“Linear” Versus “Nonlinear” O2 Responses to Exercise: Reshaping Traditional Beliefs

A number of basic tenets in traditional exercise physiology have been formulated on the assumption that pulmonary oxygen uptake (O2) adapts to changes in metabolic rate with linear, first-order response kinetics. However, questions regarding this premise have been raised for over half a century and clear contradictions have been reported. Specifically, Boltzmann's principle of superposition that defines linearity is violated for exercise transitions of different magnitudes, and the symmetry between on- and off-responses that first-order kinetics implies is not always present. Furthermore, a single exponential model does not adequately describe the O2 response to high-intensity exercise because a supplementary response compartment of delayed onset is manifest. Collectively, these findings reflect a range of nonlinear behaviors that indicate greater complexity of the O2 response, and it is imperative that these deviations be universally recognized, both to reshape our interpretation of the acute metabolic adaptation to exercise and also to provide clues regarding cellular mechanisms of respiratory control

Influence of hyperoxia on muscle metabolic responses and the power-duration relationship during severe-intensity exercise in humans: a 31P magnetic resonance spectroscopy study

addresses: School of Sport and Health Sciences, St Luke's Campus, University of Exeter, Exeter EX1 2LU, UK.types: Journal Article; Randomized Controlled TrialThis is the author's post-print version of an article published in Experimental Physiology, 2010, Vol. 95, Issue 4, pp. 528 – 540 Copyright © 2010 Wiley-Blackwell /The Physiological Society. The definitive version is available at www3.interscience.wiley.comSevere-intensity constant-work-rate exercise results in the attainment of maximal oxygen uptake, but the muscle metabolic milieu at the limit of tolerance (T(lim)) for such exercise remains to be elucidated. We hypothesized that T(lim) during severe-intensity exercise would be associated with the attainment of consistently low values of intramuscular phosphocreatine ([PCr]) and pH, as determined using (31)P magnetic resonance spectroscopy, irrespective of the work rate and the inspired O(2) fraction. We also hypothesized that hyperoxia would increase the asymptote of the hyperbolic power-duration relationship (the critical power, CP) without altering the curvature constant (W). Seven subjects (mean +/- s.d., age 30 +/- 9 years) completed four constant-work-rate knee-extension exercise bouts to the limit of tolerance (range, 3-10 min) both in normoxia (N) and in hyperoxia (H; 70% O(2)) inside the bore of 1.5 T superconducting magnet. The [PCr] (approximately 5-10% of resting baseline) and pH (approximately 6.65) at the limit of tolerance during each of the four trials was not significantly different either in normoxia or in hyperoxia. At the same fixed work rate, the overall rate at which [PCr] fell with time was attenuated in hyperoxia (mean response time: N, 59 +/- 20 versus H, 116 +/- 46 s; P < 0.05). The CP was higher (N, 16.1 +/- 2.6 versus H, 18.0 +/- 2.3 W; P < 0.05) and the W was lower (N, 1.92 +/- 0.70 versus H, 1.48 +/- 0.31 kJ; P < 0.05) in hyperoxia compared with normoxia. These data indicate that T(lim) during severe-intensity exercise is associated with the attainment of consistently low values of muscle [PCr] and pH. The CP and W parameters of the power-duration relationship were both sensitive to the inspiration of hyperoxic gas

Effects of priming and pacing strategy on VO2 kinetics and cycling performance

Copyright © 2015 Human KineticsThis is the author accepted manuscript. The final version is available from Human Kinetics via the DOI in this record.Purpose: To assess whether combining prior ‘priming’ exercise with an all-out pacing strategy was more effective at improving O2 uptake (VO2) kinetics and cycling performance than either intervention administered independently. Methods: Nine males completed target-work cycling performance trials using a self-paced or all-out pacing strategy with or without prior severe-intensity (70%Δ) priming exercise. Breath-by-breath pulmonary VO2 and cycling power output were measured during all trials. Results: Compared to the self-paced-unprimed control trial (22 ± 5 s), the VO2 mean response time (MRT) was shorter (VO2 kinetics was faster) with all-out pacing (17 ± 4 s) and priming (17 ± 3 s), with the lowest VO2 MRT observed when all-out pacing and priming were combined (15 ± 4 s) (P0.05). Conclusions: These findings suggest that combining an all-out start with severe-intensity priming exercise additively improves the VO2 MRT, but not total O2 consumption and cycling performance since these were improved by a similar magnitude in both primed trials relative to the self-paced-unprimed control condition. Therefore, these results support the use of priming exercise as a pre-competition intervention to improve oxidative metabolism and performance during short-duration high-intensity cycling exercise, independent of the pacing strategy adopted

Inorganic nitrate supplementation improves muscle oxygenation, O2 uptake kinetics and exercise tolerance at high but not low pedal rates

