modelling to training prescription and competition analysis at altitude.

Rate dependent influence of arterial desaturation on self-selected exercise intensity during cycling

The purpose of this study was to clarify if Ratings of Perceived Exertion (RPE) and self-selected exercise intensity are sensitive not only to alterations in the absolute level of arterial saturation (SPO2) but also the rate of change in SPO2. Twelve healthy participants (31.6 ± 3.9 y, 175.5 ± 7.7 cm, 73.3 ± 10.3 kg, 51 ± 7 mL·kg-1·min-1 [Formula: see text]) exercised four times on a cycle ergometer, freely adjusting power output (PO) to maintain RPE at 5 on Borg's 10-point scale with no external feedback to indicate their exercise intensity. The fraction of inspired oxygen (FIO2) was reduced during three of those trials such that SPO2 decreased during exercise from starting values (>98%) to 70%. These trials were differentiated by the time over which the desaturation occurred: 3.9 ± 1.4 min, -8.7 ± 4.2%•min-1 (FAST), 11.0 ± 3.7 min, -2.8 ± 1.3%•min-1 (MED), and 19.5 ± 5.8 min, -1.5 ± 0.8%•min-1 (SLOW) (P < 0.001). Compared to stable PO throughout the control condition (no SPO2 manipulation), PO significantly decreased across the experimental conditions (FAST = 2.8 ± 2.1 W•% SPO2-1; MED = 2.5 ± 1.8 W•% SPO2-1; SLOW = 1.8 ± 1.6 W•% SPO2-1; P < 0.001). The rates of decline in PO during FAST and MED were similar, with both greater than SLOW. Our results confirm that decreases in absolute SPO2 impair exercise performance and that a faster rate of oxygen desaturation magnifies that impairment.

Cerebral Regulation in Different Maximal Aerobic Exercise Modes

We investigated cerebral responses, simultaneously with peripheral and ratings of perceived exertion (RPE) responses, during different VO_2MAX-matched aerobic exercise modes. Nine cyclists (VO_2MAX of 57.5 ± 6.2 ml·kg⁻¹·min⁻¹) performed a maximal, controlled-pace incremental test (MIT) and a self-paced 4 km time trial (TT_4km). Measures of cerebral (COX) and muscular (MOX) oxygenation were assessed throughout the exercises by changes in oxy- (O₂Hb) and deoxy-hemoglobin (HHb) concentrations over the prefrontal cortex (PFC) and vastus lateralis (VL) muscle, respectively. Primary motor cortex (PMC) electroencephalography (EEG), VL, and rectus femoris EMG were also assessed throughout the trials, together with power output and cardiopulmonary responses. The RPE was obtained at regular intervals. Similar motor output (EMG and power output) occurred from 70% of the duration in MIT and TT_4km, despite the greater motor output, muscle deoxygenation (↓ MOX) and cardiopulmonary responses in TT_4km before that point. Regarding cerebral responses, there was a lower COX (↓ O₂Hb concentrations in PFC) at 20, 30, 40, 50 and 60%, but greater at 100% of the TT_4km duration when compared to MIT. The alpha wave EEG in PMC remained constant throughout the exercise modes, with greater values in TT_4km. The RPE was maximal at the endpoint in both exercises, but it increased slower in TT_4km than in MIT. Results showed that similar motor output and effort tolerance were attained at the closing stages of different VO_2MAX-matched aerobic exercises, although the different disturbance until that point. Regardless of different COX responses during most of the exercises duration, activation in PMC was preserved throughout the exercises, suggesting that these responses may be part of a centrally-coordinated exercise regulation.

Does altitude level of a prior time-trial modify subsequent exercise performance in hypoxia and associated neuromuscular responses?

