Inferences from pulmonary O2 uptake with respect to intramuscular [phosphocreatine] kinetics during moderate exercise in humans

Energy balance analysis suggests that lactate is not a direct cause of the slow component of oxygen uptake kinetics

PURPOSE

The mechanisms of oxygen uptake (V ˙ O 2 ) slow component in the severe exercise intensity domain are still a matter of debate. We tested the hypothesis that the rate of blood lactate ([La]) accumulation above maximal lactate steady state (MLSS) is a major cause ofV ˙ O 2 slow component.

METHODS

On 13 males exercising on a cycle-ergometer, we measured gas exchanges, heart rate, and [La] during maximal incremental exercise test to determine maximal aerobic power ( w . max) and at constant power exercise tests at 60%, 65%, 70%, and 80% of w . max.

RESULTS

MaximalV ˙ O 2 was 3.19 ± 0.37 l·min-1, w . max was 283 ± 28 W. At 60% w . max all variables attained steady state in all subjects. Power at MLSS was 177 ± 21 W. At 80% w . max a clearV ˙ O 2 slow component was observed in all subjects, exercise lasted 11.3 ± 3.1 min and [La] was 7.4 ± 2.2 mmol at 5 min and 11.5 ± 3.6 mmol at 10 min. The energy balance computed at 80% w . max resulted compatible with the principles of the energetics of muscular exercise, if we assume linear [La] increase, and thus constant metabolic power provided by [La] accumulation. Conversely, the metabolic power provided byV ˙ O 2 slow component increases with time. This contrast is incompatible with the tested hypothesis that consequently must be rejected.

CONCLUSION

This study excluded [La] accumulation as a main cause ofV ˙ O 2 slow component.

Mitochondrial efficiency in resting skeletal muscle in vivo: a novel non-invasive approach using multinuclear magnetic resonance spectroscopy in humans

Mitochondrial efficiency is a critical metabolic parameter with far-reaching implications for tissue homeostasis. However, the direct measurement of oxygen consumption (VO2) and ATP production from a large tissue sample in vivo remains challenging. Using phosphorus (31P) and proton (1H) magnetic resonance spectroscopy (MRS), this study aimed to non-invasively quantify the skeletal muscle ATP synthesis rate and VO2 to determine mitochondrial efficiency at rest and during muscle contraction in humans. We assessed mitochondrial efficiency in the plantar flexor muscles of 12 healthy adults (21 ± 1 years) using 31P and 1H MRS within a 3T MR system. MRS data were acquired at rest and during constant workloads to quantify oxidative ATP synthesis (ATPox) rate and myoglobin-derived oxygen consumption (Mb-derived VO2). At rest, ATPox was 0.85 ± 0.24 mm min-1, and Mb-derived VO2 was 0.46 ± 0.11 mm min-1, resulting in a P/O ratio of 1.95 ± 0.68. During graded exercise, end-exercise PCr concentration decreased from 29 ± 5.7 mm to 18 ± 4.8 mm, and end-exercise Mb oxygenation declined linearly to 47 ± 11%. ATPox synthesis rate increased linearly with exercise workload (r = 0.65 ± 0.31), whereas there was no significant change in Mb-derived VO2 (r = -0.19 ± 0.60), leading to non-physiological P/O values during exercise (>3). The results indicate that combined 31P/1H-MRS at rest offers a promising approach for non-invasively quantifying mitochondrial efficiency in large muscle samples, suggesting its potential as a clinical endpoint of mitochondrial function. However, further refinement is needed for use during exercise. KEY POINTS: Mitochondrial efficiency, converting chemical energy from carbon fuels into ATP, is a vital metabolic parameter for tissue homeostasis, but measuring oxygen consumption (VO2) and ATP production in vivo has been challenging. This study used phosphorus (31P) and proton (1H) magnetic resonance spectroscopy (MRS) to non-invasively quantify the skeletal muscle ATP synthesis rate and VO2 at rest and during muscle contraction in humans. At rest, the oxidative ATP synthesis (ATPox) and myoglobin-derived VO2 (Mb-derived VO2) were measured, resulting in a P/O ratio of 1.95 in the plantar flexor muscles. During exercise, the ATPox rate increased with workload, but Mb-derived VO2 did not change significantly, leading to non-physiological P/O ratios. The findings suggest that 31P/1H-MRS at rest is a promising method for assessing mitochondrial efficiency and could be used as a clinical endpoint for mitochondrial function in vivo, although further refinement is needed for exercise conditions.

Confidence intervals estimator of the kinetic parameters: do its reliability depend on the assembling method of the oxygen uptakes?

Gas exchange data acquired repeatedly under the same exercise conditions are assembled together to improve the kinetic parameters of breath-by-breath oxygen uptake. The latter are provided by the non-linear regression procedure, together with the corresponding estimators of the width of the Confidence Intervals (i.e., the Asymptotic Standard Errors; ASEs). We tested, for two different assembling procedures, whether the range of values identified by the ASE actually correspond to the 95% Confidence Interval. Ten O2 uptake responses were acquired on 10 healthy volunteers performing a square-wave moderate-intensity exercise. Kinetic parameters were estimated running the non-linear regression with a mono-exponential model on an increasingly greater number of responses (Nr, from 1 to 10), assembled together using the "stacking" and the "1-s-bins" procedures. Kinetic values obtained assembling together the 10 repetitions were assumed as "true" values. The time constant was not affected by Nr or by the assembling procedure (ANOVA; p>0.54 and p>0.16, respectively). The corresponding ASE decreased according to Nr (ANOVA; p=0.000), being significantly smaller for the "1-s-bins" procedure compared to the "stacking" one (ANOVA; p<0.001). Excluding 20s at the start of the fitting window, the range of values identified with the ASE provided by the "1-s-bins" and the "stacking" procedures included the "true" value in 85% and in 95% of cases, respectively. The "stacking" procedure should be preferred since it yielded ASEs for the time constant that provided a range of values satisfying the statistical meaning of the width of the Confidence Intervals, at the given degree of probability.

Blue plaque review series: A.V. Hill, athletic records and the birth of exercise physiology

One hundred years ago, A.V. Hill authored three manuscripts analysing athletic world records from a physiological perspective. That analysis, grounded in Hill's understanding of contemporary muscle bioenergetics, provides a fascinating sketch of the thoughts and speculations of one of the fathers of exercise physiology. In this review, we reflect on Hill's prose with the benefit of 100 years of hindsight, and illustrate how Hill was able to draw startlingly accurate conclusions from what limited data were available on the physiology of intense exercise. Hill discusses the energetics of running, swimming, rowing and cycling in both males and females, as well as addressing exercise performance in horses and the mechanics of jumping. He also considers sports nutrition, pacing strategy and ultra-endurance exercise. Perhaps most impactfully, he establishes that the speed-duration relationship has characteristics that reflect the underlying physiological basis of exercise performance. That physiology, in turn, differs depending on the duration of the event itself, providing one of the first descriptions of the task-dependent nature of mechanisms limiting exercise tolerance. A remarkable feature of Hill's papers is that they were written just a few years before a major revolution in muscle biochemistry, and yet Hill was still able to develop conceptually sound ideas about human performance. His hypotheses require only minor revision to bring them into line with current understanding. In reaching their centenary, therefore, the surprising feature of these papers is not how well they have aged, but how relevant they remain.

The effect of a 4-week, remotely administered, post-exercise passive leg heating intervention on determinants of endurance performance

PURPOSE

Post-exercise passive heating has been reported to augment adaptations associated with endurance training. The current study evaluated the effect of a 4-week remotely administered, post-exercise passive leg heating protocol, using an electrically heated layering ensemble, on determinants of endurance performance.

METHODS

Thirty recreationally trained participants were randomly allocated to either a post-exercise passive leg heating (PAH, n = 16) or unsupervised training only control group (CON, n = 14). The PAH group wore the passive heating ensemble for 90-120 min/day, completing a total of 20 (16 post-exercise and 4 stand-alone leg heating) sessions across 4 weeks. Whole-body (peak oxygen uptake, gas exchange threshold, gross efficiency and pulmonary oxygen uptake kinetics), single-leg exercise (critical torque and NIRS-derived muscle oxygenation), resting vascular characteristics (flow-mediated dilation) and angiogenic blood measures (nitrate, vascular endothelial growth factor and hypoxia inducible factor 1-α) were recorded to characterize the endurance phenotype. All measures were assessed before (PRE), at 2 weeks (MID) and after (POST) the intervention.

RESULTS

There was no effect of the intervention on test of whole-body endurance capacity, vascular function or blood markers (p > 0.05). However, oxygen kinetics were adversely affected by PAH, denoted by a slowing of the phase II time constant; τ (p = 0.02). Furthermore, critical torque-deoxygenation ratio was improved in CON relative to PAH (p = 0.03).

CONCLUSION

We have demonstrated that PAH had no ergogenic benefit but instead elicited some unfavourable effects on sub-maximal exercise characteristics in recreationally trained individuals.

The Influence of Acute Hypoxia on Oxygen Uptake and Muscle Oxygenation Kinetics During Cycling Exercise in Prepubertal Boys

PURPOSE

To examine the effect of normobaric hypoxia on pulmonary oxygen uptake (V˙O2) and muscle oxygenation kinetics during incremental and moderate-intensity exercise in children.

METHODS

Eight prepubertal boys (9-11 y) performed incremental cycle tests to exhaustion in both normoxia and hypoxia (fraction of inspired O2 of 15%) followed by repeat 6-minute transitions of moderate-intensity exercise in each condition over subsequent visits.

RESULTS

Maximal oxygen uptake (V˙O2max) was reduced in hypoxia compared with normoxia (1.69 [0.20] vs 1.87 [0.26] L·min-1, P = .028), although the gas exchange threshold was not altered in absolute terms (P = .33) or relative to V˙O2max (P = .78). During moderate-intensity exercise, the phase II V˙O2 time constant (τ) was increased in hypoxia (18 [9] vs 24 [8] s, P = .025), with deoxyhemoglobin τ unchanged (17 [8] vs 16 [6], P ≥ .28).

CONCLUSIONS

In prepubertal boys, hypoxia reduced V˙O2max and slowed V˙O2 phase II kinetics during moderate-intensity exercise, despite unchanged deoxyhemoglobin kinetics. These data suggest an oxygen delivery dependence of V˙O2max and moderate-intensity V˙O2 kinetics under conditions of reduced oxygen availability in prepubertal boys.

