Influence of phase I duration on phase II VO2 kinetics parameter estimates in older and young adults

ArticleinAJP Regulatory Integrative and Comparative Physiology 301(1):R218-24 · April 2011with20 Reads
DOI: 10.1152/ajpregu.00060.2011 · Source: PubMed
Older adults (O) may have a longer phase I pulmonary O(2) uptake kinetics (Vo(2)(p)) than young adults (Y); this may affect parameter estimates of phase II Vo(2)(p). Therefore, we sought to: 1) experimentally estimate the duration of phase I Vo(2)(p) (EE phase I) in O and Y subjects during moderate-intensity exercise transitions; 2) examine the effects of selected phase I durations (i.e., different start times for modeling phase II) on parameter estimates of the phase II Vo(2)(p) response; and 3) thereby determine whether slower phase II kinetics in O subjects represent a physiological difference or a by-product of fitting strategy. Vo(2)(p) was measured breath-by-breath in 19 O (68 ± 6 yr; mean ± SD) and 19 Y (24 ± 5 yr) using a volume turbine and mass spectrometer. Phase I Vo(2)(p) was longer in O (31 ± 4 s) than Y (20 ± 7 s) (P < 0.05). In O, phase II τVo(2)(p) was larger (P < 0.05) when fitting started at 15 s (49 ± 12 s) compared with fits starting at the individual EE phase I (43 ± 12 s), 25 s (42 ± 10 s), 35 s (42 ± 12 s), and 45 s (45 ± 15 s). In Y, τVo(2)(p) was not affected by the time at which phase II Vo(2)(p) fitting started (τVo(2)(p) = 31 ± 7 s, 29 ± 9 s, 30 ± 10 s, 32 ± 11 s, and 30 ± 8 s for fittings starting at 15 s, 25 s, 35 s, 45 s, and EE phase I, respectively). Fitting from EE phase I, 25 s, or 35 s resulted in the smallest CI τVo(2)(p) in both O and Y. Thus, fitting phase II Vo(2)(p) from (but not constrained to) 25 s or 35 s provides consistent estimates of Vo(2)(p) kinetics parameters in Y and O, despite the longer phase I Vo(2)(p) in O.
    • "where Y (t) represents VO2 at any time (t); Y Bsln is the baseline VO2 during 4 km/h/min walking; Amp is the steady-state increase in VO2 above the baseline value; τ is the time constant defined as the duration of time for VO2 to increase to 63% of the steady-state increase; and TD is the time delay (so that the model is not constrained to pass through the origin.) After excluding the initial 20 s of values, while still allowing TD to vary freely (to optimize the accuracy of parameter estimates), VO2 values were modeled from 20 s to 4 min (240 s) of the step transition; this ensured that each subject had attained a VO2 steady-state, yet did not bias the model fit during the on-transient15, 16). Model parameters were estimated by least-squares nonlinear regression (Microsoft Office Excel 2010, Microsoft Japan Co., Ltd., Tokyo, Japan) in which the best fit was defined by minimization of the residual sum of squares and minimal residual variations around the Y-axis (Y=0). "
    [Show abstract] [Hide abstract] ABSTRACT: [Purpose] The objective of this study was to determine the validity of pulmonary oxygen uptake kinetics in assessment of the ability of skeletal muscles to utilize oxygen. [Subjects] We evaluated 12 young, healthy males. [Methods] The subjects completed a series of tests to determine their peak oxygen uptake, pulmonary oxygen uptake kinetics at the onset of moderate-intensity treadmill exercise, and the rate of decline in electromyographic (EMG) mean power frequency (MPF) (EMG MPFrate) during one continuous, fatiguing, isometric muscle action of the plantar flexors until exhaustion at approximately 60% maximum voluntary contraction. We discussed the relationships between pulmonary oxygen uptake kinetics and EMG MPFrate reflecting the ability of skeletal muscles to utilize oxygen and between pulmonary oxygen uptake kinetics and peak oxygen uptake reflecting the ability to deliver oxygen to skeletal muscles. We hypothesized that pulmonary oxygen uptake kinetics may be more highly correlated with EMG MPFrate than peak oxygen uptake. [Results] Pulmonary oxygen uptake kinetics (33.9 ± 5.9 s) were more significantly correlated with peak oxygen uptake (50.6 ± 5.5 mL/kg/min) than EMG MPFrate (-14.7 ± 8.7%/s). [Conclusion] Pulmonary oxygen uptake kinetics is a noninvasive index that is mainly usable for evaluation of the ability of cardiovascular system to deliver oxygen to skeletal muscles in healthy young adults with slower pulmonary oxygen uptake kinetics (>20 s).