Copyright © 2014, Journal of Applied PhysiologyWe tested the hypothesis that inorganic nitrate (NO3-) supplementation would improve muscle oxygenation, pulmonary O2 uptake (VO2) kinetics and exercise tolerance (Tlim) to a greater extent when cycling at high compared low pedal rates. In a randomized, placebo-controlled, cross-over study, seven subjects (mean ± SD, age 21 ± 2 yr, body mass 86 ± 10 kg) completed severe-intensity step cycle tests at pedal cadences of 35 rpm and 115 rpm during separate 9 day supplementation periods with NO3--rich beetroot juice (BR; providing 8.4 mmol NO3-∙day-1) and placebo (PLA). Compared to PLA, plasma nitrite concentration increased 178% with BR (P0.05). However, when cycling at 115 rpm, muscle [O2Hb] was higher at baseline and throughout exercise, phase II VO2 kinetics was faster (47 ± 16 s vs. 61 ± 25 s; P<0.05) and Tlim was greater (362 ± 137 s vs. 297 ± 79 s; P<0.05) with BR compared to PLA. These results suggest that short-term BR supplementation can increase muscle oxygenation, expedite the adjustment of oxidative metabolism and enhance exercise tolerance when cycling at a high, but not a low, pedal cadence in healthy recreationally-active subjects. These findings support recent observations that NO3- supplementation may be particularly effective at improving physiological and functional responses in type II muscle fibers

Effect of heavy-intensity 'priming' exercise on oxygen uptake and muscle deoxygenation kinetics during moderate-intensity step-transitions initiated from an elevated work rate

We examined the effect of heavy-intensity ‘priming’ exercise on the rate of adjustment of pulmonary O2 uptake (τ 2p) initiated from elevated intensities. Fourteen men (separated into two groups: τ 2p≤25s [Fast] or τ 2p>25s [Slow]) completed step-transitions from 20W-to- 45%lactate threshold (LT; lower-step, LS) and 45%-to-90%LT (upper-step, US) performed (i) without; and (ii) with US preceded by heavy-intensity exercise (HUS). Breath-by-breath 2p and near-infrared spectroscopy-derived muscle deoxygenation ([HHb+Mb]) were measured. Compared to LS, τ 2p was greater (p0.05) from LS or Fast group US. In Slow, τ[HHb+Mb] increased (p<0.05) in US relative to HUS; this finding coupled with a reduced τ 2p indicates a priming-induced improvement in matching of muscle O2 delivery-to-O2 utilization during transitions from elevated intensities in those with Slow but not Fast 2p kinetics

Two-Scale Kirchhoff Theory: Comparison of Experimental Observations With Theoretical Prediction

We introduce a non-perturbative two scale Kirchhoff theory, in the context of light scattering by a rough surface. This is a two scale theory which considers the roughness both in the wavelength scale (small scale) and in the scales much larger than the wavelength of the incident light (large scale). The theory can precisely explain the small peaks which appear at certain scattering angles. These peaks can not be explained by one scale theories. The theory was assessed by calculating the light scattering profiles using the Atomic Force Microscope (AFM) images, as well as surface profilometer scans of a rough surface, and comparing the results with experiments. The theory is in good agreement with the experimental results.Comment: 6 pages, 8 figure

The effect of baseline metabolic rate on pulmonary O₂ uptake kinetics during very heavy intensity exercise in boys and men

addresses: Children's Health and Exercise Research Centre, College of Life and Environmental Sciences, University of Exeter, UK.Copyright © 2012 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Respiratory Physiology and Neurobiology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Respiratory Physiology and Neurobiology, 2012, 180 (2-3), pp. 223 – 229 DOI: 10.1016/j.resp.2011.11.013This study tested the hypothesis that pulmonary VO₂ kinetics would be slowed during 'work-to-work' exercise in adults but not in children. Eight boys (mean age=12.5 ± 0.5 years) and nine men completed very heavy step transitions initiated from either 'unloaded' pedalling (U→VH) or unloaded-to-moderate cycling (i.e. U→M to M→VH). The phase II τ was significantly (p<0.05) lengthened in M→VH compared to U→M and U→VH in boys (30 ± 5 vs. 19 ± 5 vs. 21 ± 5 s) and men (49 ± 14 vs. 30 ± 5 vs. 34 ± 8 s). In U→VH, a greater relative VO₂ slow component temporally coincided with an increased linear iEMG slope in men compared boys (VO₂ slow component: 16 ± 3 vs. 11 ± 4%; iEMG slope: 0.19 ± 0.24 vs. -0.06 ± 0.14%, p<0.05). These results suggest that an age-linked modulation of VO₂ kinetics might be influenced by alterations in muscle fibre recruitment following the onset of exercise

