Algorithms,modelling and $$ \dot{V}{\text{O}}_{ 2} $$ kinetics

This article summarises the pros and cons of different algorithms developed for estimating breath-by-breath (B-by-B) alveolar O₂ transfer (\( \dot{V}{\text{O}}_{{ 2 {\text{A}}}} \)) in humans. \( \dot{V}{\text{O}}_{{ 2 {\text{A}}}} \) is the difference between O₂ uptake at the mouth and changes in alveolar O₂ stores (?VO_2s), which for any given breath, are equal to the alveolar volume change at constant \( F_{{{\text{AO}}_{ 2} }} [ (F_{{{\text{A}}i{\text{O}}_{ 2} }} \Updelta {\text{V}}_{{{\text{A}}i}} ) ] \) plus the O₂ alveolar fraction change at constant volume \( [V_{{{\text{A}}i - 1}} (F_{{{\text{A}}i}} -F_{{{\text{A}}i - 1}} )_{{{\text{O}}_{ 2} }} ] \), where V _Ai?1 is the alveolar volume at the beginning of a breath. Therefore, \( \dot{V}{\text{O}}_{{ 2 {\text{A}}}} \) can be determined B-by-B provided that V _Ai?1 is: (a) set equal to the subject’s functional residual capacity (algorithm of Auchincloss, A) or to zero; (b) measured (optoelectronic plethysmography, OEP); (c) selected according to a procedure that minimises B-by-B variability (algorithm of Busso and Robbins, BR). Alternatively, the respiratory cycle can be redefined as the time between equal FO₂ in two subsequent breaths (algorithm of Grønlund, G), making any assumption of V _Ai?1 unnecessary. All the above methods allow an unbiased estimate of \( \dot{V}{\text{O}}_{ 2} \) at steady state, albeit with different precision. Yet the algorithms “per se” affect the parameters describing the B-by-B kinetics during exercise transitions. Among these approaches, BR and G, by increasing the signal-to-noise ratio of the measurements, reduce the number of exercise repetitions necessary to study \( \dot{V}{\text{O}}_{ 2} \) kinetics, compared to A approach. OEP and G (though technically challenging and conceptually still debated), thanks to their ability to track ?VO_2s changes during the early phase of exercise transitions, appear rather promising for investigating B-by-B gas exchange.     

Faster \dot{V}{\text{O}}_{ 2} kinetics after eccentric contractions is explained by better matching of O2 delivery to O2 utilization

Purpose

This study examined the impact of eccentric exercise-induced muscle damage on the rate of adjustment in muscle deoxygenation and pulmonary O₂ uptake ( \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) ) kinetics during moderate exercise.

Methods

Fourteen males (25 ± 3 year; mean ± SD) completed three step transitions to 90 % θ_L before (Pre), 24 h (Post24) and 48 h after (Post48) eccentric exercise (100 eccentric leg-press repetitions with a load corresponding to 110 % of the participant’s concentric 1RM). Participants were separated into two groups: phase II \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) time constant (τ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) ) ≤ 25 s (fast group; n = 7) or τ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\)  > 25 s (slow group; n = 7). \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) and [HHb] responses were modeled as a mono-exponential.

Results

In both groups, isometric peak torque (0°/s) at Post24 was decreased compared to Pre (p < 0.05) and remained depressed at Post48 (p < 0.05). τ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) was designed to be different (p < 0.05) at Pre between the Fast (τ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) ; 19 ± 4 s) and Slow (32 ± 6 s) groups. There were no differences among time points (τ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) : Pre, 19 ± 4 s; Post24, 22 ± 3 s; Post48, 20 ± 4 s) in the Fast group. In Slow, there was a speeding (p < 0.05) from the Pre (32 ± 6 s) to the Post24 (25 ± 6) but not Post48 (31 ± 6), resulting in no difference (p > 0.05) between groups at Post24. This reduction of τ \(\dot{V}{\text{O}}_{{2{\text{p}}}} \,\) was concomitant with the abolishment (p < 0.05) of an overshoot in the [HHb]/ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) ratio.

Conclusion

We propose that the sped \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) kinetics observed in the Slow group coupled with an improved [HHb]/ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) ratio suggest a better matching of local muscle O₂ delivery to O₂ utilization following eccentric contractions.     

