O2 uptake kinetics above and below the lactic acidosis threshold during sinusoidal exercise

Correlation of cervical-inspiratory-muscle electromyography and oxygen uptake during treadmill walking

For measurements of exercise intensity, an individual's oxygen uptake (V̇O2) is measured with an exhaled gas analyzer that involves a mask, but exercise coaching would benefit if an individual's V̇O2 could be estimated with more easily obtained predictors. We investigated the predictability of V̇O2 by electromyography (EMG) of the neck inspiratory muscles. We analyzed the EMG results of the sternocleidomastoid (EMGst) and scalene (EMGsc) muscles of 14 healthy adults who performed a treadmill exercise load test. Their V̇O2, inspiratory flow rate, and heart rate were simultaneously recorded during the exercise. The exercise load test was performed twice at a ≥2-day interval. The first visit was an incremental exercise test, and the second was a repeated two-load exercise test at levels below and above the participant's ventilatory threshold (VT) as determined in the first test. We observed that the integrated EMG values for each exercise load showed partially significant positive correlations with the EMGst and EMGsc. However, the cervical inspiratory muscle EMGs did not show as high a correlation as the minute ventilation. These results indicate that (i) EMG of the cervical inspiratory muscles could be used to estimate V̇O2, but (ii) these EMG parameters alone should be considered insufficient for estimating V̇O2.

Energetics of sinusoidal exercise below and across critical power and the effects of fatigue

PURPOSE

Previous studies investigating sinusoidal exercise were not devoted to an analysis of its energetics and of the effects of fatigue. We aimed to determine the contribution of aerobic and anaerobic lactic metabolism to the energy balance and investigate the fatigue effects on the cardiorespiratory and metabolic responses to sinusoidal protocols, across and below critical power (CP).

METHODS

Eight males (26.6 ± 6.2 years; 75.6 ± 8.7 kg; maximum oxygen uptake 52.8 ± 7.9 ml·min-1·kg-1; CP 218 ± 13 W) underwent exhausting sinusoidal cycloergometric exercises, with sinusoid midpoint (MP) at CP (CPex) and 50 W below CP (CP-50ex). Sinusoid amplitude (AMP) and period were 50 W and 4 min, respectively. MP, AMP, and time-delay (tD) between mechanical and metabolic signals of expiratory ventilation (V ˙ E ), oxygen uptake (V ˙ O 2 ), and heart rate ( f H ) were assessed sinusoid-by-sinusoid. Blood lactate ([La-]) and rate of perceived exertion (RPE) were determined at each sinusoid.

RESULTS

V ˙ O 2 AMP was 304 ± 11 and 488 ± 36 ml·min-1 in CPex and CP-50ex, respectively. Asymmetries between rising and declining sinusoid phases occurred in CPex (36.1 ± 7.7 vs. 41.4 ± 9.7 s forV ˙ O 2 tD up and tD down, respectively; P < 0.01), with unchanged tDs.V ˙ O 2 MP and RPE increased progressively during CPex. [La-] increased by 2.1 mM in CPex but remained stable during CP-50ex. Anaerobic contribution was larger in CPex than CP-50ex.

CONCLUSION

The lower aerobic component during CPex than CP-50ex associated with lactate accumulation explained lowerV ˙ O 2 AMP in CPex. The asymmetries in CPex suggest progressive decline of muscle phosphocreatine concentration, leading to fatigue, as witnessed by RPE.