We examined the influence of prior time‐trials performed at different altitudes on subsequent exercise in moderate hypoxia and associated cardiometabolic and neuromuscular responses. In normobaric hypoxia (simulated altitude 2000 m; FiO₂: 0.163), 10 healthy males performed (1) an incremental test to exhaustion (VO_{2max_2000}) and (2) a test to exhaustion at 80% of the power output associated to VO_{2max_2000} for a reference time (947 ± 336 sec). Thereafter, two sessions were conducted in a randomized order: a cycle time‐trial corresponding to the reference time (TT₁) followed 22 min later (passive rest at 2000 m) by a 6‐min cycle time‐trial (TT₂). TT₁ was either performed at 2000 or 3500 m (FiO₂: 0.135), while TT₂ was always performed at 2000 m. As expected, during TT₁, the mean power output (247 ± 42 vs. 227 ± 37 W; P < 0.001) was higher at 2000 than 3500 m. During TT₂, the mean power output (256 ± 42 vs. 252 ± 36 W) did not differ between conditions. Before and after TT₁, maximal isometric voluntary contraction torque in knee extensors (pooled conditions: −7.9 ± 8.4%; P < 0.01), voluntary activation (−4.1 ± 3.1%; P < 0.05), and indices of muscle contractility (peak twitch torque: −39.1 ± 11.9%; doublet torques at 100 Hz: −15.4 ± 8.9%; 10/100 Hz ratio: −25.8 ± 7.7%; all P < 0.001) were equally reduced at 2000 m or 3500 m. Irrespective of the altitude of TT₁, neuromuscular function remained similarly depressed after TT₁ both before and after TT₂ at 2000 m. A prior time‐trial performed at different altitude influenced to the same extent performance and associated cardiometabolic and neuromuscular responses during a subsequent exercise in moderate hypoxia.

Clamping end-tidal carbon dioxide during graded exercise with control of inspired oxygen

Exercise- and hypoxia-induced hyperventilation decreases the partial pressure of end-tidal carbon dioxide (PETCO2), which in turn exerts many physiological effects. Several breathing circuits that control PETCO2 have been previously described, but their designs are not satisfactory for exercise studies where changes in inspired oxygen (FIO2) may be desired. This study is the first report of a breathing system that can maintain PETCO2 constant within a single session of graded submaximal exercise and graded hypoxia. Thirteen fit and healthy subjects completed two bouts of exercise consisting of three 3min stages on a cycle ergometer with increasing exercise intensity in normoxia (Part A; 142±14, 167±14, 192±14W) or with decreasing FIO2 at a constant exercise intensity (Part B; 21, 18, and 14%). One bout was a control (CON) where PETCO2 was not manipulated, while during the other bout the investigator clamped PETCO2 within 2mmHg (CO2Clamp) using sequential gas delivery (SGD). During the final 30s of each exercise stage during CO2Clamp, PETCO2 was successfully maintained in Part A (43±4, 44±4, 44±3mmHg; P=0.44) and Part B (45±3, 46±3, 45±3mmHg; P=0.68) despite the increases in ventilation due to exercise. These findings demonstrate that this SGD circuit can be used to maintain isocapania in exercising humans during progressively increasing exercise intensities and changing FIO2.

Corticospinal excitability is associated with hypocapnia but not changes in cerebral blood flow

KEY POINTS

Reductions in cerebral blood flow (CBF) may be implicated in the development of neuromuscular fatigue; however, the contribution from hypocapnic-induced reductions (i.e. P ETC O2) in CBF versus reductions in CBF per se has yet to be isolated. We assessed neuromuscular function while using indomethacin to selectively reduce CBF without changes in P ETC O2 and controlled hyperventilation-induced hypocapnia to reduce both CBF and P ETC O2. Increased corticospinal excitability appears to be exclusive to reductions in P ETC O2 but not reductions in CBF, whereas sub-optimal voluntary output from the motor cortex is moderately associated with decreased CBF independent of changes in P ETC O2. These findings suggest that changes in CBF and P ETC O2 have distinct roles in modulating neuromuscular function.

