Participants
The present study consisted of two studies: Experiment 1 (Ex1) and Experiment 2 (Ex2). We used G*power software (version 3.1.5.1; Heinrich-Heine University, Dusseldorf, Germany) to calculate a sample size necessary to yield an estimated effect size of 0.8, an alpha level of 0.05, and statistical power of 0.8 for a paired t-test between the two trials [23]. This estimation revealed 15 participants as the number necessary for appropriate evaluation. Twenty-four recreationally active adults (20 males, 4 females; age, 19.8 ± 1. 2 y; range, 18–22 y; mean height, 167.9 ± 7.5 cm; mean weight, 68.6 ± 8.4 kg; mean body fat, 16.4 ± 6.1%, mean ±standard deviation [SD]) participated in Ex1, which involved a graded power test on a cycle ergometer as well as PWV measurement. Separately, 10 different recreationally active adults (7 males, 3 females; mean age, 21.2 ± 0.6 y; range, 18–22 y; mean height, 168.2 ± 3.7 cm; mean weight, 61.9 ± 4.5 kg; mean body fat, 14.9% ± 4.6%) participated in Ex2, which involved a 1500-m run as well as PWV measurement. All participants had performed recreationally activity (e.g., walking, jogging, cycling, etc.) in 1–2 sessions per week prior to starting the study. The International Physical Activity Questionnaire-Short Form was used to assess physical activity. In both experiments, all participants showed normal BP (< 130/85 mmHg). Exclusion criteria were: smoking; obesity (body mass index > 30 kg/m2); diseases or disorders affecting physical activity; pharmacotherapy compromising the cardiovascular system, including antihyperlipidemic, antihypertensive, or antihyperglycemic medications; pregnancy in females; or administration of oral contraceptives. No participants reported a history of cardiovascular disease.
Written, informed consent was obtained from all participants after receiving a full explanation of the study purpose and experimental procedures. The present study was conducted in accordance with the Declaration of Helsinki and its later amendments, and with the guidelines for experimental studies involving human participants published by our institutional review board (014-H34).
Study design
This study examined the effects of arterial stiffness on maximal aerobic capacity. We tested the hypothesis that short-term individual variation in arterial stiffness affects cardiorespiratory fitness capacity and aerobic performance. In both experiments, participants visited the laboratory twice to perform the experimental protocol. Both visits were separated by about 1-month intervals (mean, 28 days). All female participants participated in the experiments during the early follicular phase of their menstrual cycle at about 1-month intervals to minimize any potential hormonal effects. To unify the measurement period, a measurement interval of 1 month was also applied to male participants. To eliminate any potential effects of food intake, all measurements were recorded at the same time (08:00–11:00) after overnight fasting. All participants were instructed to abstain from engaging in intense physical activity or consuming alcohol within 48 h of the beginning of the study, and from consuming any caffeine-containing beverages within 24 h of the beginning of the study. The participants were also instructed to maintain their regular lifestyle and not to change their diet, engage in any activities such as massages or stretching, or take any pharmacotherapies during the experimental period. To ensure the same level of physical effort during measurements, participants remained blinded to the results until completion of the second measurement.
In consideration of the effects of diurnal variation, measurements of baPWV, brachial BP, and heart rate (HR) were taken in a quiet room at a constant temperature (23–25 °C) at around the same time. By contrast, baPWV, brachial BP, and HR measurements were obtained before each exercise protocol. After the measurements, the participants performed an aerobic performance test to assess maximal aerobic power using a graded power test on a cycle ergometer (Ex1) or a 1500-m run (Ex2). The same investigator measured all parameters. We defined the change in V̇O2max or 1500-m run time between first and second visits as its variability in this study.