The algorithm used for the calculation of gas exchange affects the estimation of O2 uptake kinetics at the onset of moderate-intensity exercise

At the start of a moderate-intensity square-wave exercise, after a short delay, breath-by-breath O2 uptake at the mouth is approximated to a mono-exponential function, whose time constant is considered matched to that of the O2 uptake of the working muscles. We compared the kinetic parameters obtained from the breath-by-breath gas exchange data yielded by the 'Independent-breath' algorithm (IND), which accounts for the changes in lung gas stores, with those obtained with the classical 'Expiration-only' algorithm (EXP). The two algorithms were applied on the same flow and gas fraction traces acquired on 10 healthy volunteers, performing 10 times the same moderate-intensity exercise transition. Repeated O2 uptake responses were stacked together and the kinetic parameters of a mono-exponential function were estimated by non-linear regression, removing the data pertaining to 1-s progressively longer initial periods (ΔTr ). Independently of ΔTr , the mean response time (time constant + time delay) obtained for the IND data was faster compared to the EXP data (∼43 s vs. ∼47 s, P < 0.001), essentially because of shorter time delays. Between ΔTr  = 16 s and ΔTr  = 29s, the time constants of the IND data decreased (30.7 s vs. 28.0 s, P < 0.05; drop = 10%), but less than those of the EXP data (32.2 s vs. 26.2 s, P < 0.001; drop = 23%); with the same ΔTr , the time constants of the two algorithms' data were not different (P > 0.07). The different decrease in the time constant, together with the different mean response time, suggests that the data yielded by the two algorithms provide a different picture of the phenomena occurring at the beginning of the exercise.

Energetic responses of head-out water immersion at different temperatures during post-exercise recovery and its consequence on anaerobic mechanical power

PURPOSE

While exercise recovery may be beneficial from a physiological point of view, it may be detrimental to subsequent anaerobic performance. To investigate the energetic responses of water immersion at different temperatures during post-exercise recovery and its consequences on subsequent anaerobic performance, a randomized and controlled crossover experimental design was performed with 21 trained cyclists.

METHOD

Participants were assigned to receive three passive recovery strategies during 10 min after a Wingate Anaerobic Test (WAnT): control (CON: non-immersed condition), cold water immersion (CWI: 20 ℃), and hot water immersion (HWI: 40 ℃). Blood lactate, cardiorespiratory, and mechanical outcomes were measured during the WAnT and its recovery. Time constant (τ), asymptotic value, and area under the curve (AUC) were quantified for each physiologic parameter during recovery. After that, a second WAnT test and 10-min recovery were realized in the same session.

RESULTS

Regardless the water immersion temperature, water immersion increased [Formula: see text] (+ 18%), asymptote ([Formula: see text]+ 16%, [Formula: see text] + 13%, [Formula: see text] + 17%, HR + 16%) and AUC ([Formula: see text]+ 27%, [Formula: see text] + 18%, [Formula: see text] + 20%, HR + 25%), while decreased [Formula: see text] (- 33%). There was no influence of water immersion on blood lactate parameters. HWI improved the mean power output during the second WAnT (2.2%), while the CWI decreased 2.4% (P < 0.01).

CONCLUSION

Independent of temperature, water immersion enhanced aerobic energy recovery without modifying blood lactate recovery. However, subsequent anaerobic performance was increased only during HWI and decreased during CWI. Despite higher than in other studies, 20 °C effectively triggered physiological and performance responses. Water immersion-induced physiological changes did not predict subsequent anaerobic performance.

Coupling of V ˙ E and V ˙ CO 2 kinetics: insights from multiple exercise transitions below the estimated lactate threshold

During a step-change in exercise power output (PO), ventilation (V ˙ E ) increases with a similar time course to the rate of carbon dioxide delivery to the lungs (V ˙ CO 2 . To test the strength of this coupling, we comparedV ˙ E andV ˙ CO 2 kinetics from ten independent exercise transitions performed within the moderate-intensity domain. Thirteen males completed 3-5 repetitions of ∆40 W step transitions initiated from 20, 40, 60, 80, 100, and 120 W on a cycle ergometer. Preceding the ∆40 W step transitions from 60, 80, 100, and 120 W was a 6 min bout of 20 W cycling from which the transitions of variable ∆PO were examined. Gas exchange (V ˙ CO 2 and oxygen uptake,V ˙ O 2 ) andV ˙ E were measured by mass spectrometry and volume turbine. The kinetics of the responses were characterized by the time constant (τ) and amplitude (ΔV ˙ E /ΔV ˙ CO 2 ). Overall,V ˙ CO 2 kinetics were consistently slower thanV ˙ O 2 kinetics (by ~ 45%) and τV ˙ CO 2 rose progressively with increasing baseline PO and with heightened ∆PO from a common baseline. Compared to τV ˙ CO 2 , τV ˙ E was on average slightly greater (by ~ 4 s). Repeated-measures analysis of variance revealed that there was no interaction between τV ˙ CO 2 and τV ˙ E in either the variable baseline (p = 0.49) and constant baseline (p = 0.56) conditions indicating that each changed in unison. Additionally, for ΔV ˙ E /ΔV ˙ CO 2 , there was no effect of either variable baseline PO (p = 0.05) or increasing ΔPO (p = 0.16). These data provide further evidence that, within the moderate-intensity domain, both the temporal- and amplitude-based characteristics of V̇E kinetics are inextricably linked to those ofV ˙ CO 2 .

Effect of recovery time on V ˙ O 2 -ON kinetics in humans at the onset of moderate-intensity cycling exercise

PURPOSE

τ of the primary phase ofV ˙ O 2 A kinetics during square-wave, moderate-intensity exercise mirrors that of PCr splitting (τPCr). Pre-exercise [PCr] and the absolute variations of PCr (∆[PCr]) occurring during transient have been suggested to control τPCr and, in turn, to modulateV ˙ O 2 A kinetics. In addition,V ˙ O 2 A kinetics may be slower when exercise initiates from a raised metabolic level, i.e., from a less-favorable energetic state. We verified the hypothesis that: (i) pre-exercise [PCr], (ii) pre-exercise metabolic rate, or (iii) ∆[PCr] may affect the kinetics of muscular oxidative metabolism and, therefore, τ.

METHODS

To this aim, seven active males (23.0 yy ± 2.3; 1.76 m ± 0.06,V ˙ O 2 max : 3.32 L min-1 ± 0.67) performed three repetitions of series consisting of six 6-min step exercise transitions of identical workload interspersed with different times of recovery: 30, 60, 90, 120, 300 s.

RESULTS

Mono-exponential fitting was applied to breath-by-breathV ˙ O 2 A , so that τ was determined. τ decays as a first-order exponential function of the time of recovery (τ = 109.5 × e(-t/14.0) + 18.9 r2 = 0.32) and linearly decreased as a function of the estimated pre-exercise [PCr] (τ = - 1.07 [PCr] + 44.9, r2 = 0.513, P < 0.01); it was unaffected by the estimated ∆[PCr].

CONCLUSIONS

Our results in vivo do not confirm the positive linear relationship between τ and pre-exercise [PCr] and ∆[PCr]. Instead,V ˙ O 2 A kinetics seems to be influenced by the pre-exercise metabolic rate and the altered intramuscular energetic state.

Heart rate and oxygen uptake kinetics obtained from continuous measurements with wearable devices during outdoor walks of patients with COPD

Objective

Continuous physiological measurements during a laboratory-based exercise test can provide physiological biomarkers, such as heart rate (HR) and oxygen uptake (V̇O₂) kinetics, that carry clinically relevant information. In contrast, it is not clear how continuous data generated by wearable devices during daily-life routines could provide meaningful biomarkers. We aimed to determine whether valid HR and V̇O₂ kinetics can be obtained from measurements with wearable devices during outdoor walks in patients with chronic obstructive pulmonary disease (COPD).

Methods

HR (Polar Belt) and V̇O₂(METAMAX3B) were measured during 93 physical activity transitions performed by eight patients with COPD during three different outdoor walks (n_tr= 77) and a 6-minute walk test (n_tr= 16). HR and V̇O₂ kinetics were calculated every time a participant started a walk, finished a walk or walked upstairs. HR and V̇O₂ kinetics were considered valid if the response magnitude and model fit were adequate, and model parameters were reliable.

Results

Continuous measurements with wearable devices provided valid HR kinetics when COPD patients started or finished (range 63%-100%) the different outdoor walks and valid V̇O₂ kinetics when they finished (range 63%-100%) an outdoor walk. The amount of valid kinetics and kinetic model performance was comparable between outdoor walks and a laboratory-based exercise test (p > .05).

Conclusion

We envision that the presented approach could improve telemonitoring applications of patients with COPD by providing regular, unsupervised assessments of HR kinetics during daily-life routines. This could allow to early identify a decline in the patients' dynamic physiological functioning, physical fitness and/or health status.

A century of exercise physiology: key concepts on coupling respiratory oxygen flow to muscle energy demand during exercise

After a short historical account, and a discussion of Hill and Meyerhof's theory of the energetics of muscular exercise, we analyse steady-state rest and exercise as the condition wherein coupling of respiration to metabolism is most perfect. The quantitative relationships show that the homeostatic equilibrium, centred around arterial pH of 7.4 and arterial carbon dioxide partial pressure of 40 mmHg, is attained when the ratio of alveolar ventilation to carbon dioxide flow ([Formula: see text]) is - 21.6. Several combinations, exploited during exercise, of pertinent respiratory variables are compatible with this equilibrium, allowing adjustment of oxygen flow to oxygen demand without its alteration. During exercise transients, the balance is broken, but the coupling of respiration to metabolism is preserved when, as during moderate exercise, the respiratory system responds faster than the metabolic pathways. At higher exercise intensities, early blood lactate accumulation suggests that the coupling of respiration to metabolism is transiently broken, to be re-established when, at steady state, blood lactate stabilizes at higher levels than resting. In the severe exercise domain, coupling cannot be re-established, so that anaerobic lactic metabolism also contributes to sustain energy demand, lactate concentration goes up and arterial pH falls continuously. The [Formula: see text] decreases below - 21.6, because of ensuing hyperventilation, while lactate keeps being accumulated, so that exercise is rapidly interrupted. The most extreme rupture of the homeostatic equilibrium occurs during breath-holding, because oxygen flow from ambient air to mitochondria is interrupted. No coupling at all is possible between respiration and metabolism in this case.

Priming Cardiac Function with Voluntary Respiratory Maneuvers and Effect on Early Exercise Oxygen Uptake

Oxygen uptake (V'O₂) at exercise onset is determined in part by acceleration of pulmonary blood flow (Q'p). Impairments in the Q'p response can decrease exercise tolerance. Prior research has shown that voluntary respiratory maneuvers can augment venous return, but the corollary impacts on cardiac function, Q'p and early-exercise V'O₂ remain uncertain. We examined a) the cardiovascular effects of 3 distinct respiratory maneuvers (abdominal, AB; rib cage, RC and deep breathing, DB) under resting conditions in healthy subjects (Protocol 1, n=13) and b) the impact of pre-exercise DB on pulmonary O₂ transfer during initiation of moderate intensity exercise (Protocol 2, n=8). In Protocol 1, echocardiographic analysis showed increased RV and LV cardiac output (RVCO and LVCO, respectively) following AB (by +23±13 and +18±15%, respectively, P<0.05), RC (+23±16; +14±15%, P<0.05) and DB (+27±21; +23±14%, P<0.05). In Protocol 2, DB performed for 12 breaths produced a pre-exercise increase in V'O₂ (+801±254 ml·min^-1 over ~ 6 s), presumably from increased Q'p followed by a reduction in pulmonary O₂ transfer during early phase exercise (first 20 s) compared to the control condition (149±51 vs 233±65 ml, P<0.05). We conclude that (1) respiratory maneuvers enhance RVCO and LVCO in healthy subjects under resting conditions, (2) AB, RC and DB have similar effects on RVCO and LVCO, and (3) DB can increase Q'p prior to exercise onset. These findings suggest that pre-exercise respiratory maneuvers may represent a promising strategy to prime V'O₂ kinetics and thereby to potentially improve exercise tolerance in patients with impaired cardiac function.

