Exercise kinetics analysis: practical applications for space flight (analogues)

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Introduction: The measurement of oxygen uptake kinetics allows a valuable understanding of the integrated physiological processes during exercise. In the exercise steady-state, pulmonary oxygen uptake (V’O2pulm) reflects the rate of oxygen consumed by the tissues (V’O2musc). However, during non-steady-state these dynamics are dissociated. The interactions of muscular gas exchange with the dynamics of the circulation and remaining body O2 capacitances means that V’O2pulm kinetics are not simply a time-shifted version of V’O2musc, with an early distortion due to the dynamics of cardiac output (Q’). The information about the distortive effects and the time delay between V’O2musc and V’O2pulm is therefore essential for a proper estimation of V’O2musc. It is hypothesized that by determining the distortive effects with a circulatory model application and time-series analysis will enable a reliable assessment of predicted V’O2musc (Hoffmann et al. Eur J Appl Physiol 113: 1745-1754, 2013). Therefore, we aimed to apply the proposed method by means of different conditions (varied body postures; before and after Space flight; before and after endurance exercise training) to highlight the necessity to distinguish between V’O2musc and V’O2pulm kinetics for proper estimations of the involved physiological systems. Methods: Three different subject groups were subjected to PRBS work rate changes between 30W and 80W for the kinetics analysis (cycle ergometry). Heart rate (HR) was assessed beat-to-beat by electrocardiography (ECG) and gas exchange was measured breath-by-breath for V’O2pulm. V’O2musc kinetics were estimated by the non-invasive approach of Hoffmann et al. (2013). The body positions group (A) was tested on a tilt table across different postures (−6°, 45°, and 75°). The astronauts group (B) performed the kinetics tests pre- (L-; [days]) and post-flight (R+; [days]) at specific points in time (L-236; L-72; R+6; R+21). The endurance training group (C) consisted of four subjects which were trained with a continuous (at 60% oxygen uptake reserve [V’O2Reserve]) and five subjects with an interval (at 90% V’O2Reserve) intervention method, each three times per week for six weeks. Given a linear, time-invariant, first order (LTI) system the cross correlation function (CCF) of work rate (WR) and a second parameter (e. g. HR, V’O2musc) indicate the kinetics responses of this parameter by the maximum (CCFmax) and its lag (CCFlag). Higher CCFmax-values denote faster system responses and greater CCFlag-values more time-delayed responses. Differences in the physiological variables were analyzed either with a two-way repeated measure ANOVA or with Wilcoxon’s ranked samples test as appropriate. The alpha level was set to 0.05 for statistical significance. Results: Group A: For V’O2pulm kinetics significant differences between −6° (CCFmax-values: 0.292 ± 0.040) and 45° (0.256 ± 0.034; p < 0.01; n = 10) as well as between −6° and 75° (0.214 ± 0.057; p < 0.05; n = 10) were detected at lag ‘40 s’ of the CCF course as interaction effects (factors: Lag × Posture). HR and V’O2musc kinetics yield no significant differences across the postures. Group B: In the astronauts group the CCFmax of V’O2musc differed significantly between L-236 and R+6 (p=0.010), between L-236 and R+21 (p=0.030) as well as between L-72 and R+6 (p=0.043). For CCFmax of V’O2pulm) a significant difference was observed (p=0.011) between L-236 and R+6. Group C: Significant differences were found between pre and post training intervention in absolute V’O2peak (3.2 ± 0.3 vs. 3.7 ± 0.2 L·min-1; p<0.05; n=9) and relative V’O2peak (37 ± 5 vs. 41 ± 4 ml·min-1·kg-1; p<0.05). Conclusion: The results of the different exercise conditions show a transient non-linear distortion in O2 exchange between muscle and lung which often results in a significant difference between V’O2musc and V’O2pulm kinetics during dynamic exercise. This is likely due to the influence of cardiac output and venous return dynamics. Accounting for these distortions enables a more reliable assessment of V’O2musc kinetics. This will improve the understanding of the altered adaptations in chronic disease and ageing, in daily life, by training and extreme environments, and will allow us to better target the involved physiological systems by therapeutic interventions to maintain health and to counteract deconditioning with appropriate training interventions. Acknowledgement: The studies were supported by the DLR (Deutsches Zentrum für Luft- und Raumfahrt), Germany (FKZ 50WB1426).
TitelAbstracts und Programmheft : 55. Wissenschaftliche Jahrestagung der Deutschen Gesellschaft für Luft- und Raumfahrtmedizin e.V.
PublikationsstatusVeröffentlicht - 16.09.2017
VeranstaltungWissenschaftliche Jahrestagung der Deutschen Gesellschaft für Luft- und Raumfahrtmedizin e.V. - Köln, Deutschland
Dauer: 14.09.201717.09.2017
Konferenznummer: 55