scholarly journals Application of Molecular Hydrogen as an Antioxidant in Responses to Ventilatory and Ergogenic Adjustments during Incremental Exercise in Humans

Nutrients ◽  
2021 ◽  
Vol 13 (2) ◽  
pp. 459
Author(s):  
Ahad Abdulkarim D. Alharbi ◽  
Naoyuki Ebine ◽  
Satoshi Nakae ◽  
Tatsuya Hojo ◽  
Yoshiyuki Fukuoka
Keyword(s):  
Gas Exchange ◽  
Molecular Hydrogen ◽  
Near Infrared ◽  
Fiber Type ◽  
Vastus Lateralis ◽  
Pulmonary Ventilation ◽  
Blood Gases ◽  
Incremental Exercise ◽  
Double Blind ◽  
O2 Delivery

We investigated effects of molecular hydrogen (H2) supplementation on acid-base status, pulmonary gas exchange responses, and local muscle oxygenation during incremental exercise. Eighteen healthy, trained subjects in a randomized, double-blind, crossover design received H2-rich calcium powder (HCP) (1500 mg/day, containing 2.544 µg/day of H2) or H2-depleted placebo (1500 mg/day) for three consecutive days. They performed cycling incremental exercise starting at 20-watt work rate, increasing by 20 watts/2 min until exhaustion. Breath-by-breath pulmonary ventilation (V˙E) and CO2 output (V˙CO2) were measured and muscle deoxygenation (deoxy[Hb + Mb]) was determined via time-resolved near-infrared spectroscopy in the vastus lateralis (VL) and rectus femoris (RF). Blood gases’ pH, lactate, and bicarbonate (HCO3−) concentrations were measured at rest and 120-, 200-, and 240-watt work rates. At rest, the HCP group had significantly lower V˙E, V˙CO2, and higher HCO3−, partial pressures of CO2 (PCO2) versus placebo. During exercise, a significant pH decrease and greater HCO3− continued until 240-watt workload in HCP. The V˙E was significantly lower in HCP versus placebo, but HCP did not affect the gas exchange status of V˙CO2 or oxygen uptake (V˙O2). HCP increased absolute values of deoxy[Hb + Mb] at the RF but not VL. Thus, HCP-induced hypoventilation would lead to lower pH and secondarily impaired balance between O2 delivery and utilization in the local RF during exercise, suggesting that HCP supplementation, which increases the at-rest antioxidant potential, affects the lower ventilation and pH status during incremental exercise. HPC induced a significantly lower O2 delivery/utilization ratio in the RF but not the VL, which may be because these regions possess inherently different vascular/metabolic control properties, perhaps related to fiber-type composition.

scholarly journals Application of Molecular Hydrogen as an Antioxidant in Responses to Ventilatory and Ergogenic Adjustments during Incremental Exercise in Humans

2020 ◽  
Author(s):  
Alharbi Ahad Abdulkarim D ◽  
Naoyuki Ebine ◽  
Satoshi Nakae ◽  
Tatsuya Hojo ◽  
Yoshiyuki Fukuoka
Keyword(s):  
Gas Exchange ◽  
Molecular Hydrogen ◽  
Fiber Type ◽  
Vastus Lateralis ◽  
Pulmonary Ventilation ◽  
Blood Gases ◽  
Work Rate ◽  
Incremental Exercise ◽  
Double Blind ◽  
O2 Delivery

We investigated effects of molecular hydrogen (H2) supplementation on acid-base status, pulmonary gas exchange responses, and local muscle oxygenation during incremental exercise. Eighteen healthy, trained subjects in a randomized, double-blind, crossover design received H2-rich calcium powder (HCP) (1500 mg/day, containing 2.544 µg/day of H2) or H2-depleted placebo (1500 mg/day) for 3 consecutive days. They performed cycling incremental exercise starting at 20-watts work rate, increasing by 20 watts/2 min until exhaustion. Breath-by-breath pulmonary ventilation (VE) and CO2 output (VCO2) were measured and muscle deoxygenation (deoxy[Hb + Mb]) was determined via time-resolved-NIRS in the vastus lateralis (VL) and rectus femoris (RF). Blood gases' pH, lactate, and HCO3− concentrations were measured at rest and 120-, 200-, and 240-watt work rates. At rest, the HCP group had significantly lower VE, VCO2, and higher HCO3−, PCO2 versus placebo. During exercise, a significant pH decrease and greater HCO3− continued until 240-watts work rate in HCP. The VE was significantly lower in HCP versus placebo, but HCP did not affect the gas exchange status of VCO2 or oxygen uptake (VO2). HCP increased absolute values of deoxy[Hb + Mb] at the RF but not VL. Thus, HCP-induced hypoventilation would lead to lower pH and secondarily impaired balance between O2 delivery and utilization in the local RF during exercise, suggesting that HCP supplementation, which increases the at-rest antioxidant potential, affects the lower ventilation and pH status during incremental exercise. HPC induced a significantly lower O2 delivery/utilization ratio in the RF but not the VL, which may be because these regions possess inherently different vascular/metabolic control properties, perhaps related to fiber-type composition.