Kinetics of cardiac myosin isoforms in mouse myocardium are affected differently by presence of myosin binding protein-C

We tested whether cardiac myosin binding protein-C (cMyBP-C) affects myosin cross-bridge kinetics in the two cardiac myosin heavy chain (MyHC) isoforms. Mice lacking cMyBP-C (t/t) and transgenic controls \(( {\text{WT}}^{\text{t/t}} )\) were fed l-thyroxine (T4) to induce 90/10 % expression of α/β-MyHC. Non-transgenic (NTG) and t/t mice were fed 6-n-propyl-2-thiouracil (PTU) to induce 100 % expression of β-MyHC. Ca²⁺-activated, chemically-skinned myocardium underwent length perturbation analysis with varying [MgATP] to estimate the MgADP release rate \(\left( {k_{ - ADP} } \right)\) and MgATP binding rate \(\left( {k_{ + ATP} } \right)\). Values for \(k_{ - ADP}\) were not significantly different between \({\text{t/t}}_{\text{T4}}\) (102.2 ± 7.0 s^?1) and \({\text{WT}}^{\text{t/t}}_{\text{T4}}\) (91.3 ± 8.9 s^?1), but \(k_{ + ATP}\) was lower in \({\text{t/t}}_{\text{T4}}\) (165.9 ± 12.5 mM^?1 s^?1) compared to \({\text{WT}}^{\text{t/t}}_{\text{T4}}\) (298.6 ± 15.7 mM^?1 s^?1, P < 0.01). In myocardium expressing β-MyHC, values for \(k_{ - ADP}\) were higher in \({\text{t/t}}_{\text{PTU}}\) (24.8 ± 1.0 s^?1) compared to \({\text{NTG}}_{\text{PTU}}\) (15.6 ± 1.3 s^?1, P < 0.01), and \(k_{ + ATP}\) was not different. At saturating [MgATP], myosin detachment rate approximates \(k_{ - ADP}\), and detachment rate decreased as sarcomere length (SL) was increased in both \({\text{t/t}}_{\text{T4}}\) and \({\text{WT}}^{\text{t/t}}_{\text{T4}}\) with similar sensitivities to SL. In myocardium expressing β-MyHC, detachment rate decreased more as SL increased in \({\text{t/t}}_{\text{PTU}}\) (21.5 ± 1.3 s^?1 at 2.2 μm and 13.3 ± 0.9 s^?1 at 3.3 μm) compared to \({\text{NTG}}_{\text{PTU}}\) (15.8 ± 0.3 s^?1 at 2.2 μm and 10.9 ± 0.3 s^?1 at 3.3 μm) as detected by repeated-measures ANOVA (P < 0.01). These findings suggest that cMyBP-C reduces MgADP release rate for β-MyHC, but not for α-MyHC, even as the number of cMyBP-C that overlap with the thin filament is reduced to zero. Therefore, cMyBP-C appears to affect β-MyHC kinetics independent of its interaction with the thin filament.     

Algorithm to improve accuracy of energy expended in a room calorimeter

The whole-room indirect calorimeter is considered as important equipment for human energy expenditure measurement, but noise reduction in the system remains a challenge. A selective filtering method (SFM) was designed to improve the accuracy of the computation of O₂ consumption rate (\( \dot{V}_{{{\text{O}}_{ 2} }} \)) and CO₂ production rate (\( \dot{V}_{{{\text{CO}}_{ 2} }} \)), based on two facts: (1) the rapid changes of \( \dot{V}_{{{\text{O}}_{ 2} }} \), \( \dot{V}_{{{\text{CO}}_{ 2} }} \) and respiratory quotient (RQ) in human should be accompanied by physical activity; (2) the oxygen consumption and the carbon dioxide production should not be negative because living humans do not generate oxygen, nor consume carbon dioxide. The performance of SFM was compared with the moving average method, the central difference method and the wavelet de-noising method. The range of \( \dot{V}_{{{\text{O}}_{ 2} }} \) and \( \dot{V}_{{{\text{CO}}_{ 2} }} \) in the empty room (the background noise) is reduced from ?130.00–146.00 ml/min to ?26.00–24.00 ml/min, and from ?20.50–12.50 ml/min to ?3.99–4.19 ml/min, by SFM. The background noise was added to simulated rectangular and sinusoidal signals that were used to evaluate the four methods over different time periods (64, 32, 16 and 8 min). The highest signal-to-noise ratio and the lowest deviation were achieved by SFM. Abnormal metabolic rates and RQs were corrected and compensated with measurement accuracy of 98.51 ± 0.3 % for 24-h alcohol burning tests. The results of the study showed that SFM can significantly improve \( \dot{V}_{{\text{O}_{2} }} \) and \( \dot{V}_{{{\text{CO}}_{ 2} }} \) measurements.     

Oxygen uptake at different intensities and sub-techniques predicts sprint performance in elite male cross-country skiers

Purpose

To investigate the relationship between sprint-prologue performance (using the classical technique) and the oxygen uptake at the lactate threshold (\( {\dot{\text V}{\rm O}} \) ₂obla), maximal oxygen uptake (\( {\dot{\text V}{\rm O}} \) ₂max), and mean oxygen uptake during double poling (\( {\dot{\text V}{\rm O}} \) ₂dp).

Methods

Eight elite male cross-country skiers [age 24.8 ± 4.8 years, (mean ± SD)] completed two treadmill roller-skiing tests using the diagonal-stride technique and a 60 s double-poling test on a ski-ergometer to determine their \( {\dot{\text V}{\rm O}} \) ₂obla, \( {\dot{\text V}{\rm O}} \) ₂max, and \( {\dot{\text V}{\rm O}} \) ₂dp. Performance data were generated from a 1.25 km sprint prologue. Power-function modelling was used to predict the skiers’ race speeds based on the oxygen-uptake variables and body mass.

