Subjects
Twenty male volunteers, aged from 19 to 21, were recruited from the surrounding districts of the university in this study. The subjects had no history of cardiovascular disease or any other medical disorder were overweight (body mass index (BMI), 30 ± 3 kgm−2) and were not involved in any regular, planned exercise program [23] during the past 3 months. Subjects were required to have swimming skills including crawl, breaststroke or both. None of the subjects had taken cardiovascular or blood pressure medicines. During the swimming intervention, three individuals withdrew, due to lack of interest in the study. A further two individuals withdrew from the detraining, due to not ceasing swimming activity. The present study was approved by the Ethics Committee, Dalian University of Technology, China. All subjects provided written informed consent before inclusion.
Experimental design
Subjects visited the lab four times during the supervised swimming training (Fig. 1), and each subject’s visit was performed at the same time. At the intervals of baseline, 4 and 8 weeks after swimming training and 4 weeks of detraining, body fat percentage and hemodynamics were measured on a resting day.
Swimming training protocol
Swimming training was organized at an indoor swimming pool with mean water temperature of 25.5 °C. Subjects completed supervised training three times per week for 8 weeks. Each training session consisted of 5 min stretching on land, a 5 min kicking exercise in the water, 30 min swimming, a 10 min cool down, and 5 min stretching. Swimming, including front crawl and breaststroke, was performed as interval training with rest times declining, as fitness improved. In the first 2 weeks, subjects swimming, exercised at 50% maximal heart rate (HRmax), and exercised at 65–80% HRmax intensity from 3 weeks to 8 weeks. HR was accessed by heart rate monitor (Geonaute 8360801, France).
Body fat percentage and hemodynamics measurement
Body fat percentage (BFP) measurement
Body fat percentage was measured by bioelectrical impedance (model TBF-418B, Tanita Corp, Japan). Subjects wore light clothing and no shoes. During the measurements, the subjects stood erect with feet shoulder-width apart.
Hemodynamics measurement
The inner arterial diameters and blood flow velocity waveforms measurements were examined using a high-resolution Doppler ultrasound (ProSound Alpha 7, Aloka). The heart rate, brachial systolic pressure (p
s_mea
), and diastolic pressure (p
d_mea
) were simultaneously assessed on the left upper arm with a cuff-type manometer (Patient Monitor PM8000, Mindray) and repeated in triplicate, and the average of the three values was calculated.
Calculation of hemodynamic variables
Blood pressure (BP)
In this study, the mean value of the carotid arterial pressure p
m
and diastolic pressure p
d
were assumed to be equal to the mean value of the brachial pressure p
m_mea
and diastolic pressure p
d_mea
, as performed in a previous investigation [10]. The mean arterial pressure (p
m
) was calculated using the following equation:
$$p_{m} = p_{m\_mea} = p_{d\_mea} + \frac{1}{3}\left( {p_{s\_mea} - p_{d\_mea} } \right)$$
(1)
Therefore, the carotid artery blood pressure waveform was calibrated using the brachial mean arterial p
m_mea
and diastolic pressure p
d_mea
. The maximal value of the carotid arterial pressure waveform was then calculated and assumed to be the systolic pressure p
s
.
Flow rate (FR)
The FR was computed as
$$Q = 2\pi R_{0}^{2} \int_{0}^{1} {y \cdot u(y) \cdot dy} ,$$
(2)
where R
0 is the time-averaged value of the carotid artery radius in one cardiac cycle, y = r/R
0 in which r is the radial coordinate, and u(y) satisfies [24]
$$u(y,t) = \sum\limits_{n = - \infty }^{ + \infty } {\frac{{J_{0} (\alpha_{n} j^{{\frac{3}{2}}} ) - J_{0} (\alpha_{n} j^{{\frac{3}{2}}} y)}}{{J_{0} (\alpha_{n} j^{{\frac{3}{2}}} ) - 1}}u(0,\omega_{n} )e^{{j\omega_{n} t}} } ,$$
(3)
where n is the harmonic number, J
0
is the 0th-order Bessel function of the first kind, and \(j = \sqrt { - 1}\), \(\alpha_{n} = R_{0} \sqrt {{{\rho \omega_{n} } \mathord{\left/ {\vphantom {{\rho \omega_{n} } \eta }} \right. \kern-0pt} \eta }}\) is the Womersley number. ρ is the density of blood, η is blood viscosity. η and ρ, in the present study, were taken as the same values for all subjects, i.e., η = 0.004 Pa·s and ρ = 1050 kg/m3, respectively. ω
n
= 2nπf is the angular frequency, and f is the base frequency. u(0, ω
n
) is the n harmonic component of the measured center-line velocities. The maximal harmonic number n was computed as 20 and satisfies
$${\text{u}}(0,t) = \sum\limits_{n = - \infty }^{ + \infty } {u\left( { 0 ,\omega_{n} } \right)e^{{j\omega_{n} t}} } .$$
(4)
V
max
, V
min
, and V
mean
are the maximal, minimal, and mean center-line velocities, after one cardiac cycle. Q
max
, Q
min
, and Q
mean
are the maximal, minimal, and mean blood flow FR, after one cardiac cycle.
β-stiffness index (β)
β was calculated as a means of adjusting arterial compliance for changes in distending pressure as follows [8]:
$$\beta = \frac{{{ \ln }\left( {{{{\text{p}}_{\text{s}} } \mathord{\left/ {\vphantom {{{\text{p}}_{\text{s}} } {{\text{p}}_{\text{d}} }}} \right. \kern-0pt} {{\text{p}}_{\text{d}} }}} \right)}}{{{\text{R}}_{\text{s}} - R_{d} }} \cdot R_{d} .$$
(5)
Peripheral resistance (RP)
$$R_{p} = \frac{{p_{mean} }}{{Q_{mean} }}$$
(6)
Wall shear stress (WSS)
The blood flowing along the vascular vessel creates a tangential friction force, known as wall shear stress (τ
w
), and was computed as [24]:
$$\tau_{w} = \frac{\eta }{{R_{0} }}\sum\limits_{n = - \infty }^{ + \infty } {\frac{{\alpha_{n} j^{{\frac{3}{2}}} J_{1} (\alpha_{n} j^{{\frac{3}{2}}} )}}{{J_{0} (\alpha_{n} j^{{\frac{3}{2}}} ) - 1}}u(0,\omega_{n} )e^{{j\omega_{n} t}} } ,$$
(7)
where J
1
is the first-order Bessel function of the first kind. τ
w_max
, τ
w_min
, and τ
w_mean
refer to the maximal, minimal, and mean shear stress waveforms, after a cardiac cycle.
Oscillatory shear index (OSI)
The OSI is an index that describes the shear stress acting in directions other than the direction of the temporal mean shear stress vector and was defined by Ku et al. [25] as
$${\text{ OSI = }}\frac{1}{2}\left( { 1{ - }\frac{{\left| {\int_{ 0}^{T} {\tau_{\text{w}} dt} } \right|}}{{\int_{0}^{T} {\left| {\tau_{\text{w}} } \right|} dt}}} \right)$$
(8)
where, T is the period of one cardiac cycle.
Statistical analysis
For data management and analysis, SPSS 20.0 software (SPSS Inc., Chicago, IL, USA) was used. All values were presented as the mean ± SD. The repeated ANOVA was used to assess differences between baseline and each measurement. When significant differences were detected, Tukey’s test was used for post hoc comparisons. The significance level was set at P = 0.05.
