Increased PIO2 at Exhaustion in Hypoxia Enhances Muscle Activation and Swiftly Relieves Fatigue: A Placebo or a PIO2 Dependent Effect?

Determinants of the maximal functional reserve during repeated supramaximal exercise by humans: The roles of Nrf2/Keap1, antioxidant proteins, muscle phenotype and oxygenation

When high-intensity exercise is performed until exhaustion a "functional reserve" (FR) or capacity to produce power at the same level or higher than reached at exhaustion exists at task failure, which could be related to reactive oxygen and nitrogen species (RONS)-sensing and counteracting mechanisms. Nonetheless, the magnitude of this FR remains unknown. Repeated bouts of supramaximal exercise at 120% of VO2max interspaced with 20s recovery periods with full ischaemia were used to determine the maximal FR. Then, we determined which muscle phenotypic features could account for the variability in functional reserve in humans. Exercise performance, cardiorespiratory variables, oxygen deficit, and brain and muscle oxygenation (near-infrared spectroscopy) were measured, and resting muscle biopsies were obtained from 43 young healthy adults (30 males). Males and females had similar aerobic (VO2max per kg of lower extremities lean mass (LLM): 166.7 ± 17.1 and 166.1 ± 15.6 ml kg LLM-1.min-1, P = 0.84) and anaerobic fitness (similar performance in the Wingate test and maximal accumulated oxygen deficit when normalized to LLM). The maximal FR was similar in males and females when normalized to LLM (1.84 ± 0.50 and 2.05 ± 0.59 kJ kg LLM-1, in males and females, respectively, P = 0.218). This FR depends on an obligatory component relying on a reserve in glycolytic capacity and a putative component generated by oxidative phosphorylation. The aerobic component depends on brain oxygenation and phenotypic features of the skeletal muscles implicated in calcium handling (SERCA1 and 2 protein expression), oxygen transport and diffusion (myoglobin) and redox regulation (Keap1). The glycolytic component can be predicted by the protein expression levels of pSer40-Nrf2, the maximal accumulated oxygen deficit and the protein expression levels of SOD1. Thus, an increased capacity to modulate the expression of antioxidant proteins involved in RONS handling and calcium homeostasis may be critical for performance during high-intensity exercise in humans.

Thirty Years of Neuroscientific Investigation of Placebo and Nocebo: The Interesting, the Good, and the Bad

Over the past 30 years there has been a surge of research on the placebo effect using a neuroscientific approach. The interesting aspects of this effort are related to the identification of several biological mechanisms of both the placebo and nocebo effects, the latter of which is defined as a negative placebo effect. Some important translational implications have emerged both in the setting of clinical trials and in routine medical practice. One of the principal contributions of neuroscience has been to draw the attention of the scientific and medical communities to the important role of psychobiological factors in therapeutic outcomes, be they drug related or not. Indeed, many biological mechanisms triggered by placebos and nocebos resemble those modulated by drugs, suggesting a possible interaction between psychological factors and drug action. Unfortunately, this new knowledge regarding placebos has the potential of being dangerously exploited by pseudoscience. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Progress Update and Challenges on V . O2max Testing and Interpretation

The maximal oxygen uptake (V.O_2max) is the primary determinant of endurance performance in heterogeneous populations and has predictive value for clinical outcomes and all-cause mortality. Accurate and precise measurement of V.O_2max requires the adherence to quality control procedures, including combustion testing and the use of standardized incremental exercise protocols with a verification phase preceded by an adequate familiarization. The data averaging strategy employed to calculate the V.O_2max from the breath-by-breath data can change the V.O_2max value by 4–10%. The lower the number of breaths or smaller the number of seconds included in the averaging block, the higher the calculated V.O_2max value with this effect being more prominent in untrained subjects. Smaller averaging strategies in number of breaths or seconds (less than 30 breaths or seconds) facilitate the identification of the plateau phenomenon without reducing the reliability of the measurements. When employing metabolic carts, averaging intervals including 15–20 breaths or seconds are preferable as a compromise between capturing the true V.O_2max and identifying the plateau. In training studies, clinical interventions and meta-analysis, reporting of V.O_2max in absolute values and inclusion of protocols and the averaging strategies arise as imperative to permit adequate comparisons. Newly developed correction equations can be used to normalize V.O_2max to similar averaging strategies. A lack of improvement of V.O_2max with training does not mean that the training program has elicited no adaptations, since peak cardiac output and mitochondrial oxidative capacity may be increased without changes in V.O_2max.

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.