Results

There were correlations between the race speed and the absolute expression of the \( {\dot{\text V}{\rm O}} \) ₂obla (r = 0.79, P = 0.021), \( {\dot{\text V}{\rm O}} \) ₂max (r = 0.86, P = 0.0069), and \( {\dot{\text V}{\rm O}} \) ₂dp (r = 0.94, P = 0.00062). The following power-function models were established for race-speed prediction: 1.09 · \( {\dot{\text V}{\rm O}} \) ₂obla^0.21, 1.05 · \( {\dot{\text V}{\rm O}} \) ₂max^0.21, and 1.19 · \( {\dot{\text V}{\rm O}} \) ₂dp^0.20; these models explained 60 % (P = 0.024), 73 % (P = 0.0073), and 87 % (P = 0.00073), respectively, of the variance in the race speed. However, body mass did not contribute to any of the models (P = 0.97, 0.88, and 0.21, respectively).

Conclusions

Oxygen uptake at different intensities and sub-techniques is an indicator of elite male sprint-prologue performance. The absolute expression of the investigated oxygen-uptake variables should be used when evaluating elite male sprint-prologue performances; if skiers oxygen uptake differs by 1 %, their performances will likely differ by 0.2 % in favour of the skier with higher oxygen uptake.

     

Influence of exercise intensity on respiratory muscle fatigue and brachial artery blood flow during cycling exercise

Purpose

During high intensity exercise, both respiratory muscle fatigue and cardiovascular reflexes occur; however, it is not known how inactive limb blood flow is influenced. The purpose of this study was to determine the influence of moderate and high exercise intensity on respiratory muscle fatigue and inactive limb muscle and cutaneous blood flow during exercise.

Methods

Twelve men cycled at 70 and 85 % \(\dot{V}{\text{O}}_{{ 2_{ {\rm max} } }}\) for 20 min. Subjects also performed a second 85 % \(\dot{V}{\text{O}}_{{ 2_{ {\rm max} } }}\) test after ingesting 1,800 mg of N-acetylcysteine (NAC), which has been shown to reduce respiratory muscle fatigue (RMF). Maximum inspiratory pressures (P _Imax), brachial artery blood flow (BABF), cutaneous vascular conductance (CVC), and mean arterial pressure were measured at rest and during exercise.

Results

Significant RMF occurred with 85 % \(\dot{V}{\text{O}}_{{ 2_{ {\rm max} } }}\) (P _Imax, ?12.8 ± 9.8 %), but not with 70 % \(\dot{V}{\text{O}}_{{ 2_{ {\rm max} } }}\) (P _Imax, ?5.0 ± 5.9 %). BABF and BA vascular conductance were significantly lower at end exercise of the 85 % \(\dot{V}{\text{O}}_{{ 2_{ {\rm max} } }}\) test compared to the 70 % \(\dot{V}{\text{O}}_{{ 2_{ {\rm max} } }}\) test. CVC during exercise was not different (p > 0.05) between trials. With NAC, RMF was reduced (p < 0.05) and BABF was significantly higher (~30 %) compared to 85 % \(\dot{V}{\text{O}}_{{ 2_{ {\rm max} } }}\) (p < 0.05).

Conclusions

These data suggest that heavy whole-body exercise at 85 % \(\dot{V}{\text{O}}_{{ 2_{ {\rm max} } }}\) leads to RMF, decreases in inactive arm blood flow, and vascular conductance, but not cutaneous blood flow.     

Pulmonary O2 uptake kinetics during moderate-intensity exercise transitions initiated from low versus elevated metabolic rates: insights from manipulations in cadence

Introduction

The rate of adjustment (τ) of phase II pulmonary O₂ uptake ( \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) ) is slower when exercise transitions are initiated from an elevated baseline work rate (WR) and metabolic rate (MR). In this study, combinations of cycling cadence (40 vs. 90 rpm) and external WR were used to examine the effect of prior MR on τ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) .

Methods

Eleven young men completed transitions from 20 W (BSL) to 90 % lactate threshold, with transitions performed as two steps of equal ?WR (LS, lower step; US, upper step), while maintaining a cadence of (1) 40 rpm, (2) 90 rpm, and (3) 40 rpm but with the WRs elevated to match the higher \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) associated with 90 rpm cycling (40_MATCH); transitions lasted 6 min. \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) was measured breath-by-breath using mass spectrometry and turbinometry; vastus lateralis muscle deoxygenation [HHb] was measured using near-infrared spectroscopy. \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) and HHb responses were modeled using nonlinear least squares regression analysis.