Heart rate and gas exchange dynamic responses to multiple brief exercise bouts (MBEB) in early- and late-pubertal boys and girls

Natural patterns of physical activity in youth are characterized by brief periods of exercise of varying intensity interspersed with rest. To better understand systemic physiologic response mechanisms in children and adolescents, we examined five responses [heart rate (HR), respiratory rate (RR), oxygen uptake (V̇O2 ), carbon dioxide production (V̇CO2 ), and minute ventilation (V̇E), measured breath-by-breath] to multiple brief exercise bouts (MBEB). Two groups of healthy participants (early pubertal: 17 female, 20 male; late-pubertal: 23 female, 21 male) performed five consecutive 2-min bouts of constant work rate cycle-ergometer exercise interspersed with 1-min of rest during separate sessions of low- or high-intensity (~40% or 80% peak work, respectively). For each 2-min on-transient and 1-min off-transient we calculated the average value of each cardiopulmonary exercise testing (CPET) variable (Y̅). There were significant MBEB changes in 67 of 80 on- and off-transients. Y̅ increased bout-to-bout for all CPET variables, and the magnitude of increase was greater in the high-intensity exercise. We measured the metabolic cost of MBEB, scaled to work performed, for the entire 15 min and found significantly higher V̇O2 , V̇CO2 , and V̇E costs in the early-pubertal participants for both low- and high-intensity MBEB. To reduce breath-by-breath variability in estimation of CPET variable kinetics, we time-interpolated (second-by-second), superimposed, and averaged responses. Reasonable estimates of τ (<20% coefficient of variation) were found only for on-transients of HR and V̇O2 . There was a remarkable reduction in τHR following the first exercise bout in all groups. Natural patterns of physical activity shape cardiorespiratory responses in healthy children and adolescents. Protocols that measure the effect of a previous bout on the kinetics of subsequent bouts may aid in the clinical utility of CPET.

The effects of sinusoidal linear drifts on the estimation of cardiorespiratory dynamic parameters during sinusoidal workload forcing: a simulation study

This study aimed at investigating whether: 1) different sinusoidal linear drifts would affect the estimation of the dynamic parameters amplitude (A) and phase lag (φ) of minute ventilation (V˙_E), oxygen uptake, carbon dioxide production and heart rate (HR) sinusoidal responses when the frequency analysis technique (F) is performed; 2) the Marquardt-Levenberg non-linear fitting technique (ML) would provide more precise estimations of A and φ of drifted sinusoidal responses compared to F. For each cardiorespiratory variable, fifteen responses to sinusoidal forcing of different sinusoidal periods were simulated by using a first-order dynamic linear model. A wide range of linear drifts were subsequently applied. A and φ were computed for all drifted and non-drifted responses by using both F (A_F and φ_F) and ML (A_ML and φ_ML). For non-drifted responses, no differences between A_F vs A_ML and φ_F vs φ_ML were found. Whereas A_F and φ_F were affected by the sinusoidal linear drifts, A_ML and φ_ML were not. Significant interaction effects (technique x drift) were found for A (P <  0.001; ƞ_P² > 0.247) and φ (P <  0.001; ƞ_P² > 0.851). Higher goodness of fit values were observed when using ML for drifted V˙_E and HR responses only. The present findings suggest ML as a recommended technique to use when sinusoidal linear drifts occur during sinusoidal exercise, and provide new insights on how to analyse drifted cardiorespiratory sinusoidal responses.

Frequency domain analysis to extract dynamic response characteristics for oxygen uptake during transitions to moderate- and heavy-intensity exercises

