Acute Autonomic Responses During and After BFR Training (HOLD TIGHT)

This is a single-center, prospective, randomized crossover trial with two parallel age groups: young adults (19-24 years) and older adults (60 years and above). The study will be conducted at the Integrative Movement in Networking Systems Laboratory (IMOVE-NET), located at the Physiotherapy Clinic of Egas Moniz School of Health & Science (Almada, Portugal), between October 2026 and June 2027.

All participants will complete three visits, each separated by at least 48 hours to allow for recovery and minimize carryover effects. The first visit serves as a familiarization session, during which participants will have baseline cardiovascular measurements taken, become acquainted with the exercise protocol, and undergo one-repetition maximum (1RM) testing for the bilateral leg press and knee extension exercises. Body composition measurements will also be collected. Participants are instructed to avoid caffeine and alcohol for 24 hours, refrain from vigorous physical activity for 24 hours, and fully void before each session.

Following the familiarization visit, participants are randomly assigned to the order in which they will receive the two experimental conditions: (1) low-intensity resistance training with blood flow restriction (LIRT-BFR), and (2) high-intensity resistance training (HIRT). The randomization sequence will be generated using a web-based tool, placed in sequentially numbered, opaque, sealed envelopes, and concealed until the day of each visit. Simple randomization will be performed separately within each age group, generating independent session-order sequences for older and younger participants.

Each experimental session begins with a 15-minute resting phase in a supine position, during which heart rate and blood pressure are recorded. Beat-to-beat systolic and diastolic blood pressure will be recorded using finger plethysmography (Finapres Nova), and R-R intervals will be sampled at 300 Hz via the integrated 5-lead electrocardiogram (ECG) module. Intermittent brachial blood pressure readings from the contralateral arm (SunTech Tango+ automated auscultatory sphygmomanometer) will validate continuous Finapres measurements, particularly during exercise conditions. Continuous 2-minute segments of artifact-free R-R interval and systolic blood pressure (SBP) time series will be constructed for analysis.

Participants then perform the assigned exercise condition for approximately 30 minutes, consisting of bilateral knee extensions and leg curls. In the HIRT condition, participants perform 3 sets of 10 repetitions at 75% of 1RM with 2-minute rest intervals. In the LIRT-BFR condition, participants perform 4 sets of 20 repetitions at 30% of 1RM with a 5 cm wide pneumatic cuff applied to the most proximal portion of both thighs, inflated to 1.3 times the individual's ankle systolic blood pressure (SBP). The cuff remains inflated continuously throughout sets and rest periods and is released immediately after the final repetition. Exercise intensity, heart rate, rate of perceived exertion (RPE) using the modified Borg scale (0-10), and perceived discomfort are continuously monitored throughout both conditions. If a participant reports a discomfort score greater than 7, cuff pressure is immediately reduced from 1.3 to 1.2 times ankle SBP. If volitional fatigue prevents completion of repetitions, pressure is reduced to facilitate completion while maintaining a safe hemodynamic profile.

After each experimental condition, participants immediately return to the examination table and recover in a supine position for 60 minutes, during which blood pressure and heart rate are continuously re-evaluated. All data acquisition and offline analyses will follow relevant task force standards for heart rate variability and baroreflex sensitivity measurement.

The intervention will be administered by physiotherapy students who have completed 20 hours of technical training in exercise protocols and autonomic evaluation using the Finapres Nova system, maintaining an investigator-to-participant ratio of 2:1 to ensure safety and precision. Predefined stopping rules will be applied in the event of any adverse event or clinically relevant safety concern. The study supervisor will randomly review and audit 15% of total sessions to identify and correct procedural errors. Protocol adherence is strictly monitored and defined as the percentage of the prescribed protocol successfully completed; a minimum threshold of 70% will be required for any individual session to be considered valid for analysis.