Results

\(\dot{V}{\text{O}}_{{2{\text{p}}}}\) at BSL, LS and US was similar for 90 rpm and 40_MATCH, but greater than in 40 rpm. Compared to 90 rpm, τ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) at 40 rpm was shorter (p < 0.05) in LS (18 ± 5 vs. 28 ± 8 s) but not in US (26 ± 8 vs. 33 ± 9 s), and at 40_MATCH, τ \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) was lower (p < 0.05) (19 ± 6 s) in LS but not in US (34 ± 13 s) despite differing external WR and ?WR.

Conclusions

A similar overall adjustment of [HHb] and \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) in LS and US across conditions suggested dynamic matching between microvascular blood flow and O₂ utilization. Prior MR (rather than external WR per se) plays a role in the dynamic adjustment of pulmonary (and muscle) \(\dot{V}{\text{O}}_{{2{\text{p}}}}\) .     

A new interpolation-free procedure for breath-by-breath analysis of oxygen uptake in exercise transients

Introduction

Interpolation methods circumvent poor time resolution of breath-by-breath oxygen uptake ( \(\dot{V}{\text{O}}_{2}\) ) kinetics at exercise onset. We report an interpolation-free approach to the improvement of poor time resolution in the analysis of \(\dot{V}{\text{O}}_{2}\) kinetics.

Methods

Noiseless and noisy (10 % Gaussian noise) synthetic data were generated by Monte Carlo method from pre-selected parameters (Exact Parameters). Each data set comprised 10 ( \(\dot{V}{\text{O}}_{2}\) )-on transitions with noisy breath distribution within a physiological range. Transitions were superposed (no interpolation, None), then analysed by bi-exponential model. Fitted model parameters were compared with those from interpolation methods (average transition after Linear or Step 1-s interpolations), applied on the same data. Experimental data during cycling were also analysed. The 95 % confidence interval around a line of parameters’ equality was computed to analyse agreement between exact parameters and corresponding parameters of fitted functions.

Results

The line of parameters’ equality stayed within confidence intervals for noiseless synthetic parameters with None, unlike Step and Linear, indicating that None reproduced Exact Parameters. Noise addition reduced differences among pre-treatment procedures. Experimental data provided lower phase I time constants with None than with Step.

Conclusion

In conclusion, None revealed better precision and accuracy than Step and Linear, especially when phenomena characterized by time constants of <30 s are to be analysed. Therefore, we endorse the utilization of None to improve the quality of breath-by-breath \(\dot{V}{\text{O}}_{2}\) data during exercise transients, especially when a double exponential model is applied and phase I is accounted for.     

The interplay between arms-only propelling efficiency,power output and speed in master swimmers

Purpose

To explore the interplay between arms-only propelling efficiency (η _p), mechanical power output (\(\dot W_{\text{tot}}\)) and swimming speed (V); these three parameters are indeed related through the following equation V ³ = 1/kη _p \(\dot W_{\text{tot}}\) (where k is the speed-specific drag; k = F/V ²); thus, the larger are η _p and \(\dot W_{\text{tot}}\) the larger is V. We furthermore wanted to test the hypothesis that a multiple linear regression between \(\dot W_{\text{tot}}\), η _p and V would have a stronger correlation coefficient than a linear regression between \(\dot W_{\text{tot}}\) and V alone.

Methods

To this aim we recruited 29 master swimmers (21 M/8F) who were asked to perform (1) an incremental protocol at the arm-ergometer (dry-land test) to determine \(\dot W_{\text{tot}}\) at \(\dot V\)O_2max (e.g. \(\dot V\) _max); (2) a maximal 200 m swim trial (with a pull buoy: arms only) during which V and η _p were determined.

Results

No relationship was found between \(\dot W_{\text{max}}\) and η _p (not necessarily the swimmers with the largest \(\dot W_{\text{max}}\) are those with the largest η _p and vice versa) whereas significant correlations were found between \(\dot W_{\text{max}}\) and V (R = 0.419, P = 0.024) and η _p and V (R = 0.741, P = 0.001); a multiple linear regression indicates that about 75 % of the variability of V can be explained by the variability of \(\dot W_{\text{max}}\) and η _p (R = 0.865, P < 0.001).

Conclusions

These findings indicate that η _p should be taken into consideration when the relationship between \(\dot W_{\text{max}}\) and V is investigated and that this allows to better explain the inter-subject variability in performance (swimming speed).

     

The validity of the Moxus Modular metabolic system during incremental exercise tests: impacts on detection of small changes in oxygen consumption

Purpose

We investigated the accuracy of the Moxus Modular Metabolic System (MOXUS) against the Douglas Bag Method (DBM) during high-intensity exercise, and whether the two methods agreed when detecting small changes in $\dot{V}{\text{O}}_{2}$ between two consecutive workloads ( $\Delta {\dot{{V}}\text{O}}_{ 2}$ ).

Methods

Twelve trained male runners performed two maximal incremental running tests while gas exchange was analyzed simultaneously by the two systems using a serial setup for four consecutive intervals of 30 s on each test. Comparisons between methods were performed for $\dot{V}{\text{O}}_{2}$ , ${\dot{{V}}}_{\text{E}}$ , fractions of expired O₂ (FeO₂) and CO₂ (FeCO₂) and $\Delta {\dot{{V}}\text{O}}_{ 2}$ .