At the onset of an exercise transition, exponential modeling to calculate a time constant (τ) is the conventional method to analyze pulmonary oxygen uptake (V̇O_2p) kinetics for moderate and heavy exercises. A new frequency domain analysis technique, mean normalized gain (MNG), has been used to analyze V̇O_2p kinetics during moderate exercise, but has not been evaluated for its ability to detect differences in kinetics between moderate and heavy exercises. This study tested the hypothesis that MNG would detect smaller amplitude V̇O_2p responses in the heavy-exercise domain compared with moderate-exercise domain. Eight young healthy adults (3 female; age: 27 ± 6 yr; peak V̇O_2p: 43 ± 6 mL·min^-1·kg^-1; means ± SD) performed three bouts of pseudorandom binary sequence (PRBS) exercise for frequency analysis, with the work rate (WR) changing between 25 W and 90% ventilatory threshold (VT; L → M_PRBS), 25 W and 50% of the difference between VT and peak V̇O_2p (Δ50%; L → H_PRBS), and VT to Δ50% (VT → H_PRBS). Step exercise tests with equivalent changes in WR to the PRBS tests were performed to facilitate the comparison between MNG and τ. MNG was the highest for L → M_PRBS (59 ± 7%), then L → H_PRBS (52 ± 6%), and the lowest for VT → H_PRBS (38 ± 7%, F_(2,14) = 129.755, P < 0.001) exercise conditions indicating slower kinetics with increasing exercise intensity that correlated strongly in repeated measures with τ from step transitions (r_rm = -0.893). These results indicate that frequency domain analysis and MNG reliably detect differences in V̇O_2p kinetics observed across exercise intensity domains.NEW & NOTEWORTHY Mean normalized gain is able to detect differences in V̇O_2p kinetics between moderate-, heavy-, and heavy-intensity exercises from a raised WR within the same individuals. This new method of kinetic analysis may be advantageous compared with conventional V̇O_2p curve fitting, as it is less sensitive to breath-by-breath noise, it can provide useful information from a single exercise testing session, and it can be applied to nonconstant work rate exercise situations.

Effect of sinusoidal leg cycling exercise period on brachial artery blood flow dynamics in humans

Purpose

To quantify the dynamics of blood flow in brachial artery (BF-BA) in response to sinusoidal work rate (WR) leg cycling exercises of 2-, 4-, and 6-min periods and to examine their relationship with the forearm skin blood flow (SBF).

Methods

Seven healthy young male subjects performed upright leg ergometer exercise with a constant WR (mean sinusoidal WR) for 30 min followed by sinusoidal WR exercise of three different periods (number of repetitions): 2 min (7), 4 min (4), and 6 min (3). The WR fluctuated from 20 W to a peak WR corresponding to 60% peak oxygen uptake (VO₂). We continuously measured pulmonary gas exchange, heart rate (HR), blood velocity and cross-sectional area of BA, and forearm SBF and sweating rate (SR).

Results

All variables were followed by the sinusoidal WR. The phases of the variables for gas exchange and central circulation, such as VO₂ and HR with WR forcing were similar (e.g., phase shift (θ) in HR [°]: 2 min, 60 ± 7; 4 min, 45 ± 10; 6 min, 37 ± 8; mean ± SD) to previous study results, that is, a longer period showed a shorter θ and larger amplitude of responses. Contrarily, the BF-BA response showed anti-phase (approximately 180°) regardless of the period, whereas the θ of forearm SBF and SR were similar to gas exchange and central circulation.

Conclusions

Inactive limb BF-BA during sinusoidal leg cycling exercise was out of phase relative to the regulation of O₂-delivery to active muscles and thermoregulation. The response of BF-BA seems to not always reflect the response of forearm SBF in the downstream area.

Brachial artery blood flow dynamics during sinusoidal leg cycling exercise in humans

To explore the control of the peripheral circulation of a nonworking upper limb during leg cycling exercise, blood flow (BF) dynamics in the brachial artery (BA) were determined using a sinusoidal work rate (WR) exercise. Ten healthy subjects performed upright leg cycling exercise at a constant WR for 30 min, followed by 16 min of sinusoidal WR consisting of 4‐min periods of WR fluctuating between a minimum output of 20 W and a maximum output corresponding to ventilatory threshold (VT). Throughout the protocol, pulmonary gas exchange, heart rate (HR), mean arterial blood pressure (MAP), blood velocity (BV), and cross‐sectional area of the BA, forearm skin BF (SBF), and sweating rate (SR) were measured. Each variable was fitted to a sinusoidal model with phase shift (θ) and amplitude (A). Nearly all variables closely fit a sinusoidal model. Variables relating to oxygen transport, such as VO₂ and HR, followed the sinusoidal WR pattern with certain delays (θ: VO₂; 51.4 ± 4.0°, HR; 41.8 ± 5.4°, mean ± SD). Conversely, BF response in the BA was approximately in antiphase (175.1 ± 28.9°) with a relatively large A, whereas the phase of forearm SBF was dissimilar (65.8 ± 35.9°). Thus, the change of BF through a conduit artery to the nonworking upper limb appears to be the reverse when WR fluctuates during sinusoidal leg exercise, and it appears unlikely that this could be ascribed exclusively to altering the downstream circulation to forearm skin.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

