## 4 Discussion
### Effects of Various Exercise Moderators on Aerobic Exercise Capacity in Patients with HF  
Subgroup analysis revealed that both aerobic exercise and combined exercise significantly improved aerobic capacity in HF patients, with combined exercise being the most effective. Combined exercise leads to greater improvements in vascular function compared to aerobic exercise alone [100], likely due to its promotion of NO synthesis and enhanced enzyme activity. An 8-week combined exercise program not only improves VO2 peak but also significantly increases lower limb muscle strength [101] and mitochondrial protein expression related to oxidative capacity [102]. Adding resistance exercise to aerobic exercise further improves the anaerobic threshold in HF patients [103]. Therefore, combined exercise enhances skeletal muscle oxidative capacity and vascular function, promoting further improvements in aerobic capacity.  
Subgroup analyses of intervention characteristics revealed that both interventions shorter than 60 min and those lasting 60 min or more improved aerobic capacity in HF patients, with the latter demonstrating superior effects. A 20-week exercise intervention with 60-min sessions performed for three times per week significantly improves VO2 peak in elderly HF patients [104]. Similarly, a 12-week combined exercise program with 60-minute sessions enhances 6-minute walking distance (6 MWD) and QOL in CHF patients [105]. HF patients often develop a fear of physical activity due to exercise intolerance, leading to fat accumulation and even metabolic disorders. Prolonged exercise (>60 min) increases plasma free fatty acids utilization [106], reduces inflammation, and activates the aerobic metabolic system, improving cardiorespiratory endurance. However, the inclusion of slower-paced exercises like yoga and Tai Chi, which involve extended breathing and meditation, may influence the statistical effects of session duration, and thus results should be interpreted with caution.  
Interventions performed at least three times per week significantly improved VO2 peak in HF patients, while lower-frequency interventions do not. The American College of Sports Medicine (ACSM) recommends that healthy adults engage in low-to-moderate intensity exercise five times per week [107]. Studies have shown that cycling interventions performed five times per week significantly improve VO2 peak in CHF patients [108], and 12-week walking programs performed five times per week significantly increases 6 MWD in HF patients [109]. Previous studies have confirmed that an exercise frequency of three or more times per week is safe and effective in the HF population. For HF patients, more -frequent exercise not only enhances treatment adherence and improves vascular health but also promotes positive changes in the nervous system [110].  
Calculating weekly exercise time by combining session duration and frequency showed that programs with 180 min or more per week demonstrated more significant improvements in aerobic capacity than those with less than 180 min. The World Health Organization (WHO) recommends that healthy adults engage in 150-300 min of moderate-intensity exercise per week [111]. Studies support the positive impact of weekly time exceeding 180 min on cardiopulmonary function and metabolic capacity in HF patients [112; 113]. However, daily exercise exceeding 180 min may cause excessive vagal activation and increase the risk of atrial fibrillation [114]. Therefore, combining a frequency of three or more sessions is recommended to achieve longer weekly time avoiding fatigue from excessively long sessions.