Influence of the fitting window on the O2 uptake kinetics at the onset of moderate intensity exercise

The O₂ uptake (V̇o₂) data at the onset of an exercise are usually fitted with a mono-exponential function, after removal of the data pertaining to a conventional initial time period (ΔTr) lasting ∼20 s. We performed a thorough quantitative analysis on the effects of removing data pertaining to different ΔTr, aiming at identifying an objective method to establish the appropriate ΔTr. Breath-by-breath O₂ uptake responses, acquired from 25 healthy adults performing a step moderate-intensity exercise, and 10⁴ simulated biexponential responses, were analyzed. For all the responses, the kinetic parameters of a mono-exponential function and the corresponding asymptotic standard errors (ASEs) were estimated by nonlinear regression, removing the data pertaining to progressively longer initial periods (1 s each) up to 60 s. Four methods to establish objectively ΔTr were compared. The minimum estimated τ was obtained for ΔTr ≅ 35 s in both the V̇o₂ and simulated data, that was about 30% lower compared with that obtained for ΔTr ≅ 0s. The average ASE values remained quite constant up to ΔTr ≅ 35 s, thereafter they increased remarkably. The τ used to generate the simulated response fell within the confidence intervals of the estimated τ in ∼85% of cases for ΔTr = 20 s ("20 s-w" method); this percentage increased to ∼92% of cases when ΔTr was established according to both the minimum τ and its narrowest confidence interval ("Mixed" method). In conclusion, the effects of removing V̇o₂ data pertaining to different ΔTr are remarkable. The "Mixed" method provided estimated parameters close to those used to generate the simulated responses and is thus endorsed.NEW & NOTEWORTHY We propose a method to objectively establish the initial time period to be removed from the fitting window when, using a mono-exponential model, the kinetics of the fundamental component is determined on breath-by-breath O₂ uptake data collected at the onset of a moderate-intensity exercise. Innovative statistical parameters ("Coverage" and "Concordance_5%," applicable on simulated responses) were used to compare its performance with that of other three methods. The proposed method yielded the best "Coverage" and "Concordance_5%."

Priming the cardiodynamic phase of pulmonary oxygen uptake through voluntary modulations of the respiratory pump at the onset of exercise

NEW FINDINGS

What is the central question of this study? The initial increase in oxygen uptake ( V ̇ O 2 ) at exercise onset results from pulmonary perfusion changes secondary to an increased venous return. Breathing mechanics contribute to venous return through abdominal and intrathoracic pressures variation. Can voluntary breathing techniques (abdominal or rib cage breathing) increase venous return and improve V ̇ O 2 at exercise onset? What is the main finding and its importance? Abdominal and rib cage breathing increase venous return and V ̇ O 2 at exercise onset. This mechanism could be clinically relevant in patients with impaired cardiac function limiting oxygen transport.

ABSTRACT

We examined how different breathing patterns can modulate venous return and alveolar gas transfer during exercise transients in humans. Ten healthy men transitioned from rest to moderate cycling while breathing spontaneously (SP) or with voluntary increases in abdominal (AB) or intrathoracic (RC) pressure swings. We used double body plethysmography to determine blood displacements between the trunk and the extremities (V_bs ). From continuous signals of airflow and O₂ fraction, we calculated breath-by-breath oxygen uptake at the mouth and used optoelectronic plethysmography to correct for lung O₂ store changes and calculate alveolar O₂ transfer ( V ̇ O 2 A ). Oesophageal (P_oes ) and gastric (P_ga ) pressures were monitored using balloon-tipped catheters. Cardiac stroke volume was measured using impedance cardiography. During the cardiodynamic phase (Φ1) of V ̇ O 2 A -on kinetics (20 s following exercise onset), AB and RC increased total alveolar oxygen transfer compared to SP (227 ± 32, P = 0.019 vs. 235 ± 27, P = 0.001 vs. 206 ± 20 ml, mean ± SD). P_ga and P_oes swings increased with AB (by 24.4 ± 9.6 cmH₂ O, P < 0.001) and RC (by 14.5 ± 5.7 cmH₂ O, P < 0.001), respectively. AB yielded a greater increase in intra-breath V_bs swings compared with RC and SP (+0.30 ± 0.14 vs. +0.16 ± 0.11, P < 0.001 vs. +0.10 ± 0.05 ml, P = 0.006) and increased the sum of stroke volumes compared to SP (4.47 ± 1.28 vs. 3.89 ± 0.96 litres, P = 0.053), while RC produced significant central blood translocation from the extremities compared with SP (by 493 ± 311 ml, P < 0.001). Our findings indicate that combining exercise onset with AB or RC increases venous return, thus increasing mass oxygen transport above metabolic consumption during Φ1 and limiting the oxygen deficit incurred.

Speeding of oxygen uptake kinetics is not different following low-intensity blood-flow-restricted and high-intensity interval training

NEW FINDINGS

What is the central question of this study? Can interval blood-flow-restricted (BFR) cycling training, undertaken at a low intensity, promote a similar adaptation to oxygen uptake ( V ̇ O 2 ) kinetics to high-intensity interval training? What is the main finding and its importance? Speeding of pulmonary V ̇ O 2 on-kinetics in healthy young subjects was not different between low-intensity interval BFR training and traditional high-intensity interval training. Given that very low workloads are well tolerated during BFR cycle training and speed V ̇ O 2 on-kinetics, this training method could be used when high mechanical loads are contraindicated.

ABSTRACT

Low-intensity blood-flow-restricted (BFR) endurance training is effective to increase aerobic capacity. Whether it speeds pulmonary oxygen uptake ( V ̇ O 2 p ), CO₂ output ( V ̇ C O 2 p ) and ventilatory ( V ̇ Ep ) kinetics has not been examined. We hypothesized that low-intensity BFR training would reduce the phase 2 time constant (τ_p ) of V ̇ O 2 p , V ̇ C O 2 p and V ̇ Ep by a similar magnitude to traditional high-intensity interval training (HIT). Low-intensity interval training with BFR served as a control. Twenty-four participants (25 ± 6 years old; maximal V ̇ O 2 46 ± 6 ml kg^-1  min^-1 ) were assigned to one of the following: low-intensity BFR interval training (BFR; n = 8); low-intensity interval training without BFR (LOW; n = 7); or high-intensity interval training without BFR (HIT; n = 9). Training was 12 sessions of two sets of five to eight × 2 min cycling and 1 min resting intervals. LOW and BFR were conducted at 30% of peak incremental power (P_peak ), and HIT was at ∼103% P_peak . For BFR, cuffs were inflated on both thighs (140-200 mmHg) during exercise and deflated during rest intervals. Six moderate-intensity step transitions (30% P_peak ) were averaged for analysis of pulmonary on-kinetics. Both BFR (pre- versus post-training τ_p  = 18.3 ± 3.2 versus 14.5 ± 3.4 s; effect size = 1.14) and HIT (τ_p  = 20.3 ± 4.0 versus 13.1 ± 2.9 s; effect size = 1.75) reduced the V ̇ O 2 p τ_p (P < 0.05). As expected, there was no change in LOW ( V ̇ O 2 p τ_p  = 17.9 ± 6.2 versus 17.7 ± 4.3 s; P = 0.9). The kinetics of V ̇ C O 2 p and V ̇ Ep were speeded only after HIT (38.5 ± 10.6%, P < 0.001 and 31.2 ± 24.7%, P = 0.004, respectively). Both HIT and low-intensity BFR training were effective in speeding moderate-intensity V ̇ O 2 p kinetics. These data support the findings of others that low-intensity cycling training with BFR increases muscle oxidative capacity.

Relationship between (non)linear phase II pulmonary oxygen uptake kinetics with skeletal muscle oxygenation and age in 11-15 year olds

NEW FINDINGS

What is the central question of this study? Do the phase II parameters of pulmonary oxygen uptake ( V ̇ O 2 ) kinetics display linear, first-order behaviour in association with alterations in skeletal muscle oxygenation during step cycling of different intensities or when exercise is initiated from an elevated work rate in youths. What is the main finding and its importance? Both linear and non-linear features of phase II V ̇ O 2 kinetics may be determined by alterations in the dynamic balance between microvascular O₂ delivery and utilization in 11-15 year olds. The recruitment of higher-order (i.e. type II) muscle fibres during 'work-to-work' cycling might be responsible for modulating V ̇ O 2 kinetics with chronological age.

ABSTRACT

This study investigated in 19 male youths (mean age: 13.6 ± 1.1 years, range: 11.7-15.7 years) the relationship between pulmonary oxygen uptake ( V ̇ O 2 ) and muscle deoxygenation kinetics during moderate- and very heavy-intensity 'step' cycling initiated from unloaded pedalling (i.e. U → M and U → VH) and moderate to very heavy-intensity step cycling (i.e. M → VH). Pulmonary V ̇ O 2 was measured breath-by-breath along with the tissue oxygenation index (TOI) of the vastus lateralis using near-infrared spectroscopy. There were no significant differences in the phase II time constant ( τ V ̇ O 2 p ) between U → M and U → VH (23 ± 6 vs. 25 ± 7 s; P = 0.36); however, the τ V ̇ O 2 p was slower during M → VH (42 ± 16 s) compared to other conditions (P < 0.001). Quadriceps TOI decreased with a faster (P < 0.01) mean response time (MRT; i.e. time delay + τ) during U → VH (14 ± 2 s) compared to U → M (22 ± 4 s) and M → VH (20 ± 6 s). The difference (Δ) between the τ V ̇ O 2 p and MRT-TOI was greater during U → VH compared to U → M (12 ± 7 vs. 2 ± 7 s, P < 0.001) and during M → VH (23 ± 15 s) compared to other conditions (P < 0.02), suggesting an increased proportional speeding of fractional O₂ extraction. The slowing of the τ V ̇ O 2 p during M → VH relative to U → M and U → VH correlated positively with chronological age (r = 0.68 and 0.57, respectively, P < 0.01). In youths, 'work-to-work' transitions slowed microvascular O₂ delivery-to-O₂ utilization with alterations in phase II V ̇ O 2 dynamics accentuated between the ages of 11 and 15 years.

VO₂FITTING: A Free and Open-Source Software for Modelling Oxygen Uptake Kinetics in Swimming and other Exercise Modalities

The assessment of oxygen uptake (VO₂) kinetics is a valuable non-invasive way to evaluate cardiorespiratory and metabolic response to exercise. The aim of the study was to develop, describe and evaluate an online VO₂ fitting tool (VO₂FITTING) for dynamically editing, processing, filtering and modelling VO₂ responses to exercise. VO₂FITTING was developed in Shiny, a web application framework for R language. Validation VO₂ datasets with both noisy and non-noisy data were developed and applied to widely-used models (n = 7) for describing different intensity transitions to verify concurrent validity. Subsequently, we then conducted an experiment with age-group swimmers as an example, illustrating how VO₂FITTING can be used to model VO₂ kinetics. Perfect fits were observed, and parameter estimates perfectly matched the known inputted values for all available models (standard error = 0; p < 0.001). The VO₂FITTING is a valid, free and open-source software for characterizing VO₂ kinetics in exercise, which was developed to help the research and performance analysis communities.

