Cardiovascular and pulmonary functions are essential indicators of overall physical fitness and endurance. Young adulthood is a critical period where establishing a foundation of cardiorespiratory health can have long-term benefits, particularly in preventing future non-communicable diseases such as hypertension, diabetes, and chronic obstructive pulmonary disease (1,2). Exercise interventions have long been studied for their effects on these parameters, and among them, High-Intensity Interval Training (HIIT) has emerged as a time-efficient and physiologically beneficial model.

HIIT involves short bursts of vigorous activity interspersed with periods of low-intensity recovery or rest. It has gained attention for promoting cardiovascular efficiency, increasing mitochondrial density, and improving aerobic and anaerobic performance (3,4). Previous studies have demonstrated significant improvements in VO₂ max, endothelial function, and metabolic markers following structured HIIT regimens (5,6). However, while the metabolic and aerobic benefits of HIIT are well-documented, its impact on specific parameters such as cardiac output and pulmonary function remains less explored, especially in the young, healthy population.

Cardiac output, a key determinant of oxygen delivery during physical activity, is influenced by both stroke volume and heart rate. Pulmonary function tests, such as Forced Vital Capacity (FVC), Forced Expiratory Volume in 1 second (FEV1), and Peak Expiratory Flow Rate (PEFR), are essential for assessing respiratory mechanics and lung health (7,8). Understanding how HIIT influences these physiological metrics can help optimize exercise prescriptions for preventive and rehabilitative health strategies in youth.

Therefore, this study aims to assess the impact of regular HIIT on cardiac output and pulmonary function among healthy young adults, thereby contributing to the growing body of literature on exercise physiology and preventive cardiopulmonary care.

A total of 60 healthy young adults aged between 18 and 25 years were recruited and divided into two groups. Group A included 30 individuals who had been regularly participating in High-Intensity Interval Training (HIIT) for a minimum of six months, at a frequency of at least three sessions per week. Group B comprised 30 age- and gender-matched sedentary individuals who had not engaged in any structured exercise for the past six months. Participants with a history of cardiovascular, respiratory, or metabolic disorders were excluded.

HIIT Protocol

The HIIT participants followed a standardized regimen consisting of a 5-minute warm-up, followed by 4 to 6 cycles of 30-second high-intensity bouts at 85–95% of maximum heart rate, interspersed with 90-second low-intensity recovery phases, and ending with a 5-minute cool-down. Exercise adherence and intensity were monitored using heart rate monitors.

Measurement of Cardiac Output

Cardiac output was measured using non-invasive Doppler echocardiography in a semi-recumbent position, performed by a trained cardiologist. Three consecutive readings were taken and averaged to minimize inter-observer variability.

Pulmonary Function Testing

Pulmonary function was assessed using a calibrated digital spirometer, following American Thoracic Society (ATS) guidelines. Parameters measured included Forced Vital Capacity (FVC), Forced Expiratory Volume in 1 second (FEV1), and Peak Expiratory Flow Rate (PEFR). Participants were instructed to perform three acceptable maneuvers, and the highest values were recorded for analysis.

Anthropometric and Baseline Data

Body mass index (BMI), resting heart rate, and blood pressure were recorded using standard protocols. Height and weight were measured using a stadiometer and digital scale, respectively.

Statistical Analysis

Data were analyzed using SPSS version 25.0 (IBM Corp., Armonk, NY, USA). Continuous variables were expressed as mean ± standard deviation. The Shapiro-Wilk test was used to assess normality. Comparisons between the two groups were performed using unpaired t-tests for normally distributed variables and Mann-Whitney U tests for non-parametric data. A p-value of <0.05 was considered statistically significant.

The study included 60 participants divided equally into two groups: HIIT-trained individuals (Group A, n=30) and sedentary controls (Group B, n=30). Baseline characteristics such as age, gender distribution, and body mass index (BMI) were comparable between the two groups (Table 1).

