## 4 Discussion  
This systematic review with meta-analysis aimed to determine if endurance HRV-g maximizes aerobic performance and/or aerobic physiological adaptation, compared to a PT program. The major findings indicate that HRV-g significantly improves VO2 max, maximum aerobic power or speed, performance at VT1 and VT2, and aerobic performance in running or cycling field test; however, these adaptations were not significantly greater than PT programs. Although these findings do not appear to support the use of HRV-g over PT programs, this study also highlights notable differences in the methodologies used between studies, which may impact the potential efficacy of HRV-g. Despite that both forms of training promote physiological adaptations and improve performance, most parameters related to aerobic performance and aerobic physiological indexes were improved further following HRV-based training. One of the key physiological variables that determines endurance performance is VO2 max [29], showing that high-level endurance athletes can achieve a large VO2 max [10]. Therefore, a common aim of endurance training programs is to improve VO2 max. This way, it was found how both training programs significantly increase cardiorespiratory fitness (VO2 max) (HRV-g: MD = 2.84, \(p<0.0001\); \(\sim\)5%) and PT (MD = 1.80, \(p=0.02\); \(\sim\)3%). It could be assumed that both types of training models led to improvements in VO2 max, because there were no significant differences between PT and HRV-g (\(p=0.36\)); but some factors could modulate the results obtained. Hence, the intensity training distribution and the fitness level of participants could play a key role in the VO2 max improvements. Although VO2 max has been improved in untrained [30], recreational [9,26], and trained athletes [10,22], the trainability and the increase of this parameter are limited in trained athletes [11]; specifically, only one study did not find an increase in VO2 max [11] and, remarkably, it used a trained cyclist sample. In addition, the intensity training distribution used by [PERSON] et al. [11] was a pyramidal distribution (\(\sim\)60% VT1, 30% VT2, 10% VO2 max). In another study [26] with low fitness level (recreational athletes), participants spent much more time in zone 1 (below VT1) (\(\sim\)80% of total training program) and less time in zone 3 (VO2 max or higher) (\(\sim\)3% of total training program). While only one study found an improvement in VO2 max  
Figure 8: (**a**) Effects of endurance training on WVT2 pre-training vs. post-training; (**b**) effects of endurance training on WVT2 HRV-g vs. PT. with trained athletes [10], that study applied an intensity distribution with a pyramidal distribution (-52% VT1, 35% VT2, 13% VO2 max). Thus, it seems that high intensity training (Z3) has to be higher in trained athletes than in untrained or recreational athletes; it is a fact that new periodization models are highlighted and defined as a crucial factor to increase athletes' performance [3,4]. For this reason, the use of HRV-g training could optimize the improvement in VO2 max (as the trend to higher increases in this training condition have shown; \(p=0.36\)), because if the high-intensity training is individualized and it is performed when the athlete is in optimal autonomic homeostasis, it could lead to an improved adaptive response to training [11]. Regarding aerobic performance measured by field test and maximum aerobic power or speed, both training programs led to a similar increase. Nevertheless, one of the characteristics of the HRV-g is the individualization of the training, making this strategy less variable, with fewer nonresponders than PT programs. For example, [PERSON] et al. [26] found that the HRV-g had lower dispersion of results in a 3000 m test (-1 to +6%) than the PT group (-4 to +8%). Similarly, [PERSON] et al. [10,11] reported less variation in their two studies of 40 km time trial following HRV-based training, compared to PT program.