## 4 Discussion
Existing research indicates that the effects of prolonged moderate- to high-intensity aerobic exercise on HDL levels are still inconsistent. Some studies suggest that exercise can directly increase HDL level [42], while others propose that exercise only enhances apolipoprotein expression without directly elevating the HDL level [43]. In our study, HIIT shows a significant effect on increasing HDL and decreasing LDL levels. Additionally, alternating between the two types of exercise interventions appears to have an even more pronounced effect. This highlights that the timing and intensity of exercise are two major factors influencing HDL level. However, it is important to note that noticeable changes in serum HDL level typically occur only when weekly exercise exceeds 2 h or caloric expenditure reaches over 900 kcal [44]. A key finding of our study is that exercising approximately 1.5 h per week, three times a week, can also result in more than a 5% increase in HDL levels. In terms of cardiovascular health, both high-intensity and low-intensity HIIT significantly improves outcomes compared to baseline, with HIIT performed at an intensity of over 70% of HRmax showing more pronounced benefits [45]. In terms of cardiac function, HIIT intervention led to significant improvements in both vascular contraction and relaxation [46]. We also demonstrated the effectiveness of HIIT, finding that different exercise interventions have certain improvements on blood pressure and heart rate. Specifically, HIIT was more effective in reducing blood pressure, with a reduction of nearly 9 mmHg. The MICT-LISS showed a more significant improvement in heart rate, reducing it by approximately 8 BPM. However, a comparative study between HIIT-MICT and MICT alone showed that MICT alone (5 days per week) and MICT combined with one weekly HIIT session (4 days per week) were both effective for cardiovascular health over 12 weeks, but there was no significant difference between the two approaches [47]. This is inconsistent with our research results, which may be attributed to the longer duration of our exercise intervention, highlighting the importance of exercise duration [48]. Cardiorespiratory endurance, as a crucial component of exercise enhancement [34], has been shown to improve VO2 max effectively in comparative studies of regular HIIT, full-body HIIT, and MICT [49]. Both self-directed HIIT and MICT exercises can enhance cardiovascular and respiratory function in individuals [50]. We found that alternating between different intensities, such as HIIT-LISS, MICT-LISS, and HIIT-MICT, leads to more pronounced improvements in cardiovascular fitness, particularly in VO2 max and 6 min walking distance, with a potential maximum increase of about 14% in VO2 max. In terms of lifestyle and mental health, no significant group differences were found among different intensity intervention methods, although the physiological benefits of HIIT have been confirmed in this study and others [51]. Different exercise intervention methods have a significant impact on participants' enjoyment levels, with HIIT showing a more pronounced effect compared to MICT [52]. However, there are also differing conclusions in the literature [53]. Our research also highlights that exercise interventions significantly improve participants' physical and psychological health, underscoring the crucial role of exercise interventions in fragmented time. Different exercise intervention methods have demonstrated varying effects on physical health indicators, cardiorespiratory function, and psychological health. Another significant finding from our study is that the HIIT-MICT intervention method is the most effective, showing the best improvement, compared to HIIT alone. In contrast, the HIIT-MICT-LISS and LISS-only interventions appear to be less effective. This also confirms that HIIT-MICTis more effective than single HIIT in fragmented time exercise [21].