• Agreements and disagreements with other studies or reviews

Based on the results from this systematic review with meta-analysis, and in response to Research Question 1, it is not surprising that the meta-analyzed results regarding improvements in athletes' VO2max were associated with both training methodologies. According to Bartlett, O'Connor, Pitchford, Torres-Ronda, and Robertson [2] and Heyward [37], adequate prescribed training should maximize

athletic performance when the specificity, overload, progression, initial level, individualization, diminishing return, and reversibility principles are followed. However, it was also found that the individual training adaptation according to the endurance athletes' daily HRV scores produced better VO2max results than the standardized prescribed training, which answers Research Question 2. As pointed out by Vesterinen et al. [4,9], not every athlete improves their VO2max after standardized group training. Similarly, Gallo, Cormack, Gabbett, Williams, and Lorenzen [38] reported that, in footballers, the internal load (perceived effort) of each athlete was different for a given external load; this definitely affects their individual performance during training and will be reflected in their individual performance improvements. Thus, daily individual HRV monitoring and training guidance balancing the sympathetic and parasympathetic autonomic nervous system leads to greater athletic performance in endurance athletes compared to standardized prescribed training. This is relevant if training optimization is the objective, supporting the idea that training should be prescribed appropriately to avoid overtraining and/or injury [38]. In the same vein, it is also interesting to point out that, according to studies such as Hulin, Gabbett, Lawson, Caputi, and Sampson [39] and Williams et al. [16], training individualization is also related to minimizing overuse and reducing the injury risk, which may be a correlative benefit in the pursuit of endurance athlete training optimization.

On the other hand, the meta-analyzed results show that VO2max improvements were greater when the sample comprised amateur endurance athletes. This answers Research Question 3. According to the initial training level principle [37], individuals with a low initial level of physical fitness should achieve more significant relative increases than those of average or high levels. This is in accordance with the results of Sanchez-Sanchez et al. [40], where greater performance improvements were obtained in lower-level football players compared to the higher-level players, concluding that the lower the athlete's initial fitness level, the higher the available window of adaptability. Conversely, in the systematic review with meta-analysis by Hammami, Gabbett, Slimani, and Bouhlel [41], the athlete's level was not a determinant variable in terms of VO2max enhancement since it improved if they were elite or amateur players. It should be noted that this review was conducted on football players and that randomized and nonrandomized controlled trials were included.

According to our meta-analyzed results, and in response to Research Question 4, there were higher effect sizes regarding VO 2max improvements when the sample was not mixed, especially in the case of women. There is controversy concerning the influence of sex on sport performance. Recent studies conducted on endurance athletes concluded that either sex was not a predictor variable of performance [42] or that performance between men and women was different in swimming, cycling, and running [43]. In the case of the present systematic review with meta-analyses, we consider that the initial level of the sample influenced the result, given that, in the Kiviniemi\_2010 study, when female samples were analyzed, the participants were amateur level athletes. Thus, a higher relative performance increment is predictable based on the athletes' level.

#### **6. Conclusions**

#### *6.1. Practical Implications*

Training optimization to enhance performance in endurance athletes is a goal that is undergoing a constant process of improvement. Finding a procedure to objectively individualize the training would be ideal for achieving this goal. The meta-analyses results considered in this review suggest that HRV is a good indicator of physiological responses to training in endurance athletes. Consequently, using daily HRV scores for training individualization and prescription is an effective method for optimizing performance in endurance athletes. This is reflected in the improved VO2max results when the training is guided by HRV, considering VO2max as one of the main performance indicators. In addition, it should be taken into account that a lower initial athlete fitness level will be relevant in achieving greater VO2max improvement. Although gender may be a variable that influences the performance gains, in our opinion, this result is primarily conditioned by the level of the athletes included in the analyzed studies. Therefore, we do not consider it to be a variable that clearly affects VO2max improvements.

#### *6.2. Research Implications*

The results from this review suggest that, while there is evidence that HRV-guided training is effective at improving VO2max in endurance athletes, there is still work to be done in terms of identifying the characteristics of the interventions that contribute to this effect and the characteristics of participants who are more likely to respond to such interventions. The most important point is that more research is required since only five studies were included in this review. Moreover, only two of the studies used samples composed of elite endurance athletes, which gave different results regarding VO2max improvement. Consequently, the research should be extended to the professional field in order to clarify the effect of guiding training on VO2max. This would also help to clarify whether the endurance sport modality is determinative of the VO2max enhancement when following this training methodology.

Using daily HRV scores to control the training load and intensity over eight weeks is enough to improve VO2max in endurance athletes. Nonetheless, the training protocol should be further standardized in terms of adjusting the number of preparation weeks or considering the measurement weeks within or around the training period, factors that determine the training duration. Moreover, the standardized training protocol used in the control groups varied between the studies, which considered low, moderate, or high training intensities, as well as different numbers of sessions per week and session durations. This might very well have influenced the VO2max results. Therefore, it is necessary to reach a consensus regarding a standardized training protocol to use in future studies. In this line, it has been recently published a protocol [44] that could clarify the studies design. Similarly, although each study in this review used the most accurate method available to obtain the cardiorespiratory data, in the future, we should consider using a measuring instrument that allows us to implement the most specific sport technique in order to minimize result variability and imprecision.

Regarding the quality of the studies, authors should consider: improving the sequence generation or allocation concealment, the blinding of the participants, personnel, and outcome assessors, the rates of follow-up loss, using statistical procedures such as intention-to-treat to minimize attrition bias, and registering their protocols before starting the randomized controlled trial.

Lastly, to reinforce knowledge regarding performance optimization in endurance athletes, a good way to supplement the effect of HRV-guided training might be to register the risk of injuries associated with overuse using tools such as the Oslo Sports Trauma Research Center Overuse Injury Questionnaire, since this considers additional aspects affecting the execution of athletes' training.

**Author Contributions:** Research concept and study design, A.G.-G. and M.C.-P.; literature review, A.G.-G., M.C.-P., and A.G.-Q.; data collection, A.G.-G., M.C.-P., and A.G.-Q.; data analysis and interpretation, A.G.-G., M.C.-P., and A.G.-Q.; statistical analyses, A.G.-G. and M.C.-P.; writing of the manuscript, A.G.-G., M.C.-P., and D.P.; reviewing/editing the draft of the manuscript, M.C.-P. and D.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

International Journal of *Environmental Research and Public Health*