THE EFFECT OF LEG PREFERENCE ON MECHANICAL EFFICIENCY DURING SINGLE-LEG EXTENSION EXERCISE

2021 ◽  
Author(s):  
Massimo Venturelli ◽  
Cantor Tarperi ◽  
Chiara Milanese ◽  
Luca Festa ◽  
Luana Toniolo ◽  
...  
Keyword(s):  
Muscle Activation ◽  
Near Infrared ◽  
Isometric Force ◽  
Fiber Type ◽  
Vastus Lateralis ◽  
Critical Role ◽  
Inverse Correlation ◽  
Mechanical Efficiency ◽  
Peak Power Output ◽  
Leg Extension

To investigate how leg preference affects net efficiency (ηnet), we examined central and peripheral hemodynamics, muscle fiber type, activation and force of preferred (PL) and non-preferred (NPL) leg. Our hypothesis was that PL greater efficiency could be explained by adaptations and interactions between central, peripheral factors and force. Fifteen young participants performed single-leg extension exercise at absolute (35W) and relative (50%peak power-output (Wpeak)) workloads with PL and NPL. Oxygen uptake, photoplethysmography, Doppler ultrasound, near-infrared-spectroscopy deoxy-hemoglobin [HHb], integrated electromyography (iEMG), maximal isometric force (MVC), rate of force development (RFD50-100) and muscle biopsies of both vastus lateralis, were studied to assess central and peripheral determinants of ηnet. During exercise executed at 35W, ηnet was 17.5±5.1% and 11.9±2.1% (p<0.01) in NP and NPL respectively, while during exercise at the 50% of Wpeak, was in PL = 18.1±5.1% and in NPL = 12.5±1.9 (p<0.01). The only parameter correlated with ηnet was iEMG which showed an inverse correlation for absolute (r=-0.83 and -0.69 for PL and NPL) and relative workloads (r=-0.92 and -0.79 for PL and NPL). MVC and RFD50-100 were higher in PL than in NPL but not correlated to ηnet. This study identified a critical role of leg preference in the efficiency during single-leg extension exercise. The whole spectrum of the central and peripheral, circulatory and muscular determinants of ηnet did not explain the difference between PL and NPL efficiency. Therefore, the lower muscle activation exhibited by the PL is likely the primary determinant of this physiological phenomenon.

scholarly journals Comparison of femoral blood gases and muscle near-infrared spectroscopy at exercise onset in humans

1999 ◽  
Vol 86 (2) ◽  
pp. 687-693 ◽  
Author(s):  
Maureen J. MacDonald ◽  
Mark A. Tarnopolsky ◽  
Howard J. Green ◽  
Richard L. Hughson
Keyword(s):  
Infrared Spectroscopy ◽  
Near Infrared Spectroscopy ◽  
Near Infrared ◽  
Vastus Lateralis ◽  
Femoral Vein ◽  
Blood Gases ◽  
Moderate Intensity ◽  
Reliable Estimate ◽  
Vastus Lateralis Muscle ◽  
Exercise Onset

