Pulmonary O2 uptake kinetics and motor unit recruitment in young people

The primary objective of this thesis was to examine the influence of alterations in muscle recruitment on pulmonary O2 uptake (VO2) kinetics during exercise above the gas exchange threshold (GET) in young people. In the first experimental chapter, the phase II time constant (τ) slowed over a 2-yr period in 14-16 yr old boys (25 ± 5 s vs. 30 ± 5 s, P = 0.002) and there was a greater relative VO2 slow component amplitude (Rel. A’2 (%)] during heavy-intensity exercise (9 ± 5% vs. 13 ± 4%, P = 0.036). In the second study, ‘work-to-work’ transitions yielded similar phase II VO2 kinetics during unloaded-to-moderate exercise (U→M) between 11-12 yr old boys and teenagers (19 ± 5 s vs. 22 ± 7 s, P = 0.32) but the phase II τ was significantly lengthened in the latter group at the onset of moderate-to-very heavy exercise (M→VH: 30 ± 5 s vs. 45 ± 11 s, P = 0.011). There were no differences in the phase II τ between teenagers and adult men during M→VH exercise (P = 0.46). In the third study, increasing pedal rate from 50 rev•min-1 to 115 rev•min-1 significantly (P 0.05). The fourth study reported that a reduced relative VO2 slow component amplitude in younger boys compared to men (11 ± 4% vs. 16 ± 3%, P = 0.015) coincided with a lower percentage change in the integrated electromyogram (iEMG) of the m. vastus lateralis from minute 2 to minute 6 of exercise (ΔiEMG6-2: 7 ± 25% vs. 49 ± 48%, P = 0.030), suggesting that alterations in motor unit recruitment might be involved in restricting the O2 cost of exercise above the primary amplitude in children compared to adults. The final experimental chapter tested this hypothesis, but no statistically significant differences were reported for the relative VO2 slow component amplitude between 10-12 yr old boys and men (15 ± 7% vs. 19 ± 4%, P = 0.145). In boys, an excess VO2 temporally coincided with a significant increase in the transverse relaxation time (T2) of the m. vastus lateralis from the VO2 slow component time delay (SCtd) to minute 6 of exercise (41.5 ± 2.4 ms vs. 45.2 ± 2.3 ms, P = 0.001), thereby consistent with the notion that delayed muscle fibre activation might contribute to the development of the VO2 slow component in youth. In conclusion, this thesis has demonstrated that maturational changes in the VO2 kinetic response to heavy-intensity exercise are extended into adolescence. During intense submaximal exercise, the recruitment of higher-order (type II) muscle fibres might be principally involved in modulating VO2 kinetics as children mature but this effect is attenuated in teenage subjects engaged in regular endurance training

Longitudinal Changes in the Oxygen Uptake Kinetic Response to Heavy-Intensity Exercise in 14- to 16-Year-Old Boys

There is another ORE record for this publication: http://hdl.handle.net/10036/3830This study examined longitudinal changes in the pulmonary oxygen uptake (p(V) over dotO(2)) kinetic response to heavy-intensity exercise in 14-16 yr old boys. Fourteen healthy boys (age 14.1 +/- 0.2 yr) completed exercise testing on two occasions with a 2-yr interval. Each participant completed a minimum of three 'step' exercise transitions, from unloaded pedalling to a constant work rate corresponding to 40% of the difference between the (p(V) over dotO(2)) (2), at the gas exchange threshold and peak (p(V) over dotO(2)) , (40% A). Over the 2-yr period a significant increase in the phase II time constant (25 5 vs. 30 +/- 5 s; p = .002, omega(2) = 0.34), the relative amplitude of the (p(V) over dotO(2)) slow component (9 +/- 5 vs. 13 +/- 4%; p = .036, omega(2) = 0.14) and the(p(V) over dotO(2)) gain at end-exercise (11.6 +/- 0.6 vs. 12.4 +/- 0.7 mL.min(-1).W-1; p < .001, omega(2) = 0.42) were observed. These data indicate that the control of oxidative phosphorylation in response to heavy-intensity cycling exercise is age-dependent in teenage boys

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

The effect of dietary nitrate supplementation on the spatial heterogeneity of quadriceps deoxygenation during heavy-intensity cycling