Effects of heavy-intensity priming exercise on pulmonary oxygen uptake kinetics and muscle oxygenation in heart failure with preserved ejection fraction

Exercise intolerance is a hallmark feature in heart failure with preserved ejection fraction (HFpEF). Prior heavy exercise ("priming exercise") speeds pulmonary oxygen uptake (V̇o_2p) kinetics in older adults through increased muscle oxygen delivery and/or alterations in mitochondrial metabolic activity. We tested the hypothesis that priming exercise would speed V̇o_2p on-kinetics in patients with HFpEF because of acute improvements in muscle oxygen delivery. Seven patients with HFpEF performed three bouts of two exercise transitions: MOD1, rest to 4-min moderate-intensity cycling and MOD2, MOD1 preceded by heavy-intensity cycling. V̇o_2p, heart rate (HR), total peripheral resistance (TPR), and vastus lateralis tissue oxygenation index (TOI; near-infrared spectroscopy) were measured, interpolated, time-aligned, and averaged. V̇o_2p and HR were monoexponentially curve-fitted. TPR and TOI levels were analyzed as repeated measures between pretransition baseline, minimum value, and steady state. Significance was P < 0.05. Time constant (τ; tau) V̇o_2p (MOD1 49 ± 16 s) was significantly faster after priming (41 ± 14 s; P = 0.002), and the effective HR τ was slower following priming (41 ± 27 vs. 51 ± 32 s; P = 0.025). TPR in both conditions decreased from baseline to minimum TPR ( P < 0.001), increased from minimum to steady state ( P = 0.041) but remained below baseline throughout ( P = 0.001). Priming increased baseline ( P = 0.003) and minimum TOI ( P = 0.002) and decreased the TOI muscle deoxygenation overshoot ( P = 0.041). Priming may speed the slow V̇o_2p on-kinetics in HFpEF and increase muscle oxygen delivery (TOI) at the onset of and throughout exercise. Microvascular muscle oxygen delivery may limit exercise tolerance in HFpEF.

Oxygen uptake on-kinetics before and after aerobic exercise training in individuals with traumatic brain injury

Objective: The high prevalence of fatigue among persons with traumatic brain injury (TBI) may be related to poor cardiorespiratory fitness observed in this population. Oxygen uptake on-kinetics is a method of assessing cardiorespiratory fitness and may be used to examine performance fatigability (decline in performance during a given activity) in persons with TBI.Purpose: To examine the effect of aerobic exercise training on oxygen uptake on-kinetics during treadmill walking in individuals with TBI.Methods: Seven ambulatory adults with chronic non-penetrating TBI performed short moderate-intensity (3-6 metabolic equivalents) walking bouts on a treadmill, prior to and following an aerobic exercise training program (clinicaltrials.gov: NCT01294332). The 12-week training program consisted of vigorous-intensity exercise on a treadmill for 30 min, 3 times a week. Breath-by-breath pulmonary gas exchange was measured throughout the bouts, and oxygen uptake on-kinetics described the time taken to achieve a steady-state response.Results: Faster oxygen uptake on-kinetics was observed after exercise training, for both the absolute and relative intensity as pre-training.Conclusions: Faster oxygen uptake on-kinetics following aerobic exercise training suggests an attenuated decline in physical performance during a standardized walking bout and improved performance fatigability in these individuals with TBI.Implications for rehabilitationSevere fatigue is a common complaint among persons with traumatic brain injury (TBI).Oxygen uptake on-kinetics may be used as an objective physiological measure of performance fatigability in persons with TBI.Faster oxygen uptake on-kinetics following aerobic exercise training suggests improved performance fatigability in these individuals with TBI.Aerobic exercise training appeared beneficial for reducing performance fatigability and may be considered as part of the rehabilitative strategy for those living with TBI.

The Bioenergetics of Exercise

Animals possess a remarkable ability to perform physical activity over a wide range of workloads and durations, reflecting both the inherent efficiency and large reserve capacity of energy transfer systems. Deciphering how different organ/physiological systems respond to the acute and chronic demands of exercise depends on a foundational understanding of the redox and bioenergetic principles that underlie the flow of electrons in living systems and its coupling to ATP synthesis. The purpose of this review is to set the stage to cover (1) the thermodynamic driving forces responsible for generating and maintaining the energy charge that establishes and sustains life for cells, and (2) how cellular energy transfer systems respond to changes in energy demand to ensure energy charge is preserved.

Comparison of metabolic adaptations between endurance- and sprint-trained athletes after an exhaustive exercise in two different calf muscles using a multi-slice 31 P-MR spectroscopic sequence

Measurements of exercise-induced metabolic changes, such as oxygen consumption, carbon dioxide exhalation or lactate concentration, are important indicators for assessing the current performance level of athletes in training science. With exercise-limiting metabolic processes occurring in loaded muscles, ³¹ P-MRS represents a particularly powerful modality to identify and analyze corresponding training-induced alterations. Against this background, the current study aimed to analyze metabolic adaptations after an exhaustive exercise in two calf muscles (m. soleus - SOL - and m. gastrocnemius medialis - GM) of sprinters and endurance athletes by using localized dynamic ³¹ P-MRS. In addition, the respiratory parameters VO₂ and VCO₂ , as well as blood lactate concentrations, were monitored simultaneously to assess the effects of local metabolic adjustments in the loaded muscles on global physiological parameters. Besides noting obvious differences between the SOL and the GM muscles, we were also able to identify distinct physiological strategies in dealing with the exhaustive exercise by recruiting two athlete groups with opposing metabolic profiles. Endurance athletes tended to use the aerobic pathway in the metabolism of glucose, whereas sprinters produced a significantly higher peak concentration of lactate. These global findings go along with locally measured differences, especially in the main performer GM, with sprinters revealing a higher degree of acidification at the end of exercise (pH 6.29 ± 0.20 vs. 6.57 ± 0.21). Endurance athletes were able to partially recover their PCr stores during the exhaustive exercise and seemed to distribute their metabolic activity more consistently over both investigated muscles. In contrast, sprinters mainly stressed Type II muscle fibers, which corresponds more to their training orientation preferring the glycolytic energy supply pathway. In conclusion, we were able to analyze the relation between specific local metabolic processes in loaded muscles and typical global adaptation parameters, conventionally used to monitor the training status of athletes, in two cohorts with different sports orientations.

Fitness Level and Not Aging per se, Determines the Oxygen Uptake Kinetics Response

Although aging has been associated to slower V˙O₂ kinetics, some evidence indicates that fitness status and not aging per se might modulate this response. The main goal of this study was to examine the V˙O₂, deoxygenated hemoglobin+myoglobin (deoxy-[Hb+Mb]) kinetics, and the NIRS-derived vascular reperfusion responses in older compared to young men of different training levels (i.e., inactive, recreationally active, and endurance trained). Ten young inactive [YI; 26 ± 5 yrs.; peak V˙O₂ (V˙O_2peak), 2.96 ± 0.55 L·min⁻¹], 10 young recreationally active (YR; 26 ± 6 yrs.; 3.92 ± 0.33 L·min⁻¹), 10 young endurance trained (YT; 30 ± 4 yrs.; 4.42 ± 0.32 L·min⁻¹), 7 older inactive (OI; 69 ± 4 yrs.; 2.50 ± 0.31 L·min⁻¹), 10 older recreationally active (OR; 69 ± 5 yrs.; 2.71 ± 0.42 L·min⁻¹), and 10 older endurance trained (OT; 66 ± 3 yrs.; 3.20 ± 0.35 L·min⁻¹) men completed transitions of moderate intensity cycling exercise (MODS) to determine V˙O₂ and deoxy-[Hb+Mb] kinetics, and the deoxy-[Hb+Mb]/V˙O₂ ratio. The time constant of V˙O₂ (τV˙O₂) was greater in YI (38.8 ± 10.4 s) and OI (44.1 ± 10.8 s) compared with YR (26.8 ± 7.5 s) and OR (26.6 ± 6.5 s), as well as compared to YT (14.8 ± 3.4 s), and OT (17.7 ± 2.7 s) (p < 0.05). τV˙O₂ was greater in YR and OR compared with YT and OT (p < 0.05). The deoxy-[Hb+Mb]/V˙O₂ ratio was greater in YI (1.23 ± 0.05) and OI (1.29 ± 0.08) compared with YR (1.11 ± 0.03) and OR (1.13 ± 0.06), as well as compared to YT (1.01 ± 0.03), and OT (1.06 ± 0.03) (p < 0.05). Similarly, the deoxy-[Hb+Mb]/ V˙O₂ ratio was greater in YR and OR compared with YT and OT (p < 0.05). There was a main effect of training (p = 0.033), whereby inactive (p = 0.018) and recreationally active men (p = 0.031) had significantly poorer vascular reperfusion than endurance trained men regardless of age. This study demonstrated not only that age-related slowing of V˙O₂ kinetics can be eliminated in endurance trained individuals, but also that inactive lifestyle negatively impacts the V˙O₂ kinetics response of young healthy individuals.

Near-infrared spectroscopy detects age-related differences in skeletal muscle oxidative function: promising implications for geroscience

Age is the greatest risk factor for chronic disease and is associated with a marked decline in functional capacity and quality of life. A key factor contributing to loss of function in older adults is the decline in skeletal muscle function. While the exact mechanism(s) remains incompletely understood, age-related mitochondrial dysfunction is thought to play a major role. To explore this question further, we studied 15 independently living seniors (age: 72 ± 5 years; m/f: 4/11; BMI: 27.6 ± 5.9) and 17 young volunteers (age: 25 ± 4 years; m/f: 8/9; BMI: 24.0 ± 3.3). Skeletal muscle oxidative function was measured in forearm muscle from the recovery kinetics of muscle oxygen consumption using near-infrared spectroscopy (NIRS). Muscle oxygen consumption was calculated as the slope of change in hemoglobin saturation during a series of rapid, supra-systolic arterial cuff occlusions following a brief bout of exercise. Aging was associated with a significant prolongation of the time constant of oxidative recovery following exercise (51.8 ± 5.4 sec vs. 37.1 ± 2.1 sec, P = 0.04, old vs. young, respectively). This finding suggests an overall reduction in mitochondrial function with age in nonlocomotor skeletal muscle. That these data were obtained using NIRS holds great promise in gerontology for quantitative assessment of skeletal muscle oxidative function at the bed side or clinic.

Oxygen uptake on-kinetics during six-minute walk test predicts short-term outcomes after off-pump coronary artery bypass surgery

PURPOSE

We aimed to investigate the ability of oxygen uptake kinetics to predict short-term outcomes after off-pump coronary artery bypass grafting.