Enhancement of Exercise Performance by 48 Hours, and 15-Day Supplementation with Mangiferin and Luteolin in Men

The natural polyphenols mangiferin and luteolin have free radical-scavenging properties, induce the antioxidant gene program and down-regulate the expression of superoxide-producing enzymes. However, the effects of these two polyphenols on exercise capacity remains mostly unknown. To determine whether a combination of luteolin (peanut husk extract containing 95% luteolin, PHE) and mangiferin (mango leave extract (MLE), Zynamite^®) at low (PHE: 50 mg/day; and 140 mg/day of MLE containing 100 mg of mangiferin; L) and high doses (PHE: 100 mg/day; MLE: 420 mg/day; H) may enhance exercise performance, twelve physically active men performed incremental exercise to exhaustion, followed by sprint and endurance exercise after 48 h (acute effects) and 15 days of supplementation (prolonged effects) with polyphenols or placebo, following a double-blind crossover design. During sprint exercise, mangiferin + luteolin supplementation enhanced exercise performance, facilitated muscle oxygen extraction, and improved brain oxygenation, without increasing the VO₂. Compared to placebo, mangiferin + luteolin increased muscle O₂ extraction during post-exercise ischemia, and improved sprint performance after ischemia-reperfusion likely by increasing glycolytic energy production, as reflected by higher blood lactate concentrations after the sprints. Similar responses were elicited by the two doses tested. In conclusion, acute and prolonged supplementation with mangiferin combined with luteolin enhances performance, muscle O₂ extraction, and brain oxygenation during sprint exercise, at high and low doses.

Mangifera indica L. Leaf Extract in Combination With Luteolin or Quercetin Enhances VO2peak and Peak Power Output, and Preserves Skeletal Muscle Function During Ischemia-Reperfusion in Humans

It remains unknown whether polyphenols such as luteolin (Lut), mangiferin and quercetin (Q) have ergogenic effects during repeated all-out prolonged sprints. Here we tested the effect of Mangifera indica L. leaf extract (MLE) rich in mangiferin (Zynamite®) administered with either quercetin (Q) and tiger nut extract (TNE), or with luteolin (Lut) on sprint performance and recovery from ischemia-reperfusion. Thirty young volunteers were randomly assigned to three treatments 48 h before exercise. Treatment A: placebo (500 mg of maltodextrin/day); B: 140 mg of MLE (60% mangiferin) and 50 mg of Lut/day; and C: 140 mg of MLE, 600 mg of Q and 350 mg of TNE/day. After warm-up, subjects performed two 30 s Wingate tests and a 60 s all-out sprint interspaced by 4 min recovery periods. At the end of the 60 s sprint the circulation of both legs was instantaneously occluded for 20 s. Then, the circulation was re-opened and a 15 s sprint performed, followed by 10 s recovery with open circulation, and another 15 s final sprint. MLE supplements enhanced peak (Wpeak) and mean (Wmean) power output by 5.0-7.0% (P < 0.01). After ischemia, MLE+Q+TNE increased Wpeak by 19.4 and 10.2% compared with the placebo (P < 0.001) and MLE+Lut (P < 0.05), respectively. MLE+Q+TNE increased Wmean post-ischemia by 11.2 and 6.7% compared with the placebo (P < 0.001) and MLE+Lut (P = 0.012). Mean VO2 during the sprints was unchanged, suggesting increased efficiency or recruitment of the anaerobic capacity after MLE ingestion. In women, peak VO2 during the repeated sprints was 5.8% greater after the administration of MLE, coinciding with better brain oxygenation. MLE attenuated the metaboreflex hyperpneic response post-ischemia, may have improved O2 extraction by the Vastus Lateralis (MLE+Q+TNE vs. placebo, P = 0.056), and reduced pain during ischemia (P = 0.068). Blood lactate, acid-base balance, and plasma electrolytes responses were not altered by the supplements. In conclusion, a MLE extract rich in mangiferin combined with either quercetin and tiger nut extract or luteolin exerts a remarkable ergogenic effect, increasing muscle power in fatigued subjects and enhancing peak VO2 and brain oxygenation in women during prolonged sprinting. Importantly, the combination of MLE+Q+TNE improves skeletal muscle contractile function during ischemia/reperfusion.

Skeletal Muscle Pyruvate Dehydrogenase Phosphorylation and Lactate Accumulation During Sprint Exercise in Normoxia and Severe Acute Hypoxia: Effects of Antioxidants

Compared to normoxia, during sprint exercise in severe acute hypoxia the glycolytic rate is increased leading to greater lactate accumulation, acidification, and oxidative stress. To determine the role played by pyruvate dehydrogenase (PDH) activation and reactive nitrogen and oxygen species (RNOS) in muscle lactate accumulation, nine volunteers performed a single 30-s sprint (Wingate test) on four occasions: two after the ingestion of placebo and another two following the intake of antioxidants, while breathing either hypoxic gas (P_IO₂ = 75 mmHg) or room air (P_IO₂ = 143 mmHg). Vastus lateralis muscle biopsies were obtained before, immediately after, 30 and 120 min post-sprint. Antioxidants reduced the glycolytic rate without altering performance or VO₂. Immediately after the sprints, Ser²⁹³- and Ser³⁰⁰-PDH-E1α phosphorylations were reduced to similar levels in all conditions (~66 and 91%, respectively). However, 30 min into recovery Ser²⁹³-PDH-E1α phosphorylation reached pre-exercise values while Ser³⁰⁰-PDH-E1α was still reduced by 44%. Thirty minutes after the sprint Ser²⁹³-PDH-E1α phosphorylation was greater with antioxidants, resulting in 74% higher muscle lactate concentration. Changes in Ser²⁹³ and Ser³⁰⁰-PDH-E1α phosphorylation from pre to immediately after the sprints were linearly related after placebo (r = 0.74, P < 0.001; n = 18), but not after antioxidants ingestion (r = 0.35, P = 0.15). In summary, lactate accumulation during sprint exercise in severe acute hypoxia is not caused by a reduced activation of the PDH. The ingestion of antioxidants is associated with increased PDH re-phosphorylation and slower elimination of muscle lactate during the recovery period. Ser²⁹³ re-phosphorylates at a faster rate than Ser³⁰⁰-PDH-E1α during the recovery period, suggesting slightly different regulatory mechanisms.

