Sedentary behaviour has become increasingly prevalent in modern societies, contributing significantly to the rise in non-communicable diseases such as obesity, type 2 diabetes, and cardiovascular disorders. Regular physical activity is known to improve cardiorespiratory fitness and metabolic health, with VO₂ max being a well-established indicator of aerobic capacity and cardiovascular function (1). However, lack of time is one of the most frequently cited barriers to engaging in regular exercise among adults (2).

High-Intensity Interval Training (HIIT), characterized by brief bouts of vigorous exercise alternated with recovery periods, has gained attention as a time-efficient and effective training strategy (3). Previous studies have demonstrated that HIIT can induce comparable or even superior improvements in VO₂ max compared to moderate-intensity continuous training (MICT), despite requiring less total exercise time (4,5). Additionally, HIIT has been associated with favourable changes in insulin sensitivity, lipid profiles, and body composition (6,7).

Despite growing evidence supporting HIIT, most studies have focused on athletic or already physically active populations. There is a need for more research examining its effectiveness in sedentary adults, who may respond differently to such high-intensity protocols (8). Furthermore, limited data exist on the combined impact of HIIT on both aerobic fitness and a comprehensive range of metabolic markers over extended intervention periods.

This study aimed to assess the effects of a 12-week HIIT program on VO₂ max and key metabolic parameters in sedentary adults, thereby evaluating its potential as a practical and efficient intervention for improving overall health.

Study Design and Participants

This 12-week randomized controlled trial was conducted to evaluate the effects of High-Intensity Interval Training (HIIT) on VO₂ max and metabolic markers in sedentary adults. Forty healthy, sedentary individuals aged 25 to 45 years were recruited through community advertisements. Inclusion criteria included self-reported physical inactivity (<30 minutes of exercise per week) for at least six months and no history of cardiovascular or metabolic disease. Participants were randomly assigned into either the HIIT group (n=20) or a control group (n=20) using computer-generated randomization.

Intervention Protocol

Participants in the HIIT group performed supervised exercise sessions four times per week. Each session consisted of a 10-minute warm-up followed by four cycles of 4-minute high-intensity cycling intervals at 85–90% of maximum heart rate (HRmax), interspersed with 3-minute active recovery periods at 60–65% HRmax. Sessions concluded with a 5-minute cooldown. Exercise intensity was monitored using heart rate monitors.

The control group was instructed to maintain their usual sedentary lifestyle and not to engage in any structured physical activity during the study period.

Outcome Measures

Primary and secondary outcomes were measured at baseline and after the 12-week intervention. VO₂ max was assessed using a graded treadmill exercise test with indirect calorimetry. Metabolic parameters including fasting blood glucose, total cholesterol, triglycerides, and high-density lipoprotein cholesterol (HDL-C) were measured from venous blood samples collected after an overnight fast.

Statistical Analysis

Data were analyzed using SPSS version 25.0. Continuous variables were presented as mean ± standard deviation (SD). Paired t-tests were used to compare pre- and post-intervention values within groups, while independent t-tests compared changes between groups. A p-value < 0.05 was considered statistically significant.

A total of 40 participants completed the study, with no dropouts or adverse events reported. Baseline characteristics were comparable between the HIIT and control groups in terms of age, body mass index (BMI), and baseline VO₂ max and metabolic parameters.

Effect on VO₂ Max

After 12 weeks, the HIIT group demonstrated a significant improvement in VO₂ max, increasing from 32.1 ± 3.8 to 39.5 ± 4.2 mL/kg/min (p < 0.001). In contrast, the control group showed no significant change (32.8 ± 4.0 to 33.1 ± 3.9 mL/kg/min, p > 0.05). The between-group difference was statistically significant (p < 0.01) (Table 1).

Effect on Metabolic Parameters

Participants in the HIIT group showed marked reductions in fasting blood glucose (102.4 ± 6.2 to 94.3 ± 5.9 mg/dL, p < 0.01), total cholesterol (198.5 ± 14.8 to 173.7 ± 12.6 mg/dL, p < 0.01), and triglycerides (152.3 ± 18.2 to 129.2 ± 15.7 mg/dL, p < 0.01), along with a significant increase in HDL-C (42.6 ± 5.1 to 46.0 ± 5.4 mg/dL, p < 0.05). The control group showed no significant changes in any of the measured markers (Table 2).

Table 1. Changes in VO₂ Max Over 12 Weeks

Table 2. Changes in Metabolic Markers Pre- and Post-Intervention

As seen in Table 1 and Table 2, the HIIT intervention resulted in significant improvements in both aerobic capacity and metabolic health markers compared to the control group.

The present study demonstrates that a 12-week high-intensity interval training (HIIT) program significantly improves cardiorespiratory fitness and several key metabolic markers in previously sedentary adults. These findings align with the growing body of literature supporting HIIT as an efficient exercise modality to enhance aerobic capacity and metabolic health, especially in time-constrained populations.

The significant improvement in VO₂ max observed in the HIIT group supports earlier studies which identified HIIT as an effective stimulus for increasing oxygen uptake and cardiovascular efficiency (1,2). The ~23% increase in VO₂ max in this study exceeds the typical improvements reported with moderate-intensity continuous training (MICT), suggesting that HIIT elicits robust physiological adaptations despite reduced time commitment (3,4). The mechanisms likely include increased mitochondrial density, enhanced capillary perfusion, and improved cardiac output (5,6).

In terms of metabolic health, significant reductions in fasting glucose, total cholesterol, and triglycerides were observed following the HIIT intervention, consistent with prior research indicating improved glycemic control and lipid metabolism in response to short-term high-intensity training (7,8). These changes can be attributed to improved insulin sensitivity, upregulation of glucose transporter-4 (GLUT-4), and enhanced lipid oxidation (9,10). In addition, a modest but statistically significant increase in HDL-C was found, echoing results from previous trials showing HDL-C improvements with HIIT (11).

Importantly, the sedentary population studied here may have experienced greater relative benefits due to their lower baseline fitness and metabolic status, a pattern observed in other intervention studies (12). The absence of adverse events highlights the feasibility and safety of implementing HIIT in a non-athletic adult population, provided proper supervision and gradual progression are ensured.

Interestingly, the control group showed no significant changes in any measured parameters, underscoring the necessity of structured physical activity for meaningful improvements in health markers. This observation reinforces public health recommendations that advocate for active interventions even in minimally active individuals (13-15).

While this study adds valuable evidence to the existing literature, certain limitations should be noted. The relatively small sample size and short duration limit generalizability and long-term inference. Furthermore, dietary intake and lifestyle variables were not strictly controlled, which may have influenced metabolic outcomes. Future studies with larger cohorts, diverse demographics, and longer follow-up periods are warranted to confirm and extend these findings.

Twelve weeks of high-intensity interval training significantly improved VO₂ max and key metabolic markers in sedentary adults. HIIT proves to be a time-efficient and effective intervention for enhancing cardiovascular fitness and metabolic health in this population.

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