The ventilatory response to sine wave variation in exercise loads and limb movement frequency

The current study's experiments tested the hypothesis that limb movement frequency is a significant determinant of exercise hyperpnoea. To this end, 19 healthy participants walked on a treadmill, where work was varied sinusiodally by alterations in either treadmill speed or grade. Measured responses were fitted with sine waves to determine their amplitudes and phase angles. Walking pace amplitude was greater during speed tests than grade tests, and phase lag relative to the treadmill smaller, as expected. Ventilation, carbon dioxide production, and oxygen uptake amplitudes were higher during speed tests than grade tests. Further, phase angle lags relative to the treadmill for these measures were shorter during speed tests than grade tests. We concluded that these findings demonstrate the presence of changes in breathing during exercise that can be attributed to changes in limb movement frequency.

Smoking-induced elevations in blood carboxyhaemoglobin levels. Effect on maximal oxygen uptake

Many people engage in physical activity to reduce their cardiovascular risk associated with smoking. These people should be made aware of the metabolic and cardiorespiratory changes induced by chronic and acute smoking and, in particular, the exercise ramifications of increased levels of blood carbon monoxide (CO). Smoking-induced elevations in the CO content of the blood can reduce exercise tolerance and maximal aerobic capacity. Smoking also increases the reliance upon glycolytic metabolism during exercise. Together, these factors contribute to earlier fatigue in smokers compared with nonsmokers who exercise. Similar effects upon exercise tolerance are noted in those who inhale environmental tobacco smoke.

O2 uptake kinetics after acetazolamide administration during moderate- and heavy-intensity exercise

Inhibition of carbonic anhydrase (CA) is associated with a lower plasma lactate concentration ([La-]pl) during fatiguing exercise. We hypothesized that a lower [La-]pl may be associated with faster O2 uptake (V(O2)) kinetics during constant-load exercise. Seven men performed cycle ergometer exercise during control (Con) and acute CA inhibition with acetazolamide (Acz, 10 mg/kg body wt iv). On 6 separate days, each subject performed 6-min step transitions in work rate from 0 to 100 W (below ventilatory threshold, <VE(T)) or to a V(O2) corresponding to approximately 50% of the difference between the work rate at VE(T) and peak V(O2) (>VE(T). Gas exchange was measured breath by breath. Trials were interpolated at 1-s intervals and ensemble averaged to yield a single response. The mean response time (MRT, i.e., time to 63% of total exponential increase) for on- and off-transients was determined using a two- (<VE(T)) or a three-component exponential model (>VE(T)). Arterialized venous blood was sampled from a dorsal hand vein and analyzed for [La-]pl. MRT was similar during Con (31.2 +/- 2.6 and 32.7 +/- 1.2 s for on and off, respectively) and Acz (30.9 +/- 3.0 and 31.4 +/- 1.5 s for on and off, respectively) for work rates <VE(T). At work rates >VE(T), MRT was similar between Con (69.1 +/- 6.1 and 50.4 +/- 3.5 s for on and off, respectively) and Acz (69.7 +/- 5.9 and 53.8 +/- 3.8 s for on and off, respectively). On- and off-MRTs were slower for >VE(T) than for <VE(T) exercise. [La-]pl increased above 0-W cycling values during <VE(T) and >VE(T) exercise but was lower at the end of the transition during Acz (1.4 +/- 0.2 and 7.1 +/- 0.5 mmol/l for <VE(T) and >VE(T) respectively) than during Con (2.0 +/- 0.2 and 9.8 +/- 0.9 mmol/l for <VE(T) and >VE(T), respectively). CA inhibition does not affect O2 utilization at the onset of <VE(T) or >VE(T) exercise, suggesting that the contribution of oxidative phosphorylation to the energy demand is not affected by acute CA inhibition with Acz.