Results

The MOXUS produced significant higher (mean ± SD, n = 54) readings for $\dot{V}{\text{O}}_{2}$ (80 ± 200 mL min^?1, p = 0.005) and ${\dot{{V}}}_{\text{E}}$ (2.9 ± 4.2 L min^?1, p < 0.0001), but not FeO₂ (?0.01 ± 0.09). Log-transformed 95 % limits of agreement for readings between methods were 94–110 % for $\dot{V}{\text{O}}_{2}$ , 97–108 % for $\dot{V}_{\text{E}}$ and 99–101 % for FeO₂. $\Delta \dot{V}{\text{O}}_{2}$ for two consecutive measurements was not different between systems (120 ± 110 vs. 90 ± 190 mL min^?1 for MOXUS and DBM, respectively, p = 0.26), but agreement between methods was very low (r = 0.25, p = 0.12).

Discussion

Although it was tested during high-intensity exercise and short sampling intervals, the MOXUS performed within the acceptable range of accuracy reported for automated analyzers. Most of the differences between equipments were due to differences in $\dot{V}_{\text{E}}$ . Detecting small changes in $\dot{V}{\text{O}}_{2}$ during an incremental test with small changes in workload, however, might be beyond the equipment’s accuracy.     

The effects of breathing a helium–oxygen gas mixture on maximal pulmonary ventilation and maximal oxygen consumption during exercise in acute moderate hypobaric hypoxia

To test the hypothesis that maximal exercise pulmonary ventilation ( $ \dot{V}{\text{E}}_{ \max } $ ) is a limiting factor affecting maximal oxygen uptake ( $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ ) in moderate hypobaric hypoxia (H), we examined the effect of breathing a helium–oxygen gas mixture (He–O₂; 20.9% O₂), which would reduce air density and would be expected to increase $ \dot{V}{\text{E}}_{ \max } $ . Fourteen healthy young male subjects performed incremental treadmill running tests to exhaustion in normobaric normoxia (N; sea level) and in H (atmospheric pressure equivalent to 2,500 m above sea level). These exercise tests were carried out under three conditions [H with He–O₂, H with normal air and N] in random order. $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ and arterial oxy-hemoglobin saturation (SaO₂) were, respectively, 15.2, 7.5 and 4.0% higher (all p < 0.05) with He–O₂ than with normal air ( $ \dot{V}{\text{E}}_{ \max } $ _, 171.9 ± 16.1 vs. 150.1 ± 16.9 L/min; $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ , 52.50 ± 9.13 vs. 48.72 ± 5.35 mL/kg/min; arterial oxyhemoglobin saturation (SaO₂), 79 ± 3 vs. 76 ± 3%). There was a linear relationship between the increment in $ \dot{V}{\text{E}}_{ \max } $ and the increment in $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ in H (r = 0.77; p < 0.05). When subjects were divided into two groups based on their $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ , both groups showed increased $ \dot{V}{\text{E}}_{ \max } $ and SaO₂ in H with He–O₂, but $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ was increased only in the high $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ group. These findings suggest that in acute moderate hypobaric hypoxia, air-flow resistance can be a limiting factor affecting $ \dot{V}{\text{E}}_{ \max } $ ; consequently, $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ is limited in part by $ \dot{V}{\text{E}}_{ \max } $ , especially in subjects with high $ \dot{V}{\text{O}}_{{ 2 {\text{max}}}} $ .     

How long does it take to achieve steady state for an accurate assessment of resting \dot{\text{V}}{\text{O}}_{2} in healthy men?

The time necessary to obtain a steady state for an accurate and reliable assessment of resting $ \dot{V}{\text{O}}_{2} $ remains unclear and was the purpose of this study. Thirty healthy men, aged 17–28 years, visited the laboratory twice for the assessment of resting $ \dot{V}{\text{O}}_{2} $ , which was assessed as follows: (a) 24 h abstention from physical exercise, alcohol, soft drinks and caffeine, (b) fasting for at least 8 h, (c) an acclimation period of 10 min, and (d) 60 min assessment in a supine position. Resting $ \dot{V}{\text{O}}_{2} $ significantly changed during the 60 min (F = 37.4, P < 0.001), exhibiting a monoexponential decrease before reaching an asymptote. Post hoc pairwise comparisons showed that significant differences existed between consecutive means until the 30 min time point, after which there were no significant differences. The $ \dot{V}{\text{O}}_{2} $ response across trials exhibited high test–retest reliability, with within-subject coefficients of variations at each time point ranging from 2.8 to 7.0 % and intraclass correlation coefficients ranging from 0.90 to 0.99. The reliability was higher from the 25 min time point onwards. Based on these findings, the following recommendations are made to promote accurate assessment of resting $ \dot{V}{\text{O}}_{2} $ : (a) initiate the resting $ \dot{V}{\text{O}}_{2} $ measurement with 10 min of acclimation to the assessment apparatus, (b) determine resting $ \dot{V}{\text{O}}_{2} $ for a minimum of 30 min, until an apparent $ \dot{V}{\text{O}}_{2} $ steady state has been achieved; and (c) determine resting $ \dot{V}{\text{O}}_{2} $ for a further 5 min, with the average of this last 5 min of data being regarding as the resting $ \dot{V}{\text{O}}_{2} $ .     

