Sedentary behavior is a major risk factor for the development of cardiovascular diseases (CVD), which remain the leading cause of morbidity and mortality globally (1). Prolonged physical inactivity is associated with increased arterial stiffness, poor cardiorespiratory fitness, elevated blood pressure, and metabolic disturbances that collectively heighten the risk of adverse cardiovascular outcomes (2). Regular aerobic exercise is widely recognized as a key non-pharmacological intervention for improving cardiovascular health. However, the optimal mode and intensity of exercise for sedentary individuals remain under debate.

Moderate-Intensity Continuous Training (MICT) has traditionally been recommended for cardiovascular conditioning and is characterized by prolonged activity at a steady-state intensity, typically between 60–70% of maximum heart rate (3). While effective, the time commitment required for MICT may reduce adherence, especially among individuals with sedentary lifestyles.

High-Intensity Interval Training (HIIT), which consists of repeated bouts of vigorous activity interspersed with periods of low-intensity recovery, has recently gained popularity due to its time efficiency and promising cardiovascular benefits (4). Several studies suggest that HIIT may produce similar or superior improvements in VO₂max, endothelial function, and blood pressure when compared to MICT, even with lower total exercise volume (5). However, despite the growing interest in HIIT, limited research has directly compared its effects with MICT in previously sedentary adults, particularly in a controlled setting.

This study aims to compare the effects of an 8-week HIIT and MICT intervention on key cardiovascular parameters, including resting heart rate, blood pressure, and VO₂max, in sedentary adults. The findings are expected to provide insights into the relative efficacy of these two training modalities for cardiovascular improvement in this population.

Study Design and Participants

This randomized controlled trial was conducted over 8 weeks and included 60 sedentary adults aged between 25 and 45 years. Participants were recruited through community advertisements and underwent screening to confirm sedentary status, defined as engaging in less than 30 minutes of structured physical activity per week for the past 6 months. Individuals with a history of cardiovascular disease, hypertension requiring medication, or musculoskeletal disorders were excluded.

Randomization and Group Allocation

Participants were randomly assigned to either the High-Intensity Interval Training (HIIT) group (n=30) or the Moderate-Intensity Continuous Training (MICT) group (n=30) using a computer-generated randomization sequence. Baseline measurements were recorded prior to the intervention.

Intervention Protocols

The HIIT group performed four repetitions of 4-minute high-intensity intervals at 85–95% of their maximum heart rate (HRmax), interspersed with 3-minute active recovery periods at 50–60% HRmax. Sessions were conducted three times a week and included a 5-minute warm-up and 5-minute cool-down.

The MICT group engaged in continuous aerobic exercise at 60–70% HRmax for 45 minutes per session, also three times weekly. Both training sessions were supervised by certified exercise professionals to ensure adherence and safety.

Outcome Measures

Primary outcomes included resting heart rate (RHR), systolic blood pressure (SBP), diastolic blood pressure (DBP), and maximal oxygen uptake (VO₂max). These parameters were assessed at baseline and after 8 weeks of intervention.

• RHR and Blood Pressure: Measured using an automated digital sphygmomanometer after 10 minutes of seated rest in a quiet room.
• VO₂max: Estimated using a submaximal treadmill test with indirect calorimetry, following the Bruce protocol.

Statistical Analysis

Data were analyzed using SPSS software version 26.0. Continuous variables were expressed as mean ± standard deviation. Paired t-tests were used to compare within-group differences, while independent t-tests assessed between-group differences. A p-value of <0.05 was considered statistically significant.

All 60 participants completed the 8-week intervention without any adverse events. Both HIIT and MICT groups demonstrated significant improvements in cardiovascular parameters, but the HIIT group showed greater changes across most measures.

Resting Heart Rate (RHR):

The HIIT group experienced a more pronounced reduction in RHR compared to the MICT group. The mean RHR decreased by 9.6 ± 2.1 bpm in the HIIT group and by 5.3 ± 1.8 bpm in the MICT group (p = 0.004 between groups).

Blood Pressure:

Systolic and diastolic blood pressure significantly declined in both groups. The HIIT group had a greater mean reduction in SBP (11.4 ± 3.2 mmHg) and DBP (7.2 ± 2.5 mmHg) than the MICT group (6.7 ± 2.9 mmHg for SBP and 4.1 ± 2.3 mmHg for DBP; p < 0.01 for both comparisons).