METHODS

Fifty-two patients aged 60.9 ± 7.8 years waiting for off-pump coronary artery bypass surgery were evaluated. The 6-min walk test distance was performed pre-operatively, while simultaneously using a portable cardiopulmonary testing device. The transition of oxygen uptake kinetics from rest to exercise was recorded to calculate oxygen uptake kinetics fitting a monoexponential regression model. Oxygen uptake at steady state, constant time, and mean response time corrected by work rate were analysed. Short-term clinical outcomes were evaluated during the early post-operative of off-pump coronary artery bypass surgery.

RESULTS

Multivariate analysis showed body mass index, surgery time, and mean response time corrected by work rate as independent predictors for short-term outcomes. The optimal mean response time corrected by work rate cut-off to estimate short-term clinical outcomes was 1.51 × 10^-3 min²/ml. Patients with slower mean response time corrected by work rate demonstrated higher rates of hypertension, diabetes, EuroSCOREII, left ventricular dysfunction, and impaired 6-min walk test parameters. The per cent-predicted distance threshold of 66% in the pre-operative was associated with delayed oxygen uptake kinetics.

CONCLUSIONS

Pre-operative oxygen uptake kinetics during 6-min walk test predicts short-term clinical outcomes after off-pump coronary artery bypass surgery. From a clinically applicable perspective, a threshold of 66% of pre-operative predicted 6-min walk test distance indicated slower kinetics, which leads to longer intensive care unit and post-surgery hospital length of stay. Implications for rehabilitation Coronary artery bypass grafting is a treatment aimed to improve expectancy of life and prevent disability due to the disease progression; The use of pre-operative submaximal functional capacity test enabled the identification of patients with high risk of complications, where patients with delayed oxygen uptake kinetics exhibited worse short-term outcomes; Our findings suggest the importance of the rehabilitation in the pre-operative in order to "pre-habilitate" the patients to the surgical procedure; Faster oxygen uptake on-kinetics could be achieved by improving the oxidative capacity of muscles and cardiovascular conditioning through rehabilitation, adding better results following cardiac surgery.

New Methods for Processing and Quantifying VO2 Kinetics to Steady State: VO2 Onset Kinetics

Current methods of oxygen uptake (VO₂) kinetics data handling may be too simplistic for the complex physiology involved in the underlying physiological processes. Therefore, the aim of this study was to quantify the VO₂ kinetics to steady state across the full range of sub-ventilatory threshold work rates, with a particular focus on the VO₂ onset kinetics. Ten healthy, moderately trained males participated in five bouts of cycling. Each bout involved 10 min at a percentage of the subject's ventilation threshold (30, 45, 60, 75, 90%) from unloaded cycling. The VO₂ kinetics was quantified using the conventional mono-exponential time constant (tau, τ), as well as the new methods for VO₂ onset kinetics. Compared to linear modeling, non-linear modeling caused a deterioration of goodness of fit (main effect, p < 0.001) across all exercise intensities. Remainder kinetics were also improved using a modified application of the mono-exponential model (main effect, p < 0.001). Interestingly, the slope from the linear regression of the onset kinetics data is similar across all subjects and absolute exercise intensities, and thereby independent of subject fitness and τ. This could indicate that there are no functional limitations between subjects during this onset phase, with limitations occurring for the latter transition to steady state. Finally, the continuing use of mono-exponential modeling could mask important underlying physiology of more instantaneous VO₂ responses to steady state. Consequently, further research should be conducted on this new approach to VO₂ onset kinetics.

Oxygen Uptake Kinetics Is Slower in Swimming Than Arm Cranking and Cycling during Heavy Intensity

Oxygen uptake (V·O2) kinetics has been reported to be influenced by the activity mode. However, only few studies have compared V·O₂ kinetics between activities in the same subjects in which they were equally trained. Therefore, this study compared the V·O₂ kinetics response to swimming, arm cranking, and cycling within the same group of subjects within the heavy exercise intensity domain. Ten trained male triathletes (age 23.2 ± 4.5 years; height 180.8 ± 8.3 cm; weight 72.3 ± 6.6 kg) completed an incremental test to exhaustion and a 6-min heavy constant-load test in the three exercise modes in random order. Gas exchange was measured by a breath-by-breath analyzer and the on-transient V·O₂ kinetics was modeled using bi-exponential functions. V·O_2peak was higher in cycling (65.6 ± 4.0 ml·kg⁻¹·min⁻¹) than in arm cranking or swimming (48.7 ± 8.0 and 53.0 ± 6.7 ml·kg⁻¹·min⁻¹; P < 0.01), but the V·O₂ kinetics were slower in swimming (τ₁ = 31.7 ± 6.2 s) than in arm cranking (19.3 ± 4.2 s; P = 0.001) and cycling (12.4 ± 3.7 s; P = 0.001). The amplitude of the primary component was lower in both arm cranking and swimming (21.9 ± 4.7 and 28.4 ± 5.1 ml·kg⁻¹·min⁻¹) compared with cycling (39.4 ± 4.1 ml·kg⁻¹·min⁻¹; P = 0.001). Although the gain of the primary component was higher in arm cranking compared with cycling (15.3 ± 4.2 and 10.7 ± 1.3 ml·min⁻¹·W⁻¹; P = 0.02), the slow component amplitude, in both absolute and relative terms, did not differ between exercise modes. The slower V·O₂ kinetics during heavy-intensity swimming is exercise-mode dependent. Besides differences in muscle mass and greater type II muscle fibers recruitment, the horizontal position adopted and the involvement of trunk and lower-body stabilizing muscles could be additional mechanisms that explain the differences between exercise modalities.

Dissociating external power from intramuscular exercise intensity during intermittent bilateral knee-extension in humans

Key points

Continuous high‐intensity constant‐power exercise is unsustainable, with maximal oxygen uptake (V˙O2 max ) and the limit of tolerance attained after only a few minutes.

Performing the same power intermittently reduces the O₂ cost of exercise and increases tolerance. The extent to which this dissociation is reflected in the intramuscular bioenergetics is unknown.

We used pulmonary gas exchange and ³¹P magnetic resonance spectroscopy to measure whole‐body V˙O2, quadriceps phosphate metabolism and pH during continuous and intermittent exercise of different work:recovery durations.

Shortening the work:recovery durations (16:32 s vs. 32:64 s vs. 64:128 s vs. continuous) at a work rate estimated to require 110% peak aerobic power reduced V˙O2, muscle phosphocreatine breakdown and muscle acidification, eliminated the glycolytic‐associated contribution to ATP synthesis, and increased exercise tolerance.

Exercise intensity (i.e. magnitude of intramuscular metabolic perturbations) can be dissociated from the external power using intermittent exercise with short work:recovery durations.

Abstract

Compared with work‐matched high‐intensity continuous exercise, intermittent exercise dissociates pulmonary oxygen uptake (V˙O2) from the accumulated work. The extent to which this reflects differences in O₂ storage fluctuations and/or contributions from oxidative and substrate‐level bioenergetics is unknown. Using pulmonary gas‐exchange and intramuscular ³¹P magnetic resonance spectroscopy, we tested the hypotheses that, at the same power: ATP synthesis rates are similar, whereas peak V˙O2 amplitude is lower in intermittent vs. continuous exercise. Thus, we expected that: intermittent exercise relies less upon anaerobic glycolysis for ATP provision than continuous exercise; shorter intervals would require relatively greater fluctuations in intramuscular bioenergetics than in V˙O2 compared to longer intervals. Six men performed bilateral knee‐extensor exercise (estimated to require 110% peak aerobic power) continuously and with three different intermittent work:recovery durations (16:32, 32:64 and 64:128 s). Target work duration (576 s) was achieved in all intermittent protocols; greater than continuous (252 ± 174 s; P < 0.05). Mean ATP turnover rate was not different between protocols (∼43 mm min⁻¹ on average). However, the intramuscular phosphocreatine (PCr) component of ATP generation was greatest (∼30 mm min⁻¹), and oxidative (∼10 mm min⁻¹) and anaerobic glycolytic (∼1 mm min⁻¹) components were lowest for 16:32 and 32:64 s intermittent protocols, compared to 64:128 s (18 ± 6, 21 ± 10 and 10 ± 4 mm min⁻¹, respectively) and continuous protocols (8 ± 6, 20 ± 9 and 16 ± 14 mm min⁻¹, respectively). As intermittent work duration increased towards continuous exercise, ATP production relied proportionally more upon anaerobic glycolysis and oxidative phosphorylation, and less upon PCr breakdown. However, performing the same high‐intensity power intermittently vs. continuously reduced the amplitude of fluctuations in V˙O2 and intramuscular metabolism, dissociating exercise intensity from the power output and work done.

Continuous high‐intensity constant‐power exercise is unsustainable, with maximal oxygen uptake (V˙O2 max ) and the limit of tolerance attained after only a few minutes.

Performing the same power intermittently reduces the O₂ cost of exercise and increases tolerance. The extent to which this dissociation is reflected in the intramuscular bioenergetics is unknown.

We used pulmonary gas exchange and ³¹P magnetic resonance spectroscopy to measure whole‐body V˙O2, quadriceps phosphate metabolism and pH during continuous and intermittent exercise of different work:recovery durations.

Shortening the work:recovery durations (16:32 s vs. 32:64 s vs. 64:128 s vs. continuous) at a work rate estimated to require 110% peak aerobic power reduced V˙O2, muscle phosphocreatine breakdown and muscle acidification, eliminated the glycolytic‐associated contribution to ATP synthesis, and increased exercise tolerance.

Exercise intensity (i.e. magnitude of intramuscular metabolic perturbations) can be dissociated from the external power using intermittent exercise with short work:recovery durations.

Sex differences in the oxygen delivery, extraction, and uptake during moderate-walking exercise transition

Previous studies in children and older adults demonstrated faster oxygen uptake (V̇O2) kinetics in males compared with females, but young healthy adults have not been studied. We hypothesized that young men would have faster aerobic system dynamics in response to the onset of exercise than women. Interactions between oxygen supply and utilization were characterized by the dynamics of V̇O2, deoxyhemoglobin (HHb), tissue saturation index (TSI), cardiac output (Q̇), and calculated arteriovenous O2 difference (a–vO2diff) in women and men. Eighteen healthy active young women and men (9 of each sex) with similar aerobic fitness levels volunteered for this study. Participants performed an incremental cardiopulmonary treadmill exercise test and 3 moderate-intensity treadmill exercise tests (at 80% V̇O2 of gas exchange threshold). Data related to the moderate exercise were submitted to exponential data modelling to obtain parameters related to the aerobic system dynamics. The time constants of V̇O2, a–vO2diff, HHb, and TSI (30 ± 6, 29 ± 1, 16 ± 1, and 15 ± 2 s, respectively) in women were statistically (p < 0.05) faster than the time constants in men (42 ± 10, 49 ± 21, 19 ± 3, and 20 ± 4 s, respectively). Although Q̇ dynamics were not statistically different (p = 0.06) between groups, there was a trend to slower Q̇ dynamics in men corresponding with the slower V̇O2 kinetics. These results indicated that the peripheral and pulmonary oxygen extraction dynamics were remarkably faster in women. Thus, contrary to the hypothesis, V̇O2 dynamics measured at the mouth at the onset of submaximal treadmill walking were faster in women compared with men.