We hypothesized that near-infrared spectroscopy (NIRS) measures of hemoglobin and/or myoglobin O2 saturation (IR-So 2) in the vascular bed of exercising muscle would parallel changes in femoral venous O2 saturation (S[Formula: see text]) at the onset of leg-kicking exercise in humans. Six healthy subjects performed transitions from rest to 48 ± 3 (SE)-W two-legged kicking exercise while breathing 14, 21, or 70% inspired O2. IR-So 2 was measured over the vastus lateralis muscle continuously during all tests, and femoral venous and radial artery blood samples were drawn simultaneously during rest and during 5 min of exercise. In all gas-breathing conditions, there was a rapid decrease in both IR-So 2 and SfvO2 at the onset of moderate-intensity leg-kicking exercise. Although SfvO2 remained at low levels throughout exercise, IR-So 2increased significantly after the first minute of exercise in both normoxia and hyperoxia. Contrary to the hypothesis, these data show that NIRS does not provide a reliable estimate of hemoglobin and/or O2 saturation as reflected by direct femoral vein sampling.

scholarly journals Blood flow occlusion-related O2 extraction “reserve” is present in different muscles of the quadriceps but greater in deeper regions after ramp-incremental test

2018 ◽  
Vol 125 (2) ◽  
pp. 313-319 ◽  
Author(s):  
Danilo Iannetta ◽  
Dai Okushima ◽  
Erin Calaine Inglis ◽  
Narihiko Kondo ◽  
Juan M Murias ◽  
...  
Keyword(s):  
Blood Flow ◽  
Near Infrared ◽  
Vastus Lateralis ◽  
Rectus Femoris ◽  
Incremental Exercise ◽  
Vastus Lateralis Muscle ◽  
Incremental Test ◽  
Time Resolved ◽  
Blood Flow Occlusion ◽  
Flow Occlusion

It was recently demonstrated that an O2 extraction reserve, as assessed by the near-infrared spectroscopy (NIRS)-derived deoxygenation signal ([HHb]), exists in the superficial region of vastus lateralis (VL) muscle during an occlusion performed at the end of a ramp-incremental test. However, it is unknown whether this reserve is present and/or different in magnitude in other portions and depths of the quadriceps muscles. We tested the hypothesis that an O2 extraction reserve would exist in other regions of this muscle but is greater in deep compared with more superficial portions. Superficial (VL-s) and deep VL (VL-d) as well as superficial rectus femoris (RF-s) were monitored by a combination of low- and high-power time-resolved (TRS) NIRS. During the occlusion immediately post-ramp-incremental test there was a significant overshoot in the [HHb] signal ( P < 0.05). However, the magnitude of this increase was greater in VL-d (93.2 ± 42.9%) compared with VL-s (55.0 ± 19.6%) and RF-s (47.8 ± 14.0%) ( P < 0.05). The present study demonstrated that an O2 extraction reserve exists in different pools of active muscle fibers of the quadriceps at the end of a ramp exercise to exhaustion. The greater magnitude in the reserve observed in the deeper portion of VL, however, suggests that this portion of muscle may present a greater surplus of oxygenated blood, which is likely due to a greater population of slow-twitch fibers. These findings add to the notion that the plateau in the [HHb] signal toward the end of a ramp-incremental exercise does not indicate the upper limit of O2 extraction. NEW & NOTEWORTHY Different portions of the quadriceps muscles exhibited an untapped O2 extraction reserve during a blood flow occlusion performed at the end of a ramp-incremental exercise. In the deeper portion of the vastus lateralis muscle, this reserve was greater compared with superficial vastus lateralis and rectus femoris. These data suggest that the O2 extraction reserve may be dependent on the vascular and/or oxidative capacities of the muscles.

Effects of acute hypoxia on cerebral and muscle oxygenation during incremental exercise

2007 ◽  
Vol 103 (1) ◽  
pp. 177-183 ◽  
Author(s):  
Andrew W. Subudhi ◽  
Andrew C. Dimmen ◽  
Robert C. Roach
Keyword(s):  
Power Output ◽  
Repeated Measures ◽  
Near Infrared ◽  
Cerebral Oxygenation ◽  
Vastus Lateralis ◽  
Acute Hypoxia ◽  
Incremental Exercise ◽  
Muscle Oxygenation ◽  
Peak Power Output ◽  
Vastus Lateralis Muscle