Cardiac Output and Pulmonary Function

Participants in the HIIT group exhibited significantly higher cardiac output (6.3 ± 0.8 L/min) compared to the sedentary group (4.9 ± 0.7 L/min, p<0.001). Pulmonary function parameters also demonstrated notable differences between the groups. Group A showed elevated mean values in FVC (4.1 ± 0.3 L), FEV1 (3.7 ± 0.3 L), and PEFR (490 ± 30 L/min), in contrast to Group B, which showed respective values of 3.4 ± 0.4 L, 3.1 ± 0.3 L, and 420 ± 35 L/min. All differences were statistically significant (p<0.01), indicating improved respiratory efficiency in the HIIT-trained group (Table 2).

Table 1. Baseline Characteristics of Study Participants

Data are presented as mean ± standard deviation or count. Statistical significance set at p<0.05.

Table 2. Comparison of Cardiac Output and Pulmonary Function Between Groups

Statistical significance determined using unpaired t-tests.

These findings suggest that HIIT is associated with significantly enhanced cardiac and pulmonary performance in young adults (Table 2). No significant differences were observed in resting heart rate or blood pressure, implying that the cardiovascular adaptations were more exercise-induced than baseline dependent (Table 1).

This study demonstrates that regular High-Intensity Interval Training (HIIT) significantly improves cardiac output and pulmonary function in healthy young adults. The observed enhancement in cardiac output among HIIT participants supports previous evidence indicating that HIIT elicits central cardiovascular adaptations, such as increased stroke volume and improved myocardial efficiency (1,2). These changes are largely attributed to repeated exposure to high workloads, which stimulate cardiac remodeling and enhance ventricular compliance (3,4).

The notable improvements in Forced Vital Capacity (FVC), Forced Expiratory Volume in 1 second (FEV1), and Peak Expiratory Flow Rate (PEFR) in the HIIT group are consistent with earlier findings suggesting that high-intensity exercise improves respiratory muscle performance and lung elasticity (5,6). Additionally, regular interval training has been linked with increased respiratory drive and improved ventilatory efficiency, which may explain the elevated spirometric values observed in our study population (7).

While traditional aerobic training has long been used to enhance pulmonary function, HIIT offers a time-efficient alternative with potentially superior outcomes. Studies comparing moderate-intensity continuous training (MICT) and HIIT have shown greater improvements in cardiorespiratory fitness parameters, including VO₂ max and cardiac output, among HIIT participants (8,9). The greater intensity and intermittent nature of HIIT might induce a more profound physiological stress, triggering adaptations at both systemic and cellular levels (10).

Our findings align with the study by Astorino et al., which demonstrated a significant increase in cardiac output and aerobic capacity following short-term HIIT protocols in healthy adults (1). Similarly, Huang et al. reported that high-intensity training improved pulmonary vascular function and reduced pulmonary afterload in sedentary men (11). These adaptations contribute to more effective oxygen transport and utilization, thus enhancing overall cardiopulmonary performance.

In terms of clinical implications, HIIT can serve as a potent non-pharmacological strategy for improving cardiovascular health in youth, especially considering the rising prevalence of sedentary behavior and associated risk factors. Moreover, the observed pulmonary improvements suggest potential utility in preventive respiratory care, even in asymptomatic populations (12,13). This is particularly relevant in light of the increasing recognition of subclinical respiratory dysfunction among young adults leading sedentary lifestyles.

It is also important to consider the safety and accessibility of HIIT protocols. Our study employed a standardized regimen with heart rate monitoring to ensure optimal training intensity without overexertion. Such structured approaches have been shown to be both feasible and effective in similar cohorts, including patients with cardiovascular risk factors (14).

Despite these promising outcomes, the study is not without limitations. Its cross-sectional design restricts causal interpretations, and the reliance on self-reported exercise adherence may introduce recall bias. Future longitudinal or interventional studies with objective training records and larger, more diverse populations are warranted to validate and expand upon these findings. Additionally, exploring gender-specific responses and the long-term sustainability of HIIT-induced benefits could provide more tailored exercise recommendations (15).

In summary, regular engagement in HIIT significantly improves cardiac output and pulmonary function in young adults. These findings support the incorporation of HIIT into exercise regimens aimed at enhancing cardiovascular and respiratory health

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