Combined spiroergometry and 31 P-MRS of human calf muscle during high-intensity exercise

Simultaneous measurements of pulmonary oxygen consumption (VO₂ ), carbon dioxide exhalation (VCO₂ ) and phosphorus magnetic resonance spectroscopy (³¹ P-MRS) are valuable in physiological studies to evaluate muscle metabolism during specific loads. Therefore, the aim of this study was to adapt a commercially available spirometric device to enable measurements of VO₂ and VCO₂ whilst simultaneously performing ³¹ P-MRS at 3 T. Volunteers performed intense plantar flexion of their right calf muscle inside the MR scanner against a pneumatic MR-compatible pedal ergometer. The use of a non-magnetic pneumotachograph and extension of the sampling line from 3 m to 5 m to place the spirometric device outside the MR scanner room did not affect adversely the measurements of VO₂ and VCO₂ . Response and delay times increased, on average, by at most 0.05 s and 0.79 s, respectively. Overall, we were able to demonstrate a feasible ventilation response (VO₂ = 1.05 ± 0.31 L/min; VCO₂ = 1.11 ± 0.33 L/min) during the exercise of a single calf muscle, as well as a good correlation between local energy metabolism and muscular acidification (τ_{PCr fast} and pH; R² = 0.73, p < 0.005) and global respiration (τ_{PCr fast} and VO₂ ; R²  = 0.55, p = 0.01). This provides improved insights into aerobic and anaerobic energy supply during strong muscular performances.

The effects of short work vs. longer work periods within intermittent exercise on V̇o2p kinetics, muscle deoxygenation, and energy system contribution

We examined the effects of inserting 3-s recovery periods during high-intensity cycling exercise at 25-s and 10-s intervals on pulmonary oxygen uptake (V̇o_2p), muscle deoxygenation [deoxyhemoglobin (HHb)], their associated kinetics (τ), and energy system contributions. Eleven men (24 ± 3 yr) completed two trials of three cycling protocols: an 8-min continuous protocol (CONT) and two 8-min intermittent exercise protocols with work-to-rest periods of 25 s to 3 s (25INT) and 10 s to 3 s (10INT). Each protocol began with a step-transition from a 20-W baseline to a power output (PO) of 60% between lactate threshold and maximal V̇o_2p (Δ60). This PO was maintained for 8 min in CONT, whereas 3-s periods of 20-W cycling were inserted every 10 s and 25 s after the transition to Δ60 in 10INT and 25INT, respectively. Breath-by-breath gas exchange measured by mass spectrometry and turbine and vastus lateralis [HHb] measured by near-infrared spectroscopy were recorded throughout. Arterialized-capillary lactate concentration ([Lac^-]) was obtained before and 2 min postexercise. The τV̇o_2p was lowest (P < 0.05) for 10INT (24 ± 4 s) and 25INT (23 ± 5 s) compared with CONT (28 ± 4 s), whereas HHb kinetics did not differ (P > 0.05) between conditions. Postexercise [Lac^-] was lowest (P < 0.05) for 10INT (7.0 ± 1.7 mM), was higher for 25INT (10.3 ± 1.9 mM), and was greatest in CONT (14.3 ± 3.1 mM). Inserting 3-s recovery periods during heavy-intensity exercise speeded V̇o_2p kinetics and reduced overall V̇o_2p, suggesting an increased reliance on PCr-derived phosphorylation during the work period of INT compared with an identical PO performed continuously.NEW & NOTEWORTHY We report novel observations on the effects of differing heavy-intensity work durations between 3-s recovery periods on pulmonary oxygen uptake (V̇o_2p) kinetics, muscle deoxygenation, and energy system contributions. Relative to continuous exercise, V̇o_2p kinetics are faster in intermittent exercise, and increased frequency of 3-s recovery periods improves microvascular O₂ delivery and reduces V̇o_2p and arterialized-capillary lactate concentration. The metabolic burden of identical intensity work is altered when performed intermittently vs. continuously.

Top 10 Research Questions Related to Youth Aerobic Fitness

Peak oxygen uptake ([Formula: see text]₂) is internationally recognized as the criterion measure of youth aerobic fitness, but despite pediatric data being available for almost 80 years, its measurement and interpretation in relation to growth, maturation, and health remain controversial. The trainability of youth aerobic fitness continues to be hotly debated, and causal mechanisms of training-induced changes and their modulation by chronological age, biological maturation, and sex are still to be resolved. The daily physical activity of youth is characterized by intermittent bouts and rapid changes in intensity, but physical activity of the intensity and duration required to determine peak [Formula: see text]₂ is rarely (if ever) experienced by most youth. In this context, it may therefore be the transient kinetics of pulmonary [Formula: see text]₂ that best reflect youth aerobic fitness. There are remarkably few rigorous studies of youth pulmonary [Formula: see text]₂ kinetics at the onset of exercise in different intensity domains, and the influence of chronological age, biological maturation, and sex during step changes in exercise intensity are not confidently documented. Understanding the trainability of the parameters of youth pulmonary [Formula: see text]₂ kinetics is primarily based on a few comparative studies of athletes and nonathletes. The underlying mechanisms of changes due to training require further exploration. The aims of the present article are therefore to provide a brief overview of aerobic fitness during growth and maturation, increase awareness of current controversies in its assessment and interpretation, identify gaps in knowledge, raise 10 relevant research questions, and indicate potential areas for future research.

Data collection, handling, and fitting strategies to optimize accuracy and precision of oxygen uptake kinetics estimation from breath-by-breath measurements

Phase 2 pulmonary oxygen uptake kinetics (ϕ2 τV̇o_2P) reflect muscle oxygen consumption dynamics and are sensitive to changes in state of training or health. This study identified an unbiased method for data collection, handling, and fitting to optimize V̇o_2P kinetics estimation. A validated computational model of V̇o_2P kinetics and a Monte Carlo approach simulated 2 × 10⁵ moderate-intensity transitions using a distribution of metabolic and circulatory parameters spanning normal health. Effects of averaging (interpolation, binning, stacking, or separate fitting of up to 10 transitions) and fitting procedures (biexponential fitting, or ϕ2 isolation by time removal, statistical, or derivative methods followed by monoexponential fitting) on accuracy and precision of V̇o_2P kinetics estimation were assessed. The optimal strategy to maximize accuracy and precision of τV̇o_2P estimation was 1-s interpolation of 4 bouts, ensemble averaged, with the first 20 s of exercise data removed. Contradictory to previous advice, we found optimal fitting procedures removed no more than 20 s of ϕ1 data. Averaging method was less critical: interpolation, binning, and stacking gave similar results, each with greater accuracy compared with analyzing repeated bouts separately. The optimal procedure resulted in ϕ2 τV̇o_2P estimates for transitions from an unloaded or loaded baseline that averaged 1.97 ± 2.08 and 1.04 ± 2.30 s from true, but were within 2 s of true in only 47-62% of simulations. Optimized 95% confidence intervals for τV̇o_2P ranged from 4.08 to 4.51 s, suggesting a minimally important difference of ~5 s to determine significant changes in τV̇o_2P during interventional and comparative studies.NEW & NOTEWORTHY We identified an unbiased method to maximize accuracy and precision of oxygen uptake kinetics (τV̇o_2P) estimation. The optimum number of bouts to average was four; interpolation, bin, and stacking averaging methods gave similar results. Contradictory to previous advice, we found that optimal fitting procedures removed no more than 20 s of phase 1 data. Our data suggest a minimally important difference of ~5 s to determine significant changes in τV̇o_2P during interventional and comparative studies.

Oral N-acetylcysteine and exercise tolerance in mild chronic obstructive pulmonary disease

Heightened oxidative stress is implicated in the progressive impairment of skeletal muscle vascular and mitochondrial function in chronic obstructive pulmonary disease (COPD). Whether accumulation of reactive oxygen species contributes to exercise intolerance in the early stages of COPD is unknown. The purpose of the present study was to determine the effects of oral antioxidant treatment with N-acetylcysteine (NAC) on respiratory, cardiovascular, and locomotor muscle function and exercise tolerance in patients with mild COPD. Thirteen patients [forced expiratory volume in 1 s (FEV₁)-to-forced vital capacity ratio < lower limit of normal (LLN) and FEV₁ ≥ LLN) were enrolled in a double-blind, randomized crossover study to receive NAC (1,800 mg/day) or placebo for 4 days. Severe-intensity constant-load exercise tests were performed with noninvasive measurements of central hemodynamics (stroke volume, heart rate, and cardiac output via impedance cardiography), arterial blood pressure, pulmonary ventilation and gas exchange, quadriceps muscle oxygenation (near-infrared spectroscopy), and estimated capillary blood flow. Nine patients completed the study with no major adverse clinical effects. Although NAC elevated plasma glutathione by ~27% compared with placebo (P < 0.05), there were no differences in exercise tolerance (placebo: 325 ± 47 s, NAC: 336 ± 51 s), central hemodynamics, arterial blood pressure, pulmonary ventilation or gas exchange, locomotor muscle oxygenation, or capillary blood flow from rest to exercise between conditions (P > 0.05 for all). In conclusion, modulation of plasma redox status with oral NAC treatment was not translated into beneficial effects on central or peripheral components of the oxygen transport pathway, thereby failing to improve exercise tolerance in nonhypoxemic patients with mild COPD.NEW & NOTEWORTHY Acute antioxidant treatment with N-acetylcysteine (NAC) elevated plasma glutathione but did not modulate central or peripheral components of the O₂ transport pathway, thereby failing to improve exercise tolerance in patients with mild chronic obstructive pulmonary disease (COPD).

MRS Evidence of Adequate O₂ Supply in Human Skeletal Muscle at the Onset of Exercise

PURPOSE

At exercise onset, intramuscular oxidative energy production responds relatively slowly in comparison with the change in adenosine triphosphate demand. To determine whether the slow kinetics of oxidative adenosine triphosphate production is due to inadequate O2 supply or metabolic inertia, we studied the kinetics of intramyocellular deoxygenation (deoxy-myoglobin (Mb)) and metabolism (phosphocreatine (PCr)) using proton (1H) and phosphorus (31P) magnetic resonance spectroscopy in six healthy subjects (33 ± 5 yr).

METHODS

Specifically, using dynamic plantarflexion exercise, rest to exercise and recovery were assessed at both 60% of maximum work rate (moderate intensity) and 80% of maximum work rate (heavy intensity).

RESULTS

At exercise onset, [PCr] fell without delay and with a similar time constant (τ) at both exercise intensities (approximately 33 s). In contrast, the increase in deoxy-Mb was delayed at exercise onset by 5-7 s, after which it increased with kinetics (moderate τ = 37 ± 9 s; heavy τ = 29 ± 6 s) that was not different from τPCr (P > 0.05). At cessation, deoxy-Mb recovered without time delay and more rapidly (τ = ∼20 s) than PCr (τ = ∼33 s) (P < 0.05).

CONCLUSIONS

Using a unique combination of in vivo magnetic resonance spectroscopy techniques with high time resolution, this study revealed a delay in intramuscular deoxygenation at the onset of exercise and rapid reoxygenation kinetics upon cessation. Together, these data imply that intramuscular substrate-enzyme interactions, and not O2 availability, determine the exercise onset kinetics of oxidative metabolism in healthy human skeletal muscles.