A Rapidly Incremented Tethered-Swimming Maximal Protocol for Cardiorespiratory Assessment of Swimmers

Incremental exercise testing is the standard means of assessing cardiorespiratory capacity of endurance athletes. While the maximal rate of oxygen consumption is typically used as the criterion measurement in this regard, two metabolic breakpoints that reflect changes in the dynamics of lactate production/consumption as the work rate is increased are perhaps more relevant for endurance athletes from a functional standpoint. Exercise economy, which represents the rate of oxygen consumption relative to performance of submaximal work, is also an important parameter to measure for endurance-athlete assessment. Ramp incremental tests comprising a gradual but rapid increase in work rate until the limit of exercise tolerance is reached are useful for determining these parameters. This type of test is typically performed on a cycle ergometer or treadmill because there is a need for precision with respect to work-rate incrementation. However, athletes should be tested while performing the mode of exercise required for their sport. Consequently, swimmers are typically assessed during free-swimming incremental tests where such precision is difficult to achieve. We have recently suggested that stationary swimming against a load that is progressively increased (incremental tethered swimming) can serve as a "swim ergometer" by allowing sufficient precision to accommodate a gradual but rapid loading pattern that reveals the aforementioned metabolic breakpoints and exercise economy. However, the degree to which the peak rate of oxygen consumption achieved during such a protocol approximates the maximal rate that is measured during free swimming remains to be determined. In the present article, we explain how this rapidly incremented tethered-swimming protocol can be employed to assess the cardiorespiratory capacity of a swimmer. Specifically, we explain how assessment of a short-distance competitive swimmer using this protocol revealed that his rate of oxygen uptake was 30.3 and 34.8 mL∙min 1∙kg-1BM at his gas-exchange threshold and respiratory compensation point, respectively.info:eu-repo/semantics/publishedVersio

‘Priming’ exercise and O2 uptake kinetics during treadmill running

We tested the hypothesis that priming exercise would speed kinetics during treadmill running. Eight subjects completed a square-wave protocol, involving two bouts of treadmill running at 70% of the difference between the running speeds at lactate threshold (LT) and max, separated by 6-min of walking at 4 km h−1, on two occasions. Oxygen uptake was measured breath-by-breath and subsequently modelled using non-linear regression techniques. Heart rate and blood lactate concentration were significantly elevated prior to the second exercise bout compared to the first. However, kinetics was not significantly different between the first and second exercise bouts (mean ± S.D., phase II time constant, Bout 1: 16 ± 3 s vs. Bout 2: 16 ± 4 s; slow component amplitude, Bout 1: 0.24 ± 0.10 L min−1vs. Bout 2: 0.20 ± 0.12 L min−1; mean response time, Bout 1: 34 ± 4 s vs. Bout 2: 34 ± 6 s; P > 0.05 for all comparisons). These results indicate that, contrary to previous findings with other exercise modalities, priming exercise does not alter kinetics during high-intensity treadmill running, at least in physically active young subjects. We speculate that the relatively fast kinetics and the relatively small slow component in the control (‘un-primed’) condition negated any enhancement of kinetics by priming exercise in this exercise modality

Muscle Oxygen Changes following Sprint Interval Cycling Training in Elite Field Hockey Players

This study examined the effects of Sprint Interval Cycling (SIT) on muscle oxygenation kinetics and performance during the 30-15 intermittent fitness test (IFT). Twenty-five women hockey players of Olympic standard were randomly selected into an experimental group (EXP) and a control group (CON). The EXP group performed six additional SIT sessions over six weeks in addition to their normal training program. To explore the potential training-induced change, EXP subjects additionally completed 5 x 30s maximal intensity cycle testing before and after training. During these tests near-infrared spectroscopy (NIRS) measured parameters; oxyhaemoglobin + oxymyoglobin (HbO2+ MbO2), tissue deoxyhaemoglobin + deoxymyoglobin (HHb+HMb), total tissue haemoglobin (tHb) and tissue oxygenation (TSI %) were taken. In the EXP group (5.34±0.14 to 5.50±0.14m.s-1) but not the CON group (pre = 5.37± 0.27 to 5.39±0.30m.s-1) significant changes were seen in the 30-15IFTperformance. EXP group also displayed significant post-training increases during the sprint cycling: ΔTSI (-7.59±0.91 to -12.16±2.70%); ΔHHb+HMb (35.68±6.67 to 69.44 ±26.48μM.cm); and ΔHbO2+ MbO2 (-74.29±13.82 to -109.36±22.61μM.cm). No significant differences were seen in ΔtHb (-45.81±15.23 to -42.93±16.24). NIRS is able to detect positive peripheral muscle oxygenation changes when used during a SIT protocol which has been shown to be an effective training modality within elite athletes