This study investigated the influence of dietary inorganic nitrate (NO3-) supplementation on pulmonary O2 uptake ( o2) and muscle deoxyhemoglobin/myoglobin (i.e. deoxy[Hb+Mb]) kinetics during submaximal cycling exercise. In a randomized, placebo-controlled, cross-over study, eight healthy and physically active male subjects completed multiple step cycle tests at a work rate equivalent to 50% of the difference between the gas exchange threshold and peak o2 over separate 4-day supplementation periods with NO3--rich (BR; providing 8.4 mmol NO3-∙day-1) and NO3--depleted (placebo; PLA) beetroot juice. Pulmonary o2 was measured breath-by-breath and time-resolved near-infrared spectroscopy was utilized to quantify absolute deoxy[Hb+Mb] and total[Hb+Mb] within the rectus femoris, vastus lateralis, and vastus medialis. There were no significant differences (P > 0.05) in the primary deoxy[Hb+Mb] mean response time or amplitude between the PLA and BR trials at each muscle site. BR significantly increased the mean (three-site) end-exercise deoxy[Hb+Mb] (PLA: 91 ± 9 vs. BR: 95 ± 12 µM, P < 0.05), with a tendency to increase the mean (three-site) area under the curve for total(Hb+Mb) responses (PLA: 3650 ± 1188 vs. BR: 4467 ± 1315 µM·s-1, P = 0.08). The o2 slow component reduction after BR supplementation (PLA: 0.27 ± 0.07 vs. BR: 0.23 ± 0.08 L·min-1, P = 0.07) correlated inversely with the mean increases in deoxy[Hb+Mb] and total[Hb+Mb] across the three muscle regions (r2 = 0.62 and 0.66, P < 0.05). Dietary NO3- supplementation increased O2 diffusive conductance across locomotor muscles in association with improved o2 dynamics during heavy-intensity cycling transitions

Beetroot juice supplementation speeds O2 uptake kinetics and improves exercise tolerance during severe-intensity exercise initiated from an elevated metabolic rate

Recent research has suggested that dietary nitrate (NO3-) supplementation might alter the physiological responses to exercise via specific effects on type II muscle. Severe-intensity exercise initiated from an elevated metabolic rate would be expected to enhance the proportional activation of higher-order (type II) muscle fibers. The purpose of this study was therefore to test the hypothesis that, compared to placebo (PL), NO3--rich beetroot juice (BR) supplementation would speed the phase II o2 kinetics (τp) and enhance exercise tolerance during severe-intensity exercise initiated from a baseline of moderate-intensity exercise. Nine healthy, physically-active subjects were assigned in a randomized, double-blind, crossover design to receive BR (140 mL/day, containing ~8 mmol of NO3-) and PL (140 mL/day, containing ~0.003 mmol of NO3-) for 6 days. On days 4, 5 and 6 of the supplementation periods, subjects completed a double-step exercise protocol that included transitions from unloaded-to-moderate intensity exercise (U→M) followed immediately by moderate-to-severe-intensity exercise (M→S). Compared to PL, BR elevated resting plasma nitrite concentration (PL: 65 ± 32 vs. BR: 348 ± 170 nM, P0.05). During M→S exercise, the faster o2 kinetics coincided with faster NIRS-derived muscle [deoxyhemoglobin] kinetics (τ; PL: 20 ± 9 vs. BR: 10 ± 3 s, P<0.05) and a 22% greater time-to-task failure (PL: 521 ± 158 vs. BR: 635 ± 258 s, P<0.05). Dietary supplementation with NO3--rich BR juice speeds o2 kinetics and enhances exercise tolerance during severe-intensity exercise when initiated from an elevated metabolic rate

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

Influence of dietary nitrate supplementation on local sweating and cutaneous vascular responses during exercise in a hot environment.

Purpose We investigated the influence of inorganic nitrate (NO−3) supplementation on local sweating and cutaneous vascular responses during exercise in hot conditions. Method Eight healthy, young subjects were assigned in a randomized, double-blind, crossover design to receive NO−3 -rich beetroot (BR) juice (140 mL/day, containing ~8 mmol of NO−3) and NO−3-depleted placebo (PL) juice (140 mL/day, containing ~0.003 mmol of NO−3) for 3 days. On day 3 of supplementation, subjects cycled at an intensity corresponding to 55% of V̇ O2max for 30 min in hot conditions (30 °C, 50% relative humidity). Chest and forearm sweat rate (SR) and skin blood flow (SkBF), were measured continuously. Cutaneous vascular conductance (CVC) was calculated by SkBF/mean arterial pressure (MAP). Results Prior to exercise, plasma NO− 3 (21±6 and 581±161 µM) and nitrite (NO− 2 , 87±28 and 336±156 nM) concentrations were higher after BR compared to PL supplementation (P≤0.011, n=6). Oesophageal, mean skin, and mean body temperatures during exercise were not different between conditions. In addition, BR supplementation did not affect SR, SkBF, and CVC during exercise. A lower MAP was found after 30 min of exercise following BR supplementation (112±6 and 103±6 mmHg for PL and BR, respectively, P=0.021). Conclusion These results suggest that inorganic NO− 3 supplementation, which increases the potential for O2-independent NO production, does not affect local sweating and cutaneous vascular responses, but attenuates blood pressure in young healthy subjects exercising in a hot environment

