Comments on point: counterpoint: the kinetics of oxygen uptake during muscular exercise do/do not manifest time-delayed phase. On the physiological issue of td determination with empirical modeling

Effects of Blood Flow Restriction on O2 Muscle Extraction and O2 Pulmonary Uptake Kinetics During Heavy Exercise

This study aimed to determine the effects of three levels of blood flow restriction (BFR) onV ˙ O 2 and O 2 extraction kinetics during heavy cycling exercise transitions. Twelve healthy trained males completed two bouts of 10 min heavy intensity exercise without BFR (CON), with 40% or 50% BFR (BFR40 and BFR50, respectively).V ˙ O 2 and tissue saturation index (TSI) were continuously measured and modelled using multiexponential functions. The time constant of theV ˙ O 2 primary phase was significantly slowed in BFR40 (26.4 ± 2.0s; p < 0.001) and BFR50 (27.1 ± 2.1s; p = 0.001) compared to CON (19.0 ± 1.1s). The amplitude of theV ˙ O 2 slow component was significantly increased (p < 0.001) with BFR in a pressure-dependent manner 3.6 ± 0.7, 6.7 ± 0.9 and 9.7 ± 1.0 ml·min-1·kg-1 for CON, BFR40, and BFR50, respectively. While no acceleration of the primary component of the TSI kinetics was observed, there was an increase (p < 0.001) of the phase 3 amplitude with BFR (CON -0.8 ± 0.3% VS BFR40 -2.9 ± 0.9%, CON VS BFR50 -2.8 ± 0.8%). It may be speculated that BFR applied during cycling exercise in the heavy intensity domain shifted the working muscles to an O 2 dependent situation. The acceleration of the extraction kinetics could have reached a plateau, hence not permitting compensation for the slowdown of the blood flow kinetics, and slowingV ˙ O 2 kinetics.

Influence of simulated hypogravity on oxygen uptake during treadmill running

Prolonged exposure to microgravity during spaceflights leads to severe deterioration in the physical performance of astronauts. To understand the effectiveness of existing in‐flight daily countermeasures and to plan exercise onboard the International Space Station, we compared supine treadmill running to traditional upright treadmill running on earth. Specifically, we assessed the cardiorespiratory responses to conventional upright running to the responses to supine treadmill running under 0.3 g, 0.6 g, and 1 g of body weight in younger (20–30 years, n = 14, 8 females) and older healthy adults (50–60 years, n = 12, 6 females). Maximal cardiorespiratory capacity was additionally evaluated by performing an incremental running protocol on each treadmill. Maximum speed was greater for 0.3 g and 0.6 g in supine than for upright running (18.5 km/h (1.1) and 15.9 (3.1) vs 13.2 (2.4) p < 0.001). In contrast, maximum oxygen uptake (V˙O2max) and maximum heart rate (HR_max) were greater in upright running than in all supine conditions (Upright treadmill running vs S1.0G vs S0.6G vs S0.3G, 41.7 ml kg⁻¹ min⁻¹ (7.2) vs 30.5 (6.6) vs 32.9 (7.0) vs 30.9 (5.2), p < 0.001 and 171 beats min⁻¹ (14) vs 152 (24) vs 155 (20) vs 152 (18), p < 0.001, respectively). The reduction in V˙O2max was remarkably similar across all three supine conditions, could not be increased by higher running speeds and can be well explained by reduced ground reaction forces (GRF). Thus, although a gravity‐related restriction of pulmonary gas exchange or perfusion of the legs when exercising in the supine position can be suspected, findings are also explicable on grounds of the vertical treadmill mechanics. Reduced loading will constitute a substantial limitation to V˙O2 in space with implications for crew health and the physical deterioration of astronauts.

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

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