To determine if fatigue at maximal aerobic power output was associated with a critical decrease in cerebral oxygenation, 13 male cyclists performed incremental maximal exercise tests (25 W/min ramp) under normoxic (Norm: 21% FiO2) and acute hypoxic (Hypox: 12% FiO2) conditions. Near-infrared spectroscopy (NIRS) was used to monitor concentration (μM) changes of oxy- and deoxyhemoglobin (Δ[O2Hb], Δ[HHb]) in the left vastus lateralis muscle and frontal cerebral cortex. Changes in total Hb were calculated (Δ[THb] = Δ[O2Hb] + Δ[HHb]) and used as an index of change in regional blood volume. Repeated-measures ANOVA were performed across treatments and work rates (α = 0.05). During Norm, cerebral oxygenation rose between 25 and 75% peak power output {Powerpeak; increased (inc) Δ[O2Hb], inc. Δ[HHb], inc. Δ[THb]}, but fell from 75 to 100% Powerpeak {decreased (dec) Δ[O2Hb], inc. Δ[HHb], no change Δ[THb]}. In contrast, during Hypox, cerebral oxygenation dropped progressively across all work rates (dec. Δ[O2Hb], inc. Δ[HHb]), whereas Δ[THb] again rose up to 75% Powerpeak and remained constant thereafter. Changes in cerebral oxygenation during Hypox were larger than Norm. In muscle, oxygenation decreased progressively throughout exercise in both Norm and Hypox (dec. Δ[O2Hb], inc. Δ [HHb], inc. Δ[THb]), although Δ[O2Hb] was unchanged between 75 and 100% Powerpeak. Changes in muscle oxygenation were also greater in Hypox compared with Norm. On the basis of these findings, it is unlikely that changes in cerebral oxygenation limit incremental exercise performance in normoxia, yet it is possible that such changes play a more pivotal role in hypoxia.

Noninvasive determination of exercise-induced hydrodgen ion threshold through direct optical measurement

2008 ◽  
Vol 104 (3) ◽  
pp. 837-844 ◽  
Author(s):  
Babs R. Soller ◽  
Ye Yang ◽  
Stuart M. C. Lee ◽  
Cassie Wilson ◽  
R. Donald Hagan
Keyword(s):  
Gas Exchange ◽  
Blood Lactate ◽  
Near Infrared ◽  
Optical Measurement ◽  
Vastus Lateralis ◽  
Ion Concentration ◽  
Flexor Digitorum Profundus ◽  
Hydrogen Ion Concentration ◽  
Gas Exchange Threshold

The intensity of exercise above which oxygen uptake (V̇o2) does not account for all of the required energy to perform work has been associated with lactate accumulation in the blood (lactate threshold, LT) and elevated carbon dioxide output (gas exchange threshold). An increase in hydrogen ion concentration ([H+]) is approximately concurrent with elevation of blood lactate and CO2 output during exercise. Near-infrared spectra (NIRS) and invasive interstitial fluid pH (pHm) were measured in the flexor digitorum profundus during handgrip exercise to produce a mathematical model relating the two measures with an estimated error of 0.035 pH units. This NIRS pHm model was subsequently applied to spectra collected from the vastus lateralis of 10 subjects performing an incremental-intensity cycle protocol. Muscle oxygen saturation (SmO2) was also calculated from spectra. We hypothesized that a H+ threshold could be identified for these subjects and that it would be different from but correlated with the LT. Lactate, gas exchange, SmO2, and H+ thresholds were determined as a function of V̇o2 using bilinear regression. LT was significantly different from both the gas exchange threshold (Δ = 0.27 ± 0.29 l/min) and H+ threshold (Δ = 0.29 ± 0.23 l/min), but the gas exchange threshold was not significantly different from the H+ threshold (Δ = 0.00 ± 0.38 l/min). The H+ threshold was strongly correlated with LT ( R2 = 0.95) and the gas exchange threshold ( R2 = 0.85). This initial study demonstrates the feasibility of noninvasive pHm estimations, the determination of H+ threshold, and the relationship between H+ and classical metabolic thresholds during incremental exercise.

Sildenafil improves microvascular O2 delivery-to-utilization matching and accelerates exercise O2 uptake kinetics in chronic heart failure

2012 ◽  
Vol 303 (12) ◽  
pp. H1474-H1480 ◽  
Author(s):  
Priscila A. Sperandio ◽  
Mayron F. Oliveira ◽  
Miguel K. Rodrigues ◽  
Danilo C. Berton ◽  
Erika Treptow ◽  
...  
Keyword(s):  
Heart Failure ◽  
Chronic Heart Failure ◽  
Exercise Tolerance ◽  
Near Infrared ◽  
Vastus Lateralis ◽  
Beneficial Effects ◽  
Subsequent Response ◽  
Constant Work Rate ◽  
No Bioavailability ◽  
O2 Delivery