The effects of creatine supplementation on thermoregulation and physical (cognitive) performance: a review and future prospects

Creatine (Cr) is produced endogenously in the liver or obtained exogenously from foods, such as meat and fish. In the human body, 95 % of Cr is located in the cytoplasm of skeletal muscle either in a phosphorylated (PCr) or free form (Cr). PCr is essential for the immediate rephosphorylation of adenosine diphosphate to adenosine triphosphate. PCr is rapidly degraded at the onset of maximal exercise at a rate that results in muscle PCr reservoirs being substantially depleted. A well-established strategy followed to increase muscle total Cr content is to increase exogenous intake by supplementation with chemically pure synthetic Cr. Most Cr supplementation regimens typically follow a well-established loading protocol of 20 g day(-1) of Cr for approximately 5-7 days, followed by a maintenance dose at between 2 and 5 g day(-1) for the duration of interest, although more recent studies tend to utilize a 0.3-g kg(-1) day(-1) supplementation regimen. Some studies have also investigated long-term supplementation of up to 1 year. Uptake of Cr is enhanced when taken together with carbohydrate and protein and/or while undertaking exercise. Cr supplementation has been shown to augment muscle total Cr content and enhance anaerobic performance; however, there is also some evidence of indirect benefits to aerobic endurance exercise through enhanced thermoregulation. While there is an abundance of data supporting the ergogenic effects of Cr supplementation in a variety of different applications, some individuals do not respond, the efficacy of which is dependent on a number of factors, such as dose, age, muscle fiber type, and diet, although further work in this field is warranted. Cr is increasingly being used in the management of some clinical conditions to enhance muscle mass and strength. The application of Cr in studies of health and disease has widened recently with encouraging results in studies involving sleep deprivation and cognitive performance.

Impact of 5-aminolevulinic acid with iron supplementation on exercise efficiency and home-based walking training achievement in older women

A reduction in exercise efficiency with aging limits daily living activities. We examined whether 5-aminolevulinic acid (ALA) with sodium ferrous citrate (SFC) increased exercise efficiency and voluntary achievement of interval walking training (IWT) in older women. Ten women [65 ± 3(SD) yr] who had performed IWT for >12 mo and were currently performing IWT participated in this study. The study was conducted in a placebo-controlled, double-blind crossover design. All subjects underwent two trials for 7 days each in which they performed IWT with ALA+SFC (100 and 115 mg/day, respectively) or placebo supplement intake (CNT), intermittently with a 2-wk washout period. Before and after each trial, subjects underwent a graded cycling test at 27.0 °C atmospheric temperature and 50% relative humidity, and oxygen consumption rate, carbon dioxide production rate, and lactate concentration in plasma were measured. Furthermore, for the first 6 days of each trial, exercise intensity for IWT was measured by accelerometry. We found that, in the ALA+SFC trial, oxygen consumption rate and carbon dioxide production rate during graded cycling decreased by 12% (P < 0.001) and 11% (P = 0.001) at every workload, respectively, accompanied by a 16% reduction in lactate concentration in plasma (P < 0.001), although all remained unchanged in the CNT trial (P > 0.2). All of the reductions were significantly greater in the ALA+SFC than the CNT trial (P < 0.05). Furthermore, the training days, impulse, and time at fast walking were 42% (P = 0.028), 102% (P = 0.027), and 69% (P = 0.039) higher during the ALA+SFC than the CNT intake period, respectively. Thus ALA+SFC supplementation augmented exercise efficiency and thereby improved IWT achievement in older women.

V̇O2 and HR kinetics before and after International Space Station missions

PURPOSE

Heart rate (HR), pulmonary and muscle oxygen uptake ([Formula: see text]O2pulm, [Formula: see text]O2musc) kinetics after changes of work rate (WR) indicate regulatory characteristics related to aerobic metabolism. We analysed whether the kinetics of HR, [Formula: see text]O2pulm and [Formula: see text]O2musc are slowed after missions to the International Space Station (ISS). The changes of the kinetics were correlated with [Formula: see text]O2peak data.

METHODS

10 astronauts [4 females, 6 males, age: 48.0 ± 3.8 years, height: 176 ± 7 cm, mass: 74.5 ± 15.9 kg (mean ± SD)] performed an incremental test to determine [Formula: see text]O2peak (before missions on L-110 days, after return on R+1/+10/+36 days), and a cardio-respiratory kinetics test (CRKT) with randomized 30-80 W WR changes to determine HR, [Formula: see text]O2pulm and [Formula: see text]O2musc kinetics by time-series analysis (L-236/-73, R+6/+21). Kinetics were summarized by maximum and related lag of cross-correlation function (CCFmax, CCFlag) of WR with the analysed parameter.

RESULTS

Statistically, significant changes were also found for CCFmax([Formula: see text]O2musc) between L-236 and R+6 (P = 0.010), L-236 and R+21 (P = 0.030), L-72 and R+6 (P = 0.043). Between pre-to-post mission change in [Formula: see text]O2peak and CCFmax(HR), a correlation was shown (r SP = 0.67, P = 0.017).

CONCLUSION

The [Formula: see text]O2musc kinetics changes indicate aerobic detraining effects which are present up to 21 days following space flight. The correlations between changes in [Formula: see text]O2peak and HR kinetics illustrate the key role of cardiovascular regulation in [Formula: see text]O2peak. The addition of CRKT to ISS flight is recommended to obtain information regarding the potential muscular and cardiovascular deconditioning. This allows a reduction in the frequency of higher intensity testing during flight.

No acetyl group deficit is evident at the onset of exercise at 90% of maximal oxygen uptake in humans

The existence of an acetyl group deficit at or above 90% of maximal oxygen uptake (VO(2max)) has proved controversial, with contradictory results likely relating to limitations in previous research. The purpose of the present study was to determine whether the "acetyl group deficit" occurs at the start of exercise at 90%VO(2max) in a well-controlled study. Eight male participants (age: 33.6 +/- 2.0 years; VO(2max): 3.60 +/- 0.21 litres . min(-1)) completed two exercise bouts at 90%VO(2max) for 3 min following either 30 min of saline (control) or dichloroacetate (50 mg . kg(-1) body mass) infusion, ending 15 min before exercise. Muscle biopsies were obtained immediately before and after exercise while continuous non-invasive measures of pulmonary oxygen uptake and muscle deoxygenation were made. Muscle pyruvate dehydrogenase activity was significantly higher before exercise following dichloroacetate infusion (control: 2.67 +/- 0.98 vs. dichloroacetate: 17.9 +/- 1.1 mmol acetyl-CoA . min(-1) . mg(-1) protein, P = 0.01) and resulted in higher pre- and post-exercise muscle acetylcarnitine (pre-exercise control: 3.3 +/- 0.95 vs. pre-exercise dichloroacetate: 8.0 +/- 0.88 vs. post-exercise control: 11.9 +/- 1.1 vs. post-exercise dichloroacetate: 17.2 +/- 1.1 mmol . kg(-1) dry muscle, P < 0.05). However, substrate-level phosphorylation (control: 125 +/- 20 vs. dichloroacetate: 113 +/- 13 mmol adenosine triphosphate . kg(-1) dry muscle) and VO(2) kinetics (control: 19.2 +/- 2.2 vs. dichloroacetate: 22.8 +/- 2.5 s), were unaltered. Furthermore, dichloroacetate infusion blunted the slow component of VO(2) and muscle deoxygenation and slowed muscle deoxygenation kinetics, possibly by enhancing oxygen delivery during exercise. These data support the hypothesis that the "acetyl group deficit" does not occur at or above 90%VO(2max).

Discrepancy between femoral and capillary blood flow kinetics during knee extension exercise

Capillary blood flow (QCAP) kinetics have previously been shown to be significantly slower than femoral artery (QFA) kinetics following the onset of dynamic knee extension exercise. If the increase in QCAP does not follow a similar time course to QFA, then a substantial proportion of the available blood flow is not distributed to the working muscle. One possible explanation for this discrepancy is that blood flow also increases to the nonworking lower leg muscles. Therefore, the present study aimed to determine if a reduction in lower limb blood flow, via arterial occlusion below the knee, alters the kinetics of QFA and QCAP during knee extension exercise, and thus provide insight into the potential mechanisms controlling the rapid increase in QFA. Subjects performed a ramp max test to determine the work rate at which gas exchange threshold (GET) occurred. At least four constant work rate trials with and without below-knee occlusion were conducted at work rates eliciting ∼ 80% GET. Pulmonary gas exchange, near-infrared spectroscopy and QFA measurements were taken continuously during each exercise bout. Muscle oxygen uptake (VO2m) and deoxy[hemoglobin+myoglobin] were used to estimate QCAP. There was no significant difference between the uncuffed and cuffed conditions in any response (P>0.05). The mean response times (MRT) of QFA were 18.7 ± 14.2s (uncuffed) and 24.6 ± 14.9s (cuffed). QCAP MRTs were 51.8 ± 23.4s (uncuffed) and 56.7 ± 23.2s (cuffed), which were not significantly different from the time constants (τ) of VO2m (39.7 ± 23.2s (uncuffed) and 46.3 ± 24.1s (cuffed). However, the MRT of QFA was significantly faster (P<0.05) than the MRT of QCAP and τVO2m. τVO2m and MRT QCAP were significantly correlated and estimated QCAP kinetics tracked VO2m following exercise onset. Cuffing below the knee did not significantly change the kinetics of QFA, QCAP or VO2m, although an effect size of 1.02 suggested that a significant effect on QFA may have been hidden by small subject number.

Muscle metabolism changes with age and maturation: How do they relate to youth sport performance?

AIM

To provide an evidence-based review of muscle metabolism changes with sex-, age- and maturation with reference to the development of youth sport performance.

METHODS

A narrative review of data from both invasive and non-invasive studies, from 1970 to 2015, founded on personal databases supported with computer searches of PubMed and Google Scholar.

RESULTS

Youth sport performance is underpinned by sex-, age- and maturation-related changes in muscle metabolism. Investigations of muscle size, structure and metabolism; substrate utilisation; pulmonary oxygen uptake kinetics; muscle phosphocreatine kinetics; peak anaerobic and aerobic performance; and fatigue resistance; determined using a range of conventional and emerging techniques present a consistent picture. Age-related changes have been consistently documented but specific and independent maturation-related effects on muscle metabolism during exercise have proved elusive to establish. Children are better equipped for exercise supported primarily by oxidative metabolism than by anaerobic metabolism. Sexual dimorphism is apparent in several physiological variables underpinning youth sport performance. As young people mature there is a progressive but asynchronous transition into an adult metabolic profile.

CONCLUSIONS

The application of recent developments in technology to the laboratory study of the exercising child and adolescent has both supplemented existing knowledge and provided novel insights into developmental exercise physiology. A sound foundation of laboratory-based knowledge has been established but the lack of rigorously designed child-specific and sport-specific testing environments has clouded the interpretation of the data in real life situations. The primary challenge remains the translation of laboratory research into the optimisation of youth sports participation and performance.

