The point of maximum curvature as a marker for physiological time series

We present a geometric analysis of the model of Stirling. In particular we analyze the curvature of a heart rate time series in response to a step like increment in the exercise intensity. We present solutions for the point of maximum curvature which can be used as a marker of physiological interest. This marker defines the point after which the heart rate no longer continues to rapidly rise and instead follows either a steady state or slow rise. These methods are then applied to find analytic solutions for a mono exponential model which is commonly used in the literature to model the response to a moderate exercise intensity. Numerical solutions are then found for the full model and parameter values presented in Stirling

The effects of acute interval exercise and strawberry intake on postprandial lipemia

YesPurpose: Raised postprandial triglycerides (TAG) and related oxidative stresses are strongly associated with increased cardiovascular disease (CVD) risk. Acute exercise and strawberry ingestion independently ameliorate postprandial lipid excursions and oxidative stress. However, the combined effects of these lifestyle interventions is unknown. We investigated whether acute exercise and strawberry consumption improved postprandial responses to an oral fat tolerance test (OFTT) in overweight/obese males. Methods: Overweight/obese adult males underwent four separate OFTT (73g fat, 33g carbohydrate) with blood sampled at baseline and hourly for 4 h after OFTT. Two OFTT contained 25g freeze-dried strawberries and two contained strawberry flavouring (placebo). Participants performed 40 minutes of submaximal high intensity interval cycling exercise (HIIE) 16 h before one strawberry and one placebo OFTT, and rested before the remaining two OFTT. Serum TAG was analysed and TAG area under curve (AUC) and incremental AUC (iAUC) were calculated. Oxidative stress markers were measured at baseline and 4 h. Differences between conditions (strawberry/placebo and exercise/rest) were assessed using repeated measures ANOVA. Results: Ten males (Age, 31.5 IQR 17.8 years; BMI, 29.9 ±1.8 kg.m-2) completed the study. TAG AUC was 1.5 mmol.4h-1.L-1 lower for the exercise conditions compared to the rest conditions (95% confidence interval [CI]= -2.3 to 0.8, p= 0.001). TAG AUC was not different between the strawberry and placebo conditions (CI= -1.3 to 0.6, p= 0.475). TAG iAUC was 0.5 mmol.4h-1.L-1 greater for the strawberry compared to the placebo conditions (CI= 0.1 to 1.0, p= 0.021). There were no changes in markers of lipid related oxidative stress (P> 0.05). Conclusion: Acute submaximal HIIE appears effective in reducing postprandial lipaemia in overweight/obese adult males. However, strawberry ingestion did not improve postprandial TAG

Reproducibility of onset and recovery oxygen uptake kinetics in moderately impaired patients with chronic heart failure

Oxygen (O2) kinetics reflect the ability to adapt to or recover from exercise that is indicative of daily life. In patients with chronic heart failure (CHF), parameters of O2 kinetics have shown to be useful for clinical purposes like grading of functional impairment and assessment of prognosis. This study compared the goodness of fit and reproducibility of previously described methods to assess O2 kinetics in these patients. Nineteen CHF patients, New York Heart Association class II–III, performed two constant-load tests on a cycle ergometer at 50% of the maximum workload. Time constants of O2 onset- and recovery kinetics (τ) were calculated by mono-exponential modeling with four different sampling intervals (5 and 10 s, 5 and 8 breaths). The goodness of fit was expressed as the coefficient of determination (R2). Onset kinetics were also evaluated by the mean response time (MRT). Considering O2 onset kinetics, τ showed a significant inverse correlation with peak- \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} \ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{2} \end{document} (R = −0.88, using 10 s sampling intervals). The limits of agreement of both τ and MRT, however, were not clinically acceptable. O2 recovery kinetics yielded better reproducibility and goodness of fit. Using the most optimal sampling interval (5 breaths), a change of at least 13 s in τ is needed to exceed normal test-to-test variations. In conclusion, O2 recovery kinetics are more reproducible for clinical purposes than O2 onset kinetics in moderately impaired patients with CHF. It should be recognized that this observation cannot be assumed to be generalizable to more severely impaired CHF patients