Measurement of V̇O2max
We conducted a graded power test using an electronically braked cycle ergometer (Corival 1000ss; Lode Co., Groningen, the Netherlands) as an aerobic performance test to assess V̇O2max. We started the test at 30 W and then increased the workload by 15 W every minute until exhaustion. V̇O2max, carbon dioxide production (V̇CO2), and respiratory exchange ratio (RER: measured as V̇CO2/V̇O2) was monitored breath-by-breath using a metabolic measurement cart (AE-310S; Minato Medical Science, Osaka, Japan). V̇O2max was defined as the highest 30-s averaged oxygen consumption when VO2 plateaued, concurrent with a RER > 1.15 [24].
1500-m run
Time trials for the 1500-m run were performed at an indoor track. In the two trials, relative humidity (i.e., between 54 and 58%) and temperature (i.e., between 18 °C and 22 °C) were similar. Before the 1500-m run, a 5-min warm-up including stretching and/or light jogging was performed. Each participant ran the 1500 m alone to remove any feeling of competition. In the two trials, participants wore the same clothing and shoes and were instructed to complete the 1500-m run in the fastest time possible. The 1500-m run time was recorded by three experienced assessors using stopwatches. Assessors were instructed to start the stopwatch by a starting signal and to stop the stopwatch when the participant crossed the finish line with the body, excluding the head, neck, arms or legs. The median value from the three assessors was used as the time for the 1500-m run.
Pulse wave velocity
We used automatic volume plethysmography (Form PWV/ABI; Fukuda-Colin Co., Tokyo, Japan) to measure baPWV. All measurements were taken after the participants had rested in the supine position for a minimum of 20 min, in reference to a previous report [25]. For each participant, electrodes for an electrocardiogram were placed on both wrists, a microphone was placed on the left edge of the sternum to detect heart sounds, and cuffs connected to a plethysmographic sensor and an oscillometric pressure sensor were wrapped around both arms and ankles to determine volume pulse forms and measure BP, respectively. We used a semiconductor pressure sensor to record the pulse volume waveforms. The sampling time was 10 s, with automatic gain analysis and quality adjustment.
We calculated the path length from the suprasternal notch to the measuring point in the brachial region (Lb) using the following equation: Lb = 0.2195 × height of the participant (cm) – 2.0734. To obtain the path length from the suprasternal notch to the ankle (La) from body surface measurements, we used the following equation: La = 0.8129 × height of the participant (cm) + 12.328. We calculated the distance between the two baPWV recording sites based on the height of the participant and anthropomorphic data for the Japanese population using the following equation:
$$ \mathrm{baPWV}=\left(\mathrm{La}-\mathrm{Lb}\right)/\mathrm{Tba}. $$
Two measurements were performed on each measurement day and the coefficient of variation for interobserver reproducibility of baPWV was 3%. In addition, the day-to-day reproducibility of measurements for PWV was 6%. These values mostly coincided with data from previous studies that identified the repeatability of PWV measurements [20,21,22, 25, 26].
Resting brachial BP and HR
We measured resting HR and systolic/diastolic BP values simultaneously using electrocardiography and an automatic oscillometric device (Form PWV/ABI; Fukuda-Colin Co.), respectively. All data were recorded in triplicate while the participants were in the supine position. The pressure signal obtained by plethysmography was calibrated by equating systolic and diastolic BP values; this was then used to calculate mean arterial pressure [25].
Statistical analysis
All data are expressed as mean ± SD. Statistical analyses were performed using Statistica software (SPSS ver. 24; SPSS, Chicago, IL). The assumption of a normal distribution was confirmed for all data using the Shapiro-Wilk test. Comparisons of these parameters were tested for significance using the paired t-test. Relationships between ΔV̇O2max, Δ1500-m run time (change from first to second visit) and ΔbaPWV were analyzed using Pearson’s correlation coefficients. The relative effect size for the performance data was calculated using Cohen’s d and defined as small (d = 0.2), medium (d = 0.5), or large (d = 0.8) [23]. In addition, 95% confidence intervals (CIs) are provided. Significance was set at the level of P < 0.05.