Effects of accumulated exercise on the stiffness and hemodynamics of the common carotid artery

Purpose: This research aims to study and compare the effects of moderate-intensity continuous exercise and accumulated exercise with different number of bouts on common carotid arterial stiffness and hemodynamic variables. Methods: Thirty healthy male adults were recruited to complete four trials in a randomized crossover design: no-exercise (CON); continuous exercise (CE, 30-min cycling); accumulated exercise including two or three bouts with 10-min rest intervals (AE15, 2 × 15-min cycling; AE10, 3 × 10-min cycling). The intensity in all the exercise trials was set at 45%–55% heart rate reserve. Blood pressure, right common carotid artery center-line velocity, and arterial inner diameter waveforms were measured at baseline and immediately after exercise (0 min), 10 min, and 20 min. Results: 1) The arterial stiffness index and pressure–strain elastic modulus of the CE and AE15 groups increased significantly at 0 min, arterial diameters decreased in AE15 and AE10, and all indicators recovered at 10 min. 2) The mean blood flow rate and carotid artery center-line velocity increased in all trials at 0 min, and only the mean blood flow rate of AE10 did not recover at 10 min. 3) At 0 min, the blood pressure in all trials was found to be increased, and the wall shear stress and oscillatory shear index of AE10 were different from those in CE and AE15. At 20 min, the blood pressure of AE10 significantly decreased, and the dynamic resistance, pulsatility index, and peripheral resistance of CE partially recovered. Conclusion: There is no significant difference in the acute effects of continuous exercise and accumulated exercise on the arterial stiffness and diameter of the carotid artery. Compared with continuous exercise, accumulated exercise with an increased number of bouts is more effective in increasing cerebral blood supply and blood pressure regulation, and its oscillatory shear index recovers faster. However, the improvement of blood flow resistance in continuous exercise was better than that in accumulated exercise.

The detected arterial diameter waveform (Figure 2 We adopted the non-invasive method that used a formula with a form factor equal to 33% to calculate carotid pressure.Recent studies show that among the different approaches used to calculate carotid pressure, the equation that the form factor equal to 33% showed the best association with the invasive measured, when the SBP does not exceed 130mmHg (Bia et al., 2023b)(Bia et al., 2023a).The mean arterial pressure (Pm) was calculated by using the following equation:

Flow Rate (Q)
The flow rate was calculated for where R0 is the average of the radius of the common carotid artery over time during a cardiac cycle.t is the period of one cardiac cycle.y = r/R0, in which r is the radial coordinate.u (y, t) satisfies: where 0 is the 0th-order Bessel function of the first kind and = −1. is the Womersley number and is the harmonic number.= 0 ω / , in which ρ is the density of blood and η is blood viscosity.Due to the limited experimental conditions, and ρ, in the present study, were taken as the same values for all subjects.= 0.004 Pa•s and ρ = 1050 kg/m 3 , respectively.ω = 2 is the angular frequency, and is the base frequency.(0, ω ) is the harmonic component of the measured center-line velocities.The maximal harmonic number was computed as 20 and satisfies:

Apparent Elastic Modulus (Ep)
Arterial elastic function reflects the degree of change in arterial volume caused by changes in blood pressure per unit.The apparent elastic modulus was computed as:

Apparent Stiffness Index (β)
was calculated as the mean of adjusting arterial compliance for changes in normal stress as follows (Rossow et al., 2010):

Wall Shear Stress (WSS)
The blood flowing along the vascular vessel creates a tangential frictional force, known as wall shear stress (τw), and was computed as: where 1 is the first-order Bessel function of the first kind.

Oscillatory Shear Index (OSI)
The oscillatory shear index is an index that describes the ratio of the retrograde shear stress to the total shear stress and was defined as:

Pulsatility Index (PI)
The pulsatility index is used to express the relationship between blood flow pulsation and arterial pulsation and was calculated by the following equation: where Vmax, Vmin, and Vmean are the maximum, minimum, and mean blood center-line velocities, respectively.

Dynamic Resistance (DR)
Dynamic resistance represents the ability of arterial regulation.The dynamic resistance was calculated as follows: where Qmax and Qmin are the maximum and minimum value of blood flow rate, respectively.
(A)) and center-line velocity (Figure 2(B)) were saved only as images.The self-compiled program in Matlab was used to extract blood vessel diameter (Figure 2(C)) and center-line velocity waveforms (Figure 2(D)).Heart rate signals were used to synchronize diameter and center-line velocity waveforms (beat-to-beat recording), then the carotid arterial blood pressure waveform (Figure 2 (E)) was calibrated using brachial pressure and arterial diameter waveforms.The calibration formula is as follows: Ps_mean and Pd_mean are the systolic and diastolic pressures of the brachial artery, Dmax and Dmin are the max and min values of carotid artery diameter.Di and Pi are the diameters and the calculated values of blood pressure at the same time.The maximum and minimum values of Pi are the calibrated SBP (PS) and DBP (Pd), respectively.