    Full-text · Article · Nov 2013
    • "LBF and HR were fit from the first data point after the start of the exercise transient until the end of the exercise bout. On the other hand, the initial 20 s of VO 2p data was excluded to avoid inclusion of data points from Phase 1 VO 2p in the fitting of Phase 2 VO 2p (Murias et al. 2011b). Moreover, the TD was allowed to vary freely to optimize the accuracy of the estimated parameters. "
    [Show abstract] [Hide abstract] ABSTRACT: The adjustment of pulmonary oxygen uptake (VO2p), heart rate (HR), limb blood flow (LBF), and muscle deoxygenation [HHb] were examined during the transition to moderate-intensity, knee-extension exercise in six older adults (70 ± 4 years) under 2 conditions: normoxia (FIO2=20.9%) and hypoxia (FIO2=15%). The subjects performed repeated step transitions from an active baseline (3 W) to an absolute work rate (21 W) in both conditions. Phase 2 VO2p, HR, LBF, and [HHb] data were fit with an exponential model. Under hypoxic conditions, no change was observed in HR kinetics, on the other hand, LBF kinetics was faster (Norm, 34±3 sec; Hypo 28±2), whereas the overall [HHb] adjustment ( ) was slower (Norm, 28±2; Hypo 33±4 sec). Phase 2 VO2p kinetics were unchanged (p<0.05). The faster LBF kinetics and slower [HHb] kinetics reflect an improved matching between O2 delivery and O2 utilization at the microvascular level, preventing the phase 2 VO2p kinetics from become slower in hypoxia. Moreover the absolute blood flow values were higher in hypoxia (1.17 ± 0.2 l*min-1) compared to normoxia (0.96 ± 0.2 l*min-1) during the steady state exercise at 21 watts. These findings support the idea that, for older adults exercising at a low work rate, an increase of limb blood flow offsets the drop in arterial oxygen content (CaO2) caused by breathing an hypoxic mixture.
    Full-text · Article · Jan 2013
    • "The initial component (i.e., cardiodynamic phase) was not modelled in this study. As a consequence, the first 20 s were removed from analysis to ensure that the early initial component did not influence the result (Murias et al. 2011a; Whipp et al. 1982). The parameters of the model were determined iteratively by minimizing the sum of the mean squares of the differences between the estimated _ VO 2 that was based on the model and the measured _ VO 2 . "
    [Show abstract] [Hide abstract] ABSTRACT: This study compares the effects of two short multiple-sprint exercise (MSE) (6 × 6 s) sessions with two different recovery durations (30 s or 180 s) on the slow component of oxygen uptake ([Formula: see text]O(2)) during subsequent high-intensity exercise. Ten male subjects performed a 6-min cycling test at 50% of the difference between the gas exchange threshold and [Formula: see text]O(2peak) (Δ50). Then, the subjects performed two MSEs of 6 × 6 s separated by two intersprint recoveries of 30 s (MSE(30)) and 180 s (MSE(180)), followed 10 min later by the Δ50 (Δ50(30) and Δ50(180), respectively). Electromyography (EMG) activities of the vastus medialis and lateralis were measured throughout each exercise bout. During MSE(30), muscle activity (root mean square) increased significantly (p ≤ 0.04), with a significant leftward-shifted median frequency of the power density spectrum (MDF; p ≤ 0.01), whereas MDF was significantly rightward-shifted during MSE(180) (p = 0.02). The mean [Formula: see text]O(2) value was significantly higher in MSE(30) than in MSE(180) (p < 0.001). During Δ50(30), [Formula: see text]O(2) and the deoxygenated hemoglobin ([HHb]) slow components were significantly reduced (-27%, p = 0.02, and -34%, p = 0.003, respectively) compared with Δ50. There were no significant modifications of the [Formula: see text]O(2) slow component in Δ50(180) compared with Δ50 (p = 0.32). The neuromuscular and metabolic adaptations during MSE(30) (preferential activation of type I muscle fibers evidenced by decreased MDF and a greater aerobic metabolism contribution to the required energy demands), but not during MSE(180), may lead to reduced [Formula: see text]O(2) and [HHb] slow components, suggesting an alteration in motor units recruitment profile (i.e., change in the type of muscle fibers recruited) and (or) an improved muscle O(2) delivery during subsequent exercise.
    Full-text · Article · Sep 2012
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