Effects of a functional knee brace for ACL insufficiency during treadmill running

PURPOSE

To assess the effects of a functional knee brace (FKB) for anterior cruciate ligament insufficiency (ACLI) on physiological and perceptual parameters during treadmill running.

METHODS

Thirteen ACLI subjects (time since injury, 5.8 +/- 5.3 yr), performed an incremental test to exhaustion and two constant load 20-min tests, one at an intensity below lactate threshold (bLT), and the other at an intensity above LT (aLT) each with and without their FKB.

RESULTS

Bracing had no effect on peak variables except for higher ratings of perceived exertion at the legs (RPE-L) at the velocities associated with a blood lactate concentration [HLa] of 4.0 mM and at peak. Bracing had no effect when exercising at bLT but did significantly alter the metabolic profile developed during the performance of the aLT tests (83 +/- 0.03% VO2peak). In particular, FKB resulted in elevated blood [HLa] (23%), VO2 (4%), VE (12%), VCO2 [corrected] (7%), and VE/VO2 (7%). HR and slow component VO2 did not differ between the brace and no brace aLT tests. RPE-L and RPE-knee were significantly elevated during aLT when the brace was worn. Suspected mechanisms include alterations in muscle recruitment patterns and/or occlusion.

CONCLUSIONS

When ACLI individuals wear a FKB during high intensity straight-ahead running exercise of long duration, physiological parameters are affected.

A review of the control of breathing during exercise

During the past 100 years many experimental investigations have been carried out in an attempt to determine the control mechanisms responsible for generating the respiratory responses observed during incremental and constant-load exercise tests. As a result of these investigations a number of different and contradictory control mechanisms have been proposed to be the sole mediators of exercise hyperpnea. However, it is now becoming evident that none of the proposed mechanisms are solely responsible for eliciting the exercise respiratory response. The present-day challenge appears to be one of synthesizing the proposed mechanisms, in order to determine the role that each mechanism has in controlling ventilation during exercise. This review, which has been divided into three primary sections, has been designed to meet this challenge. The aim of the first section is to describe the changes in respiration that occur during constant-load and incremental exercise. The second section briefly introduces the reader to traditional and contemporary control mechanisms that might be responsible for eliciting at least a portion of the exercise ventilatory response during these types of exercise. The third section describes how the traditional and contemporary control mechanisms may interact in a complex fashion to produce the changes in breathing associated with constant-load exercise, and incorporates recent experimental evidence from our laboratory.

Coupling of external to cellular respiration during exercise: the wisdom of the body revisited

The changes in cellular respiration needed to increase energy output during exercise are intimately and predictably linked to external respiration through the circulation. This review addresses the mechanisms by which lactate accumulation might influence O2 uptake (VO2) and CO2 output (VCO2) kinetics. Respiratory homeostasis (a steady state with respect to VO2 and VCO2) is achieved by 3-4 min for work rates not associated with an increase in arterial lactate. When blood lactate increases significantly above rest for constant work rate exercise, VO2 characteristically increases past 3 min (slow component) at a rate proportional to the lactate concentration increase. The development of a similar slow component in VCO2 is not evident. The divergence of VCO2 from VO2 increase can be accounted for by extra CO2 release from the cell as HCO3- buffers lactic acid. Thus the slow component of aerobic CO2 production (parallel to VO2) is masked by the increase in buffer VCO2. This CO2, and the consumption of extracellular HCO3- by the lactate-producing cells, shifts the oxyhemoglobin dissociation curve rightward (Bohr effect). The exercise lactic acidosis has been observed to occur after the minimal capillary PO2 is reached. Thus the lactic acidosis serves to facilitate oxyhemoglobin dissociation and O2 transport to the muscle cells without a further decrease in end-capillary PO2. From these observations, it is hypothesized that simultaneously measured dynamic changes in VO2 and VCO2 might be useful to infer the aerobic and anaerobic contributions to exercise bioenergetics for a specific work task.