Subjects
Forty healthy men (resistance-trained group; n = 20 and control group; n = 20) were recruited to participate in this study. The subjects in the resistance-trained group had been performing vigorous RT for > 2 years, > 5 days/week. None of the subjects in the control group regularly engaged in RT, and performed other exercise training < 3 days/ week. All of the subjects were normotensive (< 140/90 mmHg), had no history of cardiovascular diseases, diabetes, smoking, and were not taking medications such as anabolic steroids.
All of the subjects provided written informed consent to participate prior to the start of the study. All of the procedures and risks of this study were reviewed and approved by the Human Research Committee of Waseda University (approval No. 2019-102). The study adhered to the principles of the Declaration of Helsinki.
Protocol
The studies were performed following a 3-hour fast. Subjects were required to avoid caffeine intake for at least 12 hours and alcohol intake for at least 24 hours before participating in the study. The subjects were evaluated 24 hours after their last exercise session to avoid the acute effects of exercise.
All subjects underwent measurement of haemodynamic and carotid arterial variables, and cardiovagal BRS in a temperature- and humidity-controlled environment (Temperature: 22.0 ± 0.1 °C, Humidity: 50.0 ± 0.3 %) after 15 min in a resting supine position.
Body composition
Body composition was measured using bioelectrical impedance analysis (InBody 720, InBody Japan Inc., Tokyo, Japan) with the subject in the upright position.
Muscular strength
Handgrip strength was measured using a grip dynamometer (Grip-D, Takei Scientific Instruments Co., Ltd., Japan) as an index of muscular strength with the subject in the standing position. The subjects were instructed to stand and extend their arms by their sides during a hand grip execution and to grip the dynamometer with full effort for three seconds. The values (kg) were calculated as the average of two trials.
Central arterial stiffness and compliance
The β-stiffness index and arterial compliance of the carotid artery were measured as an index of central arterial stiffness and compliance, respectively. Both the β-stiffness index and arterial compliance were measured in the right carotid artery using a combination of a brightness mode ultrasonography system for the carotid artery diameter, and applanation tonometry for the carotid BP. The carotid artery diameter was obtained 1.0 – 2.0 cm proximal to the carotid bifurcation using an ultrasonography system equipped with a 10-MHz linear transducer (LOGIQ-e, GE Medical Systems, Japan). The diameter was recorded over ten cardiac cycles with the brightness mode in the longitudinal section. The images obtained were analyzed using image analysis software (ImageJ, NIH, USA), and these images were used to analyze the systolic diameter (sD) and diastolic diameter (dD).
The carotid pressure waveform was obtained in the right carotid artery. The obtained pressure waveforms were converted from a pencil-type probe incorporating a high-fidelity strain-gauge transducer (SPT-301, Millar Instruments, TX, USA) at a sampling rate of 1000 Hz through an analogue/digital converter (PowerLab/16SP, AD Instruments, Australia) and recorded in a device connected to a personal computer (Macbook, Apple, USA). Then, the obtained data were analyzed using an analysis software (LabChart5, AD Instruments, Australia). The carotid arterial pressure was calibrated by equating the carotid diastolic blood pressure (DBP) and mean arterial pressure (MAP) to the brachial artery value [21]. The β-stiffness index and arterial compliance were calculated as follows [10,11,17,18]:

Carotid arterial intima-media thickness
The carotid arterial intima-media thickness (CA IMT) was measured 1.0 – 2.0 cm proximal to the carotid bifurcation with an ultrasonography system equipped with a 10-MHz linear transducer (LOGIQ-e, GE Medical Systems, Japan). The obtained images were analyzed using image analysis software (ImageJ, NIH, USA). At least ten CA IMT measurements were taken, and the mean value was used for analysis [10,11,17,18].
Haemodynamics
HR and beat-to-beat ABP were acquired using a three-lead ECG (BSM-2401, NIHON KOHDEN, Japan) and finger photoplethysmography (Finapres Medical Systems, Amsterdam, The Netherlands), respectively. The photoplethysmograph was attached to the middle finger of the right hand. Additionally, stroke volume (SV) was calculated from the obtained ABP waveform using the Modelflow method [22,23], which incorporates age, height and weight, and simulates aortic flow waveforms from an arterial pressure signal using a nonlinear three-element model of the aortic input impedance (Beatscope, version 1.1, Finapres Medical Systems). CO and TPR were then calculated as SV × HR and MAP / CO, respectively.
Cardiovagal baroreflex sensitivity
The cardiovagal BRS was estimated with the Valsalva manoeuvre test that evaluated the slope of the R-R interval and systolic blood pressure (SBP) during phase IV of the Valsalva manoeuvre [2-4,24]. The subjects maintained an expiratory mouth pressure of 40 mmHg for 15 seconds by blowing through a short tube connected pressure gauge after deep inspiration. The pressure values were displayed to provide visual feedback to the subjects. Immediately after, subjects were instructed to maintain normal respiration, and to avoid deep respiration. The beat-to-beat R-R interval and SBP were obtained from a three-lead electrocardiogram (ECG) and a finger photoplethysmograph, respectively. The waveform of the two variables were digitally converted at a sampling rate of 1000 Hz through an analogue/digital converter and recorded in a device connected to a personal computer, and analyzed with analysis software. Furthermore, the two variables were linearly regressed from the point at which the R-R interval began to lengthen, and continued to the point of maximal SBP elevation (Fig. 1a). Then, the slope of linear correlation between the R-R interval and the SBP was assessed as a cardiovagal BRS, which was determined if the r value was > 0.8 as previously described (Fig. 1b) [25].
Statistical analysis
All values were presented as mean ± standard error of the mean (SEM). Statistical analyses were performed using statistical analysis software (SPSS version 26.0 for Mac, IBM, Japan). The mean differences in the two groups were examined using the Student’s unpaired t test. Pearson correlations were used to assess the relationship between cardiovagal BRS and β-stiffness index, and between cardiovagal BRS and arterial compliance. In all of the analyses, the level of significance for all comparisons was set at P < 0.05.