Nitric oxide (NO) can temporally and spatially match microvascular oxygen (O2) delivery (Q̇o2mv) to O2 uptake (V̇o2) in the skeletal muscle, a crucial adjustment-to-exercise tolerance that is impaired in chronic heart failure (CHF). To investigate the effects of NO bioavailability induced by sildenafil intake on muscle Q̇o2mv-to-O2 utilization matching and V̇o2 kinetics, 10 males with CHF (ejection fraction = 27 ± 6%) undertook constant work-rate exercise (70–80% peak). Breath-by-breath V̇o2, fractional O2 extraction in the vastus lateralis {∼deoxygenated hemoglobin + myoglobin ([deoxy-Hb + Mb]) by near-infrared spectroscopy}, and cardiac output (CO) were evaluated after sildenafil (50 mg) or placebo. Sildenafil increased exercise tolerance compared with placebo by ∼20%, an effect that was related to faster on- and off-exercise V̇o2 kinetics ( P < 0.05). Active treatment, however, failed to accelerate CO dynamics ( P > 0.05). On-exercise [deoxy-Hb + Mb] kinetics were slowed by sildenafil (∼25%), and a subsequent response “overshoot” ( n = 8) was significantly lessened or even abolished. In contrast, [deoxy-Hb + Mb] recovery was faster with sildenafil (∼15%). Improvements in muscle oxygenation with sildenafil were related to faster on-exercise V̇o2 kinetics, blunted oscillations in ventilation ( n = 9), and greater exercise capacity ( P < 0.05). Sildenafil intake enhanced intramuscular Q̇o2mv-to-V̇o2 matching with beneficial effects on V̇o2 kinetics and exercise tolerance in CHF. The lack of effect on CO suggests that improvement in blood flow to and within skeletal muscles underlies these effects.

Cerebral desaturation during exercise reversed by O2 supplementation

1999 ◽  
Vol 277 (3) ◽  
pp. H1045-H1052 ◽  
Author(s):  
H. B. Nielsen ◽  
R. Boushel ◽  
P. Madsen ◽  
N. H. Secher
Keyword(s):  
Infrared Spectroscopy ◽  
Near Infrared Spectroscopy ◽  
Near Infrared ◽  
Cerebral Oxygenation ◽  
Vastus Lateralis ◽  
Work Capacity ◽  
Vastus Lateralis Muscle ◽  
Double Blind ◽  
Hemoglobin Saturation ◽  
Concentration Changes

The combined effects of hyperventilation and arterial desaturation on cerebral oxygenation ([Formula: see text]) were determined using near-infrared spectroscopy. Eleven competitive oarsmen were evaluated during a 6-min maximal ergometer row. The study was randomized in a double-blind fashion with an inspired O2 fraction of 0.21 or 0.30 in a crossover design. During exercise with an inspired O2 fraction of 0.21, the arterial CO2 pressure (35 ± 1 mmHg; mean ± SE) and O2 pressure (77 ± 2 mmHg) as well as the hemoglobin saturation (91.9 ± 0.7%) were reduced ( P < 0.05).[Formula: see text] was reduced from 80 ± 2 to 63 ± 2% ( P < 0.05), and the near-infrared spectroscopy-determined concentration changes in deoxy- (ΔHb) and oxyhemoglobin (ΔHbO2) of the vastus lateralis muscle increased 22 ± 3 μM and decreased 14 ± 3 μM, respectively ( P < 0.05). Increasing the inspired O2fraction to 0.30 did not affect ventilation (174 ± 4 l/min), but arterial CO2 pressure (37 ± 2 mmHg), O2 pressure (165 ± 5 mmHg), and hemoglobin O2saturation (99 ± 0.1%) increased ( P < 0.05).[Formula: see text] remained close to the resting level during exercise (79 ± 2 vs. 81 ± 2%), and although the muscle ΔHb (18 ± 2 μM) and ΔHbO2 (−12 ± 3 μM) were similar to those established without O2 supplementation, work capacity increased from 389 ± 11 to 413 ± 10 W ( P < 0.05). These results indicate that an elevated inspiratory O2fraction increases exercise performance related to maintained cerebral oxygenation rather than to an effect on the working muscles.