The effect of priming exercise on O2 uptake kinetics, muscle O2 delivery and utilization, muscle activity, and exercise tolerance in boys

Journal ArticleCopyright © Copyright 2014 – Canadian Science PublishingThis study used priming exercise in young boys to investigate (i) how muscle oxygen delivery and oxygen utilization, and muscle activity modulate oxygen uptake kinetics during exercise; and (ii) whether the accelerated oxygen uptake kinetics following priming exercise can improve exercise tolerance. Seven boys that were aged 11.3 ± 1.6 years completed either a single bout (bout 1) or repeated bouts with 6 min of recovery (bout 2) of very heavy-intensity cycling exercise. During the tests oxygen uptake, muscle oxygenation, muscle electrical activity and exercise tolerance were measured. Priming exercise most likely shortened the oxygen uptake mean response time (change, ±90% confidence limits; -8.0 s, ±3.0), possibly increased the phase II oxygen uptake amplitude (0.11 L·min(-1), ±0.09) and very likely reduced the oxygen uptake slow component amplitude (-0.08 L·min(-1), ±0.07). Priming resulted in a likely reduction in integrated electromyography (-24% baseline, ±21% and -25% baseline, ±19) and a very likely reduction in Δ deoxyhaemoglobin/Δoxygen uptake (-0.16, ±0.11 and -0.09, ±0.05) over the phase II and slow component portions of the oxygen uptake response, respectively. A correlation was present between the change in tissue oxygenation index during bout 2 and the change in the phase II (r = -0.72, likely negative) and slow component (r = 0.72, likely positive) oxygen uptake amplitudes following priming exercise, but not for muscle activity. Exercise tolerance was likely reduced (change -177 s, ±180) following priming exercise. The altered phase II and slow component oxygen uptake amplitudes in boys following priming exercise are linked to an improved localised matching of muscle oxygen delivery to oxygen uptake and not muscle electrical activity. Despite more rapid oxygen uptake kinetics following priming exercise, exercise tolerance was not enhanced

Influence of thigh muscle activation on pulmonary O2 uptake kinetics during very heavy intensity exercise in boys and men

PublishedArticleCopyright © Springer-Verlag Berlin Heidelberg 2014During constant work rate exercise above the lactate threshold (LT ), the initial rapid phase of pulmonary oxygen uptake (˙V O2) kinetics is supplemented by an additional ˙V O2 slow component (˙V O2Sc) which reduces the efficiency of muscular work. The ˙V O2Sc amplitude has been shown to increase with maturation but the mechanisms are poorly understood. We utilized the transverse relaxation time (T2) of muscle protons from magnetic resonance imaging (MRI) to test the hypothesis that a lower ˙V O2 slow component (˙V O2Sc) amplitude in children would be associated with a reduced muscle recruitment compared to adults. Methods Eight boys (mean age 11.4 ± 0.4) and eight men (mean age 25.3 ± 3.3 years) completed repeated step transitions of unloaded-to-very heavy-intensity (U → VH) exercise on a cycle ergometer. MRI scans of the thigh region were acquired at rest and after VH exercise up to the ˙V O2Sc time delay (ScTD) and after 6 min. T2 for each of eight muscles was adjusted in relation to cross-sectional area and then summed to provide the area-weighted ΣT2 as an index of thigh recruitment. Results T here were no child/adult differences in the relative ˙VO2Sc amplitude [Boys 14 ± 7 vs. Men 18 ± 3 %, P = 0.15, effect size (ES) = 0.8] during which the change (Δ) in area-weighted ΣT2 between the ScTD and 6 min was not different between groups (Boys 1.6 ± 1.2 vs. Men 2.3 ± 1.1 ms, P = 0.27, ES = 0.6). A positive and strong correlation was found between the relative ˙V O2Sc amplitude and the magnitude of the area-weighted ΔΣT2 in men (r = 0.92, P = 0.001) but not in boys (r = 0.09, P = 0.84). Conclusions T his study provides evidence to show that progressive muscle recruitment (as inferred from T2 changes) contributes to the development of the ˙V O2Sc during intense submaximal exercise independent of age.NIHR Clinical Research Facilit