\dot{V}_{{{\text{O}}_{2} { \max }}} is not altered by self-pacing during incremental exercise

We tested the hypothesis that incremental cycling to exhaustion that is paced using clamps of the rating of perceived exertion (RPE) elicits higher $ \dot{V}_{{{\text{O}}_{2} { \max }}} $ values compared to a conventional ramp incremental protocol when test duration is matched. Seven males completed three incremental tests to exhaustion to measure $ \dot{V}_{{{\text{O}}_{2} { \max }}} $ . The incremental protocols were of similar duration and included: a ramp test at 30 W min^?1 with constant cadence (RAMP1); a ramp test at 30 W min^?1 with cadence free to fluctuate according to subject preference (RAMP2); and a self-paced incremental test in which the power output was selected by the subject according to prescribed increments in RPE (SPT). The subjects also completed a $ \dot{V}_{{{\text{O}}_{2} { \max }}} $ ‘verification’ test at a fixed high-intensity power output and a 3-min all-out test. No difference was found for $ \dot{V}_{{{\text{O}}_{2} { \max }}} $ between the incremental protocols (RAMP1 = 4.33 ± 0.60 L min^?1; RAMP2 = 4.31 ± 0.62 L min^?1; SPT = 4.36 ± 0.59 L min^?1; P > 0.05) nor between the incremental protocols and the peak $ \dot{V}_{{{\text{O}}_{2} }} $ measured during the 3-min all-out test (4.33 ± 0.68 L min^?1) or the $ \dot{V}_{{{\text{O}}_{2} { \max }}} $ measured in the verification test (4.32 ± 0.69 L min^?1). The integrated electromyogram, blood lactate concentration, heart rate and minute ventilation at exhaustion were not different (P > 0.05) between the incremental protocols. In conclusion, when test duration is matched, SPT does not elicit a higher $ \dot{V}_{{{\text{O}}_{2} { \max }}} $ compared to conventional incremental protocols. The striking similarity of $ \dot{V}_{{{\text{O}}_{2} { \max }}} $ measured across an array of exercise protocols indicates that there are physiological limits to the attainment of $ \dot{V}_{{{\text{O}}_{2} { \max }}} $ that cannot be exceeded by self-pacing.     

Influence of blood donation on the incidence of plateau at \dot{V}{\text{O}} 2max

Purpose

The purpose of this study was to examine the effects of reductions in blood volume and associated oxygen-carrying capacity on the incidence of plateau at $\dot{V}{\text{O}}$ _2max.

Methods

Fifteen well-trained athletes (age 23.3 ± 4.5; mass 77.4 ± 13.1 kg, height 180.1 ± 6.0 cm) completed three incremental cycle tests to volitional exhaustion, of which the first was defined as familiarisation, with the remaining two trials forming the experimental conditions of pre- (UBL) and post-(BLE) blood donation (~450 cm³). The work rate for the incremental tests commenced at 100 W for 60 s followed by a ramp of 0.42 W s^?1, with cadence being held constant at 80 rpm. Throughout all trials, $\dot{V}{\text{O}}$ ₂ was determined on a breath-by-breath basis using a pre-calibrated metabolic cart. The criteria for plateau determination was a ? $\dot{V}{\text{O}}$ ₂ ≤ 50 ml min^?1 over the final two consecutive 30 s sampling periods.

Results

Despite a significant (P = 0.0028) 9.4 % reduction in haemoglobin concentration and 10.8 % (P = 0.016) reduction in erythrocyte count between UBL and BLE, there was no change in plateau incidence. However, significant differences were observed for both $\dot{V}{\text{O}}$ _2max (P = 0.0059) 51.3 ± 7.6 (UBL) 48.4 ± 7.9 ml kg^?1 min^?1 (BLE) and gas exchange threshold arrival time 383.4 ± 85.2 s (UBL) 349.2 ± 71.4 s (BLE) (P = 0.0028).

Conclusion

These data suggest that plateau at $\dot{V}{\text{O}}$ _2max is unaffected by O₂ availability lending support to the notion of the plateau being dependent on the anaerobic capacity and the classically orientated concept of $\dot{V}{\text{O}}$ _2max.     

The influence of hydration status during prolonged endurance exercise on salivary antimicrobial proteins

Purpose

Antimicrobial proteins (AMPs) in saliva including secretory immunoglobulin A (SIgA), lactoferrin (SLac) and lysozyme (SLys) are important in host defence against oral and respiratory infections. This study investigated the effects of hydration status on saliva AMP responses to endurance exercise.