Pulmonary oxygen uptake and muscle deoxygenation kinetics during recovery in trained and untrained male adolescents

Previous studies have demonstrated faster pulmonary oxygen uptake ( V ˙ O 2 ) kinetics in the trained state during the transition to and from moderate-intensity exercise in adults. Whilst a similar effect of training status has previously been observed during the on-transition in adolescents, whether this is also observed during recovery from exercise is presently unknown. The aim of the present study was therefore to examine V ˙ O 2 kinetics in trained and untrained male adolescents during recovery from moderate-intensity exercise. 15 trained (15 ± 0.8 years, V ˙ O 2max 54.9 ± 6.4 mL kg−1 min−1) and 8 untrained (15 ± 0.5 years, V ˙ O 2max 44.0 ± 4.6 mL kg−1 min−1) male adolescents performed two 6-min exercise off-transitions to 10 W from a preceding “baseline” of exercise at a workload equivalent to 80% lactate threshold; V ˙ O 2 (breath-by-breath) and muscle deoxyhaemoglobin (near-infrared spectroscopy) were measured continuously. The time constant of the fundamental phase of V ˙ O 2 off-kinetics was not different between trained and untrained (trained 27.8 ± 5.9 s vs. untrained 28.9 ± 7.6 s, P = 0.71). However, the time constant (trained 17.0 ± 7.5 s vs. untrained 32 ± 11 s, P < 0.01) and mean response time (trained 24.2 ± 9.2 s vs. untrained 34 ± 13 s, P = 0.05) of muscle deoxyhaemoglobin off-kinetics was faster in the trained subjects compared to the untrained subjects. V ˙ O 2 kinetics was unaffected by training status; the faster muscle deoxyhaemoglobin kinetics in the trained subjects thus indicates slower blood flow kinetics during recovery from exercise compared to the untrained subjects

Negative accumulated oxygen deficit during heavy and very heavy intensity cycle ergometry in humans

The concept of the accumulated O2 deficit (AOD) assumes that the O2 deficit increases monotonically with increasing work rate (WR), to plateau at the maximum AOD, and is based on linear extrapolation of the relationship between measured steady-state oxygen uptake (V̇O2) and WR for moderate exercise. However, for high WRs, the measured V̇O2 increases above that expected from such linear extrapolation, reflecting the superimposition of a "slow component" on the fundamental V̇O2 mono-exponential kinetics. We were therefore interested in determining the effect of the V̇O2 slow component on the computed AOD. Ten subjects [31 (12) years] performed square-wave cycle ergometry of moderate (40%, 60%, 80% and 90% θˆL ), heavy (40%Δ), very heavy (80%Δ) and severe (110% V̇O2 peak) intensities for 10–15 min, where θˆL is the estimated lactate threshold and Δ is the WR difference between θˆL and V̇O2 peak. V̇O2 was determined breath-by-breath. Projected "steady-state" V̇O2 values were determined from sub- θˆL tests. The measured V̇O2 exceeded the projected value after ~3 min for both heavy and very heavy intensity exercise. This led to the AOD actually becoming negative. Thus, for heavy exercise, while the AOD was positive [0.63 (0.41) l] at 5 min, it was negative by 10 min [−0.61 (1.05) l], and more so by 15 min [−1.70 (1.64) l]. For the very heavy WRs, the AOD was [0.42 (0.67) l] by 5 min and reached −2.68 (2.09) l at exhaustion. For severe exercise, however, the AOD at exhaustion was positive in each case: +1.69 (0.39) l. We therefore conclude that the assumptions underlying the computation of the AOD are invalid for heavy and very heavy cycle ergometry (at least). Physiological inferences, such as the "anaerobic work capacity", are therefore prone to misinterpretation