Adaptations of Vascular and Cardiorespiratory Function to a 6-week Polarized Aerobic Training Model in Apparently Healthy Individuals

This study is a work package of the ACT-ON project and is designed to evaluate the effects of a 6-week polarized aerobic training intervention on vascular and cardiorespiratory function in apparently healthy sedentary adults. In addition, the study will investigate physiological adaptations across multiple cardiovascular, microvascular, neuromuscular, respiratory, and biochemical domains using a comprehensive multimodal assessment approach. The trial will be conducted as a single-centre, assessor-blinded, parallel-group randomized controlled trial comprising an intervention group and a non-exercising control group.

Thirty apparently healthy sedentary men and women aged 20-50 years will be recruited and randomly assigned in a 1:1 ratio to either a polarized aerobic training intervention or a non-exercising control condition. Eligible participants must not have diagnosed cardiovascular, metabolic, or renal disease, must not use medications influencing cardiovascular function, and must not engage in regular exercise training. Sedentary status will be verified using the Global Physical Activity Questionnaire (GPAQ) and current physical activity classification criteria. Participants with ambulatory blood pressure values exceeding 139 mmHg systolic and/or 89 mmHg diastolic, body mass index ≥30 kg/m², or other contraindications to exercise testing will be excluded.

Prior to enrollment, all interested individuals will attend a screening visit to confirm eligibility and ensure safe participation. The visit will be conducted in the morning and will include measurements of resting blood pressure (M7, Omron, Japan), body height (GPM, Model 101, Switzerland), body composition (InBody 720, Biospace, South Korea), and completion of the GPAQ. Female participants will additionally complete a Reproductive Status Questionnaire (RSQ) to further assess eligibility. Participants will be instructed to avoid vigorous physical activity, alcohol, and caffeine in the days preceding the visit and to arrive well rested and in a fasted state. Following the screening assessments, participants will be fitted with a 24-hour ambulatory blood pressure monitor (Boso TM-2430) to characterize habitual blood pressure status and minimize the potential influence of the white-coat effect. Written informed consent will be obtained from all participants prior to any study-related procedures.

Following enrollment and randomization, all participants will undergo comprehensive physiological assessments at the Faculty of Sport, University of Ljubljana. Baseline testing will encompass vascular and haemodynamic function, microvascular reactivity, cardiorespiratory fitness assessed by cardiopulmonary exercise testing for determination of ventilatory thresholds, skeletal muscle function, pulmonary function, respiratory muscle strength, resting blood pressure, and circulating biomarkers related to cardiovascular health and physiological adaptation to exercise.

All assessments will be performed under standardized laboratory conditions by trained investigators blinded to group allocation. Measurements of vascular and haemodynamic function will be conducted in a quiet, temperature-controlled environment (22-25 °C). Prior to testing, participants will rest in the supine position for at least 15 minutes to ensure haemodynamic stabilisation and will be instructed to refrain from caffeine, alcohol, and strenuous physical activity for at least 48 hours before each testing session. The same procedures will be repeated at least 72 hours after the final exercise session upon completion of the 6-week intervention period.