ABSTRACT

Although reductions in cerebral blood flow (CBF) may be involved in central fatigue, the contribution from hypocapnia-induced reductions in CBF versus reductions in CBF per se has not been isolated. This study examined whether reduced arterial PCO2 (P aC O2), independent of concomitant reductions in CBF, impairs neuromuscular function. Neuromuscular function, as indicated by motor-evoked potentials (MEPs), maximal M-wave (Mmax ) and cortical voluntary activation (cVA) of the flexor carpi radialis muscle during isometric wrist flexion, was assessed in ten males (29 ± 10 years) during three separate conditions: (1) cyclooxygenase inhibition using indomethacin (Indomethacin, 1.2 mg kg(-1) ) to selectively reduce CBF by 28.8 ± 10.3% (estimated using transcranial Doppler ultrasound) without changes in end-tidal PCO2 (P ETC O2); (2) controlled iso-oxic hyperventilation-induced reductions in P aC O2 (Hypocapnia), P ETC O2  = 30.1 ± 4.5 mmHg with related reductions in CBF (21.7 ± 6.3%); and (3) isocapnic hyperventilation (Isocapnia) to examine the potential direct influence of hyperventilation-mediated activation of respiratory control centres on CBF and changes in neuromuscular function. Change in MEP amplitude (%Mmax ) from baseline was greater in Hypocapnia tha in Isocapnia (11.7 ± 9.8%, 95% confidence interval (CI) [2.6, 20.7], P = 0.01) and Indomethacin (13.3 ± 11.3%, 95% CI [2.8, 23.7], P = 0.01) with a large Cohen's effect size (d ≥ 1.17). Although not statistically significant, cVA was reduced with a moderate effect size in Indomethacin (d = 0.7) and Hypocapnia (d = 0.9) compared to Isocapnia. In summary, increased corticospinal excitability - as reflected by larger MEP amplitude - appears to be exclusive to reduced P aC O2, but not reductions in CBF per se. Sub-optimal voluntary output from the motor cortex is moderately associated with decreased CBF, independent of reduced P aC O2.

Interaction between environmental temperature and hypoxia on central and peripheral fatigue during high-intensity dynamic knee extension

This study investigated causative factors behind the expression of different interaction types during exposure to multistressor environments. Neuromuscular fatigue rates and time to exhaustion (TTE) were investigated in active men (n = 9) exposed to three climates [5 °C, 50% relative humidity (rh); 23 °C, 50% rh; and 42 °C, 70% rh] at two inspired oxygen fractions (0.209 and 0.125 FiO2; equivalent attitude = 4,100 m). After a 40-min rest in the three climatic conditions, participants performed constant-workload (high intensity) knee extension exercise until exhaustion, with brief assessments of neuromuscular function every 110 s. Independent exposure to cold, heat, and hypoxia significantly (P < 0.01) reduced TTE from thermoneutral normoxia (reductions of 190, 405, and 505 s from 915 s, respectively). The TTE decrease was consistent with a faster rate of peripheral fatigue development (P < 0.01) compared with thermoneutral normoxia (increase of 1.6, 3.1, and 4.9%/min from 4.1%/min, respectively). Combined exposure to hypoxic-cold resulted in an even greater TTE reduction (-589 s), likely due to an increase in the rate of peripheral fatigue development (increased by 7.6%/min), but this was without significant interaction between stressors (P > 0.198). In contrast, combined exposure to hypoxic heat reduced TTE by 609 s, showing a significant antagonistic interaction (P = 0.003) similarly supported by an increased rate of peripheral fatigue development (which increased by 8.3%/min). A small decline (<0.4%/min) in voluntary muscle activation was observed only in thermoneutral normoxia. In conclusion, interaction type is influenced by the impact magnitude of the effect of the individual stressors' effect on exercise capacity, whereby the greater the effect of stressors, the greater the probability that one stressor will be abolished by the other. This indicates that humans respond to severe and simultaneous physiological strains on the basis of a worst-strain-takes-precedence principle.

