After screening, all eligible participants underwent a familiarization that included a blood pressure measurement, a fasting blood draw, familiarity with the exercise device, and a one-repetition maximum (1RM) strength test on knee extension exercise . Muscle mass was also determined noninvasively using dual-energy X-ray absorptiometry (DEXA) (Lunar DPX, GE Fairfield, CT). Using a standard protocol , participants underwent a maximal treadmill exercise test during the first visit to obtain individual peak oxygen consumption (VO2 peak) by using a metabolic cart (MetaLyzer 3B, Cortex, Germany). In the second visit, they also performed two core rotational exercise trials with the metabolic cart to characterize physiological responses at different cadences. To effectively address our aim, we incorporate seated knee extension exercise (KE) as the active control, which has been demonstrated to increase blood flow  and is commonly used for rehabilitation. After the screening visit, subjects performed a single bout of KE exercise followed by either fast core (FC) or slow core (SC) exercise in a randomized order.
Participants performed three sets of knee extension muscle contractions for KE, producing 90–170° arc movement, using both legs simultaneously. The exercise protocol consisted of 12 repetitions in the first two sets, followed by repetitions to fatigue with verbal encouragement in the third set. Exercise intensity was set at 80%1RM for all sets. Participants were allowed to rest for 2 min between the sets. In the core exercise sessions, participants used a core exercise machine equipped with adjustable resistance (Core X. Fitcrew Inc., Taiwan), as shown in our previous work . Participants were instructed to rotate their trunk within a set range of rotation (at least 140°). Handgrip assistance in the same position was allowed to guard against the machine’s spring resistance (13.5 kg). Hence, during exercise, participants were required to work against this spring resistance to achieve the required range of motion for 30 min. We provided a metronome cadence for participants during exercise; a cadence of 75 rpm for FC and 20 rpm for SC was set as different exercise intensities. To our knowledge, no study had determined the effects of cadence on physiological responses in this exercise mode; we chose the cadence based on our participants’ comfort from our pilot work and the previous study . All participants were able to complete all exercise protocols with verbal encouragement.
Sixteen healthy, recreational young active male adults were recruited to participate in this study after screening. Exclusion criteria included:  obesity (BMI > 30 kg/m2) , cigarette smoking within the past six months , history of diabetes (fasting blood glucose > 126 mg/dL) and heart disease , injury that may prevent him or her from completing the exercise; and  use of over-the-counter supplements or vitamins. None of the participants exercised (resistance or endurance training) more than three times a week, as determined by a physical activity questionnaire. Recruited participants had no history of hypertension (< 140/90 mmHg) or kidney dysfunction. All participants gave their written informed consent; all procedures were performed in accordance with relevant guidelines and approved by the Institutional Review Board of the school hospital (ID:201112135RIC).
Measurements were conducted before, 30, and 60 min after exercise. Participants were asked to rest in a supine position during testing sessions. All tests were performed at the same time of day for each participant. Exercise bouts were at least 24 h apart. Participants were asked to maintain their regular diet and avoid strenuous exercise between testing sessions. They were instructed to arrive at the laboratory after a 12-h overnight fast.
Impedance cardiography (ICG) was performed using a non-invasive cardiac output module (SS31LA, Biopac, Goleta CA) and an electrocardiogram (ECG) sensor (SS2L) connected to a physiological signaling processing system (MP36, Biopac, Goleta CA) to determine stroke volume (SV), heart rate (HR) and cardiac output (CO) by following the manufacturer’s instructions and the guidelines . Mean arterial blood pressure (MAP) was determined by the automatic vascular testing device (MAP = 1/3SBP + 2/3DBP). Systemic vascular resistance (SVR) was also calculated by the equation: SVR (dyne/cm2) = 80 × MAP/CO . The day-to-day coefficients of variation for BP and CO in our laboratory were less than 3% and 10%, respectively.
The participants were instructed to rest quietly in the supine position for at least 10 min before measurement. Brachial blood pressure, systemic arterial stiffness, and brachial-ankle pulse wave velocity (baPWV) were obtained using an automated vascular testing device (VP-1000 plus, Omron Healthcare). All measurements were duplicated, and average values were used for subsequent analyses.
Longitudinal images of the common carotid artery 1-2 cm proximal to the bifurcation were obtained noninvasively in the supine position using an ultrasound machine (Sonosite Ultrasound System, Bothell, WA). Vessel diameter was determined at a perpendicular angle along the scanned area axis. Blood velocity was assessed with the transducer appropriately positioned to maintain an insonation angle at 60° or less. Images were analyzed using image analysis software (ImageJ, NIH, Bethesda, MD) that included at least ten consecutive cycles. Femoral blood flow was calculated as mean blood velocity × π × (femoral diastolic diameter/2) 2 × 60, ml/min . Femoral conductance was calculated as femoral blood flow/mean arterial pressure. An independent investigator also performed all ultrasound imaging and analyses, and our laboratory’s day-to-day coefficients of variation for femoral artery diameter and velocity are 1.62 and 1%, respectively.
Statistical analyses were performed using Graph Pad Prism 8.0 (La Jolla, CA). All data are reported as mean ± SEM. Two-way repeated-measures ANOVA with a Bonferroni post hoc analysis was used to determine the effects of exercise and time on measured parameters. Significance was set a priori at p < 0.05.