Methods

Using a randomized design, 10 healthy male participants (age 23 ± 4 years, \(\dot{V}{\text{O}}_{{2{ \hbox{max} }}}\) 56.8 ± 6.5 ml/kg/min) completed 2 h cycling at 60 % \(\dot{V}{\text{O}}_{{2{ \hbox{max} }}}\) in states of euhydration (EH) or dehydration (DH) induced by 24 h fluid restriction. Unstimulated saliva samples were collected before, during, immediately post-exercise and each hour for 3 h recovery.

Results

Fluid restriction resulted in a 1.5 ± 0.5 % loss of body mass from baseline and a 4.3 ± 0.7 % loss immediately post-exercise. Pre-exercise urine osmolality was higher in DH than EH and overall, saliva flow rate was reduced in DH compared with EH (p < 0.05). Baseline SIgA secretion rates were not different between conditions; however, exercise induced a significant increase in SIgA concentration in DH (161 ± 134 to 309 ± 271 mg/L) which remained elevated throughout 3 h recovery. SLac secretion rates increased from pre- to post-exercise in both conditions which remained elevated in DH only. Overall, SLac concentrations were higher in DH than EH. Pre-exercise SLys concentrations were lower in DH compared with EH (1.6 ± 2.0 vs. 5.5 ± 6.7 mg/L). Post-exercise SLys concentrations remained elevated in DH but returned to pre-exercise levels by 1 h post-exercise in EH.

Conclusions

Exercise in DH caused a reduction in saliva flow rate yet induced greater secretion rates of SLac and higher concentrations of SIgA and SLys. Thus, DH does not impair saliva AMP responses to endurance exercise.

     

Modelling the effect of insulin on the disposal of meal-attributable glucose in type 1 diabetes

The management of postprandial glucose excursions in type 1 diabetes has a major impact on overall glycaemic control. In this work, we propose and evaluate various mechanistic models to characterize the disposal of meal-attributable glucose. Sixteen young volunteers with type 1 diabetes were subject to a variable-target clamp which replicated glucose profiles observed after a high-glycaemic-load (\(n=8\)) or a low-glycaemic-load (\(n=8\)) evening meal. [6,6-\(^{2}\hbox {H}_2\)] and [U-\(^{13}\hbox {C}\);1,2,3,4,5,6,6-\(^{2}\hbox {H}_{7}\)] glucose tracers were infused to, respectively, mimic: (a) the expected post-meal suppression of endogenous glucose production and (b) the appearance of glucose due to a standard meal. Six compartmental models (all a priori identifiable) were proposed to investigate the remote effect of circulating plasma insulin on the disposal of those glucose tracers from the non-accessible compartments, representing e.g. interstitium. An iterative population-based parameter fitting was employed. Models were evaluated attending to physiological plausibility, posterior identifiability of their parameter estimates, accuracy—via weighted fitting residuals—and information criteria (i.e. parsimony). The most plausible model, best representing our experimental data, comprised: (1) a remote effect x of insulin active above a threshold \(x_{C}\) = 1.74 (0.81–2.50) \(\cdot \,10^{-2}\) min\(^{-1}\) [median (inter-quartile range)], with parameter \(x_{C}\) having a satisfactory support: coefficient of variation CV = 42.33 (31.34–65.34) %, and (2) steady-state conditions at the onset of the experiment (\(t=0\)) for the compartment representing the remote effect, but not for the masses of the tracer that mimicked endogenous glucose production. Consequently, our mechanistic model suggests non-homogeneous changes in the disposal rates for meal-attributable glucose in relation to plasma insulin. The model can be applied to the in silico simulation of meals for the optimization of postprandial insulin infusion regimes in type 1 diabetes.     

Adaptations in muscle metabolic regulation require only a small dose of aerobic-based exercise

This study investigated the hypothesis that the duration of aerobic-based cycle exercise would affect the adaptations in substrate and metabolic regulation that occur in vastus lateralis in response to a short-term (10 day) training program. Healthy active but untrained males (n = 7) with a peak aerobic power ( $ \dot{V}{\text{O}}_{{ 2 {\text{ peak}}}} $ ) of 44.4 ± 1.4 ml kg^?1 min^?1 participated in two different training programs with order randomly assigned (separated by ≥2 weeks). The training programs included exercising at a single intensity designated as light (L) corresponding to 60 % $ \dot{V}{\text{O}}_{{ 2 {\text{ peak}}}} $ , for either 30 or 60 min. In response to a standardized task (60 % $ \dot{V}{\text{O}}_{{ 2 {\text{ peak}}}} $ ), administered prior to and following each training program, L attenuated the decrease (P < 0.05) in phosphocreatine and the increase (P < 0.05) in free adenosine diphosphate and free adenosine monophosphate but not lactate. These effects were not altered by daily training duration. In the case of muscle glycogen, training for 60 versus 30 min exaggerated the increase (P < 0.05) that occurred, an effect that extended to both rest and exercise concentrations. No changes were observed in $ \dot{V}{\text{O}}_{{ 2 {\text{ peak}}}} $ measured during progressive exercise to fatigue or in $ \dot{V}{\text{O}}_{ 2} $ and RER during submaximal exercise with either training duration. These findings indicate that reductions in metabolic strain, as indicated by a more protected phosphorylation potential, and higher glycogen reserves, can be induced with a training stimulus of light intensity applied for as little as 30 min over 10 days. Our results also indicate that doubling the duration of daily exercise at L although inducing increased muscle glycogen reserves did not result in a greater metabolic adaptation.     