Comparison of muscle near-infrared spectroscopy and femoral blood gases during steady-state exercise in humans

1996 ◽  
Vol 80 (4) ◽  
pp. 1345-1350 ◽  
Author(s):  
F. Costes ◽  
J. C. Barthelemy ◽  
L. Feasson ◽  
T. Busso ◽  
A. Geyssant ◽  
...  
Keyword(s):  
Infrared Spectroscopy ◽  
Steady State ◽  
Oxygen Saturation ◽  
Near Infrared Spectroscopy ◽  
Near Infrared ◽  
Vastus Lateralis ◽  
Femoral Vein ◽  
Blood Gases ◽  
Muscle Oxygen ◽  
Venous Oxygen Saturation

Near-infrared spectroscopy (NIRS) is a noninvasive way of measuring muscular oxygenation. We evaluated the relationship between NIRS signal [infrared muscle oxygen saturation (IR-SO2mus)] and the femoral venous oxygen saturation (SfvO2) during cycling exercise. Six healthy subjects performed a 30-min steady-state exercise at 80% maximal oxygen uptake in normoxia and hypoxia (inspired O2 fraction = 0.105). IR-So2mus was recorded continuously throughout the tests with the NIRS probe located on the vastus lateralis. During exercise, blood samples were withdrawn every 5 min from radial artery and femoral vein catheters. In normoxia, IR-So2mus initiated a transient nonsignificant decrease at 5 min, then returned to preexercise level, whereas SfvO2 showed a fast decrease, reaching 18% saturation at 10 min without further change. By contrast, in hypoxia, IR-SO2mus and SfvO2 demonstrated a parallel decrease then stabilized at 10 min. We conclude that IR-SO2mus appears to parallel SfvO2 when both the arterial and venous oxygen contents decrease during steady-state exercise in hypoxia, whereas IR-SO2mus does not follow SfvO2 change in normoxia.

scholarly journals Cerebrovascular responses to incremental exercise during hypobaric hypoxia: effect of oxygenation on maximal performance

2008 ◽  
Vol 294 (1) ◽  
pp. H164-H171 ◽  
Author(s):  
Andrew W. Subudhi ◽  
Matthew C. Lorenz ◽  
Charles S. Fulco ◽  
Robert C. Roach
Keyword(s):  
Hypobaric Hypoxia ◽  
Near Infrared ◽  
Cerebral Oxygenation ◽  
Vastus Lateralis ◽  
Acute Hypoxia ◽  
Work Rate ◽  
Incremental Exercise ◽  
Maximal Performance ◽  
Improved Performance ◽  
End Tidal

We sought to describe cerebrovascular responses to incremental exercise and test the hypothesis that changes in cerebral oxygenation influence maximal performance. Eleven men cycled in three conditions: 1) sea level (SL); 2) acute hypoxia [AH; hypobaric chamber, inspired Po2 (PiO2) 86 Torr]; and 3) chronic hypoxia [CH; 4,300 m, PiO2 86 Torr]. At maximal work rate (Ẇmax), fraction of inspired oxygen (FiO2) was surreptitiously increased to 0.60, while subjects were encouraged to continue pedaling. Changes in cerebral (frontal lobe) (COX) and muscle (vastus lateralis) oxygenation (MOX) (near infrared spectroscopy), middle cerebral artery blood flow velocity (MCA Vmean; transcranial Doppler), and end-tidal Pco2 (PetCO2) were analyzed across %Ẇmax (significance at P < 0.05). At SL, PetCO2, MCA Vmean, and COX fell as work rate rose from 75 to 100% Ẇmax. During AH, PetCO2 and MCA Vmean declined from 50 to 100% Ẇmax, while COX fell from rest. With CH, PetCO2 and COX dropped throughout exercise, while MCA Vmean fell only from 75 to 100% Ẇmax. MOX fell from rest to 75% Ẇmax at SL and AH and throughout exercise in CH. The magnitude of fall in COX, but not MOX, was different between conditions (CH > AH > SL). FiO2 0.60 at Ẇmax did not prolong exercise at SL, yet allowed subjects to continue for 96 ± 61 s in AH and 162 ± 90 s in CH. During FiO2 0.60, COX rose and MOX remained constant as work rate increased. Thus cerebral hypoxia appeared to impose a limit to maximal exercise during hypobaric hypoxia (PiO2 86 Torr), since its reversal was associated with improved performance.

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