Respiratory variability in R-R interval during sinusoidal exercise

Influence of Age on Cardiorespiratory Kinetics During Sinusoidal Walking in Humans

We sought to determine the influence of age on cardiorespiratory kinetics during sinusoidal walking in two groups: 13 healthy young subjects (YG; 7 men and 6 women, age 21 ± 2 years) and 15 healthy elderly subjects (ELD; 9 men and 6 women, age 67 ± 5 years). A treadmill’s speed was sinusoidally changed between 3 and 6 km h^-1 in the YG and between 3 and 5 km h^-1 in the ELD during periods of 1, 2, 5, and 10 min, and in a stepwise manner. We compared the groups’ heart rate (HR), ventilation (V˙_E), and gas exchange (CO₂ output (V˙CO₂) and O₂ uptake (V˙O₂)) responses. We determined the phase shift (PS) and the normalized amplitude (Amp) ratio of these kinetics in relation to the sinusoidal change in walking speed in response to the magnitude from the maximum to minimum speeds as revealed by a Fourier analysis in all cardiorespiratory variables. Both the Amp ratio and PS in the V˙_E, V˙CO₂, and V˙O₂ responses were very similar between the ELD and YG, and being independent of the periods of sinusoidal oscillations. In marked contrast, the PS of the HR kinetics was significantly slowed in the ELD compared to the YG. The Amp ratio of HR was not related to the covariance variation of HR (CVHR) at standing rest in the ELD. The HR kinetics during sinusoidal walking may not be attributable to parasympathetic nerve activity into the heart in the ELD. The slope of the Amp of V˙_E related to the Amp of V˙CO₂ (V˙_E/V˙CO₂ slope) was steeper in the ELD (0.0258) compared to the YG (0.0132), suggesting that exercise hyperpnea could be greatly induced during walking in the ELD. These findings suggest that aging influences the alterations of autonomic nervous system-dependent slower HR kinetics and exercise hyperpnea during walking in the ELD.

Effects of Low-Intensity Exercise in the Morning on Physiological Responses During Unsteady Workload Exercise in the Evening

This study examined the effects of low-intensity morning exercise (ME) on physiological response during unsteady workload evening exercise. Nine healthy men participated in the following 2 experimental conditions: 15 minutes of bicycle exercise at 40% maximum oxygen consumption (V[Combining Dot Above]O2max) in the morning (the ME condition) and rest (control [C] condition). Heart rate (HR), blood pressure (BP), temperature, oxygen uptake, and natural logarithm of high frequency, an index of cardiac parasympathetic modulation, were measured before evening exercises, which were performed for 32 minutes in 2 parts: The steady-state exercise test included three 4-min bouts of exercise at 20, 60, and 40% V[Combining Dot Above]O2max. The unsteady exercise test consisted of 4-min bouts of exercise with gradual increases and decreases in workload at 20 and 60% V[Combining Dot Above]O2max. Heart rate, BP, and oxygen uptake were measured in both experiments. Maximal and minimal values, amplitude, and phase lags were measured with each cycle of unsteady workload exercise. With steady-state exercise, HR and systolic BP at 60 and 40% V[Combining Dot Above]O2max were significantly lower in the ME condition than in the C condition. However, oxygen uptake was not significantly different between the 2 conditions. With unsteady exercise, the HR and oxygen uptake phase lags were significantly shorter and the amplitude of oxygen intake was significantly larger in the ME condition than in the C condition. There were no significant differences in physiological parameters between the conditions at rest or during recovery. The physiological response during evening exercise is enhanced by low-intensity ME, which might be an effective conditioning method on a sporting event day.

Autonomic Heart Rate Regulation during Mild Dynamic Exercise in Humans: Insights from Respiratory Sinus Arrhythmia

To better understand the neural mechanism of heart rate (HR) regulation during dynamic exercise, the responses of HR and the magnitude of respiratory R-R interval variation were examined during exercise and recovery at mild intensities in humans. Eight subjects performed 3-min constant load cycle exercises in a semi-supine position at work rates of 25, 50, and 100 W. The respiratory interval was fixed at 4 s. Peak-to-valley variation in R-R interval caused by respiration was measured breath-by-breath and standardized for tidal volume (DeltaRRst, a noninvasive index of the degree of parasympathetic cardiac control). At all work rates the HR increased significantly from 2.5 s after the beginning of exercise (p <0.05) and decreased temporarily and slightly at around 15 s, and the DeltaRRst varied almost inversely. The HR and the DeltaRRst until 12.5 s after the beginning of exercise changed independently of work rate (ANOVA, p=0.27 and p=0.08). The HR-DeltaRRst relationship at the initial phase of exercise (for 12.5 s) was almost the same at all work rates. These results suggest that the initial HR response to exercise is strongly parasympathetically regulated independently of work rate. The HR recovered slower than the DeltaRRst at 50 and 100 W. On the HR-DeltaRRst relationship, the HR during recovery was significantly higher than during exercise at 1/3, 1/2, and 2/3 levels of pre-exercise DeltaRRst at 50 and 100 W and at the 1/3 level at 25 W (p < 0.05). At 25 W, the difference in HR at the 1/3 level was 5.5 beats.min(-1), and the HR increase to exercise was 21.2 beats.min(-1). We suggest that a HR regulatory system responds slower than a cardiac parasympathetic system to exercise, a cardiac sympathetic system, is activated even during mild exercise in humans.