Males and females show distinct differences in cardiovascular function, especially during exercise. Evidence suggests that females generally have lower blood pressure due to hormonal influences that reduce blood vessel constriction. This protective effect may contribute to a lower risk of high blood pressure and cardiovascular diseases in females compared to males. Additionally, females tend to have different patterns of heart rate control and blood pressure regulation, largely due to differences in the autonomic nervous system and hormonal balance.

One key mechanism involved in blood pressure control is the baroreflex, which helps stabilize blood pressure by adjusting heart rate and blood vessel function. The sensitivity of the baroreflex (known as baroreflex sensitivity, or BRS) can vary between males and females. For instance, females may exhibit slower adjustments in heart rate in response to blood pressure changes, but they also tend to have better blood pressure regulation during moderate exercise.

The main purpose of this study is to explore whether these sex-specific differences extend to more intense exercise. Investigators will compare the responses of young, active males and females to cycling at two exercise intensities: moderate (50% of Heart Rate Reserve, HRR) and vigorous (80% HRR). As a secondary purpose, investigators we will assess the inter-day repeatability of HRV and BRS parameters measured at rest and during moderate and vigorous intensity exercise utilizing sequence and spectral indices in apparently healthy young adult males and females.

We hypothesize that:

1. Females will show higher HRV and BRS at rest compared to males, indicating better heart rate control.
2. During exercise, females will have greater cardiovagal (heart-related) activation than males, regardless of exercise intensity.
3. When exercise sessions are matched for blood pressure, there will be no significant sex differences in HRV and BRS responses.
4. Sequential methods techniques can be used to assess cardiovagal BRS and HRV changes between days during exercise in young adults, regardless of their sex. In addition, the inclusion of vigorous intensity may improve the repeatability of the outcome measure due to sympathetic outflow predominance.

Forty healthy and physically active participants (20 males and 20 females) aged 18-31 were recruited for this study. All participants reported to the laboratory in a fasted state (>4 hours) and refrained from consuming foods or drinks containing caffeine or alcohol for at least 12 hours. They also avoided strenuous exercise for at least 24 hours prior to the testing sessions. The menstrual cycle was not controlled for female participants. All participants provided written informed consent after receiving a detailed explanation of the study's aims and experimental procedures.

This study was constructed as a crossover trial. Participants attended 4 separate intervention sessions. On the first visit, participants had their body composition evaluated by a clinical bioimpedance device using 8 electrodes positioned on each hand and foot. Each participant also performed a ramp incremental cycle ergometer test to exhaustion on a recumbent stationary cycle ergometer (Excite Recline Med, TechnoGym, Italy). On the second and third visit, participants quietly rested for 15 min in the supine position to evaluate heart rate variability (HRV) using a 5-ECG lead module (Nova, Finapres Measurement Systems, Amsterdam, The Netherlands), carotid (Arietta V60 ultrasound; Aloka/Hitachi Medical Systems) and aortic (Complior Analyse software (ALAM Medical, Paris, France) arterial stiffness. Participants then performed a 6 minutes bout of moderate intensity continuous exercise (MICE) at 50% heart rate reserve [HRR= 0.5*(HRmax- HRrest) + HRrest] followed by a 6 minutes vigorous intensity continuous exercise (VICE) bout at 80% HRR on a recumbent stationary bike, while having their HRV, beat-by-beat blood pressure and carotid arterial stiffness evaluated at HR steady state at each intensity. Visits 2 and 3 provided the data for the repeatability objective and hypothesis. On the fourth visit, the participants performed an acute bout of MICE and VICE, but this time at the corresponding average blood pressure at each intensity from visits 2 and 3. As men show higher blood pressure values during exercise, in the fourth visit we matched their intensity of exercise with the average blood pressure attained by women during steady state at each intensity.

Blood pressure and HR were monitored beat-by-beat and carotid arterial stiffness was measured at BP steady-state.

All visits were conducted in the morning with each participant performing sessions at the same time of the day to minimize any potential diurnal variation.

Inclusion Criteria:

• Males and females aged 18 to 44 years old.
• Physical active as assessed by the International Physical Activity Questionnaire (IPAQ).
• Low risk to increase physical Activity as assessed by the Physical Activity Readiness Questionnaire (PAR-Q+).

Exclusion Criteria:

• Smoking
• Diagnosed with cardiac, metabolic or renal diseases
• Taking cardioactive medication
• Resting hypertension (systolic blood pressure >140 mmHg, diastolic blood pressure > 90 mmHg).

Data obtained in this study may be provided to qualified researchers with academic interest in the effects of acute exercise on the autonomic function. Data or samples shared will be coded, with no PHI included. Approval of the request and execution of all applicable agreements (i.e. a material transfer agreement) are prerequisites to the sharing of data with the requesting party.

Data requests can be submitted starting 9 months after article publication and the data will be made accessible for up to 24 months. Extensions will be considered on a case-by-case basis.

Access to trial IPD can be requested by qualified researchers engaging in independent scientific research and will be provided following review and approval of a research proposal and Statistical Analysis Plan (SAP) and execution of a Data Sharing Agreement (DSA). For more information or to submit a request, please contact xmelo@egasmoniz.edu.pt

Participants were blinded to the order of the visits until arrival at the laboratory.