Critical Life Functions: Can Placebo Replace Oxygen?

A crucial question in placebo research is related to which conditions and physiological functions are affected by placebos. Here we present evidence that critical life functions, like ventilation, oxygenation, circulation, and perfusion, can be sensitive to placebo treatments in some circumstances. Indeed, we have investigated the role of placebo effects at an altitude of 3500m, where oxygen pressure is 64% compared to the sea level. In these extreme conditions, hypoxia triggers several compensatory responses, such as hyperventilation, increased cardiac output, and increased brain perfusion. A conditioned placebo procedure was found to mimic the effects of oxygen on these compensatory responses, and these effects are still present at altitudes as high as 4500 and 5500m, where oxygen pressure is only 57% and 50%, respectively, compared to the sea level. Thus, placebo effects also take place for those functions that are critical for life and whereby oxygen is the key element.

Exercise-induced Fatigue in Severe Hypoxia after an Intermittent Hypoxic Protocol

PURPOSE

Exercise-induced central fatigue is alleviated after acclimatization to high altitude. The adaptations underpinning this effect may also be induced with brief, repeated exposures to severe hypoxia. The purpose of this study was to determine whether (i) exercise tolerance in severe hypoxia would be improved after an intermittent hypoxic (IH) protocol and (ii) exercise-induced central fatigue would be alleviated after an IH protocol.

METHODS

Nineteen recreationally active men were randomized into two groups who completed ten 2-h exposures in severe hypoxia (IH: partial pressure of inspired O2 82 mm Hg; n = 11) or normoxia (control; n = 8). Seven sessions involved cycling for 30 min at 25% peak power (W˙peak) in IH and at a matched heart rate in normoxia. Participants performed baseline constant-power cycling to task failure in severe hypoxia (TTF-Pre). After the intervention, the cycling trial was repeated (TTF-Post). Before and after exercise, responses to transcranial magnetic stimulation and supramaximal femoral nerve stimulation were obtained to assess central and peripheral contributions to neuromuscular fatigue.

RESULTS

From pre- to postexercise in TTF-Pre, maximal voluntary contraction (MVC), cortical voluntary activation (VATMS), and potentiated twitch force (Qtw,pot) decreased in both groups (all P < 0.05). After IH, TTF-Post was improved (535 ± 213 s vs 713 ± 271 s, P < 0.05) and an additional isotime trial was performed. After the IH intervention only, the reduction in MVC and VATMS was attenuated at isotime (P < 0.05). No differences were observed in the control group.

CONCLUSIONS

Whole-body exercise tolerance in severe hypoxia was prolonged after a protocol of IH. This may be related to an alleviation of the central contribution to neuromuscular fatigue.

Cerebral blood flow, frontal lobe oxygenation and intra-arterial blood pressure during sprint exercise in normoxia and severe acute hypoxia in humans

Cerebral blood flow (CBF) is regulated to secure brain O₂ delivery while simultaneously avoiding hyperperfusion; however, both requisites may conflict during sprint exercise. To determine whether brain O₂ delivery or CBF is prioritized, young men performed sprint exercise in normoxia and hypoxia (P_IO₂ = 73 mmHg). During the sprints, cardiac output increased to ∼22 L min^-1, mean arterial pressure to ∼131 mmHg and peak systolic blood pressure ranged between 200 and 304 mmHg. Middle-cerebral artery velocity (MCAv) increased to peak values (∼16%) after 7.5 s and decreased to pre-exercise values towards the end of the sprint. When the sprints in normoxia were preceded by a reduced P_ETCO₂, CBF and frontal lobe oxygenation decreased in parallel ( r = 0.93, P < 0.01). In hypoxia, MCAv was increased by 25%, due to a 26% greater vascular conductance, despite 4-6 mmHg lower PaCO₂ in hypoxia than normoxia. This vasodilation fully accounted for the 22 % lower CaO₂ in hypoxia, leading to a similar brain O₂ delivery during the sprints regardless of P_IO₂. In conclusion, when a conflict exists between preserving brain O₂ delivery or restraining CBF to avoid potential damage by an elevated perfusion pressure, the priority is given to brain O₂ delivery.