Maximal Oxygen Uptake (VO₂max):

There was a significant increase in VO₂max in both groups post-intervention. The HIIT group showed an improvement of 5.1 ± 1.3 mL/kg/min, whereas the MICT group improved by 3.2 ± 1.0 mL/kg/min (p = 0.001), indicating a superior effect of HIIT on aerobic capacity.

Table 1: Comparison of Cardiovascular Parameters Before and After 8-Week Intervention

Table 1 displays the changes in cardiovascular function parameters in both groups before and after the 8-week training program.

The findings indicate that HIIT is more effective than MICT in improving key cardiovascular markers in sedentary adults (Table 1).

Table 2: Percentage Change in Cardiovascular Parameters After 8 Weeks of Training

Table 2 shows the relative percentage improvements in cardiovascular parameters, demonstrating the greater impact of HIIT on cardiovascular health.

These percentage values provide additional clarity on the magnitude of improvement and are particularly useful for clinical interpretation and comparative research. (Table 2)

The findings of this study indicate that both High-Intensity Interval Training (HIIT) and Moderate-Intensity Continuous Training (MICT) significantly improved cardiovascular parameters in sedentary adults, including resting heart rate, blood pressure, and VO₂max. However, HIIT demonstrated greater efficacy across all measured outcomes, suggesting it may be a more time-efficient and potent intervention for improving cardiovascular health in this population.

The reduction in resting heart rate observed in the HIIT group aligns with previous research indicating that high-intensity training enhances autonomic regulation and promotes parasympathetic dominance (1,2). A lowered resting heart rate is associated with reduced cardiac workload and improved cardiovascular efficiency, both of which are protective against cardiac events (3). MICT also reduced heart rate, though the effect size was smaller, possibly due to the lower intensity stimulus failing to elicit the same level of autonomic adaptation (4).

Blood pressure reduction was another prominent outcome. The HIIT group showed significantly greater reductions in both systolic and diastolic blood pressure compared to the MICT group. These results are consistent with studies that have demonstrated the superior effects of HIIT on vascular endothelial function and arterial compliance (5,6). The mechanisms may involve greater shear stress during high-intensity bouts, stimulating nitric oxide production and improving vasodilation (7,8). Similar findings have been observed in hypertensive populations, reinforcing the relevance of HIIT for clinical prevention and management of cardiovascular diseases (9).

Improvements in VO₂max were significantly higher in the HIIT group, echoing the results of multiple meta-analyses suggesting HIIT elicits superior aerobic capacity improvements compared to continuous training (10,11). VO₂max is a critical determinant of cardiovascular fitness and overall health, and its enhancement is linked to reduced all-cause and cardiovascular mortality (12). The repeated exposure to near-maximal effort in HIIT likely challenges the cardiovascular and metabolic systems more intensely, promoting faster and more substantial physiological adaptations (13).

Despite MICT being the traditional recommendation for improving cardiovascular health, our findings support the growing body of evidence favoring HIIT, especially for individuals with limited time (14,15). It is important to note, however, that adherence and safety must be considered when prescribing high-intensity protocols, particularly in populations with comorbidities or low fitness levels. In our study, no adverse events were recorded, suggesting that with proper supervision, HIIT can be safely implemented in sedentary individuals.

Some limitations of this study should be acknowledged. The sample size, while adequate for detecting significant differences, was relatively small. Furthermore, the duration of the intervention was limited to 8 weeks. Longer studies are needed to assess the sustainability of improvements and long-term adherence. Additionally, other important cardiovascular markers such as lipid profiles and inflammatory markers were not measured.

Future studies should investigate the effects of combined HIIT and MICT protocols, explore long-term cardiovascular risk reduction, and examine psychological factors influencing adherence to different exercise modalities. Personalized exercise prescriptions based on individual fitness levels, preferences, and risk factors may further optimize cardiovascular outcomes.

Both HIIT and MICT significantly improved cardiovascular health in sedentary adults; however, HIIT produced greater enhancements in resting heart rate, blood pressure, and VO₂max. Thus, HIIT may serve as a more efficient strategy for cardiovascular improvement in time-constrained individuals.

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