Endurance training facilitates myoglobin desaturation during muscle contraction in rat skeletal muscle

At onset of muscle contraction, myoglobin (Mb) immediately releases its bound O₂ to the mitochondria. Accordingly, intracellular O₂ tension (P_mbO₂) markedly declines in order to increase muscle O₂ uptake (mO₂). However, whether the change in P_mbO₂ during muscle contraction modulates mO₂ and whether the O₂ release rate from Mb increases in endurance-trained muscles remain unclear. The purpose of this study was, therefore, to determine the effect of endurance training on O₂ saturation of Mb (S_mbO₂) and P_mbO₂ kinetics during muscle contraction. Male Wistar rats were subjected to a 4-week swimming training (Tr group; 6 days per week, 30 min × 4 sets per day) with a weight load of 2% body mass. After the training period, deoxygenated Mb kinetics during muscle contraction were measured using near-infrared spectroscopy under hemoglobin-free medium perfusion. In the Tr group, the mO₂peak significantly increased by 32%. Although the P_mbO₂ during muscle contraction did not affect the increased mO₂ in endurance-trained muscle, the O₂ release rate from Mb increased because of the increased Mb concentration and faster decremental rate in S_mbO₂ at the maximal twitch tension. These results suggest that the Mb dynamics during muscle contraction are contributing factors to faster O₂ kinetics in endurance-trained muscle.

Muscular pre-conditioning using light-emitting diode therapy (LEDT) for high-intensity exercise: a randomized double-blind placebo-controlled trial with a single elite runner

Recently, low-level laser (light) therapy (LLLT) has been used to improve muscle performance. This study aimed to evaluate the effectiveness of near-infrared light-emitting diode therapy (LEDT) and its mechanisms of action to improve muscle performance in an elite athlete. The kinetics of oxygen uptake (VO2), blood and urine markers of muscle damage (creatine kinase--CK and alanine), and fatigue (lactate) were analyzed. Additionally, some metabolic parameters were assessed in urine using proton nuclear magnetic resonance spectroscopy ((1)H NMR). A LED cluster with 50 LEDs (λ = 850 nm; 50 mW 15 s; 37.5 J) was applied on legs, arms and trunk muscles of a single runner athlete 5 min before a high-intense constant workload running exercise on treadmill. The athlete received either Placebo-1-LEDT; Placebo-2-LEDT; or Effective-LEDT in a randomized double-blind placebo-controlled trial with washout period of 7 d between each test. LEDT improved the speed of the muscular VO2 adaptation (∼-9 s), decreased O2 deficit (∼-10 L), increased the VO2 from the slow component phase (∼+348 ml min(-1)), and increased the time limit of exercise (∼+589 s). LEDT decreased blood and urine markers of muscle damage and fatigue (CK, alanine and lactate levels). The results suggest that a muscular pre-conditioning regimen using LEDT before intense exercises could modulate metabolic and renal function to achieve better performance.

An investigation into the relationship between age and physiological function in highly active older adults

KEY POINTS

The relationship between age and physiological function remains poorly defined and there are no physiological markers that can be used to reliably predict the age of an individual. This could be due to a variety of confounding genetic and lifestyle factors, and in particular to ill-defined and low levels of physical activity. This study assessed the relationship between age and a diverse range of physiological functions in a cohort of highly active older individuals (cyclists) aged 55-79 years in whom the effects of lifestyle factors would be ameliorated. Significant associations between age and function were observed for many functions. V̇O2max was most closely associated with age, but even here the variance in age for any given level was high, precluding the clear identification of the age of any individual. The data suggest that the relationship between human ageing and physiological function is highly individualistic and modified by inactivity.

ABSTRACT

Despite extensive research, the relationship between age and physiological function remains poorly characterised and there are currently no reliable markers of human ageing. This is probably due to a number of confounding factors, particularly in studies of a cross-sectional nature. These include inter-subject genetic variation, as well as inter-generational differences in nutrition, healthcare and insufficient levels of physical activity as well as other environmental factors. We have studied a cohort of highly and homogeneously active older male (n = 84) and female (n = 41) cyclists aged 55-79 years who it is proposed represent a model for the study of human ageing free from the majority of confounding factors, especially inactivity. The aim of the study was to identify physiological markers of ageing by assessing the relationship between function and age across a wide range of indices. Each participant underwent a detailed physiological profiling which included measures of cardiovascular, respiratory, neuromuscular, metabolic, endocrine and cognitive functions, bone strength, and health and well-being. Significant associations between age and function were observed for many functions. The maximal rate of oxygen consumption (V̇O2max) showed the closest association with age (r = -0.443 to -0.664; P < 0.001), but even here the variance in age for any given level was high, precluding the clear identification of the age of any individual. The results of this cross-sectional study suggest that even when many confounding variables are removed the relationship between function and healthy ageing is complex and likely to be highly individualistic and that physical activity levels must be taken into account in ageing studies.

Cardiovascular control during exercise in type 2 diabetes mellitus

Controlled studies of male and female subjects with type 2 diabetes mellitus (DM) of short duration (~3-5 years) show that DM reduces peak VO2 (L·min(-1) and mL·kg(-1)·min(-1)) by an average of 12-15% and induces a greater slowing of the dynamic response of pulmonary VO2 during submaximal exercise. These effects occur in individuals less than 60 years of age but are reduced or absent in older males and are consistently associated with significant increases in the exercise pressor response despite normal resting blood pressure. This exaggerated pressor response, evidence of exertional hypertension in DM, is manifest during moderate submaximal exercise and coincides with a more constrained vasodilation in contracting muscles. Maximum vasodilation during contractions involving single muscle groups is reduced by DM, and the dynamic response of vasodilation during submaximal contractions is slowed. Such vascular constraint most likely contributes to exertional hypertension, impairs dynamic and peak VO2 responses, and reduces exercise tolerance. There is a need to establish the effect of DM on dynamic aspects of vascular control in skeletal muscle during whole-body exercise and to clarify contributions of altered cardiovascular control and increased arterial stiffness to exertional hypertension.

A critical review of the history of low- to moderate-intensity steady-state VO2 kinetics

Research into the rate of whole-body oxygen consumption (VO2) kinetics during exercise increments to low- to moderate-intensity steady-state exercise was originally based on the theory of linear first-order VO2 kinetics, implying that the VO2 response to steady-state exercise increments is a mono-exponential response of the same time constant (tau, τ) across all steady-state intensities. Despite the acceptance of this theory for more than 30 years, early research from the 1980s documented an increasing τ with increasing steady-state exercise intensity, and recent research has confirmed such results. Today, such evidence has led to retraction of the theory of linear first-order VO2 kinetics. This history, revealing the premature acceptance of a theory, and subsequent scientific investigation using improved research design, instrumentation and data processing, has important implications for the fragility of scientific theories and the need for continual testing of theories in the search for facts and not prematurely accepted constructs. This review provides historical evidence for a critical reappraisal of the theory of linear first-order VO2 kinetics and presents data to show the need for changes in the data-processing 'standards' of the discipline to improve measurement of instantaneous VO2 kinetics and the time to steady state. For example, to date, no study of VO2 kinetics has quantified and statistically analysed the time to steady state. Furthermore, the instability of τ across different exercise increments, and for the same increment from different baseline VO2 demand, prevents τ from being a valid measure of VO2 kinetics for different exercise conditions. The concept of quantifying kinetics from a total non-linear response, when no other field of kinetics pursues this methodology, also raises concern for the methods and models used to interpret steady-state VO2 kinetics.

Oxygen kinetics during 6-minute walk tests in patients with cardiovascular and pulmonary disease

BACKGROUND

The 6-Minute Walk Test (6MWT) is representative of daily-life activities and reflects the functional capacity of patients. The change of oxygen uptake (VO2) in the initial phase of low-intensity exercise (VO2 kinetics) can be used to assess submaximal exercise performance of patients.The objective of the following study was to analyse VO2 kinetics in patients with different pulmonary and cardiovascular diseases. In addition, we investigated the extent to which VO2 kinetics at the onset of the 6MWT were associated with exercise capacity, morbidity and mortality.

METHODS

VO2 kinetics of 204 patients and 16 healthy controls were obtained using mobile telemetric cardiopulmonary monitoring during a 6MWT. A new mean response time (MRT) index (wMRT) was developed to quantify VO2 kinetics by correcting MRT for work rate. The differences in wMRT between disease categories as well as the association between wMRT and patients' exercise capacity and outcome - time to hospitalization/death- were tested.

RESULTS

The assessment of a robust wMRT was feasible in 86% (244/284) patients. wMRT was increased in patients compared to healthy controls (p <0.001). wMRT was largest in patients with pulmonary arterial hypertension (PAH). There were significant associations between wMRT and exercise capacity in all patients. High wMRT was found to be associated with a high rate of death and re-hospitalization in patients with CHF (p = 0.024). In patients with pulmonary diseases and pulmonary hypertension wMRT was not associated with outcome (p = 0.952).

CONCLUSIONS

Submaximal exercise performance of patients is reduced. O2 kinetics at the onset of exercise are associated with exercise capacity in all patients. wMRT was found to be an important prognostic factor in patients with congestive heart failure (CHF), but not with pulmonary diseases.

Skeletal muscle ATP turnover by 31P magnetic resonance spectroscopy during moderate and heavy bilateral knee extension

During constant-power high-intensity exercise, the expected increase in oxygen uptake (V̇O2) is supplemented by a V̇O2 slow component (V̇O2 sc ), reflecting reduced work efficiency, predominantly within the locomotor muscles. The intracellular source of inefficiency is postulated to be an increase in the ATP cost of power production (an increase in P/W). To test this hypothesis, we measured intramuscular ATP turnover with (31)P magnetic resonance spectroscopy (MRS) and whole-body V̇O2 during moderate (MOD) and heavy (HVY) bilateral knee-extension exercise in healthy participants (n = 14). Unlocalized (31)P spectra were collected from the quadriceps throughout using a dual-tuned ((1)H and (31)P) surface coil with a simple pulse-and-acquire sequence. Total ATP turnover rate (ATPtot) was estimated at exercise cessation from direct measurements of the dynamics of phosphocreatine (PCr) and proton handling. Between 3 and 8 min during MOD, there was no discernable V̇O2 sc (mean ± SD, 0.06 ± 0.12 l min(-1)) or change in [PCr] (30 ± 8 vs. 32 ± 7 mm) or ATPtot (24 ± 14 vs. 17 ± 14 mm min(-1); each P = n.s.). During HVY, the V̇O2 sc was 0.37 ± 0.16 l min(-1) (22 ± 8%), [PCr] decreased (19 ± 7 vs. 18 ± 7 mm, or 12 ± 15%; P < 0.05) and ATPtot increased (38 ± 16 vs. 44 ± 14 mm min(-1), or 26 ± 30%; P < 0.05) between 3 and 8 min. However, the increase in ATPtot (ΔATPtot) was not correlated with the V̇O2 sc during HVY (r(2) = 0.06; P = n.s.). This lack of relationship between ΔATPtot and V̇O2 sc , together with a steepening of the [PCr]-V̇O2 relationship in HVY, suggests that reduced work efficiency during heavy exercise arises from both contractile (P/W) and mitochondrial sources (the O2 cost of ATP resynthesis; P/O).