The verification phase and reliability of physiological parameters in peak testing of elite wheelchair athletes

The purpose of this study was (1) to examine the value of a verification phase (VER) in a peak testing protocol and (2) to assess the reliability of peak physiological variables in wheelchair athletes. On two separate days, eight tetraplegic (TETRA), eight paraplegic (PARA) and eight non-spinal cord-injured (NON-SCI) athletes performed treadmill ergometry, consisting of a graded exercise test to exhaustion (GXT) followed by a VER. Peak oxygen uptake $ \left( {\dot{V}{\text{O}}_{{ 2 {\text{peak}}}} } \right) $ was compared (1) between GXT and VER and (2) between test days. $ \dot{V}{\text{O}}_{{2{\text{peak}}}} $ did not differ between GXT and VER (P = 0.27), and coefficients of variation between GXT and VER were in the range of 2.9 and 6.4 % for all subgroups. Coefficients of variation of $ \dot{V}{\text{O}}_{{2{\text{peak}}}} $ between test days were 9.3 % (TETRA), 4.5 % (PARA) and 3.3 % (NON-SCI). It is therefore concluded that whilst a VER can be used for a more robust determination of $ \dot{V}{\text{O}}_{{2{\text{peak}}}} $ , a deviation of up to ~6 % between GXT and VER should be considered as acceptable. For between-day analyses, relatively large changes in $ \dot{V}{\text{O}}_{{2{\text{peak}}}} $ are required to confirm “true” differences, especially in TETRA athletes. This may be due to their lower aerobic capacity, which results in a larger relative variation compared with the other subgroups.     

Skeletal muscle \dot{V} {\text{O}_2} kinetics from cardio-pulmonary measurements: assessing distortions through O2 transport by means of stochastic work-rate signals and circulatory modelling

During non-steady-state exercise, dynamic changes in pulmonary oxygen uptake ( $\dot{V} {\text{O}_{\text{2pulm}}}$ ) are dissociated from skeletal muscle $ \dot{V} {\text{O}_2}$ ( $\dot{V} {\text{O}_{\text{2musc}}}$ ) by changes in lung and venous O₂ concentrations (CvO₂), and the dynamics and distribution of cardiac output (CO) between active muscle and remaining tissues ( $ \dot{Q}_{\text{rem}}$ ). Algorithms can compensate for fluctuations in lung O₂ stores, but the influences of CO and CvO₂ kinetics complicate estimation of $\dot{V} {\text{O}_{\text{2musc}}}$ from cardio-pulmonary measurements. We developed an algorithm to estimate $\dot{V} {\text{O}_{\text{2musc}}}$ kinetics from $\dot{V} {\text{O}_{\text{2pulm}}}$ and heart rate (HR) during exercise. 17 healthy volunteers (28 ± 7 years; 71 ± 12 kg; 7 females) performed incremental exercise using recumbent cycle ergometry ( $\dot{V} {\text{O}_{\text{2peak}}}$ 52 ± 8 ml min^?1 kg^?1). Participants completed a pseudo-random binary sequence (PRBS) test between 30 and 80 W. $\dot{V} {\text{O}_{\text{2pulm}}}$ and HR were measured, and CO was estimated from HR changes and steady-state stroke volume. $\dot{V} {\text{O}_{\text{2musc}}}$ was derived from a circulatory model and time series analyses, by solving for the unique combination of venous volume and the perfusion of non-exercising tissues that provided close to mono-exponential $\dot{V} {\text{O}_{\text{2musc}}}$ kinetics. Independent simulations showed that this approach recovered the $\dot{V} {\text{O}_{\text{2musc}}}$ time constant (τ) to within 7 % (R ² = 0.976). Estimates during PRBS were venous volume 2.96 ± 0.54 L; $ \dot{Q}_{\text{rem}}$ 3.63 ± 1.61 L min^?1; τHR 27 ± 11 s; τ $\dot{V} {\text{O}_{\text{2musc}}}$ 33 ± 8 s; τ $\dot{V} {\text{O}_{\text{2pulm}}}$ 43 ± 14 s; $\dot{V} {\text{O}_{\text{2pulm}}}$ time delay 19 ± 8 s. The combination of stochastic test signals, time series analyses, and a circulatory model permitted non-invasive estimates of $\dot{V} {\text{O}_{\text{2musc}}}$ kinetics. Large kinetic dissociations exist between muscular and pulmonary $\dot{V} {\text{O}_{\text{2}}}$ during rapid exercise transients.