Task Failure during Exercise to Exhaustion in Normoxia and Hypoxia Is Due to Reduced Muscle Activation Caused by Central Mechanisms While Muscle Metaboreflex Does Not Limit Performance

To determine whether task failure during incremental exercise to exhaustion (IE) is principally due to reduced neural drive and increased metaboreflex activation eleven men (22 ± 2 years) performed a 10 s control isokinetic sprint (IS; 80 rpm) after a short warm-up. This was immediately followed by an IE in normoxia (Nx, P_IO₂:143 mmHg) and hypoxia (Hyp, P_IO₂:73 mmHg) in random order, separated by a 120 min resting period. At exhaustion, the circulation of both legs was occluded instantaneously (300 mmHg) during 10 or 60 s to impede recovery and increase metaboreflex activation. This was immediately followed by an IS with open circulation. Electromyographic recordings were obtained from the vastus medialis and lateralis. Muscle biopsies and blood gases were obtained in separate experiments. During the last 10 s of the IE, pulmonary ventilation, VO₂, power output and muscle activation were lower in hypoxia than in normoxia, while pedaling rate was similar. Compared to the control sprint, performance (IS-Wpeak) was reduced to a greater extent after the IE-Nx (11% lower P < 0.05) than IE-Hyp. The root mean square (EMG_RMS) was reduced by 38 and 27% during IS performed after IE-Nx and IE-Hyp, respectively (Nx vs. Hyp: P < 0.05). Post-ischemia IS-EMG_RMS values were higher than during the last 10 s of IE. Sprint exercise mean (IS-MPF) and median (IS-MdPF) power frequencies, and burst duration, were more reduced after IE-Nx than IE-Hyp (P < 0.05). Despite increased muscle lactate accumulation, acidification, and metaboreflex activation from 10 to 60 s of ischemia, IS-Wmean (+23%) and burst duration (+10%) increased, while IS-EMG_RMS decreased (−24%, P < 0.05), with IS-MPF and IS-MdPF remaining unchanged. In conclusion, close to task failure, muscle activation is lower in hypoxia than in normoxia. Task failure is predominantly caused by central mechanisms, which recover to great extent within 1 min even when the legs remain ischemic. There is dissociation between the recovery of EMG_RMS and performance. The reduction of surface electromyogram MPF, MdPF and burst duration due to fatigue is associated but not caused by muscle acidification and lactate accumulation. Despite metaboreflex stimulation, muscle activation and power output recovers partly in ischemia indicating that metaboreflex activation has a minor impact on sprint performance.

Impact of Altitude on Power Output during Cycling Stage Racing

PURPOSE

The purpose of this study was to quantify the effects of moderate-high altitude on power output, cadence, speed and heart rate during a multi-day cycling tour.

METHODS

Power output, heart rate, speed and cadence were collected from elite male road cyclists during maximal efforts of 5, 15, 30, 60, 240 and 600 s. The efforts were completed in a laboratory power-profile assessment, and spontaneously during a cycling race simulation near sea-level and an international cycling race at moderate-high altitude. Matched data from the laboratory power-profile and the highest maximal mean power output (MMP) and corresponding speed and heart rate recorded during the cycling race simulation and cycling race at moderate-high altitude were compared using paired t-tests. Additionally, all MMP and corresponding speeds and heart rates were binned per 1000 m (<1000 m, 1000-2000, 2000-3000 and >3000 m) according to the average altitude of each ride. Mixed linear modelling was used to compare cycling performance data from each altitude bin.

RESULTS

Power output was similar between the laboratory power-profile and the race simulation, however MMPs for 5-600 s and 15, 60, 240 and 600 s were lower (p ≤ 0.005) during the race at altitude compared with the laboratory power-profile and race simulation, respectively. Furthermore, peak power output and all MMPs were lower (≥ 11.7%, p ≤ 0.001) while racing >3000 m compared with rides completed near sea-level. However, speed associated with MMP 60 and 240 s was greater (p < 0.001) during racing at moderate-high altitude compared with the race simulation near sea-level.

CONCLUSION

A reduction in oxygen availability as altitude increases leads to attenuation of cycling power output during competition. Decrement in cycling power output at altitude does not seem to affect speed which tended to be greater at higher altitudes.